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Home My WebLink AboutORDINANCE 5702ORDINANCE NO. 5702 AN ORDINANCE TO AMEND § 156.04 STORMWATER DRAINAGE AND EROSION CONTROL; CHAPTER 169 PHYSICAL ALTERATION OF LAND; CHAPTER 170 STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL, AND CHAPTER 179 LOW IMPACT DEVELOPMENT OF THE UNIFIED DEVELOPMENT CODE WHEREAS, the City's Drainage Criteria Manual has been successful in minimizing flooding in large storm events but does not address water quality, does not protect properties from damage due to smaller storm events and does not consider low impact development methods; and WHEREAS, in August 2011, the City selected and awarded a contract to FTN Associates to completely revise the City's Drainage Criteria Manual to address those concerns; and WHEREAS, adopting the new Drainage Criteria Manual will give City staff additional flexibility to allow designs that meet the intent of the code and to make changes to the Drainage Criteria Manual as technologies and methods evolve. NOW, THEREFORE, BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF FAYETTEVILLE, ARKANSAS: Section 1. That the City Council of the City of Fayetteville, Arkansas repeals § 156.04 Stormwater Drainage and Erosion Control and enacts a replacement § 156.04 Physical Alteration of Land and Stormwater Drainage and Erosion Control as shown on Exhibit "A" attached hereto. Section 2. That the City Council of the City of Fayetteville, Arkansas repeals Chapter 169 Physical Alteration of Land and enacts a replacement Chapter 169 Physical Alteration of Land as shown on Exhibit `B" attached hereto. Section 3. That the City Council of the City of Fayetteville, Arkansas repeals Chapter 170 Stormwater Management, Drainage and Erosion Control and enacts a replacement Chapter 170 Stormwater Management, Drainage and Erosion Control as shown on Exhibit "C" attached hereto. Page 2 Ordinance No. 5702 Section 4. That the City Council of the City of Fayetteville, Arkansas repeals Chapter 179 Low Impact Development and enacts a replacement Chapter 179 Low Impact Development as shown on Exhibit "D" attached hereto. Section 5. That the City Council of the City of Fayetteville, Arkansas hereby approves and adopts the revised Drainage Criteria Manual as shown on Exhibit "E" attached hereto. PASSED and APPROVED this 5th day of August, 2014. ATTEST: By: SONDRA E. SMITH, City Clerk/Treasurer 0ERK1 � TR �.�� G1T Y /•G�� AYETT V!LLE::]J= kA NSPG' ti Chris Brown Submitted By City of Fayetteville Staff Review Form 2014-0334 Legistar File ID 8/5/2014 City Council Meeting Date - Agenda Item Only N/A for Non -Agenda Item 7/18/2014 Submitted Date Action Recommendation: Engineering / Development Services Department Division / Department ADM 14-4720 Administrative Item (UPDATES TO THE DRAINAGE CRITERIA MANUAL AND UDC AMENDMENT CHAPTER 156 VARIANCES; CHAPTER 169 PHYSICAL ALTERATION OF LAND; CHAPTER 170 STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL; CHAPTER 179 LOW IMPACT DEVELOPMENT): Budget Impact: Account Number Project Number Budgeted Item? NA Current Budget Funds Obligated Current Balance Does item have a cost? No Item Cost Budget Adjustment Attached? NA Budget Adjustment Remaining Budget Fund Project Title V20140710 Previous Ordinance or Resolution # j�� Original Contract Number: Approval Date: Comments• f�lTfftE �1Z3114 y 3 CITY OF a e q1jq' ARS MEETING OF AUGUST 5T" 2014 TO: Mayor and City Council CITY COUNCIL AGENDA MEMO THRU: Don Marr, Chief of Staff Jeremy Pate, Development Services Director FROM: Chris Brown, City Engineer DATE: July 18, 2014 SUBJECT: ADM 14-4720 Administrative Item (UPDATES TO THE DRAINAGE CRITERIA MANUAL AND UDC AMENDMENT CHAPTER 156 VARIANCES; CHAPTER 169 PHYSICAL ALTERATION OF LAND; CHAPTER 170 STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL; CHAPTER 179 LOW IMPACT DEVELOPMENT RECOMMENDATION: Staff recommends approval of an ordinance to adopt the revised Drainage Criteria Manual (DCM) and updates to the Unified Development Code, Chapter 156 Variances, Chapter 169 Physical Alteration of Land, Chapter 170 Stormwater Management, Drainage, and Erosion Control, and Chapter 179 Low Impact Development BACKGROUND: The Engineering Division provides technical review and support for all development within the City including reviews for compliance with the City's storm water runoff standards and ordinances through use of the City's Drainage Criterial Manual. The current manual was developed in January 1995 and revised in 1998. Since that time, new computer design methods, drainage materials and methods, and other new technologies have made many parts of the existing manual obsolete and outdated. In August 2011, the City selected and awarded a contract to FTN Associates to completely revise the Drainage Criteria Manual including a chapter dedicated to Low Impact Development. The updated Manual better accommodates the latest engineering design practices and methods, and fully integrates LID methods into the drainage design process. DRAINAGE CRITERIA MANUAL REVISIONS: The City's current manual provides design criteria and guidance for estimating stormwater runoff quantities, for inlet and storm drainage design, culvert and bridge design, open channels, and stormwater detention for design storms larger than the 2 -year event (that event which would statistically be expected to occur once every two years). The existing manual has been a huge success in minimizing flooding in large storm events, as evidenced by the minimal flooding damage seen in areas of down that have developed since the mid -nineties. However, the existing manual does not address water quality, does not protect properties from damage due to smaller Mailing Address: 113 W. Mountain Street www.fayetteville-ar.gov Fayetteville, AR 72701 storm events, and does not consider Low Impact Development methods. The new manual addresses all of these deficiencies by establishing 4 minimum standards, as follows: 1) Minimum Standard #1 -Water Quality. This minimum standard -reduction in the average annual post -development total suspended solids loadings by 80% -is a goal rather than a requirement, in accordance with the City's stormwater permit with the Arkansas Department of Evironmental Quality (ADEQ). The manual provides guidance for meeting this standard, including using Low Impact Development strategies to capture the first 1 - inch of rainfall. If this goal as set forth by ADEQ becomes a requirement, the language of the Manual can be easily changed to comply with the new requirement. 2) Minimum Standard #2 -Channel Protection. This standard requires the capture of the increase in volume (the difference in the pre- and post -development volume) from a 1 - year storm (a 3.36 inch rainfall over 24 hours), and release of this storm over a minimum of 40 hours. This is a change from the current manual, which allows all flows up to the 2 year storm to pass directly through the detention system with no flow mitigation. This standard will more fully utilize detention ponds, many of which seldom fill up except in very large storm events, and will protect downstream properties to a much greater extent on smaller, more frequent events. 3) Minimum Standards 3 and 4, Overbank and Extreme Flood Protection. These standards are similar to the old manual, and are intended to prevent flooding to downstream properties in larger 25 to 100 year events. The major change in this standard is the removal of the "freeboard" requirement for the 100 year event. This change helps offset the potential additional cost of minimum standard #2, so the overall size of the pond will be similar to the size under the previous manual. LOW IMPACT DEVELOPMENT: The City's Low Impact Development Chapter, approved by the City Council in 2010, established goals for use of Low Impact Development, and provided guidance on specific elements that could be used on an LID project. However, the amount of runoff reduction was not quantified, and there was no guidance on how to design specific elements, and how much each element would impact the runoff on a particular project. Chapter 5 of the proposed DCM provides that guidance and design criteria, and establishes a method by which the design engineer can calculate the reduction in runoff for use in sizing the traditional storm drainage systems. In this way, the LID Chapter encourages the use of LID by providing a potential cost savings of reduced pipe and pond sizes. LID features will never replace traditional flood control systems, but the water quality benefits and mitigation of smaller storm events make LID a viable option in almost every development. ORDINANCE CHANGES: In conjunction with the new manual, staff proposes revisions to multiple chapters of the Unified Development Code (UDC) related to drainage and grading. The bulk of these changes are to remove technical requirements from the code that are now included in the DCM. This allows additional flexibility to staff to allow designs that meet the intent of the code, and to make changes to the DCM as technologies and methods evolve. Additional changes are proposed to clarify certain items and to identify ordinance intent rather than means and methods of meeting the intent. A summary of the code revisions are as follows: Chapter 156 Variances: The revisions proposed to Chapter 156 give the City Engineer the ability to administratively grant variances to Chapter 169, Physical Alteration of Land and Chapter 170, Stormwater Managements. Again, this provides flexibility to allow designs that meet the intent of these chapters but may not meet certain specific requirements. IN Chapter 169 Physical Alteration of Land: Chapter 169 is the grading ordinance. Proposed revisions to Chapter 169 are as follows: • Section 169.04 adopts the DCM. Previously the DCM was only referenced in Chapter 170. This section also clarifies that the DCM sets out erosion and sediment control requirements for all construction sites. • Sections 169.04 (A, B, D, and F) and portions of 169.06 (F and G) were removed from the ordinance because these items are addressed in the DCM. Chapter 8 of the DCM addresses requirements for sediment and erosion control on construction sites. • Items that were unnecessary in reviewing a grading plan were removed from 169.07 Grading Plan Specifications. • Requirements for master lot grading plans have been made less specific to allow for more options to achieve the goal of requiring drainage away from houses and toward the street where possible. Chapter 170 Stormwater Management, Drainage, and Erosion Control: • Section 170.05 Drainage Permit Application was relocated to 170.03 (B) and permit application packet requirements have been relocated to the DCM. • Portions of 170.04 Drainage Permit Conditions, 170.08 Maintenance Responsibility, and 170.10 Stormwater discharges From Construction Activities have been relocated to the DCM. Section 170.07 Performance Criteria: the performance criteria in the code has been revised to match the new performance criteria in the DCM. Chapter 179 Low Impact Development: The intent and general concepts presented in Chapter 179 Low Impact Development have remained; however, the technical content of the code has been removed as the DCM will be the source of low impact development guidance and criteria. A reference to Chapter 5 of the DCM is included. BUDGET/STAFF IMPACT: None Attachments: Unified Development Code Revisions: Chapter 156 Variances Chapter 169 Physical Alteration of Land; Chapter 170 Stormwater Management, Drainage, and Erosion Control Chapter 179 Low Impact Development Drainage Criteria Manual Appendix A Intrinsic GSP Specifications Appendix B GSP Specifications Appendix C Soil Infiltration and Soil Amendments Appendix D Native Plants for use in Bioretention Appendix E Detention Structural Controls Appendix F Water Quality Structural Controls Appendix G Outlet Structures Appendix H Stormwater Management Software Appendix I Construction Best Management Practices Appendix J Erosion, Runoff, and Sediment Controls ORDINANCE NO. AN ORDINANCE TO AMEND § 156.04 STORMWATER DRAINAGE AND EROSION CONTROL; CHAPTER 169 PHYSICAL ALTERATION OF LAND; CHAPTER 170 STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL, AND CHAPTER 179 LOW IMPACT DEVELOPMENT OF THE UNIFIED DEVELOPMENT CODE WHEREAS, the City's Drainage Criteria Manual has been successful in minimizing flooding in large storm events but does not address water quality, does not protect properties from damage due to smaller storm events and does not consider low impact development methods; and WHEREAS, in August 2011, the City selected and awarded a contract to FTN Associates to completely revise the City's Drainage Criteria Manual to address those concerns; and WHEREAS, adopting the new Drainage Criteria Manual will give City staff additional flexibility to allow designs that meet the intent of the code and to make changes to the Drainage Criteria Manual as technologies and methods evolve. NOW, THEREFORE, BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF FAYETTEVILLE, ARKANSAS: Section 1. That the City Council of the City of Fayetteville, Arkansas repeals § 156.04 Stormwater Drainage and Erosion Control and enacts a replacement § 156.04 Physical Alteration of Land and Stormwater Drainage and Erosion Control as shown on Exhibit "A" attached hereto. Section 2. That the City Council of the City of Fayetteville, Arkansas repeals Chapter 169 Physical Alteration of Land and enacts a replacement Chapter 169 Physical Alteration of Land as shown on Exhibit `B" attached hereto. Section 3. That the City Council of the City of Fayetteville, Arkansas repeals Chapter 170 Stormwater Management, Drainage and Erosion Control and enacts a replacement Chapter 170 Stormwater Management, Drainage and Erosion Control as shown on Exhibit "C" attached hereto. Section 4. That the City Council of the City of Fayetteville, Arkansas repeals Chapter 179 Low Impact Development and enacts a replacement Chapter 179 Low Impact Development as shown on Exhibit "D" attached hereto. Section 5. That the City Council of the City of Fayetteville, Arkansas hereby adopts the revised Drainage Criteria Manual as shown on Exhibit `B" attached hereto. PASSED and APPROVED this APPROVED: Lo LIONELD JORDAN, Mayor day of , 2014. ATTEST: LIM SONDRA E. SMITH, City Clerk/Treasurer TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 156: VARIANCES 156.01 GENERAL REQUIREMENTS............................................................................................................3 156.02 ZONING REGULATIONS..................................................................................................................3 156.03 DEVELOPMENT, PARKING AND LOADING...................................................................................3 156.04 STORMWATER DRAINAGE AND EROSION CONTROL................................................................6 156.05 SIGN REGULATIONS........................................................................................................................7 156.06 AIRPORT ZONE..............................................................................:..................................................7 156.07 LANDSCAPE REGULATIONS..........................................................................................................8 156.08-156.99 RESERVED.............................................................................................................................8 CD156:1 Fayetteville Code of Ordinances CD 156:2 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 156: VARIANCES 156.01 General Requirements (Code 1965, App. A. 5 (VII (a)); Ord. No. 2148,10-7-75; Ord. All applications for variances shall be submitted in No. 2351, 6-21-77; Ord. No. 2362, 8-2-77; Ord. No. 1747, 6- writing to the person responsible for administration of 29-70; Code. 1991, §160.038(E); Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4293, 2-20-01; Ord. 4858, 4-18-06; Ord. the referenced section. 4863, 05-02-06; Ord. 5296, 12-15-09; Ord. 5526, 9-18-12) (Ord. No. 4100, §2 (Ex. A), 6-16-98) Cross reference(s)--Administration, Ch. 152. 156.02 Zoning Regulations (A) General procedure. Variances of all, zoning regulations shall be considered and decided as shown below by the Board of Adjustment. There shall be no appeal to the City Council for zoning regulation variances. (B) Requirements for variance approval. (1) Where strict enforcement of the zoning ordinance would cause undue hardship due to circumstances unique to the individual property under consideration; and (2) Where the applicant demonstrates that the granting of the variance will be in keeping with the spirit and intent of the zoning ordinance. (C) Minimum necessary variance. The Board of Adjustment may only grant the minimum variance necessary to make possible the reasonable use of the applicant's land, building or structure. (D) Special Conditions. In granting a zoning regulation variance, the Board of Adjustment may impose whatever special conditions found necessary to ensure compliance and to protect adjacent property. (E) Non -permitted Uses. The Board of Adjustment may not grant, as a variance, any use in a zone that is not permitted under the zoning ordinance. (F) Specific Tests. The Board of Adjustment shall apply specific tests for the following variance requests: (1) Height variances in all districts. In addition to meeting all other normal requirements for a variance, an applicant seeking a height variance must establish the increased height of the proposed structure will not adversely affect adjoining or neighboring property owners, nor impair the beauty of Old Main, the historical churches on Dickson Street near East Avenue, nor otherwise impair the historic beauty and character of Fayetteville. CD156:3 156.03 Development, Parking and Loading Certain variances of the development, parking and loading regulations may be applied as follows: (A) General requirements. (1) Undue hardship. If the provisions of Development, Chapter 166, or Parking and Loading, Chapter 172 are shown by the developer to cause undue hardship as they apply to this proposed development (including, but not limited to financial, environmental, or regulatory) and that the situation is unique to the subject property, the City Planning Commission may grant a variance, on a temporary or permanent basis, to the development from such provision, so that substantial justice may be done and the public interest secured; provided that the variation will not have the effect of nullifying the intent and purpose of the development or parking and loading regulations. No variance shall be granted for any property which does not have access to an improved street. (2) Conditions and safeguards. In granting variances, the Planning Commission may prescribe appropriate conditions and safeguards to secure substantially the objectives of the standards or requirements so varied. (B) Consideration by the City Council — park land dedication. Any variation in the land dedication ratios or contribution formulas set forth in §166.04(B) shall be considered a variance and requires approval of the City Council. Upon recommendation of the Planning Commission after consultation by the commission with the Parks and Recreation Advisory Board, the City Council, upon determination that enforcement of §166.04(6) would cause unnecessary hardship, or that the problems or merits of the development reflect unique circumstances, may grant a variance of the requirements, provided: (1) Consistent with parks plan. Any dedication of land or contribution in lieu of land or combination thereof shall adequately provide for the park and recreational needs of the proposed development and be consistent with the Fayetteville Parks Plan. Fayetteville Code of Ordinances (2) Contributions of services, facilities, etc. If the developer proposes to contribute services, facilities, or equipment in lieu of a cash contribution, such a contribution shall not be accepted by the city unless the Parks and Recreation Advisory Board has been consulted and provides a recommendation as to the appropriateness and safety of such contribution. (C) Consideration by the Planning Commission. (1) Design standards. (a) Undue hardship. If the provisions of these standards are shown by the developer to cause undue hardship as they apply to his proposed development, the city Planning Commission may grant a variance to the developer from such provisions, so that substantial justice may be done and the public interest secured; provided that the variation will not have the effect of nullifying the intent and purpose of development regulations. (b) Conditions. In granting variances, the Planning Commission may impose such conditions as will, in its judgment, secure substantially the objective of the standards or requirements so varied. (2) Required off-site improvements. and the surrounding area and equally benefit said persons. (d) Improved streets or roads. The developer does not propose access to the proposed development from an existing substandard street or road, and proposes to provide access by streets or roads improved to current city or county standards. (3) Buffer strips and screening. (a) Screening. The Planning Commission shall have the authority to grant a variance from the screening requirements prescribed by §166.10. (b) Conditions. The Planning Commission may impose reasonable conditions in the granting of a variance to ensure compliance or to protect adjacent property. (4) Parking variances. (a) Number of spaces. The Planning Commission shall have the authority to vary the number of off-street parking spaces required in the Downtown Core, Main Street Center and Downtown General Districts. (b) Findings. The Planning Commission shall make findings indicating: Grounds. A developer may petition the (i) Parking generated. That the Planning Commission for a variance of off- proposed use will not generate as site improvement requirements in whole or in much parking as required under the part on one or more of the following grounds: existing standard. (a) No city plans. The city has no plans for (ii) Shared parking. That shared upgrading the substandard street or parking facilities are available; or road on which off-site improvements are proposed to be required by the (iii) On -street parking. That on -street developer. parking can satisfy intermittent and occasional demands. (b) Unfair imposition. The proposed development has primary access to (c) Conditions. All variances shall meet the improved streets or roads and the conditions listed below: portion of the development which fronts on a substandard street or road is so (i) Downtown Core, Main Street small or remote from anticipated future Center, and Downtown General traffic patterns as to cause an unfair Districts. Conditions for variances in imposition on the development. Downtown Core, Main Street Center, and Downtown General (c) Alternate off-site improvements. The Districts: developer proposes alternative off-site improvements which will protect the (a) In lieu fee. An in lieu fee of health, safety, and welfare of persons $1,200.00 for each on-site residing in the proposed development parking space shall be paid to the city. This money shall be CD 156:4 TITLE XV UNIFIED DEVELOPMENT CODE held in an interest bearing account and shall be expended for public parking facilities within the district it is collected within 10 years from the date it is collected. If said money has not been so expended within 10 years of the date collected, said money, together with the interest thereon, shall be refunded to the person or entity who made the contribution; or (b) Shared parking. For any parking space which is proposed to be shared under the provision of §172.05(C). The applicant must present a signed agreement with the owner of the property. The agreement shall address the number of spaces required for both properties, the number of spaces available together with a site plan, and any other pertinent information, such as restrictions on sharing for certain days or hours. (5) Tree preservation plan. A developer may petition the Planning Commission for a variance from the requirements of Chapter 167, Tree Preservation and Protection, in those cases where their strict application would work an injustice as applied to the proposed development due to a situation unique to the subject real property; provided that such variance shall not have the effect of nullifying the intent and purpose of the chapter. The Planning Commission's approval of said variance must be affirmed by the City Council to become effective, and a denial of the requested variance may be appealed to the City Council. (6) Flood Damage Prevention Code. The Planning Commission shall hear and decide requests for variances from the requirement of this ordinance. (a) In passing upon such applications, the Planning Commission shall consider all technical evaluations, all relevant factors, and standards specified in other sections of this ordinance. (b) Variances may be issued for the reconstruction, rehabilitation, or restoration of structures listed in the National Register of Historic Places, without regard to the procedures identified in the remainder of this CD 156:5 ordinance. Variances may only be issued for such repair, or rehabilitation if strict enforcement of the ordinance would preclude the structure's continued designation as a historic structure, and the variance is the minimum necessary to preserve the historic character and design of the structure. (c) Generally, variances may be issued for new construction and substantial improvements to be erected on a lot half acre or less in size contiguous to and surrounded by lots with existing structures constructed below the base flood level; providing items (1) through (11) of §168.03(A) have been fully considered. As the lot size increases beyond half acre, the technical justification required for issuing the variance increases. (d) Variances shall not be issued within any designated floodway if any increase in flood levels during the base flood discharge would result. (e) Variances shall only be issued upon a determination that the variance is the minimum necessary, considering the flood hazard, to afford relief. (f) Floodplain variances shall only be issued if: (i) There are exceptional or extraordinary circumstances or conditions applicable to the property involved or to the intended use of the property, which do not apply generally to other property in the same flood zone; (ii) A determination that failure to grant the variance would result in exceptional hardship to the applicant; and, (iii) A determination that the granting of a variance will not result in increased flood heights, additional threats to public safety, extraordinary public expense, create nuisances, cause fraud on or victimization of the public, or conflict with the other provisions of the Code of Fayetteville. (g) Variances may be issued for new construction and substantial improvements and for other development necessary for the conduct Fayetteville Code of Ordinances of a functionally dependent use provided that the provisions of §168.03(A) are satisfied and that the structure or other development is protected by methods that minimize flood damages during the base flood and create no additional threats to public safety. (h) Upon consideration of the factors in this section, and the purpose of this ordinance, the Planning Commission may impose conditions to the granting of floodplain variances as it deems necessary to further the purpose of this ordinance. (i) Any applicant to whom a variance is granted shall be given written notice that the structure will be permitted to be built with a lowest floor elevation below the regulatory flood elevation surcharge and that the cost of flood insurance will be commensurate with the increased risk resulting from the reduced lowest floor elevation. A copy of the notice shall be recorded by the floodplain administrator in the office of the Washington County Clerk and shall be recorded in a manner so that it appears in the chain of title of the affected parcel of land. (7) Outdoor Lighting Plan. (a) Undue Hardship. So that substantial justice may be done and the public interest secured, a developer may petition the Planning Commission for a variance from the requirements of Chapter 176: Outdoor Lighting, by showing that their strict application would cause undue hardship as applied to the proposed development; provided that such variance shall not have the effect of nullifying the intent and purpose of the chapter. (b) Conditions. In granting variances, the Planning Commission may impose such conditions as will, in its judgment, secure substantially the objectives of the requirements so varied. (8) Bicycle rack variance. The Planning Commission may modify the design standards or the requirement for a bicycle rack. (9) Streamside Protection Zones (a) Undue hardship. If the provisions of the Streamside Protection Ordinance are shown by the owner or developer to cause undue hardship as strictly applied to the owner or developer's property because of its unique characteristics, the Planning Commission may grant a variance on a permanent or temporary basis from such provision so that substantial justice may be done and the public interest protected, provided that the variance will not have the effect of nullifying the intent and purpose of the Streamside Protection regulations. (b) Consideration of alternative measures. The applicant for the variance shall establish that a reasonable rezoning by the City Council or variance request from the Board of Adjustment will not sufficiently alleviate the claimed undue hardship caused by the Streamside Protection regulations. (c) Conditions and safeguards. In granting any variance, the Planning Commission may prescribe appropriate conditions and safeguards to substantially secure the objectives and purpose for the regulations so varied and to mitigate any detrimental effects the variance may cause. The Planning Commission should consider the Streamside Protection Best Management Practices Manual and any mitigation recommendations from the City Engineer. (Ord. 4714, 6-21-05; Ord. 4930, 10-3-06; Ord. 5296, 12-15- 09; Ord. 5372, 12-7-10; Ord. 5390, 3-1-11; Ord. 5680, 4-15- 14) 156.04 Physical Alteration of Land and Stormwater Drainage And Erosion Control Variances of the requirements of Chapters 169 and 170 may be approved by the City Engineer, subject to the following criteria: CD 156:6 Criteria. Variances of the physical alteration of land and stormwater management, drainage, and erosion control regulations may be applied for as follows: (A) Chapter 169 Criteria. A variance may be granted from any requirements of the Physical Alteration of Land regulations dependent upon on the soil types encountered, planned slopes, planned vegetation, and investigative engineering reports. (B) Chapter 170 Criteria. A variance may be granted from any requirement of the stormwater management, drainage, and erosion control regulations using the following criteria: TITLE XV UNIFIED DEVELOPMENT CODE (1) Special circumstances. There are special circumstances applicable to the subject property or its intended use; and (2) Results. The granting of the variance will not result in: (a) Surface water runoff. An increase in the rate or volume of surface water runoff; (b) Adjacent property. An adverse impact on any adjacent property, wetlands, watercourse, or water body; (c) Water quality. Degradation of water quality; or (d) Objectives. Otherwise impairing attainment of the objectives of Chapters 169 and 170. (Ord. No. 4100, §2 (Ex. A), 6-16-9 156.05 Sign Regulations Consideration by the Zoning and Development Administrator. The Zoning and Development Administrator shall not grant any variance of Chapter 174, Signs, unless and until an applicant demonstrates: (A) Special conditions. That special conditions and circumstances exist which are peculiar to the land, structure, or building involved and which are not applicable to other lands, structures, or building in the same district. (B) Deprivation of rights. That literal interpretation of the provisions of the sign regulations would deprive the applicant of rights commonly enjoyed by other properties in the same district under the terms of the sign regulations. (C) Resulting actions. That the special conditions and circumstances do not result from the actions of the applicant. (D) No special privileges. That granting the variance requested will not confer on the applicant any special privilege that is denied by Chapter 174, Signs, to other lands, structures, or building in the same district. (E) Nonconforming uses. No nonconforming use of neighboring lands, structures, or buildings in the same district, and no permitted or nonconforming use of lands, structures, or buildings in other districts shall be considered grounds for the issuance of a variance. (F) Time Limitation. Any variance granted shall automatically be revoked if the applicant does not comply with the terms of the variance within 90 CD156:7 days from the granting thereof; and, the applicant shall be required to comply with the literal provisions of Chapter 174, Signs. (G) Prohibited. The Zoning and Development Administrator shall not permit as a variance any sign the erection of which or the continuance of which is prohibited by Chapter 174, nor shall any variance be granted to allow a greater number of signs than specifically set forth therein. (H) Content Neutrality; Restrictions. The Zoning and Development Administrator shall not take into account the content of any message sought to be displayed on the sign when determining whether to grant a variance. Variances can only be granted for setbacks, area, height, the proposed on-site location of the sign, or other technical requirements, and shall not exceed 15% of the Code requirement. (Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord 4652, 12-07-04) Cross reference(s)--Enforcement, Ch. 153; Signs, Ch. 174. 156.06 Airport Zone (A) Board of Adjustment. The Board of Adjustment shall have the authority to grant variances from the height limits prescribed in Chapter 165. Any person desiring to erect any structure or increase the height of any structure or permit the growth of any object of natural growth, in excess of the heights prescribed, must apply in writing to the Board of Adjustment for a variance. Such variance shall be allowed upon a showing of practical difficulty or unnecessary hardship, together with a showing that the structure or object of natural growth in question will not cause an undue hazard to aircraft operations at the airport. (B) Determination from Federal Aviation Administration. The application for a variance shall be accompanied by a determination from the Federal Aviation Administration as to the effect of a proposal on the operation of air navigation facilities and the safe, efficient use of navigable airspace. Additionally, no application for a variance may be considered by the Board of Adjustment unless a copy of the application has been furnished to the airport manager for comment as to the aeronautical effects of the variance. If the airport manager does not respond to the application within 15 days after receipt thereof, the Board of Adjustment may grant or deny said application. (C) Marking and lighting. In granting any application for any permit or variance, approval may be conditioned as to require the owner of the structure or object of natural growth in question to Fayetteville Code of Ordinances install and maintain obstruction markings or lights. (D) Findings of fact. Written findings of fact and conclusions of law shall be made by the Board of Adjustment based upon the evidence offered at the public hearing. (Ord. No. 4100, §2 (Ex. A), 6-16-98) Cross reference(s)--Notification and Public Hearings, Ch. 157. 156.07 Landscape Regulations (A) The Planning Commission shall have the authority to grant a variance from the landscaping requirements prescribed by §177. (B) Findings. The Planning Commission shall make the following findings: (1) Minimum variance. That the reasons set forth in the application justify the granting of the variance, and that the variance is the minimum variance that will make possible the reasonable use of the land, building, or structure. (2) Harmony with general purpose. The Planning Commission shall further make a finding that the granting of the variance will be in harmony with the general purpose and intent of the Landscape Regulations, §177, and will not be injurious to the neighborhood, or otherwise detrimental to the public welfare. (3) Conditions and safeguards. In granting any variance, the Planning Commission may prescribe appropriate conditions and safeguards to ensure compliance or to protect adjacent property. (4) Undue Hardship. If the provisions of the standards within Landscape Regulations, §177, are shown by the developer to cause undue hardship as they apply to his proposed development, the Planning Commission may grant a variance to the developer from such provisions, so that substantial justice may be done and the public interest secured; provided that the variation will not have the effect of nullifying the intent and purpose of development regulations. (Ord. No. 4917, 9-05-06) 156.08-156.99 Reserved CD 156:8 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 769:. PHYSICAL ALTERATION OF LAND 169.01 INTENT.............................................................................................................................................3 169.02 GENERAL REQUIREMENTS..........................................................................................................3 169.03 PERMITS REQUIRED/EXCEPTIONS.............................................................................................3 169.04 MINIMAL EROSION CONTROL REQUIREMENTS........................................................................4 169.05 ONE-TIME APPROVALS.................................................................................................................5 169.06 LAND ALTERATION REQUIREMENTS.........................................................................................5 169.07 GRADING PLAN SPECIFICATIONS.............................................................................................10 169.08 GRADING PLAN SUBMITTAL......................................................................................................11 169.09 MINOR MODIFICATIONS..............................................................................................................11 169.10 APPROVAL....................................................................................................................................11 169.11 DISCOVERY OF HISTORIC RESOURCES..................................................................................11 169.12 CERTIFICATE OF OCCUPANCY..................................................................................................11 169.13 OWNER RESPONSIBILITY...........................................................................................................11 169.14-169.99 RESERVED...........................................................:.............................................................11 CD169:1 Fayetteville Code of Ordinances CD 169:2 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 169: PHYSICAL ALTERATION OF LAND 169.01 Intent (A) Permit required. No grading, filling, excavation, or (A) It is the city's intent to safeguard the health, safety, land alteration of any kind shall take place without and welfare of Fayetteville citizens by first obtaining: implementing standards and procedures for the physical alteration of land. It is not the city's intent (1) A grading permit pursuant to this chapter to supersede federal or state regulations such as, except as specified in §169.03(B); but not limited to, the Occupational Health & Safety Act. (2) A stormwater management, drainage and erosion control permit (hereinafter referred to (B) The purpose of this chapter is to control grading, as a "drainage permit") except as specified in clearing, filling, and cutting (or similar activities) §170.03(C) and §170.03(D); and which alone or in combination cause landslides, flooding, degradation of water quality, erosion and (3) An Arkansas Department of Environmental sedimentation in storm sewer systems and water Quality Stormwater Construction Permit and storage basins. It is also the intent of this chapter incorporated Stormwater Pollution Prevention that through the implementation of the guidelines Plan, if required by state law. and regulations contained herein, the existing scenic character and quality of the neighborhood (4) A grading permit is required by the City for any and city as a whole not be diminished. development occurring within the Hillside/Hilltop Overlay District boundaries. If (Code 1991, §161.01; Ord. No. 3551, 6-4-91; Ord. No. 4100, a parcel of land is divided by the §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. 5336, 8- Hillside/Hilltop Overlay District boundary, 3-10) then only that portion of land lying within the boundary is subject to the requirements of this 169.02 General Requirements chapter. (A) Protection. Persons engaged in land alteration (B) Exceptions where no grading permit is required. activities regulated by this chapter shall take Grading permits are not required for the following: measures to protect public and private properties from damage by such activities. Adjacent and (1) Excavation below finish grade. Excavations nearby properties affected by land alterations shall below finished grade for basements, be restored in accordance with the requirements swimming pools, hot tubs, septic systems, of this Chapter. retaining walls under 4 feet in height, and like structures authorized by a valid building (B) Site conditions. Development shall generally permit. conform to the natural contours of the land, natural drainage ways, and other existing site conditions. (2) Cemetery graves. Cemetery graves. (C) Adjacent properties. All developments shall be (3) Refuse disposal. Refuse disposal sites constructed and maintained so that adjacent controlled by other regulations. properties are not unreasonably burdened with surface waters as a result of such development. (4) Single-family/duplex. Construction of one More specifically, new development may not single-family residence, or duplex not located unreasonably impede water runoff from higher within the 100 year flood plain, the properties nor may it unreasonably channel water Hillside/Hilltop Overlay District, or on a slope onto lower properties. 15 % or greater. (D) Restoration. Land shall be revegetated and (5) Building additions. Building additions of less restored as close as practically possible to its than 2,000 square feet where associated land original conditions so far as to minimize runoff and alteration activities are not beyond the scope erosion are concemed. Previously forested areas of what is necessary to construct said addition shall follow the City's Landscape Manual for and are not located within the 100 year flood mitigation of forested areas. plain, the Hillside/Hilltop Overlay District, or on a slope 15 % or greater. (Code No. 1991, §161.02; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. (6) Other minor fill or grading for maintenance 4855, 4-18-06; Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) purposes such as landscaping, yard grading, maintenance, farming, gardens, and similar 169.03 Permits Required/Exceptions activities. C D 169:3 Fayetteville Code of Ordinances (1) Where the initiation of stabilization measures (C) Grading permit application and approval. No by the 14th day after construction activity grading permit shall be issued until the grading temporarily or permanently ceases is plan, endorsed by a registered architect, precluded by snow cover, stabilization landscape architect, or engineer, is approved by measures shall be initiated as soon as the City Engineer. A separate permit shall be practicable. required for each site; it may cover both graded, developed, channeled, or physically excavations and fills. Grading permits may be (2) Where construction activity will resume on a issued jointly for parcels of land that are portion of the site within 21 days from when contiguous, so long as erosion control measures activities ceased, (e.g. the total time period are in place until project completion. Any that construction activity is temporarily application for a required grading permit under this ceased is less than 21 days) then stabilization chapter shall be submitted concurrently with the measures do not have to be initiated on that application and calculations for a drainage permit portion of the site by the 14th day after if such a drainage permit is required by §170.03., construction activity temporarily ceased. coordination with Chapter 167. Tree Preservation stormwater drainage systems to guarantee that and Protection is required. (3) Stabilization practices may include: there will be no damage from erosion or temporary seeding, permanent seeding, (D) Permit posted. A copy of the grading permit cover mulching, geotextiles, sod stabilization, page shall be posted at or near the street right -of- vegetative buffer strips, protection of trees, way line and shall be clearly visible from the street. and preservation of mature vegetation and other appropriate measures. See Chapter (Code 1991, §161.03; Ord. No. 3551, 6-5-91; Ord. No. 4100, 167 of the UDC for tree protection §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. No. requirements. 4313, 5-15-01; Ord. 4855, 4-18-06; Ord. 5336, 8-3-10) Excavation material. Excavation material shall not 169.04 Minimal Erosion Control (C) IntermittenUperennial streams. No intermittent or perennial stream, including a 25 foot perimeter Requirements strip measured from the top of the bank, shall be If exempt under 169.03, a grading permit is not graded, developed, channeled, or physically required. However, exempt as well as non-exempt altered unless adequate guarantees are made for activities shall be subject to the following minimal erosion and sedimentation control both during erosion and sedimentation control measures. construction and post construction. Likewise, cuts or fills shall be setback sufficiently from (A) The City Council hereby adopts by reference the intermittent and perennial streams and other Drainage Criteria Manual, prepared for the City of stormwater drainage systems to guarantee that Fayetteville, and adopted by Ordinance No. XXXX there will be no damage from erosion or of the City of Fayetteville and as may be amended sedimentation. Final erosion and sedimentation from time to time by the City Engineer. All minimal control measures shall be approved by the City erosion and sediment control standards contained Engineer. therein shall have the same force and effect as if printed word for word in this chapter. (D) Excavation material. Excavation material shall not Development projects also must comply with their be deposited in or so near streams and other Arkansas Department of Environmental Quality stormwater drainage systems where it may be general construction permit. All projects shall washed downstream by high water or runoff. All follow Chapter 8, Construction Site Stormwater excavation material shall be stabilized Management, of the Drainage Criteria Manual as immediately with erosion control measures. well to achieve site compliance. (E) Fording streams. Fording of streams with (B) Stabilization. A record of the dates when grading construction equipment or other activities which activities occur, when construction activities destabilize stream banks shall not be permitted. temporarily or permanently cease on a portion of the site, and when stabilization measures are (F) Debris, mud, and soil in public streets. Debris, initiated shall be included in the erosion and mud and soil shall not be allowed on public streets sediment control plan. Except as provided in (1) but if any debris, mud, or soil from development and (2) below, stabilization measures shall be sites reaches the public street it shall be initiated as soon as practicable in portions of the immediately removed via sweeping or other site where construction activities have temporarily methods of physical removal. Debris, mud, or soil or permanently ceased, but in no case more than in the street may not be washed off the street or 14 days after the construction activity in that washed into the storm drainage system. Storm portion of the site has temporarily or permanently drainage systems downstream of a development ceased. site should be protected from debris, mud, or soil CD 169:4 TITLE XV UNIFIED DEVELOPMENT CODE in the event that debris, mud, or soil reaches the (3) Existing topography. Cut or fill slopes shall be drainage system. constructed to eliminate sharp angles of intersection with the existing terrain and shall (Code 1991, §161.04; Ord. No. 3551, 6-5-91; Ord. No. 3947, be rounded and contoured to blend with the §1, 2-6-96; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, existing topography. §1, 8-18-98; Ord. 4855, 4-18-06; Ord. 5336, 8-3-10) CD169:5 (4) Setback requirements. The following setback 169.05 One -Time Approvals requirements shall be reviewed by the City Engineer for purposes of assessing safety, (A) Utilities. Public and private utility organizations stability, and drainage problems: (See may obtain a one-time approval from the City illustrations). Engineer for all routine underground electric, water, sewer, natural gas, telephone, or cable (a) Setback from top or toe of cut or fill. facilities. The approval will include a utility Buildings shall be setback from the top or organization and its contractors, agents, or toe of a cut or fill in accordance with assigns and will be permanent in nature as long as Zoning, Chapters 160 through 165; the original approved procedures are followed. Building Regulations, Chapter 173; or the approved grading plan, whichever is (B) Stockpiling materials. One-time approval may be greatest. obtained by public or private entities for the stockpiling of fill material, rock, sand, gravel, (b) Setbacks from property lines. The aggregate, or clay at particular locations, subject required setback of retaining walls, cut to Zoning, Chapters 160 through 165. slopes, and fill slopes from property lines shall be as given in the illustrations. (Code 1991, §161.05; Ord. No. 3551, 6-4-91; Ord. No. 4100, Property lines may be filled over or cut if §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10) a grading plan for the cut or fill is 169.06 Land Alteration Requirements a submitted jointly by the owner of both properties or with written permission from the adjacent property owner and if no (A) Applicability. This section shall apply to all land utility easements are involved. If utility alteration activities, including those that do not easements are involved, approval is require a grading permit. required as given in (c) below in addition to the joint submittal requirement. (B) Requirements varied. Variances of this chapter's requirements may be approved by the City (c) Setbacks from the edge of an easement. Engineer. The extent to which variations may be The required setback of retaining wall, made will depend on the soil types encountered, cut slopes, and fill slopes from the edge planned slopes, planned vegetation, and of easements shall be as given in the investigative engineering reports. In no case shall illustrations. Where no utilities are the City Engineer waive or modify any of the present in an easement, or where utilities minimum erosion control requirements as given in are planned to be relocated, and where §169.04. such action is approved by all utilities, in writing, then easements may fall within a C Cut or fill slopes. (C) p cut or fill section. (1) Finish grade. Cut or fill slopes shall have a (d) Setbacks from structures. The required finish grade no steeper than 33% (3.00 setback of retaining walls, cut slopes, horizontal to 1 vertical), unless otherwise and fill slopes from structures shall be as approved by the City Engineer. Land located given in the illustrations. If a structure within the Hillside/Hilltop Overlay District may forms an integral part of the retaining have cut or fill slopes with a finish grade no wall, then the setbacks do not apply to steeper than 50% (2.00 horizontal to 1 that structure. vertical) unless otherwise—approved by the City Engineer. (e) Calculating setbacks. For the purpose of calculating setbacks, any cut or fill (2) Maximum length. The maximum length of section which is on a slope of one to one any cut or fill slope without a terrace (as or greater shall be considered a retaining described in 169.06 (D) below) shall be 100 wall. feet as measured along the ground. The terrace shall be at least six feet (6') wide. (f) Administrative variance. Setbacks from easement lines and structures may be varied administratively by the City CD169:5 Fayetteville Code of Ordinances Engineer if geotechnical and/or structural information is provided that in the opinion of the City Engineer justifies the variance. (g) Additional information required. The City Engineer may require further geotechnical and/or structural information to show that setbacks greater than those given are not needed to protect property, utilities, or the integrity of property lines. (D) Cuts. (1) Vertical height. Cuts shall be limited to 10 feet in vertical height unless information demonstrating slope stability, erosion control, and drainage control is provided together with a re -vegetation plan. For nonsolid rock cuts, terraces shall be required for cuts greater than 10 feet in height. It is recommended that terracing be at a maximum ratio of one foot of horizontal terrace for every foot of vertical surface. (2) Maximum vertical cut. In solid rock, as determined by geotechnical and engineering data approved by the City Engineer, the maximum vertical cut shall be 30 feet. (3) Fill material. In no case shall a cut be allowed primarily for the purpose of obtaining fill material to a different site, unless the exporting site is located within an extraction district. (E) Fills. (1) Rocks1fill. All imported fill shall be free of rocks greater than 12 inches in diameter and any detrimental organic material or refuse debris. (2) Compaction. Fill shall be placed and compacted as to minimize sliding or erosion of soil. Fill compaction shall equal the compaction of undisturbed, adjacent soil, except fills covered by Building Regulations, Chapter 173, or other structural fills. The City Engineer may require soil tests during compaction work or upon its completion at the expense of the permittee. (3) Grade. Fill shall not be placed on existing slope with a grade steeper than 15% (6.67 horizontal to 1 vertical) unless keyed into steps in the existing grade and thoroughly stabilized by mechanical compaction. (4) Vertical height. Fills shall be limited to 10 feet in vertical height unless information demonstrating slope stability, erosion control, and drainage control is provided together with a re -vegetation plan. (5) Terraces. Terraces shall be required for fills greater than 10 feet in height. It is recommended that terracing be at a maximum ratio of one foot of horizontal terrace for every foot of vertical surface. (F) Erosion and sedimentation control. C D 169:6 (1) Permanent improvements. Permanent improvements such as streets, storm sewers, curb and gutters, and other features for control of runoff shall be scheduled coincidental to removing vegetative cover from the area so that large areas are not left exposed beyond the capacity of temporary control measures. (2) Phased Construction. The area of disturbance onsite at any one time shall be limited to 20 acres. An additional 20 acres (a maximum of 40 acres of disturbance at any one time) may be stripped with the permission of the City Engineer in order to balance cut and fill onsite. No additional area may be open without the permission of the City Engineer until the previously disturbed areas have been temporarily or permanently stabilized. (3) Stockpiling of top soil. Top soil may be stockpiled and protected for later use on areas requiring landscaping. All storage piles of soil, dirt or other building materials (e.g. sand) shall be located more than 25 feet from a roadway, drainage channel or stream (from top of bank), wetland, and stormwater facility. The City Engineer may also require top soil stockpiles to be located up to fifty (50) feet from a drainage channel or stream, as measured from the top of the bank to the stockpile, for established TMDL water bodies; streams listed on the State 303(d) list; an Extraordinary Resource -Water, Ecologically Sensitive Waterbody, and/or Natural and Scenic Waterbody, as defined by Arkansas Pollution Control and Ecology Commission Regulation No. 2; and/or any other uses at the discretion of the City Engineer. Topsoil piles surfaces must be immediately stabilized with appropriate stabilization measures. Stabilization practices may include: temporary seeding (i.e. annual rye or other suitable grass), mulching, and other appropriate measures. Sediment control measures such as buffer strips, wattles, or silt fence shall be provided immediately for stockpiles and remain in place until other stabilization is in place. Storm drain inlets TITLE XV UNIFIED DEVELOPMENT CODE must be protected from potential sedimentation from storage piles by silt fence or other appropriate barriers. Properly stabilized topsoil stockpiles may be used for sedimentation control. (4) Plant/water. Plant materials shall be watered or irrigated and tended. Where irrigation or regular watering is not available, only native or acclimated plant species shall be used. If the soil cannot properly sustain vegetation, it must be appropriately amended. If re - vegetation is not firmly established and healthy after one year, the urban forester shall require that it be redone in part or total. (5) Permanent erosion control. The developer shall incorporate permanent erosion control features at the earliest practical time. Temporary erosion control measures will be used to correct conditions that develop during construction that were unforeseen during the design stage, that are needed prior to installation of permanent erosion control features, or that are needed temporarily to control erosion that develops during normal construction projects, but are not associated with permanent control features on the project. (G) Percentage of land disturbance. Land disturbance percentage within the Hilltop/Hillside Overlay District shall be in accordance with the percent minimum canopy required on site per Chapter 167, Tree Preservation & Protection. (H) Required retaining wall and rock cut design. (1) Design/inspection. Any retaining wall more than four feet in height shall be designed by a registered professional engineer, and shall be field inspected by the design engineer. The design engineer shall provide proof of inspection and certify that the wall was constructed in conformance with the design. The City Engineer may require retaining walls less than four feet in height to be designed by a professional engineer. (2) Investigation/report. All proposed rock cuts and any cut or fill 10 feet or greater will require a geotechnical investigation and a formal report submitted by a registered professional engineer qualified to make such investigations. (3) Safety railings. Safety railings may be required on any retaining wall 2.5 feet or higher. The decision as to whether to require safety railing shall be based on potential pedestrian and public access to the retaining CD169:7 wall and applicable building codes. This requirement for safety rails shall also apply to vertical or near vertical rock cuts and to steep (greater than 3:1) cut or fill slopes. (Code 1991, §161.07; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. No. 4855, 4-18-06; Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) FRVPttP..V11lP. Cody of nrrlinanrac L E G E N D NOTES ----------------- EXISTING GROUND 1. WRITTEN PERMISSION FROM ADJACENT PROPERTY OWNER IS REQUIRED TO FILL OR EXCAVATE OVER EXISTING PROPERTY OR EASEMENT LINE PROPERTY LINES. 2. CONSTRUCTION LIMITS INDICATED ARE FOR INITIAL PROPOSED GROUND CONSTRUCTION; RECONSTRUCTION; OR MAINTENANCE OF CONSTRUCTION LIMITS THE STRUCTURE IN THE FUTURE. S131 MINIMUM SETBACK FROM PROPERTY LINE DOWNHILL 3. REVIEW ZONING & BUILDING SETBACK REQUIREMENTS, S132 MINIMUM SETBACK FROM PROPERTY LINE UPHILL DEFER. TO WHICHEVER. MAY BE GREATER. 4. MINIMUM SLOPE AND DISTANCE FROM THE STRUCTURE TB TIE BACK PER STATE BUILDING CODE MUST NCT BE VIOLATED. S SLOPE 5. REFER TO THE OSHA TECHNICAL MANUAL SECTION 5 Fi FOUNDATION DOWNHILL CHAPTER 2 FOR SOIL TYPES AND SLOPE EXCAVATION TO F2 FOUNDATION UPHILL DETERMINE THE LIMITS OF CONSTRUCTION. Ci LIMITS OF CONSTRUCTION—DOWNHILL C2 LIMITS OF CONSTRUCTION—UPHILL Downhill CUT SLOPE Uphill PL PL SBi = F1 +5 Downhill See RETAINING WALL CUT SLOPE PL PL C2=Limits of Construction See Note 5 SB2 = C+5' Uphill Co=Limits of Construction See Note 5 SBi = F1 +5' BLOCK RETAINING WALL SB,= C+5' CUT SLOPE Downhill Uphill CD169:8 PL SBS* j / SEE BELOW --------- SB1* FILL SLOPE SB PL 2' Flatter Than 5:1 5' 5:1 To 3:1 8' Greater Than 3:1 Downhill FILL SLOPE Uphill S131 = F; +5 Downhill RETAINING WALL FILL SLOPE C2=Limits of Construction See Note 5 SB2 = C+5' Uphill C2=Limits of Construction See Note 5 S131 = Fi +5' BLOCK RETAINING WALL S82= C+5' FILL SLOPE Downhill — Uphill TITI G X\/ I InIIFIGn nG\/pI M)KAPKIT rnnG 5132 .._ 5, Apply to ;y S8� �� Existing 5' Structures " Uphill Uphill -- 3 (max) Downhill CUT SLOPE - EXISTING STRUCTURES SB2- 2 see BELOW H Uphill - -------- ----- See S131* FILL SLOPE note 4 5' Flatter Than 3:1 (typ) 10' 3:1 To 2:1 4H/3 Greater Than 2:1 FILL SLOPE - EXISTING STRUCTURES D=5' For Existing D=2' For Proposed SB 2 S82 C2 D'* C2 5' SB SB i HF2 r/ �Fi C =Limits of See C =Limits of Fi Construction 4te noConstruction See Note 5 (typ) See Note 5 SBI = H*4/3 RETAINING WALL SB2= C+5' WITH STRUCTURES 2 SB= H RETAINING WALL SI3 - C+5' 4/3 WITH STRUCTURES 2 Downhill CUT SLOPE Uphill Downhill FILL SLOPE Uphill D=5' For Existing D=2' For Proposed 5132 'E S132 Cz 5' C2 D* SI3 SB i - ' -1 Fj Fi See C =Limits of note 4 C2=Limits of Construction (typ) Construction See Note 5 See Note 5 SBi = H*4/ 3 BLOCK RETAINING WALL SB - C+5' 2 * BLOCK RETAINING WALL SB2= C+5' SB1= H 4/3 STRUCTURES 2 WITH STRUCTURES WITH Downhill CUT SLOPE Uphill Downhill FILL SLOPE Uphill CD169:9 Fayetteville Code of Ordinances 169.07 Grading Plan Specifications (13) Underground utilities. Profiles and cross sections of streets, drainage systems, and (A) Grading plan. The applicant shall prepare a underground utilities, if they are necessary to grading plan as follows: clarify the grading plan in terms of potential erosion or runoff, or if the grading on site has (1) Site plan. Site plan at a scale no smaller than the potential of disturbing the utility line. one inch equals 50 feet, showing property lines; vicinity map; name of owner, developer (14) Treatment of slopes and benches. The and adjacent property owners. method of treatment for all slopes and benches shall be indicated. (2) Existing grades. Existing grades shall be shown with dashed line contours and proposed grades with solid line contours. Grading plans shall be required to show both the proposed grade and the undisturbed area. Contour intervals shall be a maximum of two feet. Spot elevations shall be indicated. (3) Designation of grade. Areas with 0 to 10%, 10 to 15%, 15 to 20% and more than 20% grade shall each be identified in a distinguishing manner. (4) Identify land to be disturbed. Land areas to be disturbed shall be clearly identified. (5) Engineer/architect. Seal of a registered engineer, architect, or landscape architect certifying that the plan complies with this chapter. (6) Cuts and fills. All cuts and fills, including height and slope, shall be clearly shown on the plan. (7) Streets and rights-of-way. Location and names of all existing or platted streets or rights-of-way within or adjacent to tract and location of all utilities and easements within or adjacent to the property shall all be indicated. (8) Lot/building, etc. identification. The proposed location of lots, buildings, streets, parking lots and parks, playgrounds or green space shall be indicated. Also to be indicated is any existing or proposed building within 100 feet of the site. (9) Natural features. Location of natural features such as drainage ways, ponds, rock outcroppings, and tree cover. Indication of 100 year floodplains as defined by FEMA. (10) Streets and drainage ways. Profiles and cross sections for proposed streets and drainage ways. (11) Acreage. Total acreage. (12) Surface water. Provisions for collecting and discharging surface water. [y9SL:10-Wil (15) Natural vegetation preservation. Proposals for preserving natural vegetation and description of re -vegetation or other permanent erosion control strategy. (16) Runoff/sedimentation. Specification of measures to control runoff and sedimentation during construction indicating what will be used such as straw bales, silt dams, brush check dams, lateral hillside ditches, catch basins, and the like. (17) Preliminary plat master build -out grading plan. The applicant shall prepare a master grading plan to be followed during individual lot development to convey runoff to a public drainage easement or right of way. The following shall be required for individual lot drainage design: (a) Identify lot lines and conceptual foot print of residence. (b) Indicate individual lot drainage with the use of contours and flow arrows or other indications of direction of drainage. (i) In general, drainage should be routed on the shortest practicable flow path to the public right of way or drainage easement. (c) Nonstructural grassed swales for rear lot drainage concentration is discouraged and shall not be installed in combination with a utility easement. (d) Right of way, utility easements and drainage easements shall be graded and shaped in accordance with the Master Build out Grading Plan during preliminary plat construction. (i) Utility Easements adjacent to the right of way shall be no steeper than 15%. (ii) Provisions will be considered to accommodate positive drainage until build -out occurs. TITLE XV UNIFIED DEVELOPMENT CODE (B) Preliminary grade plan. The preliminary grading plan shall have adequate detail for review. (Code 1991, §161.08; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, 8-18-98; Ord. No. 4855, 4-18-06; Ord. 5336, 8-3-10) 169.08 Grading Plan Submittal (A) Preliminary grading plan. A preliminary grading plan shall be submitted at the time of preliminary plat submission for subdivisions or plat submission for large scale development, whichever is applicable. (B) Final grading plan. No subdivision may be finalized, nor large scale development plat approved before a final grading plan has been submitted to the City Engineer and approved. The final grading plan and the final plat of land located within the Hillside/Hilltop Overlay District shall have the following plat note stating: `Property and lot owners of lands located within the Hillside/Hilltop Overlay District shall have foundation plans designed, approved and sealed by a professional architect or engineer. (C) A copy of the Stormwater Pollution Prevention Plan (SWPPP) is required to be submitted with the grading plan for sites one acre or larger. (D) In cases where neither subdivision plat, nor LSD plat is applicable, proof of notification of adjacent property owners and grading plan must be submitted simultaneously with the application for a grading permit. (Code 1991, §161.09; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4855, 4-18-06; Ord. 5308, 3-16- 10; Ord. 5336, 8-3-10) Cross reference(s)--Notification and Public Hearings, Ch. 157. 169.09 Minor Modifications Finish grades shall be allowed no more than a 0.50 foot tolerance from the grading plan. However, the City Engineer may authorize in writing minor modifications so long as they do no alter the direction of run-off and otherwise comply with the intent of this chapter. When applicable, major modifications must be brought before the Subdivision Committee for their approval. (Ord. 5336, 8-3-10) 169.10 Approval Approval of a grading permit is contingent on meeting all the requirements of this ordinance plus any set of varied requirements approved by the Planning Commission. (Code 1991, §161.10;Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, 8-18-98; Ord. 5336, 8-3- 10) 169.11 Discovery Of Historic Resources Whenever, during the conduct of grading any historical, pre -historical, or paleontological materials are discovered, grading shall cease and the City Engineer shall be notified. (Code 1991, §161.21; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10) 169.12 Certificate Of Occupancy All re -vegetation and grading plan improvements shall be in place before a certificate of occupancy shall be issued, including cleanup and restoration/revegetation of adjacent and nearby property affected by construction activities. When a property owner has finished building construction but has yet to install plant material, said owner may apply for a temporary certificate of occupancy. In evaluating whether or not to grant a temporary certificate of occupancy, the City Engineer shall consider weather conditions and temporary stabilization measures. (Code 1991, §161.15; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10, Ord. 5431, 8-16-11) 169.13 Owner Responsibility The property owner shall be responsible both for his or her employees and for all contractors and subcontractors from the onset of development until the property is fully stabilized. If property is transferred anytime between the onset of development and at the time it is fully stabilized, all responsibility and liability for meeting the terms of the chapter shall be likewise transferred to the new property owner. (Code 1991, §161.16; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10) 169.14-169,99 Reserved . ID]IMOONS TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 170:. STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL 170.01 INTENT..............................................................................................................................................3 170.02 ADOPTION OF DRAINAGE CRITERIA MANUAL...........................................................................3 170.03 PERMITS REQUIRED.......................................................................................................................3 170.04 DRAINAGE PERMIT CONDITIONS.................................................................................................4 170.05 PERMIT PROCESSING....................................................................................................................5 170.06 PERFORMANCE CRITERIA............................................................................................................5 170.07 MAINTENANCE RESPONSIBILITY.................................................................................................6 170.08 STORMWATER DISCHARGES FROM CONSTRUCTION ACTIVITIES.........................................7 170.09 PRELIMINARY PLAT, LOT REQUIREMENTS................................................................................7 170.10 1 & 2 FAMILY RESIDENTIAL REQUIREMENTS............................................................................8 170.11 STORMWATER POLLUTION PREVENTION..................................................................................8 170.12-170.99 RESERVED...........................................................................................................................9 CD170:1 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 170: STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL 170.01 Intent (A) Intent. It is the intent of this chapter to protect, maintain, and enhance the health, safety, and general welfare of the citizens of the City of Fayetteville by: (1) Preventing increases in the magnitude and frequency of stormwater runoff to prevent increases in flood flows and associated hazards and costs. (2) Controlling soil erosion and sedimentation to minimize soil deposition in streams and other receiving water bodies and storm drainage systems. (3) Requiring surface and stormwater management practices that comply with requirements of this chapter. (34) Promoting the development of stormwater facilities that are aesthetically desirable. (B) Findings of fact. The City Council finds that uncontrolled stormwater runoff from developed land adversely affects the public health, safety, and welfare because: (1) Impervious surfaces / runoff. Impervious surfaces increase the quantity and velocity of surface runoff, which reduces percolation of water through soil and increases erosion and flooding. (2) Collection and conveyance of stormwater. Improper stormwater collection and conveyance adversely affects property and increases the incidence and severity of flooding, which can endanger property and human life. (3) Erosion. Increased erosion leads to sedimentation in stormwater management systems, which decreases the system's capacity (4) Future problems. Many future problems can be avoided if land is developed in accordance with sound stormwater runoff management practices. (Code 1991, §163.03; Ord. No. 3895, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4855, 4-18-06; Ord. 4920, 9-05- 06; Ord. 5336, 8-3-10) 170.02 Adoption Of Drainage Criteria Manual The City Council hereby adopts by reference the Drainage Criteria Manual, prepared for the City of Fayetteville, and adopted by Ordinance No. XXXX of the City of Fayetteville, and as may be amended from time to time by the City Engineer. All technical procedures and design standards contained therein shall have the same force and effect as if printed word for word in this chapter. (Code 1991, §163.03; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 09-05-06; Ord. 5336, 8- 3-10) 170.03 Permits Required (A) Applicability. This chapter shall apply to all land within the corporate limits of the City of Fayetteville. No person may subdivide and develop, change to a more intensive land use, construct or reconstruct a structure, or change the size of a structure, or conduct grading, clearing, or filling activities without first obtaining a stormwater management, drainage and erosion control permit (hereinafter referred to as a "drainage permit") from the City, except as specified in §170.03(C) and §170.03 (D) below. CD170:3 (B) Permit application. Any application for a drainage permit shall be submitted to the City Engineer using appropriate forms as provided by the city for review, processing, and approval. The drainage permit application shall be submitted concurrently with the application for a grading permit, if such grading permit is required by § 169.03. A permit application shall contain sufficient information and plans to allow the City Engineer to determine whether the project complies with the requirements of this chapter. The specific items to be submitted for a permit application shall be in the form and follow the procedures as described in the Drainage Criteria Manual, Section 1.4, Drainage Report Template and Checklist. The City Engineer shall make the final determination regarding detention. (1) Fee. A nonrefundable permit application fee shall be paid when the application is submitted to help cover the cost of the plan review, administration and management of the permitting process and inspection of project implementation and operation (separate from the Physical Alteration of Land fee). (2) Issuance. If the City Engineer determines that the permit application submittal is in compliance with all provisions of this chapter, a permit may be issued. If the City Engineer Fayetteville Code of Ordinances determines that the permit submittal does not conform with all provisions of this chapter, permit issuance may be denied and a written statement as to the reasons for the denial shall be provided to the applicant. (C) Project not requiring detention. Any project that requires a drainage permit that does not require detention may receive, with the approval of the City Engineer, a grading permit prior to issuance of the drainage permit. (D) Any grading permit and/or drainage permit issued shall be subject to the following: (1) Insufficient or incomplete drainage permit application. If the drainage permit application, including the required calculations, is determined by the City Engineer to be insufficient or incomplete, it shall be revised and resubmitted by the applicant within four (4) weeks of receipt of written notice of insufficiency or incompleteness. (2) Deadline for the revised application. A stop work order for all grading on the project shall be issued by the City Engineer if a revised application is not submitted within four (4) weeks of receipt by applicant of the written notice of insufficiency or incompleteness. However, the City Engineer may delay issuance of the stop work order if the City Engineer determines that the applicant has demonstrated prior to the deadline that circumstances not reasonably foreseeable and beyond the applicant's reasonable control prevented his timely resubmission of a sufficient and complete revised drainage permit application. (E) Exceptions where no drainage permit is required. Drainage permits are not required for the following: (1) Single-family/duplex. One single-family residence or duplex. A drainage permit is not required. See Section 170.10 for building permit submittal requirements. (2) Commercialfindustrial. One commercial or industrial project built on an individual lot that is part of a larger subdivision that has been issued an approved drainage control permit when the proposed project is demonstrated to be in compliance with the overall subdivision drainage permit. (3) Existing commerciallindustrial. Existing commercial and industrial structure where additional impervious area is less than 2,000 square feet. (4) Maintenance. Maintenance or clearing activity that does not change or affect the quality, rate, volume, or location of stormwater flows on the site, or runoff from the site. (5) Agriculture. Bona fide agricultural pursuits, for which a soil conservation plan has been approved by the local Soil and Water Conservation District. (6) Emergency. Action taken under emergency conditions, either to prevent imminent harm or danger to persons, or to protect property from imminent danger of fire, violent storms, or other hazards. (F) Compliance with chapter provisions. Although a (3) Insufficient or incomplete revised application. specific permit is not required for these particular A stop work order for all grading on the project circumstances, this exception does not exempt the shall be issued by the City Engineer if the owner/developer/builder from complying with the revised application is determined by the City pollution prevention and erosion and sediment Engineer to be still insufficient or incomplete. control provisions of this chapter. (4) Stabilization and revegetation after stop work (Code 1991, §163.04; Ord. No. 3895, §1, 6-20-95; Ord. No. order. If a stop work order is issued pursuant 4100, §2 (Ex. A), 6-16-98; Ord. No. 4314, 5-15-01; Ord. 4920, to §170.03 (C) (3), the applicant shall stabilize 9-05-06; Ord. 5336, 8-3-10) and revegetate all graded and otherwise disturbed areas as set forth the Drainage 170.04 Drainage Permit Conditions Criteria Manual. Each permit issued shall be subject to the following conditions. (5) Termination of stop work order. Any stop work order issued pursuant to §170.03 (C) (2) or §170.03 (C) (3), shall expire upon the issuance of a drainage permit and compliance with any conditions contained in the drainage permit. C D 170:4 (A) Area. The development, including associated construction, shall be conducted only within the area specified in the approved permit. (B) Execution. Activities requiring a stormwater management, drainage, and erosion control permit shall not commence until the drainage permit is approved. The approved drainage permit TITLE XV UNIFIED DEVELOPMENT CODE shall be on file with the city and a copy on file with (1) Developer's expense. Building the off-site the contractor for review and inspection upon improvements at his/her own expense; request. (2) Detention. Providing detention so as to match (C) Duration. downstream capacities; or (1) Unless revoked or otherwise modified, the (3) Delay project. Delaying the project until the duration of a drainage permit issued pursuant city is able, or willing, to share in the off-site to this chapter shall be one year. costs. (2) If the permitted project is not completed prior (Code 1991, §163.07; Ord. No. 3895, §1, 6-20-95; Ord. No. to expiration, the drainage permit duration can 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8 - be extended to cover the project duration, 3-10) subject to approval of the City Engineer. 170.06 Performance Criteria (D) Modifications. If the activity authorized by the permit is not completed according to the approved (A) Performance criteria. Except as otherwise schedule and permit conditions, the City Engineer provided in this chapter, a development must be shall be notified. For revisions resulting in a designed, constructed, operated, and maintained schedule extension of more than 30 days, or if to comply with the following performance criteria: deviations from the permit conditions are expected to occur, approval of a permit modification is (1) Water Quality. Where practicable, reduce the required by the City Engineer. average annual post -development total suspended solids loadings by 80%. (E) Transfer. No transfer, assignment, or sale of the rights granted by virtue of an approved permit shall (2) Channel Protection. Capture the increased be made without prior written approval from the volume of the 1 year, 24-hour storm and City Engineer. release it over an extended period of time. (F) Special. Any additional special conditions, as (3) Overbank and Extreme Flood Protection. The deemed appropriate by the City Engineer, shall be post -development peak rate of surface established to address specific project needs or discharge must not exceed the existing circumstances. discharge for the 100 year, 24-hour storm; the 25 year, 24-hour storm; the 10 year, 24-hour (Code 1991, §163.05; Ord. 3895, §1, 6-20-95; Ord. No. 4100, storm; the 5 -year, 24-hour storm; and the 2 §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8-3-10) year, 24-hour storm, unless other discharge limits are deemed applicable for a specific site by the City Engineer. 170.05 Permit Processing (4) Low Impact Development. Use of Low Impact (A) Upon reviewed by the City Engineer, if it is Development design strategies, as described determined according to present engineering in the Drainage Criteria Manual, to attenuate practice that the proposed development will lesser storms and more closely mimic provide control of stormwater runoff in accordance predevelopment hydrology is encouraged. with the purposes, design criteria, and performance standards of these regulations and (5) Direct Discharge. Direct Discharge of a pipe will not be detrimental to the public health, safety, into streams and/or floodways is not allowed. and general welfare, the City Engineer shall A stilling basin or other structure that will approve the plan or conditionally approve the plan, collect sediment, trash, etc and that will setting forth the conditions thereof. reduce the likelihood of erosion in the receiving stream due to discharge from the (B) Off-site improvements. If it is determined that pipe shall be installed at pipe discharges into offsite drainage improvements are required, and streams and/or floodways. that such specific off-site drainage improvements are consistent with the city's current and (6) Erosion and channel stability. All stormwater established priorities, then cost sharing will be in management systems shall be evaluated accordance with "Required Off-site based on their ability to prevent erosion and Improvements." If the city is unable, or unwilling, sedimentation of the receiving waters and to contribute its share of the off-site costs, the adverse impacts on the site's natural developer shall have the option of: systems. The design engineer shall consider the on-site and downstream effects of the peak discharges and shall design both the CD170:5 Fayetteville Code of Ordinances permanent and the construction phase of the (1) Enforcement of the Lien. The lien herein stormwater management system in a manner provided for may be enforced and collected in that will not increase flooding, channel either one of the following manners: instability, or erosion downstream when considered in aggregate with other developed (a) The lien may be enforced at any time properties and downstream drainage within 18 months after work has been capacities. done, by an action in circuit court; or (7) Drainage Criteria Manual. The technical (b) The amount of the lien herein provided procedures and design standards contained may be determined at a hearing before in the Drainage Criteria Manual, prepared for the City Council held after 30 days written the City of Fayetteville, and adopted by this notice by certified mail to the owner or chapter and as may be amended from time to owners of the property, if the name and time by the City Engineer, shall be used for whereabouts of the owner or owners be guidance to determine compliance with the known, and if the name of the owner or performance criteria established by this owners cannot be determined, then only chapter. after publication of notice of such hearing in a newspaper having a bona fide (Code 1991, §163.08; Ord. No. 3895, §1, 6-20-95; Ord. No. circulation in Washington County for one 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8- insertion per week for four consecutive 3-10) weeks; the determination of the City Council shall be subject to appeal by the 170.07 Maintenance Responsibility P y property owner in circuit court; and the amount so determined at said hearing, (A) Dedication. Those stormwater management plus ten percent penalty for collection, systems approved in compliance with this chapter shall be by the City Council certified to that will function as a part of the stormwater the tax collector of the county, and by him management conveyance system shall be placed on the tax books as delinquent dedicated to the city. All areas and/or structures to taxes, and collected accordingly, and the be dedicated to the city must be dedicated by plat amount, less three percent thereof, when or separate instrument and accepted by the City so collected shall be paid to the city by Engineer. the county tax collector. (B) Maintain Stormwater Systems and Structures. (c) In case the owner of any lot or other real The owner of the property on which stormwater property is unknown or his whereabouts systems and structures have been installed shall is not known or he is a nonresident of this maintain in good condition and promptly repair and state, then a copy of the written notice restore all grade surfaces, walls, drains, dams and hereinabove referred to shall be posted structures, vegetation, erosion and sedimentation upon the premises and before any action controls, and other protective devices. to enforce such lien shall be had, the City Clerk shall make an affidavit setting out (C) Right -of -Entry for Inspection. The owner shall the facts as to unknown address or provide for the City Engineer or designee to enter whereabouts or non -residence, and the property at reasonable times and in a thereupon service of the publication as reasonable manner for the purpose of inspecting now provided for by law against stormwater systems and structures. nonresident defendants may be had, and an attorney ad litem may be appointed to (D) Failure to Maintain. If a responsible person fails or notify the defendant by registered letter refuses to meet the maintenance requirements the addressed to his last known place of City may give written notice requesting corrective residence if same can be found. action. If the conditions described in the Failure to Maintain notice are not corrected within 10 days (E) Removal and modification of Stormwater Systems after such notice is given, the mayor, or his duly and Structures. Stormwater systems and authorized representative, is hereby authorized to structures may only be modified or removed with enter upon the property and do whatever is the approval of the City Engineer, who shall necessary to correct or remove the conditions determine whether the stormwater system or described, in the notice. The costs of correcting structure does not function as a part of the said conditions shall be charged to the owner or stormwater management system. The applicant owners of the property and the city shall have a may be required to provide supporting data and lien against such property for such costs. calculations that justify the removal of the stormwater systems or structures. C D 170:6 TITLE XV UNIFIED DEVELOPMENT CODE (Code 1991, §163.09; Ord. No. 3895, §1, 6-20-95; Ord. No. (6) Concrete Truck Wash Areas. No washing of 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8- concrete trucks or chutes is allowed except in 3-10; Ord. 5431, 8-16-11) properly located and constructed concrete Cross reference(s)--Enforcement, Ch., 153; Appeals, Ch. wash pits. Proper runoff and erosion controls 155, Variances, Ch. 156; Bonds and Guarantees, Ch. 158; must be in place to retain all concrete wash Fees, Ch. 159. water. 170.8 Stormwater Discharges From Construction Activities (A) General Requirements for Construction Sites. (1) Construction Site. A construction site is a site with activity that would result in the creation of a new stormwater management system, including the building, assembling, expansion, modification, or alteration of the existing contours of the property; the erection of buildings or other structures, any part thereof; or land clearing. (2) Owner Responsibility. The owner of a site of construction activity shall be responsible for compliance with the requirements of this chapter. (3) Erosion And Sediment Control. Best Management Practices (BMPs) shall be implemented to prevent the release of airborne dust and waterborne sediment from construction sites. Disturbed areas shall be minimized, disturbed soil shall be managed and construction site entrances/exits shall be managed to prevent sediment tracking. Streets and storm inlets must be kept clean at all times and free of loose rock, mud, debris and trash. Specific inlet protection measures may be necessary, as long as they do not interfere with vehicular traffic. Mud on streets must be physically removed and not washed into inlets. (4) Construction Sites Requiring Storm Water Pollution Prevention Plans. Erosion and sediment control systems must be installed and maintained per a state approved Storm Water Pollution Prevention Plan (SWPPP) before the beginning of construction and until slope stabilization and/or vegetation is established. For sites between 1 and 5 acres, the SWPPP and Notice of Intent (NOI) must be onsite at all times. For sites over 5 acres, the SWPPP and Notice of Coverage must be onsite at all times. The site owner bears responsibility in accordance with the Arkansas Department of Environmental Quality standards and general permit. (5) Construction Exits. A stabilized rock exit is required on construction sites. (7) Dewatering. All rainwater pumped out of sumps and depressions on construction sites should be clear and free of sediment. (8) Storage of Materials. Public streets and sidewalks shall not be used for temporary storage of any containers or construction materials, especially loose gravel and topsoil. In addition to on -street storage being a violation of this chapter, all liability for any accidents and/or damages due to such storage will be the responsibility of the owner of the stored materials. (9) Dirt and Topsoil Storage. All storage piles of soil, dirt or other building materials (e.g. sand) shall be located more than 25 feet from a roadway, drainage channel or stream (from top of bank), wetland, and stormwater facility. Topsoil piles surfaces must be immediately stabilized with appropriate stabilization measures (10) Franchise and Private Utilities. The property owner or main contractor onsite will be responsible for restoring all erosion and sediment control systems and public infrastructure damaged or disturbed _ by underground private or franchise utility construction such as water and sewer service leads, telephone, gas, cable, etc. Erosion and sediment control systems must be immediately restored after each utility construction. (Ord. 4920, 9-05-06; Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) y1SKIAr1 170.09 Preliminary Plat, Lot Requirements (A) Preliminary Plats. Preliminary Plats shall include a master drainage plan for each lot related to the proposed infrastructure and adjacent lots. (B) Preliminary Plats for Residential Subdivisions. Preliminary Plats for residential subdivisions shall provide drainage information meeting the Arkansas Fire Prevention Code for building safety regulations for positive drainage of each lot. (1) The applicant shall prepare a master grading plan to be followed during individual lot Fayetteville Code of Ordinances development to convey runoff to a public drainage easement or right of way. (2) Right of way, utility easements and drainage easements shall be graded and shaped in accordance with the Master Build out Grading Plan during preliminary plat construction. (C) Rear lot drainage easements. Rear lot drainage easements for nonstructural grassed swales shall not overlap utility easements with above ground structures, ie, electric transformers, gas meters, communication junctions, etc. (D) Final Plat. The Final Plat shall include the approved master drainage plan to be filed as a supplemental document. The scale shall be legible and approved by the City Engineer. (Ord. 5336, 8-3-10) 170.10 1 & 2 Family Residential Requirements (A) 1 &2 Family Residential and Sites under One Acre. All residential lots must maintain properly installed erosion and sediment control measures from the beginning of construction until slope stabilization and/or vegetation is established in order to prevent silt and sediment from going offsite or into the street. (B) A building permit application shall contain sufficient site drainage and grading information to determine whether the project complies with the requirements of this chapter and Chapter 169, including, but not limited to: (1) Locations and types of proposed stormwater and erosion control BMPs. (2) Lot lines and conceptual foot print of building. (3) Individual lot drainage features, using contours and flow arrows. (C) If the Final Plat of the Subdivision, in which the proposed building is located, includes an approved master drainage plan, this plan shall be included in the building permit application and the individual lot drainage plan shall follow the master drainage plan unless otherwise approved by the City Engineer. (Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) 170.11 Post Construction Stormwater Management (A) Prohibitions (1) Illicit discharges are prohibited. An illicit discharge is a storm drain that has measurable flow containing pollutants and/or pathogens. No person shall discharge anything but uncontaminated stormwater, into the storm drainage system. Common stormwater contaminants include trash, yard waste, wastewater, oil, petroleum products, cleaning products, paint products, hazardous waste and sediment. (2) Illicit connections are prohibited. Illicit connections are any drain or conveyance which allows an illicit discharge to enter the storm drainage system. This prohibition includes illicit connections made in the past, regardless of whether the connection was permissible at the time of connection. (3) No person shall connect a line conveying sanitary sewage, domestic sewage or industrial waste, to the storm drainage system, or allow such a connection to continue. (B) Exemptions. The following non-stormwater discharges are acceptable and not a violation of this chapter: CD170:8 (1) A discharge authorized by an NPDES permit other than the NPDES permit for discharges from the MS4; (2) Uncontaminated waterline flushing and other infrequent discharges from potable water sources; (3) Infrequent uncontaminated discharge from landscape irrigation or lawn watering; (4) Discharge from the occasional non- commercial washing of vehicles within zoned residential areas; (5) Uncontaminated discharge from foundation, footing or crawl space drains, sump pumps and air conditioning condensation drains; (6) Uncontaminated groundwater; (7) Stream flows and natural riparian habitat or wetland flows; (8) A discharge or flow of fire protection water that does not contain oil or hazardous substances or materials. (9) De -chlorinated swimming pool water. (10) Any other non-stormwater discharge determined by the City Engineer to meet the TITLE XV UNIFIED DEVELOPMENT CODE standards and objectives of this chapter and prevent trash, debris, excessive vegetation, of the City's NPDES MS4 permit. and other obstacles from their property from entering the drainage channel or obstructing (C) Private Drainage Systems flow. (1) Private Drainage System Maintenance. A (D) Release Reporting and Cleanup. Any person private drainage system includes responsible for a release of materials which are or groundwater, drainage pipes or channels, and may result in illicit discharges to the storm any flowing or standing water not within a drainage system shall take all necessary steps to Right of Way or Drainage Easement. The ensure the discovery, containment, abatement owner of any private drainage system shall and cleanup of such release. In the event of such maintain the system to prevent or reduce the a release of a hazardous material, said person discharge of pollutants. This maintenance shall comply with all state, federal, and local laws shall include, but is not limited to, sediment requiring reporting, cleanup, containment, and any removal, bank erosion repairs, maintenance other appropriate remedial action in response to of vegetative cover, and removal of debris the release. from pipes and structures. (E) Authorization to Adopt and Impose Best (2) Minimization of Irrigation Runoff. Management Practices. The City may adopt and Concentrated flow of irrigation water to the impose a Best Management Practices Manual and storm drainage system is prohibited. Irrigation requirements identifying Best Management systems shall be managed to reduce the Practices for any activity, operation, or facility, discharge of water from a site. which may cause a discharge of pollutants to the storm drainage system. Where specific BMPs are (3) Cleaning of Paved Surfaces Required. The required, every person undertaking such activity or owner of any paved parking lot, street or drive operation, or owning or operating such facility shall shall clean the pavement as required to implement and maintain these BMPs at their own prevent the buildup and discharge of expense. pollutants. The visible buildup of mechanical fluid, waste materials, sediment or debris is a (Ord. No. 4855, 4-18-06; Ord. 4920, 9-05-06; Ord. 5336, 8 -3 - violation of this chapter. Paved surfaces shall 10) be cleaned by dry sweeping, wet vacuum sweeping, collection and treatment of wash 170.12-170.99 Reserved water or other methods in compliance with this Code. Material shall not be swept or washed into the storm drainage system. This section does not apply to pollutants discharged from construction activities. (4) Maintenance of Equipment. Any leak or spill related to equipment maintenance in an outdoor, uncovered area shall be contained to prevent the potential release of pollutants. Vehicles, machinery and equipment must be maintained to reduce leaking fluids. (5) Materials Storage. In addition to other requirements of this Code, materials shall be stored to prevent the potential release of pollutants. The uncovered, outdoor storage of unsealed containers of hazardous substances is prohibited. (6) Pesticides, Herbicides and Fertilizers. Pesticides, herbicides and fertilizers shall be applied in accordance with manufacturer recommendations and applicable laws. Excessive application shall be avoided. (7) Open Drainage Channel Maintenance. Every person owning or occupying property through which an open drainage channel passes shall C D 170:9 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 179: LOW IMPACT DEVELOPMENT 179.01 PURPOSE..........................................................................................................................................3 179.02 APPLICABILITY.................................................................................................................................3 179.03 LID SITE DESIGN STRATEGIES......................................................................................................3 179.04 MAINTENANCE OF LID SYSTEMS AND STRUCTURES................................................................4 179.05-179.99 RESERVED.............................................................................................................................4 CD179:1 Fayetteville Code of Ordinances C D 179:2 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 179: LOW IMPACT DEVELOPMENT 179.01 Purpose Some of the existing natural site features typically protected through the use of LID techniques are: wetlands, floodplains, forested areas, hillsides, riparian corridors and existing soils. There are a variety of LID design alternatives that allow professionals in the land development field the flexibility to implement LID stormwater design elements. The various LID practices can be used alone or in series to maximize benefits to the site. In most cases, some type of structural drainage systems will also be required to be implemented in conjunction with LID element. (A) Objectives. The objectives of this ordinance are (1) To establish criteria by which a LID strategy can be measured and implemented through use of the criteria in the LID chapter of the Drainage Criteria Manual. (2) To strive to maintain and restore natural rainwater absorption and infiltration processes; (3) To strive to maintain pre -development hydrologic conditions; (4) To filter pollutants from stormwater runoff thereby improving water quality and positively impacting the region's lakes, streams and groundwater; (5) To reduce stormwater runoff intensity and velocity; (6) To preserve riparian banks and beds, and reduce sedimentation that impairs water quality; (7) To promote the widespread use of LID practices integrated with conventional stormwater engineering; (8) To protect the safety and welfare of citizens, property owners, and businesses by minimizing the negative impacts of stormwater discharge from land development. (B) LID Principles. (2) Minimize impervious surfaces such as streets, driveways and parking areas. (3) Minimize direct connection of impervious areas which convey runoff directly to wetlands or water courses. (4) Attenuate stormwater flow through a diverse system of.collection and infiltration. (Ord. 5316, 4-20-10) 179.02 Applicability (A) Development approval. The standards and guidelines contained in Chapter 5 of the Drainage Criteria Manual shall apply in all cases where a land developer chooses to utilize LID to obtain Administrative, Planning Commission or City Council approval for their project. (B) Engineering approval. The City Engineer, or their designee, will administer this chapter and shall be responsible for final approval of all LID systems and structures. With the approval of the City Engineer, LID systems and structures may be implemented. (C) Drainage Criteria Manual. The Drainage Criteria Manual integrates LID design principles throughout the manual. Submittal requirements for LID projects are found within the submittal requirements for a drainage report. Criteria for the design of specific LID elements, criteria for receiving credit for those elements through reductions in traditional stormwater infrastructure, and maintenance requirements are detailed in Chapter 5 of the DCM. (Ord. 5316, 4-20-10) 179.03 LID Site Design Strategies (A) Definition. For the purposes of this chapter Low Impact Development (LID) is a stormwater management strategy concerned with maintaining, restoring or replicating the natural hydrologic functions of a site, where possible, by employing a variety and combination of natural and built features that reduce the volume and velocity of stormwater runoff, filter out its pollutants, and facilitate the infiltration of water into the ground. (1) Define and locate critical resource areas (B) Site design strategies. Generally, site design during the project planning stage, such as; strategies will address the arrangement of wetlands, riparian zones and soils with buildings, roads, parking areas, and other infiltration capacities. features, and the conveyance of stormwater runoff across the site. LID site design strategies are intended to complement the natural and built CD 179:3 Fayetteville Code of Ordinances environment while minimizing the generation of runoff. Site design strategies should address some or all of the following considerations: (1) Necessary grading and land disturbance should be designed to encourage sheet flow and lengthen stormwater flow paths. (2) Natural drainage divides should be maintained to keep flow paths dispersed. (3) Areas of impervious surfaces should be separated and stormwater should be conveyed across vegetated areas. This assists runoff filtration and encourages infiltration. (4) Distribute small-scale LID strategies across the development site in order to maximize benefits. (5) To the maximum extent possible, treat pollutant loads where they are generated. (6) Preserve naturally vegetated areas and soil types that slow runoff, filter pollutants and facilitate infiltration. (7) LID systems and structures should be integrated into the natural and built landscape with attention to flow paths, infiltration areas and the use of appropriate native plant materials. (C) Site Design Elements. In addition to water quality impacts, LID site design elements when successfully implemented, perform three necessary functions; filtration and infiltration, capture and re -use and reductions in impervious surfaces. 179.04 Maintenance of LID Systems and Structures (A) Removal and modification of LID systems and structures. LID systems and structures may only be modified or removed with the approval of the City Engineer, who shall determine the LID system or structure does not function as a part of the stormwater management system. The applicant may be required to provide supporting data and calculations that justify the removal of the LID systems or structures. (B) Exemptions from maintenance agreements and inspections. LID systems and structures that are not designed as part of a development and are instead utilized on a site by site basis (i.e., use of a rain barrel at a single family home, or individual rain gardens or filter strips on a site) shall not be required to submit a formal maintenance and inspection agreement, unless the function of the C D 179:4 LID system or structure is found to be essential to accommodating the stormwater needs of the property or surrounding properties by the City Engineer. 179.05-179.99 Reserved TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 156: VARIANCES 156.01 GENERAL REQUIREMENTS ............................................... 156.02 ZONING REGULATIONS...................................................... 156.03 DEVELOPMENT, PARKING AND LOADING ....................... 156.04 STORMWATER DRAINAGE AND EROSION CONTROL... 156.05 SIGN REGULATIONS........................................................... 156.06 AIRPORT ZONE.................................................................... 156.07 LANDSCAPE REGULATIONS ............................................. 156.08-156.99 RESERVED................................................................ CD156:1 ...................................................... 3 ...................................................... 3 ...................................................... 3 ...................................................... 6 ...................................................... 7 ...................................................... 7 ...................................................... 8 ...................................................... 8 Fayetteville Code of Ordinances CD156:2 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 156: VARIANCES 156.01 General Requirements (Code 1965, App. A. 5 (VII (a)); Ord. No. 2148, 10-7-75; Ord. All applications for variances shall be submitted in No. 2351, 6-21-77; Ord. No. 2362, 8-2-77; Ord. No. 1747, 6- writing to the person responsible for administration of 29-70; Code. 1991, §160.038(E); Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4293, 2-20-01; Ord. 4858, 4-18-06; Ord. the referenced section. 4863, 05-02-06; Ord. 5296, 12-15-09; Ord. 5526, 9-18-12) (Ord. No. 4100, §2 (Ex. A), 6-16-98) Cross reference(s)--Administration, Ch. 152. 156.02 Zoning Regulations (A) General procedure. Variances of all zoning regulations shall be considered and decided as shown below by the Board of Adjustment. There shall be no appeal to the City Council for zoning regulation variances. (B) Requirements for variance approval. (1) Where strict enforcement of the zoning ordinance would cause undue hardship due to circumstances unique to the individual property under consideration; and (2) Where the applicant demonstrates that the granting of the variance will be in keeping with the spirit and intent of the zoning ordinance. (C) Minimum necessary variance. The Board of Adjustment may only grant the minimum variance necessary to make possible the reasonable use of the applicant's land, building or structure. (D) Special Conditions. In granting a zoning regulation variance, the Board of Adjustment may impose whatever special conditions found necessary to ensure compliance and to protect adjacent property. (E) Non -permitted Uses. The Board of Adjustment may not grant, as a variance, any use in a zone that is not permitted under the zoning ordinance. (F) Specific Tests. The Board of Adjustment shall apply specific tests for the following variance requests: (1) Height variances in all districts. In addition to meeting all other normal requirements for a variance, an applicant seeking a height variance must establish the increased height of the proposed structure will not adversely affect adjoining or neighboring property owners, nor impair the beauty of Old Main, the historical churches on Dickson Street near East Avenue, nor otherwise impair the historic beauty and character of Fayetteville. 156.03 Development, Parking and Loading Certain variances of the development, parking and loading regulations may be applied as follows: C D 156:3 (A) General requirements. (1) Undue hardship. If the provisions of Development, Chapter 166, or Parking and Loading, Chapter 172 are shown by the developer to cause undue hardship as they apply to this proposed development (including, but not limited to financial, environmental, or regulatory) and that the situation is unique to the subject property, the City Planning Commission may grant a variance, on a temporary or permanent basis, to the development from such provision, so that substantial justice may be done and the public interest secured; provided that the variation will not have the effect of nullifying the intent and purpose of the development or parking and loading regulations. No variance shall be granted for any property which does not have access to an improved street. (2) Conditions and safeguards. In granting variances, the Planning Commission may prescribe appropriate conditions and safeguards to secure substantially the objectives of the standards or requirements so varied. (B) Consideration by the City Council — park land dedication. Any variation in the land dedication ratios or contribution formulas set forth in §166.04(6) shall be considered a variance and requires approval of the City Council. Upon recommendation of the Planning Commission after consultation by the commission with the Parks and Recreation Advisory Board, the City Council, upon determination that enforcement of §166.04(6) would cause unnecessary hardship, or that the problems or merits of the development reflect unique circumstances, may grant a variance of the requirements, provided: (1) Consistent with parks plan. Any dedication of land or contribution in lieu of land or combination thereof shall adequately provide for the park and recreational needs of the proposed development and be consistent with the Fayetteville Parks Plan. Fayetteville Code of Ordinances and the surrounding area and equally (2) Contributions of services, facilities, etc. If benefit said persons. the developer proposes to contribute services, facilities, or equipment in lieu of a (d) Improved streets or roads. The cash contribution, such a contribution shall developer does not propose access to not be accepted by the city unless the Parks the proposed development from an and Recreation Advisory Board has been existing substandard street or road, and consulted and provides a recommendation proposes to provide access by streets or as to the appropriateness and safety of such roads improved to current city or county contribution. standards. (C) Consideration by the Planning Commission. (3) Buffer strips and screening. (1) Design standards. (a) Screening. The Planning Commission shall have the authority to grant a (a) Undue hardship. If the provisions of variance from the screening these standards are shown by the requirements prescribed by §166.10. developer to cause undue hardship as they apply to his proposed development, (b) Conditions. The Planning Commission the city Planning Commission may grant may impose reasonable conditions in a variance to the developer from such the granting of a variance to ensure provisions, so that substantial justice compliance or to protect adjacent may be done and the public interest property. secured; provided that the variation will not have the effect of nullifying the intent (4) Parking variances. and purpose of development regulations. (a) Number of spaces. The Planning Commission shall have the authority to (b) Conditions. In granting variances, the vary the number of off-street parking Planning Commission may impose such spaces required in the Downtown Core, conditions as will, in its judgment, Main Street Center and Downtown secure substantially the objective of the General Districts. standards or requirements so varied. (b) Findings. The Planning Commission (2) Required off-site improvements. shall make findings indicating: Grounds. A developer may petition the (i) Parking generated. That the Planning Commission for a variance of off- proposed use will not generate as site improvement requirements in whole or in much parking as required under the part on one or more of the following grounds: existing standard. (a) No city plans. The city has no plans for (ii) Shared parking. That shared upgrading the substandard street or parking facilities are available; or road on which off-site improvements are proposed to be required by the (iii) On -street parking. That on -street developer. parking can satisfy intermittent and occasional demands. (b) Unfair imposition. The proposed development has primary access to (c) Conditions. All variances shall meet the improved streets or roads and the conditions listed below: portion of the development which fronts on a substandard street or road is so (i) Downtown Core, Main Street small or remote from anticipated future Center, and Downtown General traffic patterns as to cause an unfair Districts. Conditions for variances in imposition on the development. Downtown Core, Main Street Center, and Downtown General (c) Alternate off-site improvements. The Districts: developer proposes alternative off-site improvements which will protect the (a) In lieu fee. An in lieu fee of health, safety, and welfare of persons $1,200.00 for each on-site residing in the proposed development parking space shall be paid to the city. This money shall be CD156:4 TITLE XV UNIFIED DEVELOPMENT CODE held in an interest bearing ordinance. Variances may only be account and shall be expended issued for such repair, or rehabilitation if for public parking facilities strict enforcement of the ordinance within the district it is collected would preclude the structure's continued within 10 years from the date it designation as a historic structure, and is collected. If said money has the variance is the minimum necessary not been so expended within to preserve the historic character and 10 years of the date collected, design of the structure, said money, together with the interest thereon, shall be (c) Generally, variances may be issued for refunded to the person or entity new construction and substantial who made the contribution; or improvements to be erected on a lot half acre or less in size contiguous to and (b) Shared parking. For any surrounded by lots with existing parking space which is structures constructed below the base proposed to be shared under flood level, providing items (1) through the provision of §172.05(C). (11) of §168.03(A) have been fully The applicant must present a considered. As the lot size increases signed agreement with the beyond half acre, the technical owner of the property. The justification required for issuing the agreement shall address the variance increases. number of spaces required for both properties, the number of (d) Variances shall not be issued within any spaces available together with designated floodway if any increase in a site plan, and any other flood levels during the base flood pertinent information, such as discharge would result. restrictions on sharing for certain days or hours. (e) Variances shall only be issued upon a determination that the variance is the (5) Tree preservation plan. A developer may minimum necessary, considering the petition the Planning Commission for a flood hazard, to afford relief. variance from the requirements of Chapter 167, Tree Preservation and Protection, in (f) Floodplain variances shall only be those cases where their strict application issued if: would work an injustice as applied to the proposed development due to a situation (i) There are exceptional or unique to the subject real property; provided extraordinary circumstances or that such variance shall not have the effect conditions applicable to the of nullifying the intent and purpose of the property involved or to the intended chapter. The Planning Commission's use of the property, which do not approval of said variance must be affirmed apply generally to other property in by the City Council to become effective, and the same flood zone; a denial of the requested variance may be appealed to the City Council. (ii) A determination that failure to grant the variance would result in (6) Flood Damage Prevention Code. The exceptional hardship to the Planning Commission shall hear and decide applicant; and, requests for variances from the requirement of this ordinance. (iii) A determination that the granting of a variance will not result in (a) In passing upon such applications, the increased flood heights, additional Planning Commission shall consider all threats to public safety, technical evaluations, all relevant extraordinary public expense, factors, and standards specified in other create nuisances, cause fraud on or sections of this ordinance. victimization of the public, or conflict with the other provisions of the (b) Variances may be issued for the Code of Fayetteville. reconstruction, rehabilitation, or restoration of structures listed in the (g) Variances may be issued for new National Register of Historic Places, construction and substantial without regard to the procedures improvements and for other identified in the remainder of this development necessary for the conduct CD156:5 Fayetteville Code of Ordinances of a functionally dependent use provided that the provisions of §168.03(A) are satisfied and that the structure or other development is protected by methods that minimize flood damages during the base flood and create no additional threats to public safety. (h) Upon consideration of the factors in this section, and the purpose of this ordinance, the Planning Commission may impose conditions to the granting of floodplain variances as it deems necessary to further the purpose of this ordinance. (i) Any applicant to whom a variance is granted shall be given written notice that the structure will be permitted to be built with a lowest floor elevation below the regulatory flood elevation surcharge and that the cost of flood insurance will be commensurate with the increased risk resulting from the reduced lowest floor elevation. A copy of the notice shall be recorded by the floodplain administrator in the office of the Washington County Clerk and shall be recorded in a manner so that it appears in the chain of title of the affected parcel of land. (7) Outdoor Lighting Plan. (a) Undue Hardship. So that substantial justice may be done and the public interest secured, a developer may petition the Planning Commission for a variance from the requirements of Chapter 176: Outdoor Lighting, by showing that their strict application would cause undue hardship as applied to the proposed development; provided that such variance shall not have the effect of nullifying the intent and purpose of the chapter. (b) Conditions. In granting variances, the Planning Commission may impose such conditions as will, in its judgment, secure substantially the objectives of the requirements so varied. (8) Bicycle rack variance. The Planning Commission may modify the design standards or the requirement for a bicycle rack. (9) Streamside Protection Zones (a) Undue hardship. If the provisions of the Streamside Protection Ordinance are shown by the owner or developer to CD156:6 cause undue hardship as strictly applied to the owner or developer's property because of its unique characteristics, the Planning Commission may grant a variance on a permanent or temporary basis from such provision so that substantial justice may be done and the public interest protected, provided that the variance will not have the effect of nullifying the intent and purpose of the Streamside Protection regulations. (b) Consideration of alternative measures. The applicant for the variance shall establish that a reasonable rezoning by the City Council or variance request from the Board of Adjustment will not sufficiently alleviate the claimed undue hardship caused by the Streamside Protection regulations. (c) Conditions and safeguards. In granting any variance, the Planning Commission may prescribe appropriate conditions and safeguards to substantially secure the objectives and purpose for the regulations so varied and to mitigate any detrimental effects the variance may cause. The Planning Commission should consider the Streamside Protection Best Management Practices Manual and any mitigation recommendations from the City Engineer. (Ord. 4714, 6-21-05; Ord. 4930, 10-3-06; Ord. 5296, 12-15- 09; Ord. 5372, 12-7-10; Ord. 5390, 3-1-11; Ord. 5680, 4-15- 14) 156.04 Physical Alteration of Land and Stormwater Drainage And Erosion Control Variances of the requirements of Chapters 169 and 170 may be approved by the City Engineer, subject to the following criteria: Criteria. GrVariances of the physical alteration of land and stormwater management, drainage, and erosion control regulations may be applied for as follows: (A) Chapter 169 Criteria. A variance may be granted from any requirements of the Physical Alteration of Land regulations dependent upon on the soil types encountered, planned slopes, planned vegetation, and investigative engineering reports. (B) Chapter 170 Criteria. A variance may be granted from any requirement of the stormwater management, drainage, and erosion control regulations using the following criteria: TITLE XV UNIFIED DEVELOPMENT CODE (1) Special circumstances. There are special circumstances applicable to the subject property or its intended use; and (2) Results. The granting of the variance will not result in: (a) Surface water runoff. An increase in the rate or volume of surface water runoff; (b) Adjacent property. An adverse impact on any adjacent property, wetlands, watercourse, or water body; (c) Water quality. Degradation of water quality; or (d) Objectives. Otherwise impairing attainment of the objectives of Chapters 169 and 170. (Ord. No. 4100, §2 (Ex. A), 6-16-9 156.05 Sign Regulations Consideration by the Zoning and Development Administrator. The Zoning and Development Administrator shall not grant any variance of Chapter 174, Signs, unless and until an applicant demonstrates: (A) Special conditions. That special conditions and circumstances exist which are peculiar to the land, structure, or building involved and which are not applicable to other lands, structures, or building in the same district. (B) Deprivation of rights. That literal interpretation of the provisions of the sign regulations would deprive the applicant of rights commonly enjoyed by other properties in the same district under the terms of the sign regulations. (C) Resulting actions. That the special conditions and circumstances do not result from the actions of the applicant. (D) No special privileges. That granting the variance requested will not confer on the applicant any special privilege that is denied by Chapter 174, Signs, to other lands, structures, or building in the same district. (E) Nonconforming uses. No nonconforming use of neighboring lands, structures, or buildings in the same district, and no permitted or nonconforming use of lands, structures, or buildings in other districts shall be considered grounds for the issuance of a variance. (F) Time Limitation. Any variance granted shall automatically be revoked if the applicant does not C D 156:7 comply with the terms of the variance within 90 days from the granting thereof; and, the applicant shall be required to comply with the literal provisions of Chapter 174, Signs. (G) Prohibited. The Zoning and Development Administrator shall not permit as a variance any sign the erection of which or the continuance of which is prohibited by Chapter 174, nor shall any variance be granted to allow a greater number of signs than specifically set forth therein. (H) Content Neutrality; Restrictions. The Zoning and Development Administrator shall not take into account the content of any message sought to be displayed on the sign when determining whether to grant a variance. Variances can only be granted for setbacks, area, height, the proposed on-site location of the sign, or other technical requirements, and shall not exceed 15% of the Code requirement. (Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord 4652, 12-07-04) Cross reference(s)--Enforcement, Ch. 153; Signs, Ch. 174. 156.06 Airport Zone (A) Board of Adjustment. The Board of Adjustment shall have the authority to grant variances from the height limits prescribed in Chapter 165. Any person desiring to erect any structure or increase the height of any structure or permit the growth of any object of natural growth, in excess of the heights prescribed, must apply in writing to the Board of Adjustment for a variance. Such variance shall be allowed upon a showing of practical difficulty or unnecessary hardship, together with a showing that the structure or object of natural growth in question will not cause an undue hazard to aircraft operations at the airport. (B) Determination from Federal Aviation Administration. The application for a variance shall be accompanied by a determination from the Federal Aviation Administration as to the effect of a proposal on the operation of air navigation facilities and the safe, efficient use of navigable airspace. Additionally, no application for a variance may be considered by the Board of Adjustment unless a copy of the application has been furnished to the airport manager for comment as to the aeronautical effects of the variance. If the airport manager does not respond to the application within 15 days after receipt thereof, the Board of Adjustment may grant or deny said application. (C) Marking and lighting. In granting any application for any permit or variance, approval may be conditioned as to require the owner of the Fayetteville Code of Ordinances structure or object of natural growth in question to install and maintain obstruction markings or lights. (D) Findings of fact. Written findings of fact and conclusions of law shall be made by the Board of Adjustment based upon the evidence offered at the public hearing. (Ord. No. 4100, §2 (Ex. A), 6-16-98) Cross reference(s)--Notification and Public Hearings, Ch. 157. 156.07 Landscape Regulations (A) The Planning Commission shall have the authority to grant a variance from the landscaping requirements prescribed by §177. (B) Findings. The Planning Commission shall make the following findings: (1) Minimum variance. That the reasons set forth in the application justify the granting of the variance, and that the variance is the minimum variance that will make possible the reasonable use of the land, building, or structure. (2) Harmony with general purpose. The Planning Commission shall further make a finding that the granting of the variance will be in harmony with the general purpose and intent of the Landscape Regulations, §177, and will not be injurious to the neighborhood, or otherwise detrimental to the public welfare. (3) Conditions and safeguards. In granting any variance, the Planning Commission may prescribe appropriate conditions and safeguards to ensure compliance or to protect adjacent property. (4) Undue Hardship. If the provisions of the standards within Landscape Regulations, §177, are shown by the developer to cause undue hardship as they apply to his proposed development, the Planning Commission may grant a variance to the developer from such provisions, so that substantial justice may be done and the public interest secured; provided that the variation will not have the effect of nullifying the intent and purpose of development regulations. (Ord. No. 4917, 9-05-06) 156.08-156.99 Reserved C D 156:8 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 169: PHYSICAL ALTERATION OF LAND 169.01 INTENT............................................................................................................................................ 3 169.02 GENERAL REQUIREMENTS......................................................................................................... 3 169.03 PERMITS REQUIRED/EXCEPTIONS.............................................................................................3 169.04 MINIMAL EROSION CONTROL REQUIREMENTS.......................................................................4 169.05 ONE-TIME APPROVALS................................................................................................................5 169.06 LAND ALTERATION REQUIREMENTS......................................................................................... 5 169.07 GRADING PLAN SPECIFICATIONS............................................................................................11 169.08 GRADING PLAN SUBMITTAL.....................................................................................................12 169.09 MINOR MODIFICATIONS.............................................................................................................12 169.10 APPROVAL...................................................................................................................................12 169.11 DISCOVERY OF HISTORIC RESOURCES..................................................................................13 169.12 CERTIFICATE OF OCCUPANCY.................................................................................................13 169.13 OWNER RESPONSIBILITY..........................................................................................................13 169.14-169.99 RESERVED........................................................................................................................13 CD169:1 Fayetteville Code of Ordinances CD169:2 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 169: PHYSICAL ALTERATION OF LAND 169.01 Intent (A) It is the city's intent to safeguard the health, 169.03 Permits Required/Exceptions safety, and welfare of Fayetteville citizens by (A) Permit required. No grading, filling, excavation, implementing standards and procedures for the or land alteration of any kind shall take place physical alteration of land. It is not the city's without first obtaining: intent to supersede federal or state regulations such as, but not limited to, the Occupational (1) A grading permit pursuant to this chapter Health & Safety Act. except as specified in §169.03(8); (B) The purpose of this chapter is to control grading, (2) A stormwater management, drainage and clearing, filling, and cutting (or similar activities) erosion control permit (hereinafter referred to which alone or in combination cause landslides, as a "drainage permit") except as specified in flooding, degradation of water quality, erosion §170.03(C) and §170.03(D); and and sedimentation in storm sewer systems and water storage basins. It is also the intent of this (3) An Arkansas Department of Environmental chapter that through the implementation of the Quality Stormwater Construction Permit and guidelines and regulations contained herein, the incorporated Stormwater Pollution existing scenic character and quality of the Prevention Plan, if required by state law. neighborhood and city as a whole not be diminished. (4) A grading permit is required by the City for any development occurring within the (Code 1991, §161.01; Ord. No. 3551, 6-4-91; Ord. No. 4100, Hillside/Hilltop Overlay District boundaries. If §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. 5336, a parcel of land is divided by the 8-3-10) Hillside/Hilltop Overlay District boundary, 169.02 General Requirements then only that portion of land lying within the boundary is subject to the requirements of (A) Protection. Persons engaged in land alteration this chapter. activities regulated by this chapter shall take (B) Exceptions where no grading permit is required. measures to protect public and private properties Grading permits are not required for the from damage by such activities. Adjacent and following: nearby properties affected by land alterations shall be restored in accordance with the (1) Excavation below finish grade. Excavations requirements of this Chapter. below finished grade for basements, (B) Site conditions. Development shall generally swimming pools, hot tubs, septic systems, conform to the natural contours of the land, retaining walls under 4 feet in height, and natural drainage ways, and other existing site like structures authorized by a valid building conditions. permit. (C) Adjacent properties. All developments shall be (2) Cemetery graves. Cemetery graves. constructed and maintained so that adjacent (3) Refuse disposal. Refuse disposal sites properties are not unreasonably burdened with controlled by other regulations. surface waters as a result of such development. More specifically, new development may not (4) Single-family/duplex. Construction of one unreasonably impede water runoff from higher single-family residence, or duplex not properties nor may it unreasonably channel water located within the 100 year flood plain, the onto lower properties. Hillside/Hilltop Overlay District, or on a slope (D) Restoration. Land shall be revegetated and 15 % or greater. restored as close as practically possible to its (5) Building additions. Building additions of less original conditions so far as to minimize runoff than 2,000 square feet where associated and erosion are concerned. Previously forested land alteration activities are not beyond the areas shall follow the City's Landscape Manual scope of what is necessary to construct said for mitigation of forested areas. addition and are not located within the 100 (Code No. 1991, §161.02; Ord. No. 3551, 6-4-91; Ord. No. year flood plain, the Hillside/Hilltop Overlay 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. District, or on a slope 15 % or greater. 4855, 4-18-06; Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) CD169:3 Fayetteville Code of Ordinances (6) Other minor fill or grading for maintenance purposes such as landscaping, yard grading, (B) Stabilization. A record of the dates when grading maintenance, farming, gardens, and similar activities occur, when construction activities activities. temporarily or permanently cease on a portion of the site, and when stabilization measures are (C) Grading permit application and approval. No initiated shall be included in the erosion and grading permit shall be issued until the grading sediment control plan. Except as provided in (1) plan, endorsed by a registered architect, and (2) below, stabilization measures shall be landscape architect, or engineer, is approved by initiated as soon as practicable in portions of the the City Engineer. A separate permit shall be site where construction activities have required for each site; it may cover both temporarily or permanently ceased, but in no excavations and fills. Grading permits may be case more than 14 days after the construction issued jointly for parcels of land that are activity in that portion of the site has temporarily contiguous, so long as erosion control measures or permanently ceased. are in place until project completion. Any application for a required grading permit under (1) Where the initiation of stabilization measures this chapter shall be submitted concurrently with by the 14t' day after construction activity the application and calculations for a drainage temporarily or permanently ceases is permit if such a drainage permit is required by precluded by snow cover, stabilization §170.03., coordination with Chapter 167. Tree measures shall be initiated as soon as Preservation and Protection is required. practicable. (D) Permit posted. A copy of the grading permit (2) Where construction activity will resume on a cover page shall be posted at or near the street portion of the site within 21 days from when right-of-way line and shall be clearly visible from activities ceased, (e.g. the total time period the street. that construction activity is temporarily ceased is less than 21 days) then (Code 1991, §161.03; Ord. No. 3551, 6-5-91; Ord. No. 4100, stabilization measures do not have to be §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. No. initiated on that portion of the site by the 141h 4313, 5-15-01; Ord. 4855, 4-18-06; Ord. 5336, 8-3-10) day after construction activity temporarily 169.04 Minimal Erosion Control ceased. Requirements (3) Stabilization practices may include: If exempt under 169.03, a grading permit is not temporary seeding, permanent seeding, required. However, exempt as well as non-exempt mulching, geotextiles, sod stabilization, activities shall be subject to the following minimal vegetative buffer strips, protection of trees, erosion and sedimentation control measures. and preservation of mature vegetation and other appropriate measures. See Chapter (A) The City Council hereby adopts by reference the 167 of the UDC for tree protection Drainage Criteria Manual, prepared for the City of requirements. Fayetteville, and adopted by Ordinance No. XXXX of the City of Fayetteville and as may be (C) Intermittent/perennial streams. No intermittent or amended from time to time by the City Engineer. perennial stream, including a 25 foot perimeter All minimal erosion and sediment control strip measured from the top of the bank, shall be standards contained therein shall have the same graded, developed, channeled, or physically force and effect as if printed word for word in this altered unless adequate guarantees are made for chapter. Development projects also must comply erosion and sedimentation control both during with their Arkansas Department of Environmental construction and post construction. Likewise, Quality general construction permit. All projects cuts or fills shall be setback sufficiently from shall follow Chapter 8, Construction Site intermittent and perennial streams and other Stormwater Management, of the Drainage stormwater drainage systems to guarantee that Criteria Manual as well to achieve site there will be no damage from erosion or compliance. sedimentation. Final erosion and sedimentation control measures shall be approved by the City Engineer. (D) Excavation material. Excavation material shall not be deposited in or so near streams and other vi stormwater drainage systems where it may be washed downstream by high water or runoff. All to the rete^+�^ sf natural V9g8tRtiAA—en excavation material shall be stabilized H+lls+d-A-11=1611taps. immediately with erosion control measures. CD169:4 TITLE XV UNIFIED DEVELOPMENT CODE horizontal to 1 vertical), unless otherwise (E) Fording streams. Fording of streams with approved by the City Engineer. Land construction equipment or other activities which located within the Hillside/Hilltop Overlay destabilize stream banks shall not be permitted. District may have cut or fill slopes with a finish grade no steeper than 50% (2.00 (F) Debris, mud, and soil in public streets. Debris, horizontal to 1 vertical) unless otherwise mud and soil shall not be allowed on public approved by the City Engineer. streets but if any debris, mud, or soil from development sites reaches the public street it (2) Maximum length. The maximum length of shall be immediately removed via sweeping or any cut or fill slope without a terrace (as other methods of physical removal. Debris, mud, described in 169.06 (D) below) shall be 100 or soil in the street may not be washed off the feet as measured along the ground. The street or washed into the storm drainage system. terrace shall be at least six feet (6') wide. Storm drainage systems downstream of a development site should be protected from (3) Existing topography. Cut or fill slopes shall debris, mud, or soil in the event that debris, mud, be constructed to eliminate sharp angles of or soil reaches the drainage system. intersection with the existing terrain and shall be rounded and contoured to blend with the (Code 1991, §161.04; Ord. No. 3551, 6-5-91; Ord. No. 3947, existing topography. §1, 2-6-96; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. 4855, 4-18-06; Ord. 5336, 8-3-10) (4) Setback requirements. The following 169.05 One -Time Approvals PP setback requirements shall be reviewed by the City Engineer for purposes of assessing safety, stability, and drainage problems: (A) Utilities. Public and private utility organizations (See illustrations). may obtain a one-time approval from the City Engineer for all routine underground electric, (a) Setback from top or toe of cut or fill. water, sewer, natural gas, telephone, or cable Buildings shall be setback from the top facilities. The approval will include a utility or toe of a cut or fill in accordance with organization and its contractors, agents, or Zoning, Chapters 160 through 165; assigns and will be permanent in nature as long Building Regulations, Chapter 173; or as the original approved procedures are followed. the approved grading plan, whichever is greatest. (B) Stockpiling materials. One-time approval may be obtained by public or private entities for the (b) Setbacks from property lines. The stockpiling of fill material, rock, sand, gravel, required setback of retaining walls, cut aggregate, or clay at particular locations, subject slopes, and fill slopes from property to Zoning, Chapters 160 through 165. lines shall be as given in the illustrations. Property lines may be filled (Code 1 §161.05; Ord. 1, 6-4-91; Ord. No. 4100, over or cut if a grading plan for the cut §2 (Ex. A)),, 6-16-98; Ord. 533636,, 8-3--3-10) or fill is submitted jointly by the owner of 169.06 Land Alteration Requirements both properties or with written permission from the adjacent property (A) Applicability. This section shall apply to all land owner and if no utility easements areinvolved. If utility easements are easements alteration activities, including those that do not involved, approval is required given require a grading permit. in (c) below in addition to the joint (B) Requirements varied. Variances of this chapter's submittal requirement. requirements may be approved by the City (c) Setbacks from the edge of an Engineer. The extent to which variations may be easement. The required setback of made will depend on the soil types encountered, retaining wall, cut slopes, and fill slopes planned slopes, planned vegetation, and from the edge of easements shall be as investigative engineering reports. In no case given in the illustrations. Where no shall the City Engineer waive or modify any of the utilities are present in an easement, or minimum erosion control requirements as given where utilities are planned to be in §169.04. relocated, and where such action is (C) Cut or fill slopes. approved by all utilities, in writing, then easements may fall within a cut or fill (1) Finish grade. Cut or fill slopes shall have a section. finish grade no steeper than 33% (3.00 CD169:5 Fayetteville Code of Ordinances (d) Setbacks from structures. The required setback of retaining walls, cut slopes, and fill slopes from structures shall be as given in the illustrations. If a structure forms an integral part of the retaining wall, then the setbacks do not apply to that structure. (e) Calculating setbacks. For the purpose of calculating setbacks, any cut or fill section which is on a slope of one to one or greater shall be considered a retaining wall. (f) Administrative variance. Setbacks from easement lines and structures may be varied administratively by the City Engineer if geotechnical and/or structural information is provided that in the opinion of the City Engineer justifies the variance. (g) Additional information required. The City Engineer may require further geotechnical and/or structural information to show that setbacks greater than those given are not needed to protect property, utilities, or the integrity of property lines. (D) Cuts. (1) Vertical height. Cuts shall be limited to 10 feet in vertical height unless information demonstrating slope stability, erosion control, and drainage control is provided together with a re -vegetation plan. For nonsolid rock cuts, terraces shall be required for cuts greater than 10 feet in height. It is recommended that terracing be at a maximum ratio of one foot of horizontal terrace for every foot of vertical surface. (2) Maximum vertical cut. In solid rock, as determined by geotechnical and engineering data approved by the City Engineer, the maximum vertical cut shall be 30 feet. (3) Fill material. In no case shall a cut be allowed primarily for the purpose of obtaining fill material to a different site, unless the exporting site is located within an extraction district. (E) Fills. (1) Rocks/fill. All imported fill shall be free of rocks greater than 12 inches in diameter and any detrimental organic material or refuse debris. CD169:6 (2) Compaction. Fill shall be placed and compacted as to minimize sliding or erosion of soil. Fill compaction shall equal the compaction of undisturbed, adjacent soil, except fills covered by Building Regulations, Chapter 173, or other structural fills. The City Engineer may require soil tests during compaction work or upon its completion at the expense of the permittee. (3) Grade. Fill shall not be placed on existing slope with a grade steeper than 15% (6.67 horizontal to 1 vertical) unless keyed into steps in the existing grade and thoroughly stabilized by mechanical compaction. (4) Vertical height. Fills shall be limited to 10 feet in vertical height unless information demonstrating slope stability, erosion control, and drainage control is provided together with a re -vegetation plan. (5) Terraces. Terraces shall be required for fills greater than 10 feet in height. It is recommended that terracing be at a maximum ratio of one foot of horizontal terrace for every foot of vertical surface. (F) Erosion and sedimentation control. (1) Permanent improvements. Permanent improvements such as streets, storm sewers, curb and gutters, and other features for control of runoff shall be scheduled coincidental to removing vegetative cover from the area so that large areas are not left exposed beyond the capacity of temporary control measures. (2) Phased Construction. The area of disturbance onsite at any one time shall be limited to 20 acres. An additional 20 acres (a maximum of 40 acres of disturbance at any one time) may be stripped with the permission of the City Engineer in order to balance cut and fill onsite. No additional area may be open without the permission of the City Engineer until the previously disturbed areas have been temporarily or permanently stabilized. (3) Stockpiling of top soil. Top soil may be stockpiled and protected for later use on areas requiring landscaping. All storage piles of soil, dirt or other building materials (e.g. sand) shall be located more than 25 feet from a roadway, drainage channel or stream (from top of bank), wetland, and stormwater facility. The City Engineer may also require top soil stockpiles to be located up to fifty (50) feet from a drainage channel or stream, as measured from the top of the TITLE XV UNIFIED DEVELOPMENT CODE bank to the stockpile, for established TMDL water bodies; streams listed on the State 303(d) list; an Extraordinary Resource Water, Ecologically Sensitive Waterbody, and/or Natural and Scenic Waterbody, as defined by Arkansas Pollution Control and Ecology Commission Regulation No. 2; and/or any other uses at the discretion of the City Engineer. Topsoil piles surfaces must be immediately stabilized with appropriate stabilization measures. Stabilization practices may include: temporary seeding (i.e. annual rye or other suitable grass), mulching, and other appropriate measures. Sediment control measures such as buffer strips, wattles, or silt fence shall be provided immediately for stockpiles and remain in place until other stabilization is in place. Storm drain inlets must be protected from potential sedimentation from storage piles by silt fence or other appropriate barriers. Properly stabilized topsoil stockpiles may be used for sedimentation control. 0 gFade� Re vagetatiGR Shall __age that MiRiFR4496 Aresiop sea69RG. C D 169:7 (46) Plant/water. Plant materials shall be watered or irrigated and tended. Where irrigation or regular watering is not available, only native or acclimated plant species shall be used. If the soil cannot properly sustain vegetation, it must be appropriately amended. If re -vegetation is not firmly established and healthy after one year, the urban forester shall require that it be redone in part or total. (58) Permanent erosion control. The developer shall incorporate permanent erosion control features at the earliest practical time. Temporary erosion control measures will be used to correct conditions that develop during construction that were unforeseen during the design stage, that are needed prior to installation of permanent erosion Fayetteville Code of Ordinances control features, or that are needed temporarily to control erosion that develops during normal construction projects, but are not associated with permanent control features on the project. (G) Percentage of land disturbance within the Hilltop/Hillside Overlay District shall be in accordance with the percent minimum canopy required on site per Chapter 167, Tree Preservation & Protection. requirements. IR the development of Fesidential 10 to 15 persal# 40-serse-nt 15 to 20 perseat ,-20 Sereeat 64Perseet (H) Required retaining wall and rock cut design. (1) Design/inspection. Any retaining wall more than four feet in height shall be designed by a registered professional engineer, and shall be field inspected by the design engineer. The design engineer shall provide proof of inspection and certify that the wall was constructed in conformance with the design. The City Engineer may require retaining walls less than four feet in height to be designed by a professional engineer. (2) Investigation/report. All proposed rock cuts and any cut or fill 10 feet or greater will require a geotechnical investigation and a formal report submitted by a registered professional engineer qualified to make such investigations. (3) Safety railings. Safety railings may be required on any retaining wall 2.5 feet or higher. The decision as to whether to require safety railing shall be based on potential pedestrian and public access to the retaining wall and applicable building codes. This requirement for safety rails shall also apply to vertical or near vertical rock cuts and to steep (greater than 3:1) cut or fill slopes. C D 169:8 (Code 1991, §161.07; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, §1, 8-18-98; Ord. No. 4855, 4-18-06; Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) L E G E N D NOTES EXISTING GROUND 1. WRITTEN PERMISSION FROM ADJACENT PROPERTY OWNER IS REQUIRED TO FILL OR EXCAVATE OVER EXISTING PROPERTY Lu LINES OR SETBACKS. o PROPERTY OR EASEMENT LINE 2 CONSTRUCTION LIMITS INDICATED ARE FOR INITIAL CONSTRUCTION; RECONSTRUCTION; OR MAINTENANCE OF THE CL STRUCTURE IN THE FUTURE. PROPOSED GROUND 3. REVIEW ZONING & BUILDING SETBACK REQUIREMENTS, DEFER CONSTRUCTION LIMITS TO WHICHEVER MAY BE GREATER. 4. MINIMUM SLOPE AND DISTANCE FROM THE STRUCTURE PER S131 MINIMUM SETBACK FROM PROPERTY LINE DOWNHILL STATE BUILDING CODE MUST NOT BE VIOLATED. SB2 MINIMUM SETBACK FROM PROPERTY LINE UPHILL 5. REFER TO THE OSHA TECHNICAL MANUAL SECTION 5 CHAPTER 2 TB TIE BACK FOR SOIL TYPES AND SLOPE EXCAVATION TO DETERMINE THE S SLOPE LIMITS OF CONSTRUCTION. F1 FOUNDATION DOWNHILL 6. APPLICABLE FOR RETAINING WALLS MORE THAN 4 FT. IN HEIGHT. F2 FOUNDATION UPHILL C1 LIMITS OF CONSTRUCTION -DOWNHILL C2 LIMITS OF CONSTRUCTION -UPHILL SB2 PL or EL S62 - B22" 2" I PL or EL I SEE I BELOW f I PL I SBI I 1' i _ Z PL � SEI 1 FILL SLOPE 2' Flatter Than 5:1 5' 5:1 To 3:1 Uphill 8' Greater Than 3:1 Downhill CUT SLOPE Downhill FILL SLOPE Uphill PL or EL PL or EL I I PL or EL PL or EL S8, SB2 I I SB, S8a 51 C2 5' C2 1' I g C2 1'I See Note 1 f I I See Note 1 F. /{EI SBZ C2+1' Fi FZ r Uphill, : r: SB2= C2+1 SBi= F1+5' SBi= F1+5' Uphill Downhill Downhill RETAINING WALL RETAINING WALL CUT SLOPE FILL SLOPE PL or EL PL or EL PL or EL PL or EL SBS SB2 I I 5' C2 f SBti SB2 l 5' C2 1.1 I TB. TB See Note 1 �- Fi F — - -- — SIB 2=c 2+ t 1, . SB 2= C2+1. Uphill' Uphill SBi= F1+5' Downhill SBi= F1+5' Downhill BLOCK RETAINING WALL BLOCK RETAINING WALL CUT SLOPE FILL SLOPE SBz 5' 8 8I�� Apply to Existing Structures F5' _ - UphVll 3M+ax1 Downhill CUT SLOPE- EXISTING STRUCTURES SBS 2 e SerTII B I ser: BELOW }, Uphill SBI* FILL SLOPE 5' Flatter Than 3:1 10' 3:1 To 2:1 41-1/3 Greater Than 2:1 Downhill FILL SLOPE - EXISTING STRUCTURES * D=5' For Existing " D=2' For Proposed �— 58zS32 Cz •0 B El I B H Fz S Uphill '- � z+❑ Uphill Se, = N'413 F Downhill SB = H*4/3 Downhill RETAINING WALL RETAINING WALL WITH STRUCTURES WITH STRUCTURES CUT SLOPE FILL SLOPE * D=5' For Existing * D=2' For Proposed 58z - SB2 c L 51C- 'a El Ca •❑ e TB T SB2=C2+5' SBz =C +0 H _r_.._. _ Uphill _ H Uphill F — , _. F SB, = H*4/3 SB, = H*4/3 Downhill Downhill BLOCK RETAINING WALL BLOCK RETAINING WALL WITH STRUCTURES WITH STRUCTURES CUT SLOPE FILL SLOPE 11URW"VIN APMM1101UAl:4il,1 101101 169.07 Grading Plan Specifications (A) Grading plan. The applicant shall prepare a grading plan as follows: (1) Site plan. Site plan at a scale no smaller than one inch equals 50 feet, showing property lines; vicinity map; name of owner, developer and adjacent property owners. (2) Existing grades. Existing grades shall be shown with dashed line contours and proposed grades with solid line contours. Grading plans shall be required to show both the proposed grade and the undisturbed area. Contour intervals shall be a maximum of two feet. Spot elevations shall be indicated. (3) Designation of grade. Areas with 0 to 10%, 10 to 15%, 15 to 20% and more than 20% grade shall each be identified in a distinguishing manner. (4) Identify land to be disturbed. Land areas to be disturbed shall be clearly identified. (5) Engineer/architect. Seal of a registered engineer, architect, or landscape architect certifying that the plan complies with this chapter. (6) Cuts and fills. All cuts and fills, including height and slope, shall be clearly shown on the plan. (7) Streets and rights-of-way. Location and names of all existing or platted streets or rights-of-way within or adjacent to tract and location of all utilities and easements within or adjacent to the property shall all be indicated. (8) Lot/building, etc. identification. The proposed location of lots, buildings, streets, parking lots and parks, playgrounds or green space shall be indicated. Also to be indicated is any existing or proposed building within 100 feet of the site. (-94 909 We. Sam' tyPes shall -he id-e-Atifie-d- (948) Natural features. Location of natural features such as drainage ways, ponds, rock outcroppings, and tree cover. Indication of 100 year floodplains as defined by FEMA. (1044) Streets and drainage ways. Profiles and cross sections for proposed streets and drainage ways. CD169:11 (112-) Acreag%. Total acreage and (123) Surface water. Provisions for collecting and discharging surface water. (134) Underground utilities. Profiles and cross sections of streets, drainage systems, and underground utilities, if they are necessary to clarify the grading plan in terms of potential erosion or runoff, or if the grading on site has the potential of disturbing the utility line. (145) Treatment of slopes and benches. The method of treatment for all slopes and benches shall be indicated. (156) Natural vegetation preservation. Proposals for preserving natural vegetation and description of re -vegetation or other permanent erosion control strategy. (167-) Runoff/sedimentation. Specification of measures to control runoff and sedimentation during construction indicating what will be used such as straw bales, silt dams, brush check dams, lateral hillside ditches, catch basins, and the like. (178) Preliminary plat master build -out grading plan. The applicant shall prepare a master grading plan to be followed during individual lot development to convey runoff to a public drainage easement or right of way. 4R te the requirements ef 1.69.07 A, Jhe following shall be required for individual lot drainage design: (a) Identify lot lines and conceptual foot print of residence. 173R'-'Ild!ng—Re961latiOR6 and the (bs) Indicate individual lot drainage with the use of contours spet elevatffiGRs and flow arrows or other indications of direction of drainage. (i) Thom Slope of the flew path at a UPR of 20/ fn, n boa (ii) In general, drainage should be routed on the shortest practicable Fayetteville Code of Ordinances flow path to the public right of way or drainage easement. (cd) Nonstructural grassed swales for rear lot drainage concentration is discouraged and shall not be installed in combination with a utility easement. (de) Right of way, utility easements and drainage easements shall be graded and shaped in accordance with the Master Build out Grading Plan during preliminary plat construction. (i) Utility Easements adjacent to the right of way shall be no steeper than 15%. shall be gFaded and shaped OR aGGA_rd_;;AG_A_ thO during— PF616MIRa;T— plat seastF, GtiAA (ii) Provisions will be considered to accommodate positive drainage until build -out occurs. (B) Preliminary grade plan. The preliminary grading plan shall have adequate detail for review4pAkWe (14) above. In ad-d-ition to the a-heve itemG, the (Code 1991, §161.08; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, 8-18-98; Ord. No. 4855, 4-18-06; Ord. 5336, 8-3-10) 169.08 Grading Plan Submittal (A) Preliminary grading plan. A preliminary grading plan shall be submitted at the time of preliminary plat submission for subdivisions or plat submission for large scale development, whichever is applicable. (B) Final grading plan. No subdivision may be finalized, nor large scale development plat approved before a final grading plan has been submitted to the City Engineer and approved. The final grading plan and the final plat of land located within the Hillside/Hilltop Overlay District shall have the following plat note stating: 'Property and lot owners of lands located within the Hillside/Hilltop Overlay District shall have foundation plans designed, approved and sealed by a professional architect or engineer. (C) A copy of the Stormwater Pollution Prevention Plan (SWPPP) is required to be submitted with the grading plan for sites one acre or larger. (D) In cases where neither subdivision plat, nor LSD plat is applicable, proof of notification of adjacent property owners and grading plan must be submitted simultaneously with the application for a grading permit. (Code 1991, §161.09; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4855, 4-18-06; Ord. 5308, 3- 16-10; Ord. 5336, 8-3-10) Cross reference(s)--Notification and Public Hearings, Ch. 157. 169.09 Minor Modifications Finish grades shall be allowed no more than a 0.50 foot tolerance from the grading plan. However, the City Engineer may authorize in writing minor modifications so long as they do no alter the direction of run-off and otherwise comply with the intent of this chapter. When applicable, major modifications must be brought before the Subdivision Committee for their approval. (Ord. 5336, 8-3-10) 169.10 Approval Approval of a grading pta44 permit is contingent on meeting all the requirements of this ordinance plus any set of varied requirements approved by the Planning Commission. (Code 1991, §161.10;Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4113, 8-18-98; Ord. 5336, 8-3- 10) CD169:12 TITLE XV UNIFIED DEVELOPMENT CODE 169.11 Discovery Of Historic Resources Whenever, during the conduct of grading any historical, pre -historical, or paleontological materials are discovered, grading shall cease and the City Engineer shall be notified. (Code 1991, §161.21; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10) 169.12 Certificate Of Occupancy All re -vegetation and grading plan improvements shall be in place before a certificate of occupancy shall be issued, including cleanup and restoration/revegetation of adjacent and nearby property affected by construction activities.- When a property owner has finished building construction but has yet to install plant material, said owner may apply for a temporary certificate of occupancy. In evaluating whether or not to grant a temporary certificate of occupancy, the City Engineer shall consider weather conditions and temporary stabilization measures. (Code 1991, §161.15; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10, Ord. 5431, 8-16-11) 169.13 Owner Responsibility The property owner shall be responsible both for his or her employees and for all contractors and subcontractors from the onset of development until the property is fully stabilized. If property is transferred anytime between the onset of development and at the time it is fully stabilized, all responsibility and liability for meeting the terms of the chapter shall be likewise transferred to the new property owner. (Code 1991, §161.16; Ord. No. 3551, 6-4-91; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 5336, 8-3-10) 169.14-169.99 Reserved CD169:13 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 170: STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL 170.01 INTENT............................................................................................................................................. 3 170.02 ADOPTION OF DRAINAGE CRITERIA MANUAL.......................................................................... 3 170.03 PERMITS REQUIRED......................................................................................................................3 170.04 DRAINAGE PERMIT CONDITIONS................................................................................................. 4 170.0-5- -DRAINAGE PERRAIT APPLICATION ...............................................................................................5 170.055 elieMIS-SWON, REVIEW, AND APPROVAL OF nl eucPERMIT PROCESSING ........................6 170.07-6 PERFORMANCE CRITERIA..........................................................................................................6 170.075 MAINTENANCE RESPONSIBILITY.............................................................................................. 7 170.0-9 nnAWAr_e PERMIT PROCESSING 170.84$ STORMWATER DISCHARGES FROM CONSTRUCTION ACTIVITIES ...................................... 9 170.944 PRELIMINARY PLAT, LOT REQUIREMENTS...........................................................................11 170.1021 & 2 FAMILY RESIDENTIAL REQUIREMENTS........................................................................11 170.113 STORMWATER POLLUTION PREVENTION.............................................................................12 170.124-170.99 RESERVED......................................................................................................................13 CD170:1 Fayetteville Code of Ordinances I14491will K TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 170: STORMWATER MANAGEMENT, DRAINAGE AND EROSION CONTROL 170.01 Intent (A) Intent. It is the intent of this chapter to protect, maintain, and enhance the health, safety, and general welfare of the citizens of the City of Fayetteville by: (1) Preventing increases in the magnitude and frequency of stormwater runoff to prevent increases in flood flows and associated hazards and costs. (2) Controlling soil erosion and sedimentation to minimize soil deposition in streams and other receiving water bodies and storm drainage systems. (3) Requiring surface and stormwater management practices that comply with requirements of this chapter. (34) Promoting the development of stormwater facilities that are aesthetically desirable. (B) Findings of fact. The City Council finds that uncontrolled stormwater runoff from developed land adversely affects the public health, safety, and welfare because: (1) Impervious surfaces / runoff. Impervious surfaces increase the quantity and velocity of surface runoff, which reduces percolation of water through soil and increases erosion and flooding. (2) Collection and conveyance of stormwater. Improper stormwater collection and conveyance adversely affects property and increases the incidence and severity of flooding, which can endanger property and human life. (3) Erosion. Increased erosion leads to sedimentation in stormwater management systems, which decreases the system's capacity (4) Future problems. Many future problems can be avoided if land is developed in accordance with sound stormwater runoff management practices. (Code 1991, §163.03; Ord. No. 3895,6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4855, 4-18-06; Ord. 4920, 9-05- 06; Ord. 5336, 8-3-10) 170.02 Adoption Of Drainage Criteria Manual CD170:3 The City Council hereby adopts by reference the Drainage Criteria Manual, prepared for the City of Fayetteville, and adopted by Ordinance No. XXXX39" of the City of Fayetteville, and as may be amended from time to time by the City Engineer. All technical procedures and design standards contained therein shall have the same force and effect as if printed word for word in this chapter. (Code 1991, §163.03; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 09-05-06; Ord. 5336, 8- 3-10) 170.03 Permits Required (A) Applicability. This chapter shall apply to all land within the corporate limits of the City of Fayetteville. No person may subdivide and develop, change to a more intensive land use, construct or reconstruct a structure, or change the size of a structure, or conduct grading, clearing, or filling activities without first obtaining a stormwater management, drainage and erosion control permit (hereinafter referred to as a "drainage permit") from the City, except as specified in §170.03(C) and §170.03 (D) below. (B) Permit application. Any application for a -drainage it shall he submitted acnerdiRg te §1:7().g5 If sudh .,n mit row hu PS 169 03 T4F6 a drainage permit shall be submitted to the City Engineer using appropriate forms as provided by the city for review, processing, and approval. The drainage permit application shall be submitted concurrently with the application for a grading permit, if such grading permit is required by § 169.03. A permit application shall contain sufficient information and plans to allow the City Engineer to determine whether the project complies with the requirements of this chapter. The specific items to be submitted for a permit application shall be in the form and follow the procedures as described in the Drainage Criteria Manual, Section 1.4, Drainage Report Template and Checklist. The City Engineer shall make the final determination regarding detention. (1) Fee. A nonrefundable permit application fee shall be paid when the application is submitted to help cover the cost of the plan Fayetteville Code of Ordinances review, administration and management of the permitting process and inspection of project implementation and operation (separate from the Physical Alteration of Land fee). (2) Issuance. If the City Engineer determines that the permit application submittal is in compliance with all provisions of this chapter, a permit may be issued. If the City Engineer determines that the permit submittal does not conform with all provisions of this chapter, permit issuance may be denied and a written statement as to the reasons for the denial shall be provided to the applicant. (C) Project not requiring detention. Any project that requires a drainage permit that does not require detention may receive, with the approval of the City Engineer, a grading permit prior to issuance of the drainage permit. (D) Any grading permit and/or issued prioF to the drainage permit issued shall be subject to the following: (1) Insufficient or incomplete drainage permit application. If the drainage permit application, including the required calculations, is determined by the City Engineer to be insufficient or incomplete, it shall be revised and resubmitted by the applicant within four (4) weeks of receipt of written notice of insufficiency or incompleteness. (2) Deadline for the revised application. A stop work order for all grading on the project shall be issued by the City Engineer if a revised application is not submitted within four (4) weeks of receipt by applicant of the written notice of insufficiency or incompleteness. However, the City Engineer may delay issuance of the stop work order if the City Engineer determines that the applicant has demonstrated prior to the deadline that circumstances not reasonably foreseeable and beyond the applicant's reasonable control prevented his timely resubmission of a sufficient and complete revised drainage permit application. disturbed areas as set forth in §169.96(F)(6) and the Drainage Criteria Manual. (5) Termination of stop work order. Any stop work order issued pursuant to §170.03 (C) (2) or §170.03 (C) (3), shall expire upon the issuance of a drainage permit and compliance with any conditions contained in the drainage permit. (E) Exceptions where no drainage permit is required. Drainage permits are not required for the following: (1) Single-family/duplex. One single-family residence or duplex. A drainage permit is not required. See Section 170.8910 for building permit submittal requirements. (2) Commercial/industrial. One commercial or industrial project built on an individual lot that is part of a larger subdivision that has been issued an approved drainage control permit when the proposed project is demonstrated to be in compliance with the overall subdivision drainage permit. (3) Existing commercial/industrial. Existing commercial and industrial structure where additional impervious area is 6#61stWat less than 2,000 square feet. (4) Maintenance. Maintenance or clearing activity that does not change or affect the quality, rate, volume, or location of stormwater flows on the site, or runoff from the site. (5) Agriculture. Bona fide agricultural pursuits, for which a soil conservation plan has been approved by the local Soil and Water Conservation District. (6) Emergency. Action taken under emergency conditions, either to prevent imminent harm or danger to persons, or to protect property from imminent danger of fire, violent storms, or other hazards. (F) Compliance with chapter provisions. Although a (3) Insufficient or incomplete revised application. specific permit is not required for these particular A stop work order for all grading on the project circumstances, this exception does not exempt the shall be issued by the City Engineer if the owner/developer/builder from complying with the revised application is determined by the City pollution prevention and erosion and sediment Engineer to be still insufficient or incomplete. control provisions of this chapter. (4) Stabilization and revegetation after stop work order. If a stop work order is issued pursuant to §170.03 (C) (3), the applicant shall stabilize and revegetate all graded and otherwise CD170:4 (Code 1991, §163.04; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No, 4314, 5-15-01; Ord. 4920, 9-05-06; Ord. 5336, 8-3-10) 170.04 Drainage Permit Conditions TITLE XV UNIFIED DEVELOPMENT CODE Each permit issued shall be subject to the following conditions. (A) Area. The development, including associated construction, shall be conducted only within the area specified in the approved permit. (B) Execution. Activities requiring a stormwater management, drainage, and erosion control permit shall not commence until the drainage permit is approved. The approved drainage permit shall be on file with the city and a copy on file with the contractor for review and inspection upon request. permit.itting shall be established as GoAditiaps te4he (CD) Duration. (1) Unless revoked or otherwise modified, the duration of a drainage permit issued pursuant to this chapter shall be one year. (2) If the permitted project dirGhaFgG 6#61AWF8 is not completed prior to expiration, the drainage permit duration can be extended to cover the project duration, subject to approval of the City Engineer. Gity F= �T (Dt=)Modifications. If the activity authorized by the permit is not completed according to the approved schedule and permit conditions, the City Engineer shall be notified. For revisions resulting in a schedule extension of more than 30 days, or if deviations from the permit conditions are expected to occur, approval of a permit modification is required by the City Engineer. (E6) Transfer. No transfer, assignment, or sale of the rights granted by virtue of an approved permit shall be made without prior written approval from the City Engineer. (F44)Special. Any additional special conditions, as deemed appropriate by the City Engineer, shall be established to address specific project needs or circumstances. (Code 1991, §163.05; Ord. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8-3-10) CD170:5 Fayetteville Code of Ordinances (Code 1991, §163.06; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. No. 4855, 4-18-06; Ord. 4920, 9-05-06; Ord. 5336, 8-3-10) 170.05-6 C,,bm0ggiGn Review And ApPFGval Of Plans Permit Processing 1010 1111 (A) Upon reviewed• by the City Engineer, if it is determined according to present engineering practice that the proposed development will provide control of stormwater runoff in accordance with the purposes, design criteria, and performance standards of these regulations and will not be detrimental to the public health, safety, and general welfare, the City Engineer shall approve the plan or conditionally approve the plan, setting forth the conditions thereof. (BF)Off--site improvements. If it is determined that offsite drainage improvements are required, and that such specific off-site drainage improvements are consistent with the city's current and established priorities, then cost sharing will be in accordance with "Required Off-site Improvements." If the city is unable, or unwilling, to contribute its share of the off-site costs, the developer shall have the option of: (1) Developer's expense. Building the off-site improvements at his/her own expense; (2) Detention. Providing detention so as to match downstream capacities; or (3) Delay project. Delaying the project until the city is able, or willing, to share in the off-site costs. (Code 1991, §163.07; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8- 3-10) 170.067- Performance Criteria (AR) Performance criteria. Except as otherwise provided in this chapter, a development must be designed, constructed, operated, and maintained to comply with the following performance criteria: (1) Water Quality. Where practicable, reduce the average annual post -development total suspended solids loadings by 80%. €lee6i Damage Prevention a0do fAF TITLE XV UNIFIED DEVELOPMENT CODE (2) Channel Protection. Capture the increased volume of the 1 year, 24-hour storm and release it over an extended period of time. (3) Overbank and Extreme Flood Protection. The post -development peak rate of surface discharge must not exceed the existing discharge for the 100 year, 24-hour storm; the 25 year, 24-hour storm; the 10 year, 24-hour storm; the 5 -year, 24-hour storm; and the 2 year, 24-hour storm, unless other discharge limits are deemed applicable for a specific site by the City Engineer. (4 3) Low Impact Development. Use of Low Impact Development design strategies, as described in Ghapter-179 the Drainage Criteria Manual, to attenuate lesser storms and more closely mimic predevelopment hydrology is encouraged. (5 4) Direct Discharge. Direct Discharge of a pipe into streams and/or floodways is not allowed. A stilling basin or other structure that will collect sediment, trash, etc and that will reduce the likelihood of erosion in the receiving stream due to discharge from the pipe shall be installed at pipe discharges into streams and/or floodways. (6 5) Erosion and channel stability. All stormwater management systems shall be evaluated based on their ability to prevent erosion and sedimentation of the receiving waters and adverse impacts on the site's natural systems. The design engineer shall consider the on-site and downstream effects of the peak discharges and shall design both the permanent and the construction phase of the stormwater management system in a manner that will not increase flooding, channel instability, or erosion downstream when considered in aggregate with other developed properties and downstream drainage capacities. (5) ClAfonod Ag; '5AfLqtI;;Adr" ;;Pd "floodways" by th49 ntity Rd quality from ate -! Ion!! d9VeIGPFRGRt. (7) Drainage Criteria Manual. The technical procedures and design standards contained in the Drainage Criteria Manual, prepared for the City of Fayetteville, and adopted by this chapter and as may be amended from time to time by the City Engineer, shall be used for C D 170:7 guidance to determine compliance with the performance criteria established by this chapter. (Code 1991, §163.08; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8- 3-10) 170.078 Maintenance Responsibility (A) Dedication. Those stormwater management systems approved in compliance with this chapter that will function as a part of the stormwater management conveyance system shall be dedicated to the city. All areas and/or structures to be dedicated to the city must be dedicated by plat or separate instrument and accepted by the City Engineer. (B) Agreengent to Maintain Stormwater Systems and Structures. The owner of the property on which stormwater systems and structures have been installed shall agree-te maintain in good condition and promptly repair and restore all grade surfaces, walls, drains, dams and structures, vegetation, erosion and sedimentation controls, and other protective devices. Fayetteville Code of Ordinances CD170:8 known, and if the name of the owner or owners cannot be determined, then only after publication of notice of such hearing in a newspaper having a bona fide circulation in Washington County for one insertion per week for four consecutive pepeFt. Gress data shall be at weeks; the determination of the City the Q -.Ad- n-fthe fourth year. if the designed Council shall be subject to appeal by the GapaG ity 06 not available the owner shall property owner in circuit court; and the amount so determined at said hearing, n the Anal drainage FepeFt. plus ten percent penalty for collection, shall be by the City Council certified to Any ether that no -Irl affent the the tax collector of the county, and by him placed on the tax books as delinquent taxes, and collected accordingly, and the amount, less three percent thereof, when so collected shall be paid to the city by the county tax collector. ^riocarrvArtakAn (c) In case the owner of any lot or other real property is unknown or his whereabouts After the 214 IRspeGti9R R8P9FtiG F9G96Yed, the Git is not known or he is a nonresident of this Engineer will .,,Ue dAtt;r.,,onafi n who+hor state, then a copy of the written notice hereinabove referred to shall be posted upon the premises and before any action to enforce such lien shall be had, the City Clerk shall make an affidavit setting out (C9) Right -of -Entry for Inspection. The owner shall the facts as to unknown address or provide for the City Engineer or designee to enter whereabouts or non -residence, and the property at reasonable times and in a thereupon service of the publication as reasonable manner for the purpose of inspecting now provided for by law against stormwater systems and structures. nonresident defendants may be had, and an attorney ad litem may be appointed to (D) Failure to Maintain. If a responsible person fails or notify the defendant by registered letter refuses to meet the maintenance requirements of addressed to his last known place of the residence if same can be found. City may shat4 give written notice requesting corrective action. If the conditions described in the (E)Removal and modification of Stormwater Systems Failure to Maintain notice are not corrected within and Structures. Stormwater systems and 10 days after such notice is given, the mayor, or structures may only be modified or removed with his duly authorized representative, is hereby the approval of the City Engineer, who shall authorized to enter upon the property and do determine t4a whether the stormwater system or whatever is necessary to correct or remove the structure does not function as a part of the conditions described, in the notice. The costs of stormwater management system. The applicant correcting said conditions shall be charged to the may be required to provide supporting data and owner or owners of the property and the city shall calculations that justify the removal of the have a lien against such property for such costs. stormwater systems or structures. (1) Enforcement of the Lien. The lien herein (Code 1991, §163.09; Ord. No. 3895, §1, 6-20-95; Ord. No. provided for may be enforced and collected in 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8 - either one of the following manners: 3-10; Ord. 5431, 8-16-11) Cross reference(s)--Enforcement, Ch., 153; Appeals, Ch. (a) The lien may be enforced at any time 155, Variances, Ch. 156; Bonds and Guarantees, Ch. 158; within 18 months after work has been Fees, Ch. 159. done, by an action in circuit court; or Drainage Dermmt Drar-P (b) The amount of the lien herein provided song 170.09 may be determined at a hearing before the City Council held after 30 days written notice by certified mail to the owner or owners of the property, if the name and a� whereabouts of the owner or owners be CD170:8 'TITLE XV UNIFIED DEVELOPMENT CODE (Code 1991, §163.10; Ord. No. 3895, §1, 6-20-95; Ord. No. 4100, §2 (Ex. A), 6-16-98; Ord. 4920, 9-05-06; Ord. 5336, 8- 3-10) 170.849 Stormwater Discharges From Construction Activities (A) General Requirements for Construction Sites. (1) Construction Site. A construction site is a site with activity that would result in the creation of a new stormwater management system, including the building, assembling, expansion, modification, or alteration of the existing contours of the property; the erection of buildings or other structures, any part thereof; or land clearing. (2) Owner Responsibility. The owner of a site of construction activity shall be responsible for compliance with the requirements of this chapter. (3) Erosion And Sediment Control. Best Management Practices (BMPs) shall be implemented to prevent the release of airborne dust and waterborne sediment from construction sites. Disturbed areas shall be minimized, disturbed soil shall be managed and construction site entrances/exits shall be managed to prevent sediment tracking. Streets and storm inlets must be kept clean at all times and free of loose rock, mud, debris and trash. Specific inlet protection measures may be necessary, as long as they do not interfere with vehicular traffic. Mud on streets must be physically removed and not washed into inlets. (4) Construction Sites Requiring Storm Water Pollution Prevention Plans. Erosion and sediment control systems must be installed and maintained per a state approved Storm CD170:9 Water Pollution Prevention Plan (SWPPP) before the beginning of construction and until slope stabilization and/or vegetation is established. For sites between 1 and 5 acres, the SWPPP and Notice of Intent (NOI) must be onsite at all times. For sites over 5 acres, the SWPPP and Notice of Coverage must be onsite at all times. The site owner bears responsibility in accordance with the Arkansas Department of Environmental Quality standards and general permit. (5) Construction Exits. A stabilized rock exit is required on construction sites.Rork +ts FG6id9RtiaI) 9F 59' 'ORO (all Gth@F up to it from the street. TernpepaF:y wood (6) Concrete Truck Wash Areas. No washing of concrete trucks or chutes is allowed except in properly located and constructed concrete wash pits. Proper runoff and erosion controls must be in place to retain all concrete wash water. (7) Dewatering. All rainwater pumped out of sumps and depressions on construction sites should be clear and free of sediment–and ppablems. (8) Storage of Materials. Public streets and sidewalks shall not be used for temporary storage of any containers or construction materials, especially loose gravel and topsoil. In addition to on -street storage being a violation of this chapter, all liability for any accidents and/or damages due to such storage will be the responsibility of the owner of the stored materials. (9) Dirt and Topsoil Storage. All storage piles of soil, dirt or other building materials (e.g. sand) shall be located more than 25 feet from a roadway, drainage channel or stream (from top of bank), wetland, and stormwater facility. ftem the top A -f the -bank to the stOGkpile, lis -ted- e the—Stat@3 ) Gst; a Fayetteville Code of Ordinances qualified persons; The owner ar thei Regulation No. 2; and/or any other ---ses at the representative shall provide upon the &.411212P nn rwtg ;;Ad shall be prepared t Topsoil piles surfaces must be immediately stabilized with appropriate stabilization measures.ppaGtIG96 may ndude: temporary seeding (i.e. annual rye or (b) A qualified iRSP@GtGr (provided by nthef R-0tahlo grass), m, Jr,hinn and n+hor ........ that are exposed to pr9GiPitAtiGA thR plaG8 until other stabilization plaGe. 1}avt; been r—finally stabilized, ono (10) Franchise and Private Utilities. The property owner or main contractor onsite will be responsible for restoring all erosion and mw6t be in6peGtA-d- to determine sediment control systems and public infrastructure damaged or disturbed by underground private or franchise utility construction such as water and sewer service leads, telephone, gas, cable, etc. Erosion d-eviRstre-arA must be iRspeGted and sediment control systems must be immediately restored after each utility praGt!Gable. The MI-Ilst be, construction. on any site, the party GWRer and generally FepeFted iR the ViGiRity Of t146 Subsequent propept;ew„AFs—will he site. A FaiR gauge must be maintaiRed ea -site AFePEA Shal�nPr red f, GF � h R6p9 +Gn name(s) title(s) Storm MIRter 0oW ition Oroyeptinn Dlon /Qld/ODDI Gr.r -All n no+n,n+inn oi+oo whore r nc+nit+inn n prpwpn+inn Plan /CINDDD\ for then on+ m ic+ hoFequired (when ,;Ghgd- -led and mplemented by the GenrstFUntion rite AwAeF as W IAWS , The site 9-M.A.M(ar bears the re6penrsibility-fGF RSPeGtGr. The reports shall be retained mpllemnntation nf the S%A/DDP.a from the rL++o the isite 06 fi Rally S h rli�orl '.., rl Ph'.II he m;.rin ;; oilahle /nl Wdr finofn S. Ba Sunil GR Speon+'v fS Gity (1) lrnPl8Fn9RtRtiQR C1AIDDD vii" -he �n if at o Y time (a) installation l II aintenonno. -R.N.4DS ch.olhhn AFtAllPd nnrl intaiRe d h„ CD170:10 TITLE XV UNIFIED DEVELOPMENT CODE (Ord. 4920, 9-05-06; Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) 170.944 Preliminary Plat, Lot Requirements (A) Preliminary Plats. Preliminary Plats shall include a master drainage plan for each lot related to the proposed infrastructure and adjacent lots. (B) Preliminary Plats for Residential Subdivisions. Preliminary Plats for residential subdivisions shall provide detailed drainage information 41461d+aO meeting the Arkansas Fire Prevention Code for building safety regulations for positive drainage of each lot. (1) The applicant shall prepare a master grading plan to be followed during individual lot development to convey runoff to a public drainage easement or right of way (2) Right of way, utility easements and drainage easements shall be graded and shaped in accordance with the Master Build out Grading Plan during preliminary plat construction. (C) Rear lot drainage easements. Rear lot drainage easements for nonstructural grassed swales shall not overlap utility easements with above ground structures, ie, electric transformers, gas meters, communication junctions, etc. (D) Final Plat. The Final Plat shall include the approved master drainage plan to be filed as a supplemental document. The scale shall be legible and approved by the City Engineer. (Ord. 5336, 8-3-10) 170.1042 1 & 2 Family Residential Requirements (A) 1 &2 Family Residential and Sites under One Acre. All residential lots must maintain properly installed erosion and sediment control measures from the beginning of construction until slope stabilization and/or vegetation is established in order to prevent silt and sediment from going offsite or into the street. (B) A building permit application shall contain sufficient site drainage and grading information to determine whether the project complies with the requirements of this chapter and Chapter 169, including, but not limited to: (1) Locations and types of proposed stormwater and erosion control BMPs. (2) Lot lines and conceptual foot print of building. (34) Individual lot drainage features, using contours spet-elevatieRs and flow arrows. (C) If the Final Plat of the Subdivision, in which the proposed building is located, includes an approved master drainage plan, this plan shall be included in the building permit application and the individual lot drainage plan shall follow the master drainage plan unless otherwise approved by the City Engineer. (Ord. 5336, 8-3-10; Ord. 5431, 8-16-11) CD170:11 Fayetteville Code of Ordinances 170.113 Post Construction Stormwater Management Pollution Prevention (A) Prohibitions (1) Illicit discharges are prohibited. An illicit discharge is a storm drain that has measurable flow containing pollutants and/or pathogens. No person shall discharge anything but uncontaminated stormwater, into the storm drainage system. Common stormwater contaminants include trash, yard waste, wastewater, oil, petroleum products, cleaning products, paint products, hazardous waste and sediment. (2) Illicit connections are prohibited. Illicit connections are any drain or conveyance which allows an illicit discharge to enter the storm drainage system. This prohibition includes illicit connections made in the past, regardless of whether the connection was permissible at the time of connection. (3) No person shall connect a line conveying sanitary sewage, domestic sewage or industrial waste, to the storm drainage system, or allow such a connection to continue. (B) Exemptions. The following non-stormwater discharges are acceptable and not a violation of this chapter: (1) A discharge authorized by an NPDES permit other than the NPDES permit for discharges from the MS4; (2) Uncontaminated waterline flushing and other infrequent discharges from potable water sources; (3) Infrequent uncontaminated discharge from landscape irrigation or lawn watering; (4) Discharge from the occasional non- commercial washing of vehicles within zoned residential areas; (5) Uncontaminated discharge from foundation, footing or crawl space drains, sump pumps and air conditioning condensation drains; (6) Uncontaminated groundwater; (7) Diverted F; flows and natural riparian habitat or wetland flows; (8) A discharge or flow of fire protection water that does not contain oil or hazardous substances or materials. (9) De -chlorinated swimming pool water. (109) Any other non-stormwater discharge determined by the City Engineer to meet the standards and objectives of this chapter and of the City's NPDES MS4 permit. (C) Private Drainage Systems Requirergents AppliGablo to Q -0-4-9i.9 _IX8GhaFg9_9 CD170:12 (1) Private Drainage System Maintenance. A private drainage system includes groundwater, drainage pipes or channels, and any flowing or standing water not within a Right of Way or Drainage Easement. The owner of any private drainage system shall maintain the system to prevent or reduce the discharge of pollutants. This maintenance shall include, but is not limited to, sediment removal, bank erosion repairs, maintenance of vegetative cover, and removal of debris from pipes and structures. (2) Minimization of Irrigation Runoff. Concentrated flow of irrigation water to the storm drainage system is prohibited. Irrigation systems shall be managed to reduce the discharge of water from a site. (3) Cleaning of Paved Surfaces Required. The owner of any paved parking lot, street or drive shall clean the pavement as required to prevent the buildup and discharge of pollutants. The visible buildup of mechanical fluid, waste materials, sediment or debris is a violation of this chapter. Paved surfaces shall be cleaned by dry sweeping, wet vacuum sweeping, collection and treatment of wash water or other methods in compliance with this Code. Material shall not be swept or washed into the storm drainage system. This section does not apply to pollutants discharged from construction activities. (4) Maintenance of Equipment. Any leak or spill related to equipment maintenance in an outdoor, uncovered area shall be contained to prevent the potential release of pollutants. Vehicles, machinery and equipment must be maintained to reduce leaking fluids. (5) Materials Storage. In addition to other requirements of this Code, materials shall be stored to prevent the potential release of pollutants. The uncovered, outdoor storage of unsealed containers of hazardous substances is prohibited. (6) Pesticides, Herbicides and Fertilizers. Pesticides, herbicides and fertilizers shall be applied in accordance with manufacturer TITLE XV UNIFIED DEVELOPMENT CODE recommendations and applicable laws. Excessive application shall be avoided. (7) Open Drainage Channel Maintenance. Every person owning or occupying property through which an open drainage channel passes shall prevent trash, debris, excessive vegetation, and other obstacles from their property from entering the drainage channel or obstructing flow. (D) Release Reporting and Cleanup. Any person responsible for a release of materials which are or may result in illicit discharges to the storm drainage system shall take all necessary steps to ensure the discovery, containment, abatement and cleanup of such release. In the event of such a release of a hazardous material, said person shall comply with all state, federal, and local laws requiring reporting, cleanup, containment, and any other appropriate remedial action in response to the release. (E) Authorization to Adopt and Impose Best Management Practices. The City may adopt and impose a Best Management Practices Manual and requirements identifying Best Management Practices for any activity, operation, or facility, which may cause a discharge of pollutants to the storm drainage system. Where specific BMPs are required, every person undertaking such activity or operation, or owning or operating such facility shall implement and maintain these BMPs at their own expense. (Ord. No. 4855, 4-18-06; Ord. 4920, 9-05-06; Ord. 5336, 8-3- 10) 170.124-170.99 Reserved CD170:13 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 179: LOW IMPACT DEVELOPMENT 179.01 PURPOSE..........................................................................................................................................3 179.02 APPLICABILITY................................................................................................................................3 179.03 LID SITE DESIGN STRATEGIES..................................................................................................... 3 179.0-4 SUBMITTAL REQUIREMENTS 179051 111 CREDITS .....................................................................................................................................7 179.046 MAINTENANCE OF LID SYSTEMS AND STRUCTURES.............................................................. 8 179.057--179.99 RESERVED..........................................................................................................................10 CD179:1 Fayetteville Code of Ordinances CD179:2 TITLE XV UNIFIED DEVELOPMENT CODE CHAPTER 179: LOW IMPACT DEVELOPMENT Some of the existing natural site features typically protected through the use of LID techniques are: wetlands, floodplains, forested areas, hillsides, riparian corridors and existing soils. There are a variety of LID design alternatives that allow professionals in the land development field the flexibility to implement LID stormwater design elements. The various LID practices can be used alone or in series to maximize benefits to the site. In most cases, some type of structural drainage systems will also be required to be implemented in conjunction with LID element. (A) Objectives. The objectives of this ordinance are: (1) To establish criteria by which a LID strategy can be measured and implemented through use of the criteria in the LID chapter of the Drainage Criteria Manual. (2) To strive to maintain and restore natural rainwater absorption and infiltration processes; (3) To strive to maintain pre -development hydrologic conditions; (4) To filter pollutants from stormwater runoff thereby improving water quality and positively impacting the region's lakes, streams and groundwater; (5) To reduce stormwater runoff intensity and velocity; (6) To preserve riparian banks and beds, and reduce sedimentation that impairs water quality; (7) To promote the widespread use of LID practices integrated with conventional stormwater engineering; (8) To protect the safety and welfare of citizens, property owners, and businesses by minimizing the negative impacts of stormwater discharge from land development. (B) LID Principles. (1) Define and locate critical resource areas during the project planning stage, such as; wetlands, riparian zones and soils with infiltration capacities. (2) Minimize impervious surfaces such as streets, driveways and parking areas. (3) Minimize direct connection of impervious areas which convey runoff directly to wetlands or water courses. (4) Attenuate stormwater flow through a diverse system of collection and infiltration. (Ord. 5316, 4-20-10) 179.02 Applicability (A) Development approval. The standards and guidelines contained in Chapter 5 of the Drainage Criteria Manual this Ghaptef shall apply in all cases where a land developer chooses to utilize LID to obtain Administrative, Planning Commission or City Council approval for their project. CD179:3 (B) Engineering approval. The City Engineer, or their designee, will administer this chapter and shall be responsible for final approval of all LID systems and structures. With the approval of the City Engineer, LID systems and structures may be implemented teFeplaGe OF 661ppleffleR+ The use of thesesystemsmustalse be ,.; +D „+Der ,.A -,+e . „+G611Gh GIGGapiRg fire a.,,,__ a+, Systems that are appFGyed shall 138 ir.+eg Fat8d ,.,Dore +her deG*gR f Wnn+iGR diGtat c (C) Drainage Criteria Manual. The Drainage Criteria Manual integrates LID design principles throughout the manual. Submittal requirements for LID projects are found within the submittal requirements for a drainage report. Criteria for the design of specific LID elements, criteria for receiving credit for those elements through reductions in traditional stormwater infrastructure, and maintenance requirements are detailed in Chapter 5 of the DCM. (Ord. 5316, 4-20-10) 179.03 LID Site Design Strategies Fayetteville Code of Ordinances (A) Definition. For the purposes of this chapter Low Impact Development (LID) is a stormwater management strategy concerned with maintaining, restoring or replicating the natural hydrologic functions of a site, where possible, by employing a variety and combination of natural and built features that reduce the volume and velocity of stormwater runoff, filter out its pollutants, and facilitate the infiltration of water into the ground. (B) Site design strategies. Generally, site design strategies will address the arrangement of buildings, roads, parking areas, and other features, and the conveyance of stormwater runoff across the site. LID site design strategies are intended to complement the natural and built environment while minimizing the generation of runoff. Site design strategies should address some or all of the following considerations: (1) Necessary grading and land disturbance should be designed to encourage sheet flow and lengthen stormwater flow paths. (2) Natural drainage divides should be maintained to keep flow paths dispersed. (3) Areas of impervious surfaces should be separated and stormwater should be conveyed across vegetated areas. This assists runoff filtration and encourages infiltration. (4) Distribute small-scale LID strategies across the development site in order to maximize benefits. (5) To the maximum extent possible, treat pollutant loads where they are generated. (6) Preserve naturally vegetated areas and soil types that slow runoff, filter pollutants and facilitate infiltration. (7) LID systems and structures should be integrated into the natural and built landscape with attention to flow paths, infiltration areas and the use of appropriate native plant materials. (C) Site Design Elements. In addition to water quality impacts, the #eIIGWieg LID site design elements when successfully implemented, perform three necessary functions; filtration and infiltration, capture and re -use and reductions in impervious surfaces. SPGGifiG site—desig„ elements ap eutlined belew- C D 179:4 TITLE XV UNIFIED DEVELOPMENT CODE GhaRnel Gapae.f.i��''TT C..•alp m int hp c zerl fn .man rflnnlae of ilhe .len inn sfefn4 S046 Rn m.naL.�l�i., ni iL.a •.nil .•ill determine whether to Infllkratmar. Tre 44 he.c S \A /arks L.enf 'n mid fe higL. CD179:5 6eaera;—Besiga .—Soz-e of the Geasidwaiieas •,wed \/egetat'. of the •ateml.e.i nrl Po^d �vea?oRrnl e#eGtn and vaFmati9R Co'I re.i'l.'l't.. anef ..flim+'n rate mmcracrvi-�-acc StOFFn Gharan GS Fayetteville Code of Ordinances C D 179:6 the ;Am',; are GAR4paGtod-. Sub grade Materia's And DFamRage Ra4iGLl r-Gar$&hould•be Of the R de.l.: n GP may be URSUitabIG for thi .nederw;;I An n.der,dra•n s st bs-r diad. Base Gewme The dine anal .depth of the . hn should he designed hase.d OR the storm evepk C D 179:6 TITLE XV UNIFIED DEVELOPMENT CODE Show exiGtiRg 6001 GlaGGifiGatiGRS for the (n) Additional I In Site DBS'nn Clements Additional 1=19 site elements determined to he hnnefinial aed that m nt the site design StFategies may he appFGvnd by the Git.. Engineer. eFGGieR problems OR 61te OF WithiR 3W /r\ Qheu. the PFGPGGed let laya h for a PFG ed hdi..isie OF deyelenmen+ P'a - CD179:7 Fayetteville Code of Ordinances 179.06 Maintenance of LID Systems and Structures TITLE XV UNIFIED DEVELOPMENT CODE CD179:9 Fayetteville Code of Ordinances (H) Removal and modification of LID systems and structures. LID systems and structures may only be modified or removed with the approval of the City Engineer, who shall determine the LID system or structure does not function as a part of the stormwater management system. The applicant may be required to provide supporting data and calculations that justify the removal of the LID systems or structures. (1) Exemptions from maintenance agreements and inspections. LID systems and structures that are not designed as part of a development and are instead utilized on a site by site basis (i.e., use of a rain barrel at a single family home, or individual rain gardens or filter strips on a site) shall not be required to submit a formal maintenance and inspection agreement, unless the function of the LID system or structure is found to be essential to accommodating the stormwater needs of the property or surrounding properties by the City Engineer. 179.07-179.99 Reserved CD179:10 Drainage Criteria Manual ARKANSAS July 1, 2014 TABLE OF CONTENTS CHAPTER 1. MINIMUM STORMWATER STANDARDS AND SUBMITTAL REQUIREMENTS.....................................1-1 SECTION1.1. GENERAL...............................................................................................................................................................1-1 1.1.1 Stormwater Management, Drainage, and Erosion Control Ordinance..............................1-1 SECTION 1.2. ADDITIONAL REGULATORY REQUIREMENTS..................................................................................1-2 1.2.1 Arkansas Department of Environmental Quality.........................................................................1-2 1.2.2 U.S. Army Corps of Engineers..................................................................................................................1-2 1.2.3 Floodplain Development Permits.........................................................................................................1-2 SECTION 1.3. SUBMITTAL PROCEDURES..........................................................................................................................1-2 1.3.1 Conceptual Review.......................................................................................................................................1-2 1.3.2 Technical Plat Review.................................................................................................................................1-2 1.3.3 Construction Plan Review.........................................................................................................................1-3 1.3.4 Waivers...............................................................................................................................................................1-3 SECTION 1.4. GRADING AND DRAINAGE PERMIT APPLICATION REQUIREMENTS...................................1-3 1.4.1 Transmittal Letter.........................................................................................................................................1-3 1.4.2 Preliminary Grading and Drainage Report......................................................................................1-3 1.4.3 Final Grading and Drainage Report.....................................................................................................1-4 1.4.4 Plans and Specifications.............................................................................................................................1-9 1.4.4.1 Title Sheet..........................................................................................................................................1-9 1.4.4.2 General Layout Sheet.................................................................................................................1-10 1.4.4.3 Other Requirements for Plans and Specifications......................................................1-10 1.4.5 Project Closeout and Final Acceptance............................................................................................1-12 SECTION 1.5. PERTINENT FAYETTEVILLE ORDINANCES......................................................................................1-13 CHAPTER 2. STORMWATER SIZING CRITERIA, PLANNING, AND REGULATIONS....................................................... 2-1 SECTION 2.1. STORMWATER SIZING CRITERIA............................................................................................................2-1 2.1.1 Introduction..................................................................................................................................................... 2-1 2.1.2 Minimum Standard #1- Water Quality(WQv)...............................................................................2-3 2.1.3 Minimum Standard #2 - Channel Protection(CPv)...................................................................... 2-4 2.1.4 Minimum Standard #3 - Overbank Flood Protection (Qpzs)................................................... 2-5 2.1.5 Minimum Standard #4 - Extreme Flood Protection(Qf)...........................................................2-5 SECTION 2.2. REFERENCES...................................................................................................................................... 2-6 I� F Table of Contents You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) CHAPTER 3. METHODS FOR ESTIMATING STORM WATER RUNOFF................................................................................. 3-1 3.1 SECTION 3.1. GENERAL..............................................................................................................................................3-1 3.2 SECTION 3.2. PRECIPITATION DATA.................................................................................................................. 3-1 3.2.1 Precipitation Data and Rainfall Intensity.......................................................................................... 3-2 3.3 SECTION 3.3. SCS CURVE NUMBER METHOD................................................................................................. 3-4 3.3.1 Equations and Concepts............................................................................................................................. 3-5 3.3.2 Runoff Factor................................................................................................................................................... 3-5 3.3.3 Travel Time Estimation.............................................................................................................................. 3-9 3.3.3.1 Overland Flow................................................................................................................................. 3-9 3.3.3.2 Shallow Concentrated Flow....................................................................................................3-10 3.3.3.3 Open Channels...............................................................................................................................3-10 3.3.3.4 In-line Detention Check............................................................................................................3-13 SECTION 3.4. RATIONAL METHOD.....................................................................................................................................3-14 SECTION 3.5. HEC -HMS METHODS.....................................................................................................................................3-16 SECTION 3.6. STORMWATER RUNOFF ANALYSIS SOFTWARE............................................................................3-17 SECTION 3.7. REFERENCES.....................................................................................................................................................3-17 CHAPTER4 WATER QUALITY.................................................................................................................................................................. 4-1 SECTION 4.1. THE NATURE OF POLLUTANTS IN STORMWATER RUNOFF....................................................4-1 4.1.1 Stormwater Pollution Sources................................................................................................................4-1 4.1.2 Areas with High Pollutant Discharge Potential..............................................................................4-2 SECTION 4.2. WATER QUALITY MANAGEMENT CRITERIA.....................................................................................4-2 4.2.1 Overview of Water Quality Criteria..................................................................................................... 4-2 SECTION 4.3. MEETING THE WATER QUALITY SIZING CRITERIA REQUIREMENTS WITH TOTAL SUSPENDED SOLIDS REDUCTION METHOD(TRM).....................................................4-4 4.3.1 Site Design as the First Step in Addressing Requirements......................................................4-4 4.3.2 Site Design Stormwater Credits.............................................................................................................4-4 4.3.2.1 Site Design Credit #1: Natural Area Conservation.......................................................4-5 4.3.2.2 Site Design Credit #2: Stream Buffers.................................................................................4-6 4.3.2.3 Site Design Credit #3: Vegetated Channels...................................................................... 4-7 4.3.2.4 Site Design Credit #4: Overland Flow Filtration/Groundwater RechargeZones..............................................................................................................................4-8 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) 4.3.3 Structural Stormwater Control Practices.........................................................................................4-9 4.3.3.1 Structural Stormwater Control Pollutant Removal Capabilities .........................4-11 SECTION 4.4. USING STRUCTURAL STORMWATER CONTROLS IN SERIES..................................................4-13 4.4.1 Stormwater Treatment Trains.............................................................................................................4-13 4.4.2 Use of Multiple Structural Controls in Series................................................................................4-13 4.4.3 Calculation of Pollutant Removal for Structural Controls in Series..................................4-16 4.4.4 Routing with WQv Removed..................................................................................................................4-17 SECTION 4.5. STORMWATER QUALITY BMP LONG TERM MAINTENANCE.................................................4-18 SECTION 4.6. REFERENCES.....................................................................................................................................................4-19 CHAPTER 5. LOW IMPACT DEVELOPMENT...................................................................................................................................... 5-1 SECTION 5.1. INTRODUCTION................................................................................................................................................. 5-1 5.1.1 Background and Purpose..........................................................................................................................5-1 5.1.2 Overview: LID and Stormwater Management................................................................................ 5-2 5.1.3 Chapter Components................................................................................................................................... 5-2 5.1.4 How Does this Chapter Relate to the LID Ordinance, the Fayetteville Drainage Criteria Manual, and other Ordinances? ............................................ 5-3 5.1.5 How to Use this Chapter?.......................................................................................................................... 5-4 SECTION 5.2. PLANNING AND SITE DESIGN....................................................................................................................5-5 5.2.1 Introduction and Design Principles.....................................................................................................5-5 5.2.2 Applying Intrinsic Green Stormwater Practices............................................................................ 5-8 5.2.2.1 Initial Intrinsic GSP Site Considerations............................................................................ 5-9 5.2.2.2 Intrinsic GSP Selection Process.............................................................................................. 5-9 5.2.2.3 Implementing a Stormwater Sensitive Site Design....................................................5-11 5.2.3 Green Stormwater Practices Selection Criteria...........................................................................5-13 5.2.3.1 Site Feasibility Factors..............................................................................................................5-15 5.2.3.2 Practice Feasibility Factors.....................................................................................................5-17 SECTION 5.3. THE RUNOFF REDUCTION METHOD....................................................................................................5-19 5.3.1 Introduction...................................................................................................................................................5-19 5.3.1.1 Background.....................................................................................................................................5-19 5.3.1.2 Objectives.........................................................................................................................................5-20 5.3.1.3 Conceptual Design Steps in the Runoff Reduction Method....................................5-20 You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) 5.3.2 Technical Design Procedure..................................................................................................................5-21 5.3.2.1 STEP 1: Land Use Rv Values...................................................................................................5-21 5.3.2.2 STEPS 2 AND 3: Green Stormwater Practice Rv Values...........................................5-23 5.3.2.3 Rv Values for GSPs in Series...................................................................................................5-24 5.3.2.4 Sizing of Media -Based GSPs....................................................................................................5-25 5.3.2.5 Calculation of Curve Numbers with Volume Removed............................................5-26 5.3.2.6 Calculation of Rainfall Removal Based on Capture Depth......................................5-29 SECTION 5.4. GREEN STORMWATER PRACTICES......................................................................................................5-30 5.4.1 Overview..........................................................................................................................................................5-30 5.4.2 Implementing GSPs....................................................................................................................................5-31 5.5 SECTION 5.5. REFERENCES....................................................................................................................................5-32 CHAPTER 6. STORM DRAINAGE SYSTEM DESIGN.........................................................................................................................6-1 SECTION 6.1. STORMWATER DRAINAGE DESIGN OVERVIEW.............................................................................. 6-1 6.1.1 Stormwater Drainage System Design.................................................................................................6-1 6.1.1.1 Drainage System Components................................................................................................ 6-1 6.1.1.2 Checklist for Drainage Planning and Design.................................................................... 6-1 6.1.2 Design Considerations................................................................................................................................6-2 6.1.2.1 General Drainage Design Considerations and Requirements ................................. 6-2 6.1.2.2 Inlets and Drains............................................................................................................................ 6-3 6.1.2.3 Storm Drain Pipe Systems (Storm Sewers)......................................................................6-3 6.1.2.4 Open Channels................................................................................................................................. 6-3 6.1.2.5 Energy Dissipaters........................................................................................................................ 6-4 SECTION 6.2. MINOR DRAINAGE SYSTEM DESIGN......................................................................................................6-4 6.2.1 Introduction..................................................................................................................................................... 6-4 6.2.1.1 General Criteria...............................................................................................................................6-4 6.2.2 Bypass Flow......................................................................................................................................................6-5 6.2.3 Symbols and Definitions............................................................................................................................6-5 6.2.4 Street and Roadway Gutters....................................................................................................................6-6 6.2.4.1 Design Procedure...........................................................................................................................6-6 6.2.5 Stormwater Inlets......................................................................................................................................... 6-7 6.2.6 Curb Inlet Design........................................................................................................................................... 6-8 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) 6.2.6.1 Curb Inlets on Grade.....................................................................................................................6-8 6.2.6.2 Curb Inlets in Sump...................................................................................................................... 6-8 6.2.6.3 Design Steps...................................................................................................................................... 6-8 6.2.7 Grate Inlet Design.......................................................................................................................................... 6-9 6.2.7.1 Grate Inlets on Grade................................................................................................................... 6-9 6.2.7.2 Grate Inlets in Sag..........................................................................................................................6-9 6.2.8 Combination Inlets.....................................................................................................................................6-10 6.2.8.1 Combination Inlets On Grade.................................................................................................6-10 6.2.8.2 Combination Inlets In Sump...................................................................................................6-10 6.2.9 Storm Drain Pipe Systems......................................................................................................................6-11 6.2.9.1 Introduction....................................................................................................................................6-11 6.3.4.2 Length and Slope..........................................................................................................................6-17 6.2.9.2 General Design Procedure.......................................................................................................6-11 6.3.4.3 Headwater Limitations.............................................................................................................6-17 6.2.9.3 Design Criteria...............................................................................................................................6-11 6.2.9.4 Capacity Calculations.................................................................................................................6-12 ........................................................................................................6-18 6.2.9.5 Hydraulic Grade Lines...............................................................................................................6-13 6.2.9.6 Junctions and Manholes............................................................................................................6-13 6.2.9.7 Minimum Grade............................................................................................................................6-14 SECTION 6.3. CULVERT and BRIDGE DESIGN...............................................................................................................6-14 6.3.1 Overview..........................................................................................................................................................6-15 6.3.2 Protected Streams.......................................................................................................................................6-15 6.3.3 Symbols and Definitions..........................................................................................................................6-16 6.3.4 Design Criteria..............................................................................................................................................6-17 6.3.4.1 Velocity Limitations....................................................................................................................6-17 6.3.4.2 Length and Slope..........................................................................................................................6-17 6.3.4.3 Headwater Limitations.............................................................................................................6-17 6.3.4.4 Tailwater Considerations ........................................................................................................6-18 6.3.4.5 Culvert Inlets..................................................................................................................................6-19 6.3.4.6 Inlets with Headwalls................................................................................................................6-19 6.3.4.7 Wingwalls and Aprons..............................................................................................................6-19 6.3.4.8 Material Selection........................................................................................................................6-19 6.3.4.9 Culvert Skews................................................................................................................................6-19 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) 6.3.4.10 Culvert Sizes.................................................................................................................................6-19 6.3.4.11 Outlet Protection.......................................................................................................................6-19 6.3.5 Design Procedures......................................................................................................................................6-21 6.3.5.1 Types of Flow Control................................................................................................................6-21 6.3.5.2 Procedures.......................................................................................................................................6-22 6.3.5.3 Design Procedure.........................................................................................................................6-22 6.3.5.4 Performance Curves - Roadway Overtopping...............................................................6-23 6.3.5.5 Multibarrel Installations..........................................................................................................6-24 SECTION 6.4. OPEN CHANNEL DESIGN............................................................................................................................6-25 6.4.1 Overview..........................................................................................................................................................6-2 5 6.4.1.1 Introduction....................................................................................................................................6-25 6.4.1.2 Considerations for Use of Open Channels.......................................................................6-25 6.4.2 Open Channel Types..................................................................................................................................6-26 6.4.3 Symbols and Definitions..........................................................................................................................6-27 6.4.4 Design Criteria..............................................................................................................................................6-28 6.4.4.1 General Criteria.............................................................................................................................6-28 6.4.4.2 Velocity Limitations....................................................................................................................6-29 6.4.4.3 Channel Cross Section Requirements................................................................................6-30 6.4.4.4 Channel Drops...............................................................................................................................6-31 6.4.4.5 Baffle Chutes...................................................................................................................................6-31 6.4.4.6 Computation and Software.....................................................................................................6-31 6.4.5 Manning's n Values.....................................................................................................................................6-32 6.4.6 Uniform Flow Calculations.....................................................................................................................6-35 6.4.6.1 Channel Discharge - Manning's Equation.......................................................................6-35 6.4.7 Vegetative Design Requirements........................................................................................................6-36 6.4.8 Riprap Design................................................................................................................................................6-36 6.4.9 Gradually Varied Flow - Backwater Modeling and Data Requirements .........................6-37 SECTION 6.5. ENERGY DISSIPATION DESIGN...............................................................................................................6-37 6.5.1 Overview..........................................................................................................................................................6-37 6.5.1.1 Introduction....................................................................................................................................6-37 6.5.1.2 General Criteria.............................................................................................................................6-37 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) 6.5.1.3 Recommended Energy Dissipators.....................................................................................6-38 6.5.2 Symbols and Definitions..........................................................................................................................6-38 6.5.3 Baffled Outlets...............................................................................................................................................6-39 6.5.3.1 Description......................................................................................................................................6-39 6.5.3.2 Design Procedure.........................................................................................................................6-39 6.5.4 Outfall Protection........................................................................................................................................6-40 SECTION 6.6. REFERENCES.....................................................................................................................................................6-42 CHAPTER 7. STORMWATER DETENTION.......................................................................................................................................... 7-1 SECTION7.1. GENERAL............................................................................................................................................................... 7-1 7.1.1 Introduction..................................................................................................................................................... 7-1 7.1.2 Volume of Detention.................................................................................................................................... 7-1 7.1.3 Design Criteria................................................................................................................................................7-1 SECTION 7.2. DETENTION DESIGN PROCEDURES........................................................................................................ 7-1 7.2.1 Introduction..................................................................................................................................................... 7-1 7.2.2 Estimating Detention Volume................................................................................................................. 7-2 7.2.3 Detention Basin Design Procedure...................................................................................................... 7-4 SECTION 7.3. METHODS OF DETENTION.......................................................................................................................... 7-5 7.3.1 Structural Controls Appropriate for Detention............................................................................. 7-5 7.3.1.1 Stormwater Ponds......................................................................................................................... 7-6 7.3.1.2 Stormwater Wetlands.................................................................................................................. 7-6 7.3.1.3 Dry Detention / Dry ED Basins............................................................................................... 7-6 7.3.1.4 Multi-purpose Detention Areas.............................................................................................. 7-7 7.3.1.5 Underground Detention.............................................................................................................7-7 SECTION 7.4. DETENTION DESIGN STANDARDS..........................................................................................................7-8 7.4.1 General................................................................................................................................................................7-8 7.4.2 Dry Detention / Dry ED Basins............................................................................................................... 7-8 7.4.3 Stormwater Ponds........................................................................................................................................ 7-8 7.4.4 Parking lots....................................................................................................................................................... 7-9 7.4.5 Low Impact Development Practices.................................................................................................... 7-9 7.4.6 Underground Detention............................................................................................................................. 7-9 7.4.7 Wetlands............................................................................................................................................................ 7-9 �. I�I4LF[ viii You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) 7.4.8 Other Methods..............................................................................................................................................7-10 7.4.9 Verification of Adequacy..........................................................................................................................7-10 7.4.10 Outlet Works..................................................................................................................................................7-10 7.4.11 Discharge Systems......................................................................................................................................7-10 7.4.12 Ownership of Stormwater Detention Ponds .................................................................................7-10 7.4.13 Easements.......................................................................................................................................................7-10 7.4.14 Maintenance...................................................................................................................................................7-11 SECTION 7.5. DOWNSTREAM HYDROLOGIC ASSESSMENT...................................................................................7-11 7.5.1 Introduction...................................................................................................................................................7-11 7.5.2 Reasons for Downstream Problems..................................................................................................7-12 7.5.3 The Ten -Percent Rule................................................................................................................................7-13 7.5.4 Example Problem........................................................................................................................................7-14 SECTION 7.6. STORMWATER DETENTION ANALYSIS SOFTWARE...................................................................7-15 SECTION 7.7. REFERENCES.....................................................................................................................................................7-15 CHAPTER 8. CONSTRUCTION SITE STORMWATER MANAGEMENT...................................................................................8-1 SECTION 8.1. PERMITS AND PLANS.....................................................................................................................................8-1 8.1.1 Stormwater Pollution Prevention Plan..............................................................................................8-1 8.1.2 Grading and Drainage Permits............................................................................................................... 8-1 8.1.3 Phased Construction.................................................................................................................................... 8-1 8.1.4 Installation and Maintenance.................................................................................................................. 8-1 8.1.4.1 Stormwater Pollution Prevention Plans............................................................................ 8-1 8.1.5 Qualified Inspector.......................................................................................................................................8-2 8.1.6 Modifications...................................................................................................................................................8-2 8.1.7 Stabilization......................................................................................................................................................8-2 SECTION 8.2. EROSION, RUNOFF, AND SEDIMENT CONTROLS FOR CONSTRUCTION SITES................................................................................................................................................8-3 8.2.1 Erosion Control...............................................................................................................................................8-3 8.2.2 Runoff Control................................................................................................................................................. 8-5 8.2.3 Sediment Control........................................................................................................................................... 8-6 8.2.4 Good Housekeeping...................................................................................................................................... 8-8 I�I4LF[ �iiii You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) SECTION 8.3. UNDERGROUND UTILITY CONSTRUCTION - PLANNING AND IMPLEMENTATION....................................................................................................................................................... 8-9 SECTION 8.4. POST -CONSTRUCTION SITE STABILIZATION STANDARDS.......................................................8-9 SECTION 8.5. REFERENCES.....................................................................................................................................................8-10 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) TABLE OF CONTENTS (CONTINUED) LIST OF APPENDICES APPENDIX A: Intrinsic GSP Specifications APPENDIX B: GSP Specifications APPENDIX C: Soil Infiltration and Soil Amendments APPENDIX D: Native Plants for Use In Bioretention APPENDIX E: Detention Structural Controls APPENDIX F: Water Quality Structural Controls APPENDIX G: Detention Outlet Structure Design APPENDIX H: Stormwater Software APPENDIX I: Construction Stormwater BMPs APPENDIX 1: EPA NPDES Fact Sheets Erosion Control Runoff Control Sediment Control Good Housekeeping Table of Contents You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) LIST OF TABLES Table 2.1 Summary of the stormwater sizing criteria for stormwater control andmitigation................................................................................................................................................................... 2-1 Table 3.1 Rainfall intensity for Fayetteville, Arkansas..................................................................................................... 3-2 Table 3.1.a Design rainfall for Fayetteville, Arkansas........................................................................................................... 3-4 Table3.2 Runoff curve numbers.................................................................................................................................................. 3-8 Table 3.3 Roughness coefficients (Manning's n) for sheet flow ................................................................................3-10 Table 3.4 Manning's roughness coefficient...........................................................................................................................3-11 GSP cost selection factors.........................................................................................................................................5-19 Table 3.5 Runoff coefficients for various land uses..........................................................................................................3-15 Table 5.7 Table 3.6 Frequency factors for the Rational Formula...................................................................................................3-16 Media volume -based specifications.....................................................................................................................5-25 Table 4.1 Summary of urban stormwater pollutants........................................................................................................4-1 Table 4.2 Summary of site design practices that receive site design stormwater credits.............................4-5 Table 4.3 Design pollutant removal efficiencies for structural stormwater controls.........................................4-12 Table 5.1 Stormwater management goals: Traditional design vs. LID..................................................................... 5-2 Table 5.2 Questionnaire for designer on implementing stormwater better site designpractices.............................................................................................................................................................5-11 Table 5.3 Areas of high pollution potential..........................................................................................................................5-17 Table5.4 GSP selection factors...................................................................................................................................................5-18 Table 5.5 GSP cost selection factors.........................................................................................................................................5-19 Table 5.6 Site cover runoff coefficients...................................................................................................................................5-22 Table 5.7 Green stormwater practices runoff reduction credit percentages......................................................5-23 Table 5.8 Media volume -based specifications.....................................................................................................................5-25 Table 5.9 Fayetteville 24-hour rainfall depths....................................................................................................................5-27 Table 5.10 Effectiveness of GSPs in meeting stormwater management objectives ...........................................5-31 Table 5.11 Green stormwater practice lands use and land area selection matrix..............................................5-32 Table 6.1 Flow spread limits & ponding depths for inlets - streets and parking (10 -year design storm)................................................................................................................................................ 6-5 Table6.2 Symbols and Definitions.............................................................................................................................................. 6-6 Table6.3 Gutter report output file.............................................................................................................................................. 6-7 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) LIST OF TABLES (CONTINUED) Table6.4 Inlet report output file.................................................................................................................................................. 6-9 Table 6.5 Manhole sizes and spacing.......................................................................................................................................6-14 Table 6.6 Culvert and bridge sizing requirements based on roadway type........................................................6-15 Table6.7 Symbols and definitions............................................................................................................................................6-17 Table6.8 Inlet coefficients.............................................................................................................................................................6-20 Table6.9 Manning's n values.......................................................................................................................................................6-21 Table 6.10 Sample culvert output file.........................................................................................................................................6-23 Table 6.11 Symbols and definitions............................................................................................................................................6-28 Table 6.12 Maximum velocities for comparing lining materials..................................................................................6-29 Table 6.13 Maximum velocities for vegetative channel linings....................................................................................6-30 Table 6.14 Channel report output file........................................................................................................................................6-32 Table 6.15 Manning's roughness coefficients (n) for artificial lined channels .....................................................6-33 Table 6.16 Uniform flow values of roughness coefficient n............................................................................................6-33 Table 6.17 Symbols and definitions............................................................................................................................................6-38 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) LIST OF FIGURES Figure 2.1 Representation of the stormwater sizing criteria.......................................................................................... 2-2 Figure 2.2 Sizing criteria water surface elevations in a stormwater pond..............................................................2-2 Figure 4.1 Generalized stormwater treatment train.........................................................................................................4-13 Figure 4.2 Examples of structural controls used in series..............................................................................................4-14 Figure 4.3 Example treatment train - commercial development...............................................................................4-15 Figure 4.4 Curve number adjustment factor..........................................................................................................................4-18 Figure 5.1 LID implementation process..................................................................................................................................... 5-5 Figure 5.2 Green stormwater practice selection factors.................................................................................................5-13 Figure 5.3 Treatment train example - commercial development...............................................................................5-15 Figure 5.4 Runoff reduction design process..........................................................................................................................5-20 Figure 5.5 Site example with land uses....................................................................................................................................5-22 Figure 5.6 Capture depth vs. rainfall capture........................................................................................................................5-30 Figure6.1 Culvert flow conditions..............................................................................................................................................6-21 Figure6.2 Riprap sizing curve.......................................................................................................................................................6-36 Figure 6.3 Schematic of baffled outlet.......................................................................................................................................6-41 Figure 6.4 Storm drain outlet protection.................................................................................................................................6-42 Figure 7.1 Approximate detention basin routing for Type III rainfall distribution ............................................. 7-3 Figure 7.3 Online versus offline storage.................................................................................................................................... 7-5 Figure7.4 Detention timing example........................................................................................................................................7-12 Figure 7.5 Effect of increased post -development runoff volume with detention on a downstreamhydrograph..........................................................................................................................................7-13 Figure 7.6 Example of the ten -percent rule............................................................................................................................7-14 Table of Contents �t— ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 1. MINIMUM STORMWATER STANDARDS AND SUBMITTAL REQUIREMENTS SECTION 1.1. GENERAL 1.1.1 Stormwater Management, Drainage, and Erosion Control Ordinance This document adopted by ordinance No. 3895 of the City of Fayetteville, provides technical procedures and design standards to support Chapter 170: Stormwater Management, Drainage, and Erosion Control of the Title XV Unified Development Code (UDC). All development projects meeting the applicability criteria, as stated in Chapter 170.03 of the UDC, shall obtain a Grading and Drainage Permit. A Stormwater Management, Drainage and Erosion Control Permit is required from the City of Fayetteville for all activities which develop, change to a more intensive land use, construct or reconstruct a structure, or change the size of a structure, or conduct grading, clearing, or filling activities within the corporate limits of the City of Fayetteville. Exceptions where no drainage permit is required are as follows: • One single-family residence or duplex (see Section 170.10 of the UDC for one- and two-family residential requirements). • One commercial or industrial project built on an individual lot that is part of a larger subdivision that has been issued an approved drainage permit when the proposed project is demonstrated to be in compliance with the overall subdivision drainage permit. • Existing commercial or industrial structure where additional structural improvements are less than 2,000 square feet. • Maintenance or clearing activity that does not change or affect the quality, rate, volume, or location of stormwater flows on the site or runoff from the site. • Bona fide agricultural pursuits for which a soil conservation plan has been approved by Washington County Soil and Water Conservation District. • Action taken under emergency conditions, either to prevent imminent harm or danger to persons, or to protect property from imminent danger of fire, flooding, or other hazards. The application for a Grading and Drainage Permit shall be prepared by the Engineer of Record, who is a licensed professional engineer of the State of Arkansas, and shall be submitted in accordance with the submittal procedures described in this chapter. The Grading and Drainage Permit application shall consist of a Transmittal Letter, the Final Drainage Report, and the Grading and Drainage Design Plans and Specifications (Plans and Specifications). The Final Drainage Report Checklist in Section 1.4.3 shall be filled out and signed and sealed by the Engineer of Record. Chapter 1– Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) SECTION 1.2. ADDITIONAL REGULATORY REQUIREMENTS 1.2.1 Arkansas Department of Environmental Quality A NPDES Construction Stormwater General Permit (Permit No. ARR150000) is required for discharges from large and small construction activities that result in a total land disturbance of equal to or greater than one acre, where those discharges enter waters of the State or a municipal separate storm sewer system (MS4). Small construction sites (disturbing one acre or more and less than five acres) have automatic coverage under the Construction Stormwater General Permit. Under automatic coverage for small sites it is not necessary to submit any documents to ADEQ and there is no fee. However, the automatic Notice of Coverage (NOC) must be posted at the site prior to commencing construction and a Stormwater Pollution Prevention Plan (SWPPP) must be prepared and made available at the site prior to commencing construction. Large Construction Sites (disturbing five acres or more) must submit a Notice of Intent (NOI), a Stormwater Pollution Prevention Plan (SWPPP) and pay a fee to the Arkansas Department of Environmental Quality (ADEQ) in order to obtain coverage for discharges of stormwater associated with construction activity at any site or common plan of development or sale that will result in the disturbance of five (5) or more acres of total land area. Additional information may be found at: htt12: / /www.adeq.state.ar.us/water 1.2.2 U.S. Army Corps of Engineers Section 404 of the Clean Water Act requires a permit from the U.S. Army Corps of Engineers (USACE) prior to discharging dredged or fill material into waters of the United States, including wetlands. Activities in waters of the United States regulated under this program include fill for development, water resources projects (such as dams and levees), infrastructure development (such as highways and airports), streambank restoration, and mining projects. 1.2.3 Floodplain Development Permits Any development within or bordering a Special Flood Hazard Area, as portrayed on FEMA Flood Insurance Rate Maps (FIRMS), or bordering a Protected Stream, as portrayed on the Protected Streams Map, is required to obtain a Floodplain Development Permit. Permit requirements and application procedures can be found in Section 168.07 of the City of Fayetteville Unified Development Code. SECTION 1.3. SUBMITTAL PROCEDURES 1.3.1 Conceptual Review A preliminary meeting (prior to technical plat review or development of construction documents) with the engineering staff is suggested before developing site improvement plans. 1.3.2 Technical Plat Review A Grading and Drainage Permit application consisting of a Transmittal Letter, Preliminary Grading and Drainage Report (see Subsection 1.4.2) and Preliminary Grading, Drainage, and Erosion Control Plans shall Chapter 1— Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) be submitted to the Planning Department for Technical Plat Review for all site development submittals. The Technical Review meeting will be scheduled by the City of Fayetteville with representatives of the developer, including the Engineer, to review the overall concepts included in the preliminary Grading and Drainage Permit application. The purpose of this review is to assess the overall stormwater management concept for the proposed development and to review criteria and design parameters that shall apply to final design of the project. The Urban Forester's review of the Tree Preservation Ordinance, for example, can impact the grading and stormwater design. Additional submittals may be required for subsequent meetings with the Subdivision Committee and Planning Commission. 1.3.3 Construction Plan Review Following project approval by the Planning Department, the final civil site package consisting of a Transmittal Letter, Final Grading and Drainage Report (see Subsection 1.4.3) including checklist, grading, drainage, and erosion control plans and specifications shall be submitted to the Engineering Coordinator in .pdf format for review. Developments within the floodplain shall also gain approval from the Floodplain Administrator prior to issuance of a Grading and Drainage Permit. Additional submittals may be necessary. The application submittal shall be in accordance with Section 1.4, Grading and Drainage Permit Application Requirements. Once the final package has been reviewed for compliance and compliance has been confirmed, a conditional approval letter will be issued to the design engineer. The conditional approval letter allows the contractor to install perimeter erosion control measures. Issuance of the Grading and Drainage Permit is also dependent upon the review and approval of the Urban Forester. Before issuance of the Grading and Drainage Permit the perimeter erosion controls and tree protection measures must be inspected and the preconstruction conference held. Final approved plans shall be submitted to reviewing divisions for other development permits as needed. 1.3.4 Waivers Only the City Engineer can grant waivers to the Minimum Standards or any other requirement of this Drainage Criteria Manual, if adequate documentation and supporting calculations are provided that demonstrate such a waiver is warranted. Proof of receipt of such waivers shall be provided to the City as part of subsequent submittals throughout the remainder of the project application process. SECTION 1.4. GRADING AND DRAINAGE PERMIT APPLICATION REQUIREMENTS 1.4.1 Transmittal Letter A cover letter shall be included with each submittal. The cover letter should include the Planning Project number assigned by the Planning Department and the project name. 1.4.2 Preliminary Grading and Drainage Report A Preliminary Drainage Report will be required at the time of the Technical Plat Review for site development projects. The Preliminary Grading and Drainage Report shall follow the Final Grading and Drainage Report Chapter 1— Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Template provided in Section 1.4.3, items 1-15. Also submit preliminary grading and drainage drawings. Items 16 - 25 are not required for the Preliminary Report. 1.4.3 Final Grading and Drainage Report A Final Grading and Drainage Report, following the Final Grading and Drainage Report Template as provided below, shall be included in the Final Grading and Drainage Permit Application. Computer input and output information shall also be provided as part of the Final Grading and Drainage Report. An example of input and output to report may be found at the end of Chapter 6, and a sample output sheet is provided in Appendix H. A Grading and Drainage Permit will not be issued until the Final Report has been submitted, reviewed, and approved. The City Engineer may request a more detailed drainage study prior to the approval of the Final Grading and Drainage Permit application and issuance of the permit. If hydrologic and hydraulic studies reveal that the proposed development would cause increased frequency of flooding, increased depth of inundation of structures, or inundation of unprotected structures not previously subject to inundation, then the permit application shall be denied unless one or more of the following mitigation measures are used: (1) onsite storage, (2) offsite storage, or (3) offsite drainage system improvements. If it is determined by the City Engineer that offsite drainage improvements are required, then cost sharing will be in accordance with City ordinances (UDC Chapter 166.04 and UDC Chapter 170.06.17). If the City is unable to contribute its share of the offsite costs, the developer shall have the option of: a) building the offsite improvements at his own expense, b) providing detention so as to match pre -development downstream capacities, or c) delaying the project until the City is able to share in the offsite costs. Final Drainage Report Template and Checklist The City of Fayetteville, Arkansas Project name Engineer of Record Planning Project Number Revision no. Date 1. _ PROJECT TITLE & DATE 2. _ PROJECT LOCATION - Include street address and Vicinity Map. 3. _ PROJECT DESCRIPTION - Brief description of the proposed project. Chapter 1- Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 4. _ NAME, ADDRESS, TELEPHONE NUMBER, AND EMAIL of the owner and developer of the property to be permitted. 5. _ NARRATIVE SUMMARY - The summary shall include a description of the methods used to meet the requirements of the Minimum Standards. This includes at a minimum a description of the treatment train for Minimum Standard #1 and a description of the detention strategy used to meet the requirements of Minimum Standards #2 through #4. Also include a description of the off-site areas, onsite areas, condition of the downstream receiving areas, existing problems, changes to flows and flow volume, proposed improvements, detention, areas with potential for high pollutant loading, and final conclusions. 6. _ EXISTING DRAINAGE AREA MAP - Existing drainage area map on a 1 -inch = 200 -feet minimum scale plan drawing, with 2 foot contours (1 foot contours on "flat" sites), that includes: study points at property lines, time of concentration path, bar scale, and the following information: a. —Aerial photograph of the project vicinity, covering the project area and the total lands that contribute runoff; b. _ Existing drainage areas and flow patterns to downstream property line, establishing the study points; c. _ Upstream and downstream drainage flow paths for all areas that contribute runoff to the existing site or receive runoff from the site. The downstream area(s) shall be shown as necessary to document the receiving conveyance system; and d. _ Existing land use conditions for the drainage areas that contribute runoff. 7. _ SOIL MAP - Provide the most recent U.S. Soil Conservation Service soils and vegetation information for both the project area and the drainage area that contributes runoff on a separate map from the Existing Drainage Area Map. 8. _ PROPOSED DRAINAGE AREA MAP - Proposed drainage area site map on a 1 -inch = 200 -feet minimum scale plan drawing, with 2' contours (1 foot contours on "flat" sites), that include: study points, time of concentration path, bar scale, and the following information: a. _ Proposed drainage areas and flow patterns and, if applicable, natural feature protection areas, green stormwater practice and infiltration areas; b. _ Upstream and downstream drainage flow paths for all areas that contribute runoff to the proposed development site or receive runoff from the site. The downstream area(s) shall be shown as necessary to document the receiving conveyance system; c. _ Proposed land use conditions for the development site and drainage areas that contribute runoff; and d. _ Proposed locations of grading and placement of fill material within the project area and drainage areas that contribute runoff. 9. _ MINIMUM STANDARD #1, Water Quality - Calculations and documentation indicating the percentage of the total suspended solids that the proposed stormwater management system is capable of removing from the stormwater flows EXCEEDING predevelopment levels. a. _ If the TSS Reduction Method described in Chapter 4 is used to meet this standard, Chapter 1- Minimum Stormwater Standards t— And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) provide calculations for each structural control indicating the corresponding level of treatment; and ii. provide a map showing the impervious area and structural controls b. _ If Green Stormwater Practices (GSPs) described in Chapter 5, Low -Impact Development, are used to meet this standard, provide calculations determining the total Volumetric Runoff Coefficient (Rv) for the site. The printed output from the Low Impact Development calculation spreadsheets provided by the City of Fayetteville shall be submitted to confirm the level of treatment provided by the design; and ii. provide a map showing the impervious areas and GSPs. c. _ If the goal of 80% removal is not practicable, provide a description explaining why 80% removal is not achievable. d. _ FLOW TABLE - Provide a summary of peak discharges table that lists the pre -development, the post -development without mitigation, and the post -development with mitigation flow rates for the 2-, 10-, 25-, and 100 -year storm events for each study point. 10. _ MINIMUM STANDARD #2, Channel Protection - Provide calculations and documentation indicating compliance with Minimum Standard #2 such that the 1 -year post development site flow is captured. The calculations shall include the following information: a. _ Calculations of the predevelopment 1 -yr, 24-hour peak discharge and whether it is >2cfs. If below 2 cfs, the CPv requirement shall not apply. b. _ Calculations and documentation showing that the 1 -year, 24-hour storm volume is captured and released over a period of 40 hours. C. _ The calculated discharge velocity for the 10 -year, 24-hour storm event. If the discharge velocity exceeds or is near to the erosion velocity of the downstream channel system, then energy dissipation, erosion prevention measures, and/or velocity control measures shall be designed to control the velocity of or mitigate erosion potential from the 10 -year, 24-hour storm event. d. _ Calculations for the energy dissipation measures, if required, to reduce the discharge velocity calculated above. e. —The design shall also comply with all requirements of the City of Fayetteville Streamside Protection Ordinance (UDC Chapter 168.12). 11. _ MINIMUM STANDARD #3, Overbank Flood Protection - Provide calculations and documentation indicating compliance with Minimum Standard #3 such that the post -development peak discharge rate does not exceed the predevelopment rate for the 2 -year, 5 -year, 10 -year, and 25 -year, 24-hour storm events. The calculations shall include the following information: a. _ A summary table of runoff discharge flows for the 2 -year, 5 -year, 10 -year and 25 -year, 24- hour storm events for the pre -development and post -development conditions for each study point. The summary shall include the existing and proposed flows along with supporting calculations for all of the discharge points to the receiving system. This includes the flow Chapter 1- Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) entering each drainage area and the flow generated within each drainage area on the site (do not separate onsite and offsite flows). 12. _ MINIMUM STANDARD #4, Extreme Flooding - Provide calculations and documentation indicating compliance with Minimum Standard #4, such that the post -development peak discharge rate does not exceed the predevelopment rate for the 100 -year, 24-hour storm event. The calculations shall include the following information: a. _ Calculations and results of the extreme flood analysis showing that the 100 -year, 24-hour storm event has been controlled as required by Minimum Standard #4 such that the post - development peak discharge rate does not exceed the pre -development rate for this event. b. _ The effects of the 100 -year, 24-hour storm event (Qf) on the stormwater management system, adjacent property, and downstream facilities and property shall be evaluated. The Qf shall be controlled through the use of structural stormwater controls to protect existing downstream property with no increase in the existing base flood elevation, or calculations shall be provided to indicate that the on-site conveyance system will safely pass Qf and allow it to discharge into receiving waters where the floodplain is of capacity sufficient to accommodate significant additional discharges without causing damage. C. _ A summary table of discharges for the 100 -year 24-hour storm event for the pre - development and post -development conditions for each study point. 13. _ CHECK FOR EXISTING DOWNSTREAM FLOODING - Describe the existing downstream capacity of each receiving area (study point). Provide documentation of an assessment of downstream conditions a minimum of 1/4 mile downstream of the proposed development in accordance with Section 7.5. Documentation shall include photographs of the existing structures downstream of the development as well as a map showing the locations and distances of downstream structures from the development. 14. _ FLOW TABLE - List in a summary table the pre -development, post -development without mitigation, and post -development with mitigation flow rates for the 2-, 10-, 25-, and 100 -year, 24- hour storm events for each study point. 15. _ STORMWATER DETENTION DESIGN - If detention is required to comply with the Minimum Standards, include all computations and backup/support data including: a. _ Detention basin size requirement computations (using an approved method). b. _ Release structure design computations including design Water Surface Elevations for the 1- (where required), 10-, 25-, and 100 -year storms. If extended detention of 1 -year is not provided, design computations for the 2- and 5 -year storm are also required. c. _ Stage -Storage and Stage -Discharge curves for the detention facility. d. _ A summary hydrograph of the effect of the detention facility for relevant storms, incorporated with bypass. e. _ Overflow structure(s) size and location(s); f. _ Outfall structure(s), location(s), and orifice size(s). g. _ Emergency overflow path. Chapter 1- Minimum Stormwater Standards t -Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) h. _ Results of downstream analysis. 16. _ PAVEMENT DRAINAGE DESIGN - Include a table listing street classification, width, allowable spread and actual spread for 10 and 100 -year, 24-hour storm. 17. _ STORM SEWER INLET DESIGN - Include all computations for the 10 and 100 -year, 24-hour storm. Reference table in Chapter 6 for allowable spread and depth. 18. _ INLET DRAINAGE AREA MAP - Provide a separate map showing the inlet layout and design including the drainage areas. The map should include the proposed design, drainage areas, time of concentrations paths, runoff coefficients, and bar scale. 19. _ STORM SEWER DESIGN - Include all computations and hydraulic profiles for the 10 and 100 -year, 24-hour storms. 20. _ CULVERT DESIGN - Include all computations, hydraulic profile, and energy transition to channel. 21. _ OPEN CHANNEL FLOW DESIGN - Include computations for normal depth and velocity. 22. _ FEDERAL AND STATE REQUIREMENTS (Answer Yes or No if required). a. _ Wetlands determination (if wetlands are present on the site). b. _ 404 permit required (include letter from USACE as an exhibit). C. _ NPDES Construction Stormwater "Notice of Intent" (ADEQ)(include as an exhibit if required). d. _ ANRC permit/review for "dams" (required if a stormwater impoundment qualifies as a dam per ANRC regulations). e. _ Other 23. _ EXHIBITS - Attach the following exhibits to the final drainage report. a. _ Grading and drainage construction drawings. b. _ Landscaping Plan. c. _ Operations and maintenance plan (see Section 7.4.12). d. Letter from USACE if answered Yes to 23.b. above. e. _ Notice of Coverage (NOC) and completed SWPPP (sites 1 acre or larger). f. Master Drainage Plan. 24. The following paragraph with relevant information included: "I, , Registered Professional Engineer No. in the State of Arkansas, hereby certify that the drainage studies, reports, calculations, designs, and specifications contained in this report have been prepared in accordance with sound engineering practice and principles, and the requirements of the City of Fayetteville. Further, I hereby acknowledge that the review of the drainage studies, reports, calculations, designs, and specifications by the City of Fayetteville or its representatives cannot and does not relieve me from any professional responsibility or liability." Signed & Sealed by Professional Engineer Chapter 1- Minimum Stormwater Standards t -Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 25. ARKANSAS REGISTERED ENGINEER SEAL 1.4.4 Plans and Specifications Grading and Drainage Plans and Specifications are to be signed and sealed by a professional engineer registered in the State of Arkansas in accordance with applicable state statutes and State PELS board licensure requirements. Because plans, specifications, and calculations may be retained by the City for use as permanent records, neatness, clarity and completeness are very important, and lack of these qualities will be considered sufficient basis for submittal rejection. A complete legend shall be included in the set of Plans. Plan sheet size will be 22 inches x 34 inches with all sheets in a given set of plans the same size. Plan drawings will be prepared with a maximum horizontal scale of 1 inch = 100 feet unless otherwise approved by the City Engineer. Profile drawings for storm sewers should be drawn at a suggested horizontal scale of 1 inch = 20 feet, with a minimum scale of 1 inch = 50 feet and a maximum vertical scale of 1 inch = 5 feet. Drainage ditch profiles should be drawn at a suggested horizontal scale of 1 inch = 20 feet, with a minimum scale of 1 inch = 50 feet and a maximum vertical scale of 1 inch = 5 feet. Special cases may warrant use of larger or smaller scale drawings for increased clarity or conciseness of the plans and may be used with prior permission from the City Engineer. Each sheet in a set of Plans shall contain a sheet number, the total number of sheets in the Plans, proper project identification, and the date. Revised sheets submitted must contain a revision block with identifying notations and dates for revisions. Plans and Specifications for the proposed improvements will be submitted in the following format during the project application process, where pertinent, and shall include at a minimum: (1) Title Sheet, (2) General Layout Sheet, (3) Grading, Drainage, Paving, and/or Building Plans, (4) Erosion and Sedimentation Control Plan, (5) Plan and Profile Sheet(s), and (6) Standard and Special Detail Sheets. 1.4.4.1 Title Sheet Title sheet shall include: 1. The designation of the project, which includes the nature of the project, legal description, the name or title, city, and state; 2. Planning Department project number; 3. Index of sheets; 4. Vicinity map showing project location in relation to streets, railroads, and physical features. The map shall have a north arrow and appropriate scale; 5. A project control benchmark identified including the location and elevation with notation referencing City monument(s) used to establish the Project Benchmark; 6. Reference to horizontal and vertical datum for the project; Chapter 1— Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 7. The name, address, telephone number, and email address of the owner of the project and the name, address, telephone number, and email address of the engineer preparing the plans; 8. Floodplain statement identifying the FIRM panel, date, and flood zone; and, 9. Engineer's seal (every sheet). 1.4.4.2 General Layout Sheet The General Layout Sheet shall include: 1. North arrow and scale. 2. Legend of symbols that will apply to all sheets. 3. Name of subdivision, if applicable, and all street names. Unplatted tracts should have an accurate tie to at least one quarter -section corner. 4. Boundary line or project area. 5. Location and description of existing major drainage facilities within or adjacent to the project area. 6. Location of major proposed drainage facilities. 7. Location and dimensions of proposed drainage and utility easements. 8. Name of each utility within or adjacent to the project area. 9. Standard notes. 10. If more than one General Layout Sheet is required, a match line should be used to show continuation of coverage from one sheet to the next sheet. 1.4.4.3 Other Requirements for Plans and Specifications 1. Topographic sheet size will be 22 inches x 34 inches with all sheets in a given set of topographic surveys the same size. Topographic drawings will be drawn to a recommended horizontal scale of 1 inch = 20 feet having a contour interval of 1 foot, with a maximum horizontal scale of 1 inch = 100 feet and a maximum contour interval of 2 feet. Special cases may warrant use of larger or smaller scale drawings for increased clarity or conciseness of the site topography and may be used with prior permission of the City Engineer. 2. All topographic surveys shall meet those specifications found in Section E, NSPS MODEL STANDARDS FOR TOPOGRAPHIC SURVEYS as approved March 12, 2002. The horizontal datum shall be NAD83, Arkansas State Plane, North Zone. The units shall be U.S. Survey Foot. 4. The vertical datum shall be North American Vertical Datum of 1988 (NAVD88). 5. At least two horizontal control monuments shall be shown on each sheet. At least one benchmark shall be shown on each sheet. A horizontal and vertical tie to at least one City of Fayetteville GPS monument shall be made and the results provided to the City Surveyor. Chapter 1– Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6. Proposed Grading, Drainage, Paving, and Building Plan showing details of proposed grading, drainage, paving, and buildings. The required details for all plans, including street plans, water and sewer plans, grading and drainage plans shall include all information deemed necessary by the City Engineer for review. 7. The drainage plan (and/or street and drainage plan) shall include plan and profiles of streets and storm drainage, street cross-sections, and details of all drainage systems and appurtenances. 8. Include the Master Residential Lot Grading Plans as an Exhibit to the Plans. 9. Green Stormwater Practices (GSPs) shall be shown on the Plans and Specifications and shall be designed and constructed in accordance with the standards and criteria in Chapter 5, Low Impact Development, and relevant Appendices of the Drainage Criteria Manual. 10. Erosion and Sediment Control Plan identifying the type, location, and schedule for implementing erosion and sediment control measures, including appropriate provisions for maintenance and disposition of temporary measures. 11. Operation and Maintenance Plan, included as a separate exhibit, prepared by a registered professional engineer, describing the activities and schedule required to operate and maintain the permitted facilities. The plan should include specific details such as when and what to mow and when to remove sediment. 12. Elevations on profiles, sections, and plans shall have the vertical datum designated. At least one permanent bench mark in the vicinity of each project shall be noted on the first drawing of each project, and the location and elevation of each benchmark shall be clearly defined. 13. The top of each page shall be either north or east. The stationing of street plans and profiles shall be ascending from left to right and downstream to upstream in the case of channel improvement/construction projects, unless otherwise approved by the City Engineer. 14. Each project shall show topographic data extending a minimum of 20 feet beyond each side of the project area or property boundary. Topographic data extending a minimum of 50 feet beyond the property boundary shall be shown in areas of channel or overbank flow. Provide finished floor elevations (FFE) of existing structures within the extent of topographic data. Any proposed changes, including utilities, telephone installations, etc., shall be shown on the plans and profiles. 15. Revisions to drawings shall be indicated above the title block in a revision block. The revisions shall be annotated and the revision block shall describe the nature and date of the revision made. 16. Standard symbols for engineering plans shall be used and a legend of symbols provided. Existing utilities, telephone installations, sanitary and storm sewers, pavements, curbs, inlets, culverts, etc., shall be shown with a broken line; proposed facilities with a solid line; and land, lot, and property lines with a slightly lighter dashed line. 17. Easements, lot lines, and dimensions shall be shown where applicable. Drainage easements shall be provided in accordance with the following requirements: a. Drainage easements shall be a minimum of 20 feet. b. For pipe or culverts less than 36 -inch in diameter or width, the drainage easement shall be measured from the center of the pipe or culvert. For pipes or culverts greater than 36 -inch Chapter 1– Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) diameter or width, the easement shall be a minimum of 10 feet from the outside edges of the pipe or culvert. c. For channels, a minimum drainage easement of 20 feet shall be provided. However, in addition to the minimum drainage easement width, 10 feet shall be provided from one side top of bank for access to the channel. The 10 -feet access may be provided within the minimum 20 feet, if space permits. No fences shall be permitted in the drainage easement. 18. The proposed water surface elevation (WSEL) and corresponding boundaries of inundation resulting from the 100 -year storm for all overland flow, including flow in easements, streets, parking lots, swales and between lots shall be calculated and shown on the construction drawings and must also be included in the final plat. 19. Minimum floor elevation shall be shown a minimum of 2 feet above the computed 100 -year base flood elevation (BFE) on each lot for which any of the following apply: where located in a designated floodplain, where in a localized low area due to surrounding topographic relief (existing or proposed), and where flooding is known to occur or where City Flood Damage Prevention Code (Ordinance 168) applies. Minimum floor elevations for other areas shall be a minimum of 1 foot above the calculated 100 -year WSEL of open channels or swales or overland flow. Within designated regulatory floodplain areas or where City Floodplain Ordinance 168 applies, comply with all relevant FEMA regulatory requirements. 20. It shall be understood that the requirements outlined in these standards are only minimum requirements and shall only be applied when conditions, design criteria, and materials conform to the City's specifications and are normal and acceptable to the City Engineer. When unusual subsoil or drainage conditions are suspected, an investigation should be made and a special design prepared consistent with good engineering practice. 1.4.5 Drainage Criteria for Subdivisions 1. Preliminary Plats shall include a master drainage plan for each lot related to the proposed infrastructure and adjacent lots. 2. Preliminary Plats for residential subdivisions shall provide detailed drainage information including flow arrows and design spot elevations including the proposed finish floor elevation meeting the Arkansas Fire Prevention Code for building safety regulations for positive drainage of each lot. 3. Rear lot drainage easements for nonstructural grassed swales shall not overlap utility easements with above ground structures, i.e., electric transformers, gas meters, communication junctions, etc. 4. The Final Plat shall include the approved master drainage plan to be filed as a supplemental document. The scale shall be legible and approved by the City Engineer. 1.4.6 Project Closeout and Final Acceptance 1. At the completion of a project and prior to final acceptance by the City, the Engineer of Record shall submit the following information for approval by the City Engineer. a. As -built survey drawings. The as -built drawings shall include all storm drainage structures including elevations of the top of structures, inverts, pipe sizes, and any other critical design elevations. The as -built shall also include cross-sections of any detention or retention basins. The Chapter 1– Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) cross-sections shall provide enough information to calculate the volume of detention provided by the basin. b. Revised Drainage Report per as -built changes stamped and signed by the Engineer of Record in .pdf format. The Revised Drainage Report shall confirm the material and sizes of drainage pipes and structures, the general grading pattern, and verify drainage boundaries. c. Certification of Detention supported by field data (cross-sections and as -built measurements). d. Copies of reviewed and approved construction material or product submittals in .pdf format. Submittals must be reviewed and approved prior to installation. e. Inspection reports and test reports submitted weekly. f. Construction costs of public infrastructure for maintenance bond. g. Final inspection by Design Engineer identifying all deficiencies. h. Operation and maintenance plan, executed by financially responsible party. SECTION 1.5. PERTINENT FAYETTEVILLE ORDINANCES Refer to the following chapters of the UDC and additional documents for information as it relates to development and stormwater management within the City of Fayetteville. Chapter 161- Zoning Regulations Chapter 166 - Development Chapter 167 - Tree Preservation & Protection Chapter 168 - Flood Damage Prevention Code Chapter 169 - Physical Alteration of Land Chapter 170 - Stormwater Management, Drainage, & Erosion Chapter 171- Streets and Sidewalks Chapter 172 - Parking & Loading Chapter 173 - Building Regulations Chapter 177 - Landscape Regulations Chapter 179 - Low Impact Development City Plan 2030 City of Fayetteville Drainage Criteria Manual Appendix H Exhibits City of Fayetteville Landscape Manual City of Fayetteville Streamside Protection BMP Manual City of Fayetteville Minimum Street Standards City of Fayetteville Water and Sewer Specifications Chapter 1- Minimum Stormwater Standards t—Nis And Submittal Requirements ,l You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 2. STORMWATER SIZING CRITERIA, PLANNING, AND REGULATIONS SECTION 2.1. STORMWATER SIZING CRITERIA 2.1.1 Introduction Development projects applying for a Grading and Drainage Permit shall meet the following four Minimum Standards related to stormwater runoff and protection of existing water bodies and properties. For the purposes of this Drainage Manual, predevelopment is defined as the existing conditions of the site at the time of development. The City of Fayetteville Stormwater Sizing Criteria Flow Chart should be used in conjunction with this manual to design stormwater management systems, to: • Remove stormwater runoff pollutants and improve water quality (Minimum Standard #1); • Prevent downstream streambank and channel erosion (Minimum Standard #2); • Reduce downstream overbank flooding (Minimum Standard #3); and • Control the peak flow rate of runoff from extreme storm events (Minimum Standard #4). For these objectives, the following stormwater sizing criteria have been developed which are used to size and design structural stormwater controls. Table 2.1. briefly summarizes the criteria. Tableof the stormwater. . control . mitigation. Sizing Criteria Description 1. TSS Reduction Method - Provide water quality treatment for the runoff resulting from a rainfall depth of 1.2 inches (where practicable) (Chapter 4), or Water Quality 2. Runoff Reduction Method - Capture 1.0 inch of rainfall using Low Impact Development strategies. (Chapter 5). Methods are intended to reduce the average annual post -development total suspended solids loadings by 80%. Provide extended detention of the increased volume of the 1 -year storm Channel Protection event released over a period of 40 hours to reduce flows and protect downstream channels from erosive velocities and unstable conditions. Post -development flows shall not exceed the predevelopment flows. Provide peak discharge control of the 2 -year, 5 -year, 10 -year, and 25- Overbank Flood Protection year storm event such that the post -development peak rate does not exceed the predevelopment rate. Extreme Flood Protection Provide peak discharge control of the 100 -year storm event such that the post -development peak rate does not exceed the predevelopment rate. Each stormwater sizing criterion is intended to be used in conjunction with the others to address the overall stormwater impacts from a development site. Used as a set, the criteria control a range of hydrologic events, from the smallest runoff -producing rainfalls to the 100 -year storm. hapter 2. — Stormwater Planning And Regulations You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Figure 2.1 illustrates the relative volume requirements of each stormwater sizing criterion and demonstrates that successive criteria are "stacked" upon the previous requirement - i.e., the extreme flood protection volume requirement also contains the overbank flood protection volume, the channel protection volume, and the water quality treatment volume. Figure 2.2 shows how these volumes would be stacked in a typical stormwater pond designed to handle all four criteria. Figure 2.1. Representation of the stormwater sizing criteria. EMBANKMENT *SAFETY RISER BENCH Q EXTREME FLOOD PROTECTION (100 Year) LEVEL V OVERBANK FLOOD PROTECTION (25 -Year) LEVEL CHANNEL PROTECTION LEVEL 0 0 WATER QUALITY VOLUME LEVEL AQUATIC BENCH STABLE INFLOW —� f 0 PERMANENT POOL OUTFALL III I I FOREBAY OVERFLOW POND DRAIN SPILLWAY REVERSE PIPE BARREL I, 2- MAX. ANTI -SEEP COLLAR OR DPES FILTER DIAPHRAGM ALLUVVtU wlVIING OUT OF WATER Figure 2.2. Sizing criteria water surface elevations in a stormwater pond. hapter 2. — Stormwater Planning And Regulations You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) The following text describes the four sizing criteria and presents information on how to properly compute and apply the required stormwater storage volumes. Additional detailed guidance regarding the computations is provided in Appendix E. 2.1.2 Minimum Standard #1- Water Quality (WQ„) In accordance with the City's Municipal Separate Storm Sewer System (MS4) general permit under NPDES Permit Tracking No. ARR040010, the stormwater management system should be designed to remove at least 80% of the total suspended solids (TSS) from stormwater flows which exceed predevelopment levels (where practicable) and be able to meet any other additional watershed- or site-specific water quality requirements. If a development project has an increase in impervious area, the stormwater management system shall provide (where practicable) a water quality treatment system that removes at least 80% of the TSS from the runoff from the site. There are two methods that can be used to remove 80% of the TSS from stormwater runoff generated by the post -development impervious area and meet this Minimum Standard: • The TSS Reduction Method may be used to select appropriate structural stormwater controls and design a system or "treatment train" that removes 80% of the TSS from 1.2 inches of rainfall, the Water Quality Treatment Volume (WQv). Refer to Chapter 4 for information on the TSS Reduction Method and structural stormwater controls. • Low Impact Development strategies and the Runoff Reduction Method (RRM) may be used to design a system that captures and treats the runoff volume resulting from the first 1 -inch of rainfall onto a site. Refer to Chapter 5 for information on Low Impact Development and the RRM. Appendices A, B, C, and D provide supporting information regarding Chapter 5 and Low Impact design. The Water Quality sizing criterion specifies a treatment volume, denoted WQ,,, required to size structural stormwater controls to meet Minimum Standard #1 using the TSS Reduction Method. For the City of Fayetteville, this value is computed as 1.2 inches of rainfall over the catchment area multiplied by the runoff coefficient (Rv). The Water Quality Volume is calculated using the formula below: _ 1.2RVA W QV 12 Where: WQv = water quality volume (acre-feet) Rv = 0.05 + 0.009(I) where I isep rcent impervious cover within the project area (post -development) A = project area (acres) Refer to Chapter 4 for detailed design guidance regarding water quality treatment. The equation above describes the sizing criteria for the TSS Reduction Method. The calculations and requirements for using the Runoff Reduction Method are described in Chapter S. hapter 2. — Stormwater Planning And Regulations You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 2.1.3 Minimum Standard #2 - Channel Protection (CPQ) Channel protection shall be provided to both downstream and on-site channels by meeting the following three requirements: 1. Capture the increased runoff volume from the 1 -year, 24-hour return frequency storm event (3.36 inches in 24 hours) so the post -development volume does not exceed the pre - development volume, and release that volume of runoff over a period of 40 hours. The volume required to detain the 1 -year storm over this extended period of time is called the Channel Protection Volume (CPQ). The CPQ is one measure of the stormwater sizing criteria that are used to size and design stormwater management facilities. If the TSS Reduction Method is being used to meet the Water Quality (WQv) minimum standard then the WQv may be subtracted from the total CPQ volume calculated; a. If the post -development 1 -year 24-hour peak discharge flow rate is less than 2.0 cubic feet per second (cfs) then the CPQ detention is not required, but the post -development 1 -year, 24-hour peak discharge rate shall not exceed the predevelopment rate. 2. Provide energy dissipation at outfalls to limit the velocity for the 10 -year, 24-hour return frequency storm event to a non-erosive velocity from the outfall to the receiving channel; and 3. Preserve the applicable stream buffer in compliance with the City of Fayetteville streamside protection provisions (UDC Chapter 168.12). Minimum Standard #2 may be waived for sites that discharge directly into the White River, the West Fork of the White River, Lake Sequoyah, and Lake Fayetteville or as approved by the City Engineer. To support the waiver, a No Downstream Impact Certification Statement must be signed and sealed by a Professional Engineer and shall be submitted to the City Engineer as part of the Grading and Drainage Permit Application. The City Engineer will review the request and either approve or deny the waiver request. Refer to Appendix E -Detention Structural Controls and Appendix G -Outlet Structures for technical design guidance for extended detention facilities. The use of nonstructural site design practices that reduce the total amount of runoff will also reduce the required channel protection volume by a proportional amount. Determining the Channel Protection Volume (CPvj • CP„ Calculation Methods: The SCS TR -55 method is used to calculate the CPQ storage volume required for a site (see Chapter 3). • Rainfall Depths: The rainfall depth of the 1 -year, 24-hour storm in the City of Fayetteville is 3.50 inches in 24 hours. For more details, refer to Table 3.1.a in Chapter 3. • Hydrograph Generation: The SCS TR -55 hydrograph methods provided in Chapter 3 can be used to compute the runoff hydrograph for the 1 -year, 24-hour storm. hapter 2. - Stormwater Planning And Regulations You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Multiple Drainage Areas: When a development project contains or is divided into multiple drainage areas, CPQ shall be distributed proportionally to each drainage area. • Off-site Drainage Areas: Off-site drainage areas should be modeled as "existing condition" for the 1 -year storm event. A structural stormwater control, if located in-line, will need to safely bypass any off-site flows. • Routing/Storage Requirements: The required storage volume for the CPQ may be provided above the WQv storage in stormwater ponds and wetlands with appropriate hydraulic control structures for each storage requirement. • Control Orifices: Orifice diameters for CPQ control of less than 3 inches should be in accordance with City details for small outlets. 2.1.4 Minimum Standard #3 - Overbank Flood Protection (Qp25) Downstream overbank flood protection shall be provided by controlling the post -development peak discharge rate to not exceed the predevelopment rate for the 2, 5, 10, and 25 -year, 24-hour return frequency storm event (Qp25). The use of nonstructural site design practices that reduce the total amount of runoff will also reduce Qp2s by a proportional amount. If a waiver has been approved for the 1 -year, 24-hour storm under Minimum Standard #2, then for overbank flood protection, the peak flow of the 1 -year (Qpl), 2 -year (Qp2), 5 -year (Qps), 10 -year (Qpio), and 25 -year (Qp2s) return frequency storm events must be controlled to not exceed the corresponding predevelopment rate. The 24-hour rainfall depths that correspond to these events are 3.50, 3.92, 4.65, 5.31, and 6.27 inches, respectively. Determining the Overbank Flood Protection Volume (01225). • Peak -Discharge and Hydrograph Generation: The SCS TR -55 hydrograph method provided in Chapter 3 shall be used to compute the peak discharge rate and runoff for the 2, 5, 10, and 25 -year, 24-hour storm. 2.1.5 Minimum Standard #4 - Extreme Flood Protection (Qf) Extreme flood protection shall be provided by controlling and/or safely conveying the 100 -year, 24-hour return frequency storm event (Qf). This is accomplished by: 1. Controlling the post -development peak discharge rate to not exceed the predevelopment rate for the 100 -year, 24-hour return frequency storm event (Qf), of 7.91 inches in 24 hours, through the use of on-site or regional structural stormwater controls, or 2. With permission of the City Engineer, performing a downstream hydrologic assessment as described in Section 7.5 of the Drainage Manual to determine if detention of the 100 -year, 24-hour return frequency storm event will cause an increase in peak flow rates when combined with the flow in the downstream system. If, upon analysis, the downstream hydrologic assessment indicates that detaining the 100 -year event will cause the peak flows hapter 2. — Stormwater Planning And Regulations You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) downstream to increase, and the City Engineer accepts the determination, then the following requirement shall be met: a. Size the on-site conveyance system to safely convey the Qf and only discharge into receiving waters where the floodplain is demonstrated to have sufficient capacity to accommodate additional discharges without causing damage, even under fully built -out conditions. Existing floodplain areas should be preserved to the extent possible. At the discretion of the City Engineer, analysis of floodplain impacts and additional detention or reduction in post -development peak discharge rates may be required for developments. Determining the Extreme Flood Protection Criteria (Of). • Peak -Discharge and Hydrograph Generation: The SCS TR -55 hydrograph method provided in Chapter 3 shall be used to compute the peak discharge rate and runoff for the 100 -year, 24-hour storm. Downstream Analysis: Peak discharges at downstream locations shall be checked and evaluated for any increase in peak flow above pre -development conditions. The downstream check shall extend to the point where the developed site area comprises no more than 10% of the total drainage area checked (see Chapter 7, Section 7.5). If the post - developed discharges at the downstream checkpoints exceed pre -development conditions, mitigation measures may be required by the City Engineer. System Check: As a final check, Qfshall be used in the routing of the 100 -year, 24-hour runoff through the drainage system and stormwater management facilities to determine the effects on the facilities, adjacent property, and downstream, and to confirm adequacy of finished floor elevations for structures (See Chapter 1 final submittal checklist as applicable). Emergency spillways for structural stormwater controls should be designed to safely pass the resulting flows. (Additional drainage easements may be required.) SECTION 2.2. REFERENCES Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 1: Chapter 6, Floodplain Management. Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf. 1k,hapter 2. — Stormwater Planning And Regulations You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 3. METHODS FOR ESTIMATING STORM WATER RUNOFF SECTION 3.1. GENERAL The accepted approaches for drainage analysis within the City of Fayetteville are listed below. However, the City Engineer may approve other engineering methods for calculation of stormwater runoff when they are shown to be comparable to the required methods. The area limits and/or allowed ranges and applicability for the analysis methods are: Rational Method 0 to 40 acres. May be used only for inlet, culvert, gutter, storm sewer design. May not be used for detention. SCS TR -55 / TR -20 up to 2,000 acres (maximum drainage area (preferred method) per sub -basin) with no cumulative upper limit. May be used for pre -development / post -development comparisons and detention, channel, culvert, gutter, inlet, and storm sewer designs. HEC -HMS (or other Corps of up to 2,000 acres (maximum drainage area Engineers or FEMA -authorized per sub -basin) with no cumulative upper methods) limit. Recommended within designated regulatory floodplain, and for design of open channels / waterways. The design of detention areas shall be based on proposed site design and existing conditions upstream of proposed detention. Inlet design shall be based on fully built -out conditions in accordance with the zoning designation, as shall the conveyance systems downstream of detention features. The required design storm frequencies and durations to be computed shall be based on the applicable Minimum Standards as provided in Chapter 2. SECTION 3.2. PRECIPITATION DATA Once the drainage basin is defined, the next step in the hydrologic analysis is an estimation of the rainfall that will fall on the basin for a given time period. The duration, depth, and intensity of the rainfall are defined below: • Duration (hours) - Length of time over which rainfall (storm event) occurs. • Depth (inches) - Total amount of rainfall occurring during the storm duration. • Intensity (inches per hour) - Depth divided by the duration. The frequency of a rainfall event is the recurrence interval of storms having the same duration and volume (depth). This can be expressed in terms of annual chance or return period. �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Annual Chance - Percent chance that a storm event having the specified duration and volume will be exceeded in one year/years (e.g., a "10 -year" storm has a 10 -percent -annual -chance of occurring in a given year). Return Period - Average length of time between events that have the same duration and volume (e.g., 10 -year event). Thus, if a storm event with a specified duration and volume has a 1 percent chance of occurring in any given year, it may be termed a 1 -percent -annual chance event. The use of the phrase "return period" is discouraged because it gives a false impression that storm events cannot occur more frequently than the corresponding return periods. 3.2.1 Precipitation Data and Rainfall Intensity The precipitation data chart provided for the City of Fayetteville, Arkansas is based on rainfall atlas data from NOAA publications TP -40 and Hydro -35 (US Department of Commerce, 1961 and 1977). Rainfall intensity is the design rainfall rate in inches per hour for a particular drainage basin or subbasin. The available data for a given event varies slightly based on location, however the variation within the City of Fayetteville is so small that the value for a single event may be assumed to be constant within the corporate limits. Rainfall intensity is selected on the basis of the design rainfall duration and frequency of occurrence. The design duration is equal to the time of concentration for a drainage area under consideration. Once the time of concentration is known, the design intensity of rainfall may be determined from the rainfall intensity chart provided as Table 3.1. The frequency of occurrence is a statistical variable. The frequencies of occurrence to be used for drainage system design in the City of Fayetteville are established by the minimum standards provided in Chapter 2. Where a design time of concentration for a watershed sub -basin exceeds 30 minutes, the applicability of the Rational Method shall be justified with documentation if it is used. In sub - basins with significant channel or overland storage, errors may be introduced by the use of the Rational Method. If desired for ease of reference, the data in Table 3.1 may be graphed to create a family of Intensity -Duration - Frequency (I -D -F) curves for the City of Fayetteville locale. Such a graph provides the rainfall intensity on the vertical axis, as a function of the time of concentration for the drainage area under consideration, for each storm frequency. The majority of drainage sub -basins within the City of Fayetteville have a relatively short time of concentration. In general, basins with computed times of concentration in excess of 90 minutes (maximum) should be subdivided to create smaller sub -basins for more accurate computation of peak discharge. Duration Table 1 year 3.1. Rainfall 2 year intensity 5 year for Fayetteville, 10 year Arkansas. 25 year 50 year 100 year (minutes) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) 5 5.46 5.54 6.58 7.34 8.46 9.34 10.22 6 5.21 5.34 6.35 7.08 8.17 9.02 9.87 7 4.96 SAS 6.12 6.83 7.88 8.70 9.52 8 4.71 4.95 5.89 6.57 7.58 8.38 9.17 9 4.46 4.76 5.66 6.32 7.29 8.05 8.81 10 4.21 4.56 5.43 6.06 7.00 7.73 8.46 11 4.07 4.42 5.27 5.88 6.79 7.51 8.22 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Duration Table 1 year 3.1. Rainfall 2 year intensity 5 year for Fayetteville, 10 year Arkansas. 25 year 50 year 100 year (minutes) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) 12 3.94 4.29 5.11 5.71 6.59 7.28 7.97 13 3.81 4.15 4.95 5.53 6.39 7.06 7.73 14 3.68 4.02 4.79 5.35 6.18 6.84 7.48 15 3.54 3.88 4.63 5.18 5.98 6.61 7.24 16 3.47 3.81 4.55 5.09 5.88 6.50 7.12 17 3.40 3.74 4.47 5.00 5.78 6.39 7.00 18 3.33 3.67 4.39 4.91 5.68 6.29 6.89 19 3.25 3.60 4.31 4.83 5.58 6.18 6.77 20 3.18 3.54 4.23 4.74 5.48 6.07 6.65 21 3.11 3.47 4.15 4.65 5.39 5.96 6.53 22 3.04 3.40 4.07 4.57 5.29 5.85 6.41 23 2.97 3.33 3.99 4.48 5.19 5.74 6.30 24 2.89 3.26 3.91 4.39 5.09 5.64 6.18 25 2.82 3.19 3.84 4.30 4.99 5.53 6.06 26 2.75 3.12 3.76 4.22 4.89 5.42 5.94 27 2.68 3.05 3.68 4.13 4.79 5.31 5.82 28 2.60 2.98 3.60 4.04 4.69 5.20 5.71 29 2.53 2.92 3.52 3.96 4.59 5.09 5.59 30 2.46 2.85 3.44 3.87 4.49 4.98 5.47 31 2.43 2.81 3.40 3.82 4.44 4.93 5.41 32 2.40 2.77 3.36 3.78 4.39 4.87 5.35 33 2.37 2.74 3.32 3.73 4.34 4.81 5.29 34 2.34 2.70 3.27 3.69 4.29 4.76 5.23 35 2.31 2.67 3.23 3.64 4.24 4.70 5.16 36 2.28 2.63 3.19 3.60 4.18 4.65 5.10 37 2.25 2.60 3.15 3.55 4.13 4.59 5.04 38 2.21 2.56 3.11 3.51 4.08 4.53 4.98 39 2.18 2.52 3.07 3.46 4.03 4.48 4.92 40 2.15 2.49 3.03 3.41 3.98 4.42 4.86 41 2.12 2.45 2.99 3.37 3.93 4.36 4.80 42 2.09 2.42 2.94 3.32 3.88 4.31 4.73 43 2.06 2.38 2.90 3.28 3.82 4.25 4.67 44 2.03 2.34 2.86 3.23 3.77 4.19 4.61 45 2.00 2.31 2.82 3.19 3.72 4.14 4.55 46 1.97 2.27 2.78 3.14 3.67 4.08 4.49 47 1.94 2.24 2.74 3.10 3.62 4.02 4.43 48 1.91 2.20 2.70 3.05 3.57 3.97 4.37 49 1.88 2.16 2.66 3.01 3.51 3.91 4.31 50 1.85 2.13 2.61 2.96 3.46 3.85 4.24 51 1.82 2.09 2.57 2.92 3.41 3.80 4.18 52 1.79 2.06 2.53 2.87 3.36 3.74 4.12 53 1.75 2.02 1 2.49 2.83 3.31 3.68 4.06 54 1.72 1.99 2.45 2.78 3.26 3.63 4.00 55 1.69 1.95 2.41 2.73 3.20 3.57 3.94 56 1.66 1.91 2.37 2.69 3.15 3.52 3.88 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Duration Table 1 year 3.1. Rainfall 2 year intensity 5 year for Fayetteville, 10 year Arkansas. 25 year 50 year 100 year (minutes) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) (in/hr) 57 1.63 1.88 2.33 2.64 3.10 3.46 3.81 58 1.60 1.84 2.28 2.60 3.05 3.40 3.75 59 1.57 1.81 2.24 2.55 3.00 3.35 3.69 60 1.54 1.77 2.20 2.51 2.95 3.29 3.63 120 0.98 1.10 1 1.40 1.62 1.87 2.10 2.34 180 0.69 0.78 1.00 1.17 1.38 1.54 1.73 6 -hr 0.40 0.48 0.62 0.73 0.83 0.93 1.03 12 -hr 0.24 0.29 0.38 0.44 0.50 0.57 0.62 24 -hr 0.14 1 0.17 0.22 0.25 0.30 0.33 0.37 Source: U.S. Dept. of Commerce (1961, 1977). Table 3.1.a provides total rainfall for 24-hour storms for the design frequencies. SECTION 3.3. SCS CURVE NUMBER METHOD The Soil Conservation Service hydrologic method is based on a synthetic unit hydrograph. The SCS TR -55 approach for runoff determination was developed specifically for use in urbanized and urbanizing areas. Multiple software programs are available that accommodate the SCS hydrologic method and several are listed in Appendix H, Stormwater Software. A detailed examination of the capabilities and limitations of various software is required to ensure that the appropriate software is used. In general, the SCS approach considers time distribution of rainfall, initial rainfall losses (infiltration and depression storage), and allows for varying infiltration throughout the storm interval. Further details are provided in the National Engineering Handbook (NRCS, 2004). The SCS method directly relates runoff to rainfall amounts through use of curve numbers (CNs) based on Hydrologic Soil Group (HSG) soil type and on land use. A typical application of the SCS method includes the following basic steps: • Determine curve numbers for different land uses and soil types within the drainage area. • Calculate time of concentration to the study point. • Use the Type III rainfall distribution to determine excess rainfall. • Develop the direct runoff hydrograph for the drainage basin. This method can be used both to estimate stormwater runoff peak discharges and to generate hydrographs for routing stormwater flows. This method may be used for design applications including open channels, 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) small drainage ditches, energy dissipation, storm drain systems, storm sewer networks, inlet and outlet structures, and storage facilities. Note that design should be based on the highest peak discharge and not necessarily the longest time of concentration. Design rainfall may be input into various programs that use the SCS method. For the purpose of pre- and post -development runoff comparisons, the following design storm data shall be used: Rainfall amounts for 24-hour storm durations with recurrence intervals of 1, 2, 10, 25, 100 years. The appropriate rainfall distribution for the City of Fayetteville is Type III. 3.3.1 Equations and Concepts Rainfall -Runoff Equation - The following SCS runoff equation is used to estimate direct runoff depth from a 24-hour storm duration. The equation is: Q = P - I z (P-I.)+S Eq. 3.1 Where: Q = accumulated direct runoff depth (inches) P = accumulated rainfall (potential maximum runoff) (inches) I, = initial abstraction including surface storage, interception, evaporation, and infiltration prior to runoff (inches) S = potential maximum soil retention (inches) An empirical relationship used in the SCS method to estimate Ia is: Ia = 0.2S Eq. 3.2 Substituting 0.2S for Ia in Equation 3.1, the equation becomes: Q = P-0.25 z Eq. 3.3 (P + 0.85) Where S = (1000/CN) -10 Eq. 3.3a Equation 3.3 can be rearranged so that the curve number can be estimated if rainfall and runoff volume are known. The equation then becomes (Pitt, 1994): CN = 1000/[10 + 5P + 10Q -10(Qz + 1.25QP)1/2] Eq. 3.4 3.3.2 Runoff Factor The principal physical watershed characteristics affecting the relationship between rainfall and runoff are land use, land treatment, soil types, and land slope. The SCS method uses a combination of soil conditions and land uses (ground cover) to assign a runoff factor to an area. These runoff factors, called runoff curve numbers (CN), indicate the runoff potential of an area. The higher the CN, the higher the runoff potential. Soil �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) properties influence the relationship between runoff and rainfall since soils have differing rates of infiltration. Based on infiltration rates, the SCS has divided soils into four hydrologic soil groups (HSGs). Group A Soils having a low runoff potential due to high infiltration rates. These soils consist primarily of deep, well -drained sands and gravels. Group B Soils having a moderately low runoff potential due to moderate infiltration rates. These soils consist primarily of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. Group C Soils having a moderately high runoff potential due to slow infiltration rates. These soils consist primarily of soils in which a layer exists near the surface that impedes the downward movement of water or soils with moderately fine to fine texture. Group D Soils having a high runoff potential due to very slow infiltration rates. These soils consist primarily of clays with high swelling potential, soils with permanently high water tables, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious parent material. Embankments designated or identified as "hillside" in the City shall be classified as Hydrologic Soil Group D. There are no identified HSG A soils with the City of Fayetteville, based on the latest available NRCS soil survey data (USDA, SSURGO). Spatial analysis of the hydrologic soil group distribution indicated HSG B soils comprise approximately 25% of the City's soils, while HSG C comprises approximately 50%, and HSG D or combined HSG B and D soils constitute the remaining 25%. In areas indicated as combined HSG B and D soils, the more conservative number shall be assumed for purposes of design, unless the use of different numbers can be justified to the City Engineer. For use in hydrologic computations, the most recent soil distribution data can be viewed online and downloaded from the NRCS Web Soil Survey (USDA NRCS). The effects of urbanization on the natural hydrologic soil group should be accounted for in design. Runoff curve numbers for different land uses are provided in Table 3.2. In all areas disturbed by heavy equipment use during construction or where grading will mix the surface and subsurface soils, the curve numbers shall be shifted to the next higher HSG for design, except as noted in Table 3.2. Composite curve numbers shall be calculated and used in the analysis based on variations in soil type and land use. It should be noted that when composite curve numbers are used, the analysis does not take into account the location of the specific land uses. The drainage area is assigned a composite uniform land use represented by the composite curve number. However, if the spatial distribution of land use is important to the hydrologic analysis, then sub -basins corresponding to the distribution (to the extent possible) should be developed and separate sub -basin hydrographs developed and routed to the study point. The curve numbers in Table 3.2 are based on directly connected impervious area. An impervious area is considered directly connected if runoff from it flows directly into the drainage system, or occurs as concentrated shallow flow that runs over pervious areas then into a drainage system. It is possible that curve number values from urban areas could be reduced by disconnecting impervious areas and allowing such runoff to sheet flow over additional significant pervious areas prior to entering the �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) drainage system. Additional information on this approach is described in Chapter 5. The CNs provided for various land cover types were developed for typical land use relationships based on specific assumed percentages of impervious area. These CN values were developed on the assumptions that: • Pervious areas that are not disturbed by construction equipment are equivalent to pasture in good hydrologic condition, and • Impervious areas have a CN of 98 and are directly connected to the drainage system. If Low Impact Development, or Green Infrastructure, stormwater controls or practices are implemented in design, the impact of such features in reducing overall stormwater runoff may be accounted for. Practices resulting in increased infiltration will decrease overall runoff and this can be addressed by modifying the Curve Number through the use of Equation 3.4 above where the runoff depth Q has been reduced to reflect the effects of infiltration. The procedure for this approach is provided together with an example in Chapter 5, Section 5.3. In cases where such practices do not reduce overall runoff but delay the timing, reductions in runoff rate must be accounted for by routing. For example, EPA SWMM version 5.0 explicitly accommodates routing methods for LID controls that may be used to compute timing -based reductions in discharge. If the actual impervious area percentage for the proposed design exceeds the proportion assumed for land uses in Table 3.2, a composite CN shall be computed based on actual percentage rather than using the table values. For purposes of Table 3.2, all compacted earthen fill areas shall be classified as Hydrologic Soil Group D. �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 1. Antecedent Moisture Condition II, and la = 0.2S. 2. Areas of compacted earthen fill shall be classified as Hydrologic Soil Group D. 3. The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. 4. CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type. 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.2. Runoff curve numbers'. Cover Description Curve numbers for hydrologic soil groups Cover type and hydrologic condition' Average percent 3 B impervious area C D Cultivated land: Without conservation treatment 81 88 91 With conservation treatment 71 78 81 Pasture or range land: Poor condition 79 86 89 Good condition 61 74 80 Meadow Good condition 58 71 78 Wood or forest land: Thin stand, poor cover 66 77 83 Good cover 55 70 77 Open space (lawns, parks, golf courses, cemeteries, etc. )4 Poor condition (grass cover <50%) 79 86 89 Fair condition (grass cover 50% to 75%) 69 79 84 Good condition (grass cover > 75%) 61 74 80 Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way) 98 98 98 Streets and roads Paved; curbs and storm drains (excluding right-of-way) 98 98 98 Paved; open ditches (including right-of-way) 89 92 93 Gravel (including right-of-way) 85 89 91 Dirt (including right-of-way) 82 87 89 Urban districts: Commercial and business 85% 92 94 95 Industrial 72% 88 91 93 Residential districts by average lot size: 1/8 acre or less (town houses) 65% 85 90 92 1/4 acre 38% 75 83 87 1/3 acre 30% 72 81 86 1/2 acre 25% 70 80 85 1 acre 20% 68 79 84 2 acres 12% 65 77 82 Developing urban areas and newly graded areas 86 91 94 (pervious areas only, no vegetation). 1. Antecedent Moisture Condition II, and la = 0.2S. 2. Areas of compacted earthen fill shall be classified as Hydrologic Soil Group D. 3. The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. 4. CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type. 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 3.3.3 Travel Time Estimation Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these. The type of flow that occurs is a function of the conveyance system and is best determined by field inspection. Travel time (Tt) is the time it takes water to travel from one location to another within a watershed subarea, through the various components of the drainage system. Time of concentration (tc) is computed as the total time for a particle of water to travel through consecutive components of the drainage conveyance system from the hydraulically most distant point of the watershed to the downstream point of interest within the watershed subarea, typically for the 2 -year 24-hour or 50 -percent -annual -chance frequency event. Total tc can also be described as the time required for a particle of water to travel from the most hydraulically distant point of the watershed to the downstream point of interest subarea under sheet (or overland) flow (to), shallow concentrated flow (tconc), and open channel flow (toc) conditions to the downstream point of interest. Therefore, tc = to + tconc + toc Eq. 3.5 The design engineer shall update the most hydraulically distant point and travel path information as required between pre- and post -development computations. Minimum allowed tc = 5 minutes Travel time is also the ratio of flow length for a particular type of flow to flow velocity: Tt = L Eq. 3.6 3600V Where: Tt = travel time (hr) L = flow length (ft) V = flow velocity (ft/s) 3600 = conversion factor from seconds to hours 3.3.3.1 Overland Flow The overland flow time can be calculated using the following formula: to = 0.007 (nQ0.8 Eq. 3.7 (P2)0.S(S)0.4 Where: to = overland flow travel time (hr) n = Manning's roughness coefficient (see Table 3.3) L = flow length (ft), limited to 150 feet maximum (NRCS, 2010) Pz = 2 -year, 24-hour rainfall depth (in) S = land slope (ft/ft) �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.3. Roughness Surface Description n Smooth surfaces Concrete, asphalt, gravel, or bare soil 0.011 Fallow (no residue) 0.05 Cultivated soils Residue cover < 20% 0.06 Residue cover > 20% 0.17 Grass Short grass prairie 0.15 Dense grasses' 0.24 Bermuda grass 0.41 Range (Natural) 0.13 Woods3 Light underbrush 0.40 Dense underbrush (Use only where slopes <2% and deep forest litter present) 0.80 Then values area composite of information by Engman (1986) and are not to be used for other flow conditions. Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native grass mixtures. When selecting n, consider cover to a height of about 0.1 foot. This is the only part of the plant cover that will obstruct sheet flow. Source: SCS, TR -55, Second Edition, June 1986. 3.3.3.2 Shallow Concentrated Flow After a maximum of 150 feet (NRCS, 2010), sheet flow usually becomes shallow concentrated flow. The average velocity for this type of flow may be determined using the equations below: Unpaved V = 16.13(S)O.s Eq. 3.8 Paved V = 20.33(S)O.s Eq. 3.9 Where: V = average velocity (ft/s) S = slope of hydraulic grade line (watercourse slope, ft/ft) Equation 3.6 may be used to compute travel time for the shallow concentrated flow segment. Allowable maximum shallow concentrated flow velocity for post -developed conditions (for any design storm event) is 6 feet/sec (unpaved) and 10 feet/sec (paved). 3.3.3.3 Open Channels A chart of Manning's roughness coefficients for open channel flow is provided as Table 3.4. Travel time for open channel flow may be computed using Equation 3.6. In watersheds with storm sewers, carefully identify the hydraulically most distant location and the appropriate flow path to estimate tc. Allowable velocities for open channels with engineered or vegetated linings are provided in Section 6.4. �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.4. Manning's roughness coefficient - "n" Type of Channel and Description Minimum Normal Maximum A. Lined or Built-up Channels Al. Metal a. Smooth steel surface 1. Unpainted 0.011 0.012 0.014 2. Painted 0.012 0.013 0.017 b. Corrugated 0.021 0.025 0.030 A2. Nonmetal a. Cement 1. Neat, surface 0.010 0.011 0.013 2. Mortar 0.011 0.013 0.015 b. Wood 1. Planed, untreated 0.010 0.012 0.014 2. Planed, creosoted 0.011 0.012 0.015 3. Unplaned 0.011 0.013 0.015 4. Plank with battens 0.012 0.015 0.018 S. Lined with roofing paper 0.010 0.014 0.017 c. Concrete 1. Trowel finish 0.011 0.013 0.015 2. Float finish 0.013 0.015 0.016 3. Finished, with gravel on bottom 0.015 0.017 0.020 4. Unfinished 0.014 0.017 0.020 S. Gunite, good section 0.016 0.019 0.023 6. Gunite, wavy section 0.018 0.022 0.025 7. On good excavated rock 0.017 0.020 -- 8. On irregular excavated rock 0.022 0.027 -- d. Concrete bottom float finished with sides of 1. Dressed stone in mortar 0.015 0.017 0.020 2. Random stone in mortar 0.017 0.020 0.024 3. Cement rubble masonry, plastered 0.016 0.020 0.024 4. Cement rubble masonry 0.020 0.025 0.030 S. Dry rubble or riprap 0.020 0.030 0.035 e. Gravel bottom with sides of 1. Formed concrete 0.017 0.020 0.025 2. Random stone in mortar 0.020 0.023 0.026 3. Dry rubble or riprap 0.023 0.033 0.036 f. Brick 1. Glazed 0.011 0.013 0.015 2. In cement mortar 0.012 0.015 0.018 g. Masonry 1. Cemented rubble 0.017 0.025 0.030 2. Dry rubble 0.023 0.032 0.035 h. Dressed ashlar 0.013 0.015 0.017 i. Asphalt 1. Smooth 0.013 0.013 -- 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.4. Manning's roughness coefficient - "n" Type of Channel and Description Minimum Normal Maximum 2. Rough 0.016 0.016 -- j. Vegetal lining 0.030 -- 0.050 B. Excavated or Dredged a. Earth, straight and uniform 1. Clean, recently completed 0.016 0.018 0.020 2. Clean, after weathering 0.018 0.022 0.025 3. Gravel, uniform section, clean 0.022 0.025 0.030 4. With short grass, few weeds 0.022 0.027 0.033 b. Earth, winding and sluggish 1. No vegetation 0.023 0.025 0.030 2. Grass, some weeds 0.025 0.030 0.033 3. Dense weeds or aquatic plants in deep channels 0.030 0.035 0.040 4. Earth bottom and rubble sides 0.028 0.030 0.035 5. Stone bottom and weedy banks 0.025 0.035 0.040 6. Cobble bottom and clean sides 0.030 0.040 0.050 c. Dragline - excavated or dredged 1. No vegetation 0.025 0.028 0.033 2. Light brush or banks 0.035 0.050 0.060 d. Rock cuts 1. Smooth and uniform 0.025 0.035 0.040 2. Jagged and irregular 0.035 0.040 0.050 e. Channels not maintained, weeds and brush uncut 1. Dense weeds, high as flow depth 0.050 0.080 0.120 2. Clean bottom, brush on sides 0.040 0.050 0.080 3. Same, highest stage of flow 0.045 0.070 0.110 4. Dense brush, high stage 0.080 0.100 0.140 C. Natural Streams C1. Minor streams (top width at flood stage <100 feet) a. Streams on plain 1. Clean, straight, full stage, no rifts or deep pools 0.025 0.030 0.033 2. Same as above, but more stones and weeds 0.030 0.035 0.040 3. Clean, winding, some pools and shoals 0.033 0.040 0.045 4. Same as above, but some weeds and stones 0.035 0.045 0.050 5. Same as above, lower stages, more ineffective slopes and sections 0.040 0.048 0.055 6. Same as 4, but more stones 0.045 0.050 0.060 7. Sluggish reaches, weedy, deep pools 0.050 0.070 0.080 8. Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush 0.075 0.100 0.150 b. Mountain streams, no vegetation in channel, banks usually steep, trees and brush along banks submerged at high stages 1. Bottom: gravels, cobbles, and few boulders 0.030 0.040 0.050 2. Bottom: cobbles with large boulders 0.040 0.050 0.070 C2. Floodplains a. Pasture, no brush 1. Short grass 0.025 0.030 0.035 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.4. Manning's roughness coefficient - "n" Type of Channel and Description Minimum Normal Maximum 2. High grass 0.030 0.035 0.050 b. Cultivated areas 1. No crop 0.020 0.030 0.040 2. Mature row crops 0.025 0.035 0.045 3. Mature field crops 0.030 0.040 0.050 c. Brush 1. Scattered brush, heavy weeds 0.035 0.050 0.070 2. Light brush and trees, in winter 0.035 0.050 0.060 3. Light brush and trees, in summer 0.040 0.060 0.080 4. Medium to dense brush, in winter 0.045 0.070 0.110 5. Medium to dense brush, in summer 0.070 0.100 0.160 d. Trees 1. Dense willows, summer, straight 0.110 0.150 0.200 2. Cleared land with tree stumps, no sprouts 0.030 0.040 0.050 3. Same as above, but with heavy growth of sprouts 0.050 0.060 0.080 4. Heavy stand of timber, a few down trees, little undergrowth, flood stage below branches 0.080 0.100 0.120 5. Same as above, but with flood stage reaching branches 0.100 0.120 0.160 C3. Major streams (top width at flood stage >100 feet). The "n" value is less than that for minor streams of similar description because banks offer less effective resistance a. Regular section with no boulders or brush 0.025 -- 0.060 b. Irregular and rough section 0.035 -- 0.100 Source: Chow (1959), Open -Channel Hydraulics. 3.3.3.4 In-line Detention Check A culvert or bridge can often act as an in-line detention structure if there is significant upstream storage due to the constriction of discharges. Water surface profiles through the culvert or bridge structures should be checked and if more than 2 feet of hydraulic head change occurs from the downstream to upstream side of a culvert or bridge for the 10 -percent -annual -chance (10 -year, 24-hour storm) event, or at the discretion of the City Engineer, detailed storage routing procedures shall be used to determine the outflow through the culvert or bridge. The potential loss of this storage should also be accounted for, and the corresponding change to downstream discharges computed with respect to potential downstream impacts, in the case of structure replacements where a larger opening is proposed. 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) SECTION 3.4. RATIONAL METHOD The Rational Method formula is: Q=(C*I*A) Eq. 3.10 where Q is defined as the peak rate of runoff in cubic feet per second (cfs). Actually, Q is in units of acre -inches per hour, but the unit conversion difference is less than 1 percent and is therefore neglected. C is the dimensionless coefficient of runoff representing the ratio of runoff to rainfall I is the average intensity of rainfall (inch/hr) for a duration equal to the time of concentration, or tc A is the drainage area (acres) that contributes to runoff at the point of design or the point under consideration. Basic assumptions associated with use of the Rational Method are as follows: 1. The computed peak rate of runoff to the design point is a function of the average rainfall rate during the time of concentration to that point. 2. The time of concentration is the critical value in determining the design rainfall intensity and is equal to the time required for water to flow from the hydraulically most distant point in the watershed to the point of design. 3. The ratio of runoff to rainfall, C, is uniform during the entire duration of the storm event. 4. The rainfall intensity, I, is uniform for the entire duration of the storm event and is uniformly distributed over the entire watershed area. Precautions to be considered when using this method: The value of C assigned for the drainage area should account for the proposed land use within the project area. Typical values of C are provided in Table 3.5 for various land use conditions, slopes and hydrologic soil types. In addition to land use, slope and soil type, the value of C applied should be based on consideration of other variables including surface infiltration, proportion of impervious surface, localized topography, rainfall intensity, proximity to groundwater and vegetation characteristics. These values and Equation 3.10 as shown above are appropriate for storm events of 10 -percent -annual -chance or greater frequency (design storms up to and including the 10 -year). �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.5. Runoff coefficients Land Use Description for various Slope, % land useS1, 2,3,4. Hydrologic Soil Group B C D Lawns 0-2 0.15 0.25 0.35 2-7 0.25 0.35 0.4 > 7 0.3 0.35 0.45 Unimproved areas Forest 0.15-.2 0.2-.25 0.2-0.3 Meadow 0.2-0.4 0.25-0.45 0.3-0.55 Row crops 0.25-0.6 0.35-0.75 0.4-0.8 Business Downtown areas 0.7 0.8 0.9 Neighborhood areas 0.5 0.6 0.7 Residential 8 lots / acre 0.67 0.71 0.76 4 lots / acre 0.46 0.52 0.61 3 lots/ acre 0.4 0.47 0.57 2 lots / acre 0.35 0.43 0.54 Suburban (1 lot/ acre) 0.3 0.38 0.5 Multi -units, detached 0.7 0.75 0.8 Multi -units, attached 0.75 0.8 0.85 Apartments 0.65 0.7 0.75 Industrial Light areas 0.6 0.75 0.85 Heavy areas 0.8 0.85 0.9 Parks, cemeteries 0.25 0.35 0.45 Schools, Churches 0.7 0.75 0.8 Railroad yard areas 0.2 0.35 0.5 Asphalt & Concrete Pavements, Roofs. 0.95 Brick Pavement or Gravel (compacted subgrade) 0.85 Graded or no plant cover 0-2 0.25 0.3 0.35 2-7 0.35 0.45 0.55 >7 0.5 0.65 0.8 1 State of Georgia (2001). 2 Oregon Dept. of Transportation (2005). 3 Arkansas Highway and Transportation Department (1982). 4 Virginia Department of Transportation (2002). For storm events of less than 10 -percent -annual -chance, or storm frequencies exceeding the 10 -year event, a multiplier referred to as Cf shall be used, unless site-specific supporting calculations are provided to address the other variables described above. Table 3.6 provides the multiplier Cf to be used for storm events of various frequency. In these cases, Equation 3.10 is modified to become Q=Cf*(C*I*A) Eq. 3.11 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 3.6. Frequency factors for the Return period years (% annual chance) Rational Formula. Multiplier, Cf 2 — 10 (50% - 10%) 1.0 25(4%) 1.1 50(2%) 1.2 100(1%) 1.25 Cf * C shall not exceed 1.0. Selection of design intensity in the Rational Method shall be based on the required design frequency and depends on the time of concentration being consistent with the duration of the rainfall event. The rate of runoff is equal to the rainfall excess when the rainfall event lasts long enough for the entire watershed to contribute to drainage. Where drainage areas have relatively high proportions of impervious area, the drainage area should be subdivided to reflect this so that peak discharge computations appropriately reflect the highest peak discharge and lowest time of concentration for the subdivided portions. Drainage area computations for runoff estimation should be based on the best available data. Where more recent and more detailed site-specific topographic data is not available, the most recent publicly available topographic contour data should be used. SECTION 3.5. HEC -HMS METHODS HEC -HMS (USACE, 2000) is a free hydrologic modeling software available from the USACE. It accommodates significant complexity and a wide variety of options are available; therefore it is included as an available method. This method may be applied for developing peak discharge and hydrograph information to use in open channel design. For runoff computations, the model provides several options for the following components: • Various precipitation models - observed conditions, frequency -based, upper limit event; • Runoff volume estimation models; • Direct runoff models that account for overland flow, storage, and energy losses; • Hydrologic routing models; • Modeling of natural confluences and bifurcations; and • Water -control measures including diversions and storage facilities. These models are similar to what was provided in HEC -1, the predecessor to HEC -HMS. Other additional features are also available in HEC -HMS. If HEC -HMS is used to compute runoff, the preferred method for estimation of runoff volume is the SCS Curve Number method. However, other methods may be accepted at the discretion of the City Engineer. Also, the selection of the routing model should consider channel slope, the influence of backwater and whether there is a need for the model method to account for in-line channel storage. �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) SECTION 3.6. STORMWATER RUNOFF ANALYSIS SOFTWARE Computer software shall be used for stormwater runoff analyses in conformance with design criteria to meet the design standards of the City of Fayetteville and this Drainage Criteria Manual. A list of software that may be used is provided in Appendix H, Stormwater Management Software. Within Special Flood Hazard Areas, FEMA -approved hydrologic models should be used. Regardless of software type, output data provided shall be clearly and concisely labeled based on percent -annual -chance event or design storm, and organized in a consistent fashion. A spatial file or schematic shall be provided for reference that identifies sub -basins for which data are computed. Minimum output data required shall correspond to the Drainage Report requirements detailed in Chapter 6. Example tables depicting required input/output data to be reported are provided within Chapter 6. SECTION 3.7. REFERENCES Arkansas Highway and Transportation Department, 1982. Roadway Design Drainage Manual, Little Rock, AR. Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook, Atlanta, GA. Chow, Ven Te, 1959. Open Channel Hydraulics. McGraw Hill. City of Fayetteville, updated March 2011. Title XV Unified Development Code, Chapter 168: Flood Damage Prevention Code. City of Fayetteville, updated August 2010. Title XV Unified Development Code, Chapter 169: Physical Alteration of Land. City of Fayetteville, updated August 2010. Title XV Unified Development Code, Chapter 170: Stormwater Management & Drainage. Fort Bend County Drainage District, revised 2011. Drainage Criteria Manual, Fort Bend County, Texas. Oregon Department of Transportation, Highway Division. 2005. Hydraulics Manual, Chapter 7, Appendix F. U.S. Army Corps of Engineers, 2000. Hydrologic Modeling System Technical Reference Manual. Hydrologic Engineering Center, Davis, CA. http://www.hec.usace.army.mil/software/ U.S. Department of Agriculture, Natural Resources Conservation Service, 2004. Part 630, National Engineering Handbook, Chapter 10, Estimation of Direct Runoff from Storm Rainfall, Washington, D.C. U.S. Department of Agriculture, Natural Resources Conservation Service, 2010. Part 630, National Engineering Handbook, Chapter 15, Time of Concentration, Washington, D.C. U.S. Department of Agriculture, Natural Resources Conservation Service, 2009. WIN TR -20 Watershed Hydrology, Washington, D.C. U.S. Department of Agriculture, Natural Resources Conservation Service. Soil Survey Geographic (SSURGO) Database. U.S. Department of Agriculture, Soil Conservation Service, 1986. Urban Hydrology for Small Watersheds, Technical Release 55, Washington, D.C. http://www.nres.usda.gov/wps/portal/nres/main/national/home 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) U.S. Department of Commerce, 1961. Rainfall Frequency Atlas of the United States, Technical Paper No. 40, Washington, D.C. U.S. Department of Commerce, 1977. Five- to 60 -Minute Precipitation Frequency for the Eastern and Central United States, NOAA Technical Memo, NWS HYDRO -35, Washington, D.C. U.S. Environmental Protection Agency, 2008. Storm Water Management Model User's Manual, Version 5.0, Washington, D.C. http://www.epa.gov/nrmrl/wswrd/wq/models/swmm/#Downloads Virginia Department of Transportation, 2002 Drainage Manual. �t— ,l 3. Methods for Estimating Storm Water Runoff You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 4 WATER QUALITY SECTION 4.1. THE NATURE OF POLLUTANTS IN STORMWATER RUNOFF Nonpoint source pollution, which is a primary cause of polluted stormwater runoff and water quality impairment, can come from many sources—many of which are the result of human activities within a watershed. Development can concentrate and increase the amount of these nonpoint source pollutants. As stormwater runoff moves across the land surface, it has the potential to pick up and carry away both natural and human -made pollutants, depositing them into Fayetteville's water resources. In urbanizing watersheds, the potential for water quality degradation occurs as a result of development and other human activities. Erosion from construction sites and other disturbed areas can contribute large amounts of sediment to streams. As construction and development proceed, impervious surfaces replace the natural land cover and pollutants from human activities begin to accumulate on these surfaces. During storm events, these pollutants could be washed off into the streams. Excess stormwater also has the potential to cause discharges from sewer overflows and leaching from septic tanks. There are a number of other causes of potential nonpoint source pollution in urban areas that are not specifically related to wet weather events including leaking sewer pipes, sanitary sewage spills, illegal dumping of pollutants into streams or storm drains by individuals, and illicit discharge of commercial/industrial wastewater and wash waters to storm drains. 4.1.1 Stormwater Pollution Sources For effective stormwater management, it is important to understand the nature and sources of urban stormwater pollution. Table 4.1 summarizes the major stormwater pollutants and their effects. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) urbanTable 4.1. Summary of pollutants. Constituents Effects Stream turbidity Sediments—Total Suspended Solids (TSS), Dissolved Habitat changes Recreation/aesthetic loss Solids, Turbidity Contaminant transport Filling of lakes and reservoirs Algae blooms Nutrients—Nitrate, Nitrite, Ammonia, Organic Nitrogen, Eutrophication Phosphate, Total Phosphorus Ammonia and nitrate toxicity Recreation/aesthetic loss Microbes—Total and Fecal Coliforms, Fecal Streptococci, Ear/Intestinal infections Viruses, E.Coli, Enterocci Recreation/aesthetic loss Organic Matter—Vegetation, Sewage, Other Oxygen Dissolved oxygen depletion Demanding Materials Odors Fish kills Toxic Pollutants—Heavy Metals (cadmium, copper, lead, Human & aquatic toxicity zinc), Organics, Hydrocarbons, Pesticides/Herbicides Bioaccumulation in the food chain Thermal Pollution Dissolved oxygen depletion Habitat changes Trash and debris Recreation/aesthetic loss Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 4.1.2 Areas with High Pollutant Discharge Potential Areas with high pollutant discharge potential are areas of the urban landscape that often produce higher concentrations of certain pollutants, such as hydrocarbons or heavy metals, than are normally found in urban runoff. These areas merit special management and the use of specific pollution prevention activities and/or structural stormwater controls. Examples of areas with high pollutant discharge potential include: • Gas / fueling stations • Vehicle maintenance areas • Vehicle washing / steam cleaning • Auto recycling facilities • Outdoor material storage areas • Loading and transfer areas • Landfills • Construction sites • Industrial sites • Industrial rooftops SECTION 4.2. WATER QUALITY MANAGEMENT CRITERIA 4.2.1 Overview of Water Quality Criteria This section presents an integrated approach for meeting the stormwater runoff management requirements of Minimum Standard #1 (see Section 1.2). There are two complementary approaches to meet the water quality objectives of Minimum Standard #1: (1) Runoff Reduction Methodology (RRM) using Green Infrastructure and other Low Impact Development (LID) practices to capture and hold on-site runoff from a 1 -inch rainfall; and, (2) the Total Suspended Solids (TSS) Reduction Method (TRM), a more traditional approach focusing on removal of 80% TSS from a selected rainfall runoff - 1.2 inches in the case of Fayetteville. The RRM is explained in Chapter S. TRM is covered in this chapter. TSS Reduction Method (TRM) The TSS Reduction Method follows the philosophy of removing pollutants and at least 80% of the TSS "where practicable" through the use of a percentage removal performance goal. The approach provides treatment of the Water Quality Volume (WQv) from a site to reduce post -development TSS loadings by 80%, as measured on an average annual basis. This performance goal is based on the ADEQ NPDES small MS4 permit in accordance with U.S. EPA guidance. The WQv is used to size structural control facilities that work to remove pollutants from the runoff. The WQv is roughly equal to the runoff from the first 1.2 inches of rainfall within the catchment area. A stormwater management system designed to treat the WQv will treat the runoff from storm events of 1.2 inches or less, as well as the first 1.2 inches of runoff for larger storm events. Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) The volumetric runoff coefficient (Rv) was derived from a regression analysis performed on rainfall runoff volume data from a number of cities nationwide and is a shortcut method considered adequate for runoff volume calculation for the type of small storms considered in stormwater quality calculations. The Water Quality Volume (WQv) is equal to a rainfall depth of 1.2 inches multiplied by the volumetric runoff coefficient (Rv) and the site area, and is calculated using Equation 4.1 below: WQv = PRA Equation 4.1 12 Where: WQv = water quality volume (ac -ft) Rv = 0.05 + 0.009(I) where I isep rcent impervious cover (i.e., 50% impervious is 50 not 0.5) A = site area (acres) P = 1.2 inches Determining the Water Quality Volume (WWQJ • Measuring Impervious Area: The area of impervious cover shall be based on the proposed project plans and independent of pre -construction conditions. • Multiple Drainage Areas: When a development project contains or is divided into multiple drainage areas, WQv should be calculated and addressed separately for each drainage area. • Off-site Drainage Areas: Off-site existing impervious areas are excluded from the calculation of the WQvvolume. • Credits for Site Design Practices: The use of certain site design practices may allow the WQv volume to be reduced through the subtraction of a site design "credit." These site design credits are described in Section 4.3.2. • Determining the Peak Discharge for the Water Quality Storm: When designing off-line structural control facilities, the peak discharge of the water quality storm (Qv„a) can be determined using the SCS method provided in Chapter 3. The water quality storm is equivalent to 1.2 inches of rainfall in 24 hours. • Extended Detention of the Water Quality Volume: The water quality treatment requirement can be met by providing a 24-hour drawdown of a portion of WQv in a stormwater pond or wetland system. Referred to as water quality ED (extended detention), it is different than providing extended detention of the 1 -year 24-hour storm for the channel protection volume (CPQ). Where used, the ED portion of the WQv may be included when routing the CPQ. • WQv can be expressed in cubic feet by multiplying by 43,560. WQv can also be expressed in watershed -inches as simply PRS by removing the area (A) and the 12 from Equation 4.1. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) SECTION 4.3. MEETING THE WATER QUALITY SIZING CRITERIA REQUIREMENTS WITH TOTAL SUSPENDED SOLIDS REDUCTION METHOD (TRM) There are two primary approaches for managing stormwater runoff and addressing the water quality (and quantity -based) criteria requirements on a development site: • The use of site design practices to reduce the amount of stormwater runoff and pollutants generated and/or provide for natural treatment and control of runoff; and • The use of structural stormwater controls to provide treatment and control of stormwater runoff 4.3.1 Site Design as the First Step in Addressing Requirements Using the site design process to reduce stormwater runoff and pollutants should always be the first consideration of the site designer and engineer in the planning of the stormwater management system for a development. Site design concepts can be used as both water quantity and water quality management tools and can reduce the size and cost of required structural stormwater controls. The site design approach can result in a more natural and cost-effective stormwater management system that better mimics the natural hydrologic conditions of the site and has a lower maintenance burden. 4.3.2 Site Design Stormwater Credits A set of stormwater "credits" has been developed to provide developers and site designers an incentive to use site design practices that can reduce the volume of stormwater runoff and minimize the pollutant loads from a site. The credit system directly translates into cost savings to the developer by reducing the size of structural stormwater control and conveyance facilities. The credit system recognizes the water quality benefits of certain site design practices by allowing for a reduction in the water quality volume (WQv). If a developer incorporates one or more of the credited practices in the design of the site, the volume of stormwater required to be captured and treated to meet Minimum Standard #1 will be reduced. The following credits are only for use with the TRM method. Runoff Reduction credits are discussed in Low Impact Development -Chapter S. The site design practices that provide stormwater credits are listed in Table 4.2. Site-specific conditions will determine the applicability of each credit. For example, stream buffer credits cannot be taken on upland sites that do not contain perennial or intermittent streams. It should be noted that site design practices and techniques that reduce the overall impervious area on a site implicitly reduce the total amount of stormwater runoff generated by a site (and thus reduce WQv) and are not further credited in the TRM method. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 4.2. Summary of site design practices that receive site design stormwater credits. Practice Description Undisturbed natural areas are conserved on a site, thereby Natural Area Conservation retaining their pre -development hydrologic and water quality characteristics. Stormwater runoff is treated by directing sheet flow runoff Stream buffers through a naturally vegetated or forested buffer as overland flow. Use of vegetated channels Vegetated channels are used to provide stormwater treatment. Overland flow filtration/infiltration Overland flow filtration/infiltration zones are incorporated into the site design to receive runoff from rooftops and other small zones impervious areas. Environmentally sensitive large lot A group of site design techniques are applied to low and very subdivisions low density residential development. For each potential credit, there is a minimum set of criteria and requirements which identify the conditions or circumstances under which the credit may be applied. Site designers are encouraged to utilize as many credits as they can on a site. Greater reductions in stormwater storage volumes can be achieved when many credits are combined. However, credits cannot be combined by applying any portion of multiple practices to the same area. For practices that do not apply to the same area - for example, Natural Area Conservation and Overland flow filtration/infiltration zones - the site design stormwater credits may be applied cumulatively, by adding the reduction in required water quality volume computed for each practice individually and subtracting the sum of the reductions from the original total that would be required if no practices were applied. 4.3.2.1 Site Design Credit #1: Natural Area Conservation A stormwater credit can be taken when undisturbed natural areas are conserved on a site. Under this credit, a designer would be able to subtract conservation areas from total site area when computing water quality volume requirements. Additionally, the post -development peak discharges will be smaller, and hence water quantity control volumes (CPQ, Qpzs, and Qf) will be reduced due to lower post -development curve numbers or rational formula "C" values. Rule: Water quality volume requirements may be reduced by the proportion of the Natural Conservation Area to the total site area. Criteria: • Conservation area cannot be disturbed during project construction; • Area shall be protected by limits of disturbance clearly shown on all construction drawings; Area shall be located within an acceptable conservation easement instrument that ensures perpetual protection of the proposed area. The easement must clearly specify how the natural area vegetation shall be managed and boundaries will be marked [Note: managed turf (e.g., playgrounds, regularly maintained open areas) is not an acceptable form of vegetation management]; Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Area shall have a minimum contiguous area requirement of 10,000 square feet; and • Rv is kept constant when calculating WQv. Example: Residential Subdivision; Total Area = 38 acres Natural Conservation Area = 7 acres Impervious Area = 13.8 acres Rv = 0.05 + 0.009 (I) = 0.05 + 0.009 (36.3) = 0.377 Credit: 7.0 acres in natural conservation area New drainage area = 38 - 7 = 31 acres WQv = (1.2)(0.377)(38)/12 = 1.43 ac -ft With credit: WQv = 1.43 * 31/38 = 1.17 ac -ft (18% reduction in required water quality volume) 4.3.2.2 Site Design Credit #2: Stream Buffers This credit can be taken when stormwater runoff is effectively treated by directing overland flow through a naturally vegetated or forested stream buffer. The areas draining via overland flow to the buffer may be subtracted from the total site area when computing water quality volume requirements. In addition, the volume of runoff draining to the buffer can be subtracted from the channel protection volume. The design of the stream buffer treatment system must use appropriate methods for conveying flows above the annual recurrence (1 -yr storm) event. Rule: Water quality volume requirements may be reduced by the proportion of the area draining via overland flow to the buffer to the total site area. Criteria: • The minimum undisturbed buffer width shall be 50 feet. • The maximum contributing length shall be 150 feet for pervious surfaces and 75 feet for impervious surfaces. (Areas with lengths exceeding this criteria shall not be counted.) • The average contributing slope shall be 3% maximum unless a flow spreader is used. • Runoff shall enter the buffer as overland sheet flow. A flow spreader can be supplied to ensure this, or if average contributing slope criteria cannot be met. • Not applicable if overland flow filtration/groundwater recharge credit is already being taken. • Buffers shall remain unmanaged other than routine debris removal. • Rv is kept constant when calculating WQv. Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Residential Subdivision; Total Area = 38 acres Impervious Area = 13.8 acres Area Draining to Buffer = 5 acres Rv = 0.95 + 0.009 (I) = 0.05 + 0.009 (36.3) = 0.377 Credit: 5.0 acres draining to buffer Ne drainage area = 38 - 5 = 33 acres WQv = (1.2)(0.377)(38)/12 = 1.43 ac -ft With credit: WQv = 1.43 ac -ft * 33/38 = 1.24 ac -ft (13% reduction in water quality volume) 4.3.2.3 Site Design Credit #3: Vegetated Channels This credit may be taken when vegetated (grass) channels constructed in accordance with Appendix B of this manual are used for water quality treatment. A designer is able to subtract the areas draining to a grass channel from total site area when computing water quality volume requirements. A vegetated channel can fully meet the water quality volume requirements for certain kinds of low-density residential development. An added benefit will be that the post -development peak discharges will likely be lower due to a longer time of concentration for the area draining to the grass channel. This credit cannot be taken if grass channels are used as a structural stormwater control towards meeting the 80% TSS removal goal for WQv treatment. That is, the vegetated channels credit may reduce the area to be treated or be used toward the 80% TSS removal goal for stormwater treatment, but not both. Rule: Water quality volume requirements may be reduced by the proportion of the area draining to a grass channel to the total site area. Criteria: • The credit shall only be applied to moderate or low density residential land uses (three dwelling units per acre maximum). Maximum 5 acres may drain to a single channel. • The maximum flow velocity for water quality design storm shall be less than or equal to 1.0 feet/s. • The minimum channel residence time for the water quality storm shall be 5 minutes, with a minimum 300 feet channel length. • The bottom width shall be a maximum of 6 feet. If a larger channel is needed use of a compound cross section (with low -flow channel and bench on at least one side for higher flows) is required. • The side slopes shall be 3:1 (horizontal:vertical) or flatter. • The channel slope shall be 3% or less. • Rv is kept constant when calculating WQv. Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Residential Subdivision; Total Area = 38 acres Impervious Area = 13.8 acres Rv = 0. 5 + 0.009 (I) = 0.05 + 0.009 (36.3) = 0.377 Credit: 12.5 acres meet grass channel criteria New drainage area = 38 - 12.5 = 25.5 acres Be . WQv _ (1.2)(0.377)(38)/12 = 1.43 ac -ft With credit: WQv = 1.43 ac -ft * 25.5/38 = 0.96 ac -ft (33% reduction in water quality volume) 4.3.2.4 Site Design Credit #4: Overland Flow Filtration/Groundwater Recharge Zones This credit can be taken when "overland flow filtration/infiltration zones" are incorporated into site design to receive runoff from rooftops or other small impervious areas (e.g., driveways, small parking lots, etc.). This can be achieved by grading the site to promote overland vegetative filtering or by providing infiltration or "rain garden" areas. If impervious areas are adequately disconnected, they can be deducted from total site area when computing the water quality volume requirements. An added benefit will be a longer time of concentration for the area draining into the feature. Rule: Water quality volume requirements may be reduced by the proportion of the adequately disconnected impervious area draining to an infiltration area, to the total site area. Criteria: • Relatively permeable soils (generally, hydrologic soil group B) should be present. • Runoff shall not come from an area with high pollutant discharge potential. • The maximum contributing impervious flow path length shall be 75 feet. • Downspouts shall be at least 10 feet away from the nearest impervious surface with drainage diverted away to discourage "re -connections". • The disconnection shall drain continuously through a vegetated channel, Swale, or filter strip to the property line or structural stormwater control. • The length of the "disconnection" shall be equal to or greater than the contributing length. • The entire vegetative "disconnection" shall be on a slope less than or equal to 3%. • The surface impervious area directed to any one discharge location shall not exceed 5,000 square feet. • For those areas draining directly to a buffer, either the overland flow filtration credit -or- the stream buffer credit can be used. Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • R, is kept constant when calculating WQ,. Example: Site Area = 3.0 acres Impervious Area = 1.9 acres (or 63.3% impE "Disconnected" Impervious Area = 0.5 acres Rv = 0.05 + 0.009 (I) = 0.05 + 0.009 (63.3) = 0.62 Credit: 0.5 acres of surface imperviousness hydrologically d -s New drainage area = 3 - 0.5 = 2.5 acres WQv = (1.2)(0.62)(3)/12 = 0.19 ac -ft With credit: WQv = 0.19 ac -ft *2.5/3 = 0.16 ac -ft (17% reduction in water quality volume) 4.3.3 Structural Stormwater Control Practices cover) -onnected Structural stormwater controls (sometimes referred to as structural best management practices or BMPs) are constructed stormwater management facilities designed to treat stormwater runoff and/or mitigate the effects of increased stormwater runoff peak rate, volume, and velocity due to urbanization. This Manual recommends a number of water quality structural stormwater controls that can be implemented to help meet the stormwater management Minimum Standards. The recommended water quality controls are divided into two categories: general application and limited application controls. These controls are targeted at 80% TSS pollution reduction and only incidentally reduce the runoff volume. Green Stormwater Practices (GSPs) are discussed in detail at the end of Chapter 5 and are not presented in this section. GSPs are targeted at runoff volume reduction and assume that all pollutants contained within the reduced volume are 100% removed. Detention structural controls are discussed in Chapter 7. General Application Controls General application structural controls are recommended for use with a wide variety of land uses and development types. These structural controls have a demonstrated ability to effectively treat the Water Quality Volume (WQv) and are presumed to be able to remove 80% of the total annual average TSS load in typical post -development urban runoff when designed, constructed and maintained in accordance with recommended specifications. Several of the general application structural controls can also be designed to provide water quantity control; i.e., downstream channel protection (CPQ), overbank flood protection (Qpzs) and/or extreme flood protection (Qf). General application controls are the recommended stormwater management facilities for a site wherever feasible and practical. There are six types of general application controls, which are summarized below. They are broken up into two categories, water quality structural controls and low impact structural controls. Detailed descriptions of the water quality structural controls along with design criteria and procedures are provided in Appendix F, Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Water Quality Structural Controls. Detailed descriptions of the low -impact structural controls along with design criteria and procedures are provided in Appendix B, GSP Specifications. Water Quality Structural Controls Stormwater Ponds Stormwater ponds are constructed stormwater detention basins that have a permanent pool (or micropool) of water. Runoff from each rain event is detained and treated in the pool. Pond design variants include: • Wet Pond, • Wet Extended Detention Pond, • Micropool Extended Detention Pond, and • Multiple Pond Systems. Stormwater Wetlands Stormwater wetlands are constructed wetland systems used for stormwater management. Stormwater wetlands consist of a combination of shallow marsh areas, open water and semi -wet areas above the permanent water surface. Wetland design variants include: • Shallow Wetland, • Extended Detention Shallow Wetland, • Pond/Wetland Systems, and • Pocket Wetland. Sand Filters Sand filters are multi-chamber structures designed to treat stormwater runoff through filtration, using a sand bed as the primary filter media. Filtered runoff may be returned to the conveyance system, or allowed to fully or partially exfiltrate into the soil. The two sand filter design variants are: • Surface Sand Filter, and • Perimeter Sand Filter. Low Impact Structural Controls Bioretention Areas Bioretention areas are shallow stormwater basins or landscaped areas that utilize engineered soils and vegetation to capture and treat stormwater runoff. Runoff may be returned to the conveyance system, or allowed to fully or partially exfiltrate into the soil. Infiltration Trenches An infiltration trench is an excavated trench filled with stone aggregate used to capture and allow infiltration of stormwater runoff into the surrounding soils from the bottom and sides of the trench. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Enhanced Swales Enhanced swales are vegetated open channels that are explicitly designed and constructed to capture and treat stormwater runoff within dry or wet cells formed by check dams or other means. The two types of enhanced swales are: • Dry Swale, and • Wet Swale/Wetland Channel. Limited Application Controls Limited application structural controls are those that are recommended only for limited use or for special site or design conditions. Generally, these practices: (1) cannot alone achieve the 80% TSS removal target, (2) are intended to address specific land use constraints or conditions, and/or (3) may have high or special maintenance requirements that may preclude their use. Limited application controls are typically used for water quality treatment only. Some of these controls can be used as a pretreatment measure or in series with other structural controls to meet pollutant removal goals. Limited application structural controls should be considered primarily for commercial, industrial or institutional developments, and not residential developments. The following limited application controls are provided for consideration in this Manual. Each is discussed in detail with appropriate application guidance in Appendix F, Water Quality Structural Controls. Filtering Practices • Organic Filter, and • Underground Sand Filter. Wetland Systems • Submerged Gravel Wetland. Hydrodynamic Devices • Gravity (Oil -Grit) Separator. Proprietary Systems • Commercial Stormwater Controls. 4.3.3.1 Structural Stormwater Control Pollutant Removal Capabilities General and limited application structural stormwater controls are intended to provide water quality treatment for stormwater runoff. Though each of these structural controls provides pollutant removal capabilities, the relative capabilities vary between structural control practices and for different pollutant types. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Pollutant removal capabilities for a given structural stormwater control practice are based on a number of factors including the physical, chemical and/or biological processes that take place in the structural control and the design and sizing of the facility. In addition, pollutant removal efficiencies for the same structural control type and facility design can vary widely depending on the tributary land use and area, incoming pollutant concentration, rainfall pattern, time of year, maintenance frequency and numerous other factors. Table 4.3 provides design removal efficiencies for each of the general and limited application control practices. It should be noted that these values are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. A structural control design may be capable of exceeding these performances, however the values in the table are generally reasonable values that can be assumed to be achieved when the structural control is sized, designed, constructed and maintained in accordance with recommended specifications in this Manual. Where the pollutant removal capabilities of an individual structural stormwater control are not deemed sufficient for a given site application, additional controls may be used in series in a "treatment train" approach. More detail on using structural stormwater controls in series are provided in the next section. For additional information and data on the range of pollutant removal capabilities for various structural stormwater controls, the refer to the National Pollutant Removal Performance Database (2nd Edition) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org Table 4.3. Design pollutant Structural Control Total Suspended Solids Total Phosphorus Total Nitrogen Fecal Coliform Metals General Application Structural Controls Stormwater Ponds 80 50 30 70* 50 Stormwater Wetlands 80 40 30 70* 50 Bioretention Areas 80 60 50 --- 80 Sand Filters 80 50 2S 40 SO Infiltration Trench 80 60 60 90 90 Enhanced Dry Swale 80 SO SO --- 40 Enhanced Wet Swale 80 2S 40 --- 20 Limited Application Structural Controls Organic Filter 80 60 40 SO 7S Underground Sand Filter 80 SO 2S 40 SO Submerged Gravel Wetland 80 SO 20 70 SO Gravity (Oil -Grit) Separator 40 S S --- --- Proprietary Systems *** *** *** *** *** * If no resident waterfowl population present. *** The performance of specific proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third -party sources and data. --- Insufficient data to provide design removal efficiency. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) SECTION 4.4. USING STRUCTURAL STORMWATER CONTROLS IN SERIES 4.4.1 Stormwater Treatment Trains A stormwater "treatment train" is an integrated planning and design approach with components that work together to limit the adverse impacts of urban development on downstream waters and riparian areas. When considered comprehensively a treatment train consists of all the design concepts and nonstructural and structural controls that work together to attain water quality and quantity goals. This is illustrated in Figure 4.1. Runoff & Load Primary Treatment Generation Pretreatment and/or Quantity Control Figure 4.1. Generalized stormwater treatment train. Runoff and Load Generation - The initial part of the "train" is located at the source of runoff and pollutant load generation, and consists of better site design and pollution prevention practices that reduce runoff and stormwater pollutants. Pretreatment - The next step in the treatment train consists of pretreatment measures. These measures typically do not provide sufficient pollutant removal to meet the 80% TSS reduction goal, but do provide calculable water quality benefits that may be applied towards meeting the WQv treatment requirement. These measures include: • The use of stormwater better site design practices and site design credits to reduce the water quality volume (WQv), • Limited application structural controls that provide pretreatment, and • Pretreatment facilities such as sediment forebays on general application structural controls. Primary Treatment and/or Quantity Control - The last step is primary water quality treatment and/or quantity (channel protection, overbank flood protection, and/or extreme flood protection) control. This is achieved through the use of: • General application structural controls, • Limited application structural controls, and • Detention structural controls. 4.4.2 Use of Multiple Structural Controls in Series Many combinations of structural controls in series may exist for a site. Figure 4.2 provides a number of hypothetical examples of how the stormwater sizing criteria may be addressed by using structural stormwater controls. Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Water --- 110. Channel --- 110. Overbank --- ,, Extreme Quality , Protection 0 Flooding I Flood A Stormwater Ponds / Stormwater Wetlands B Other GA Control Extended Det. Detention C I Site Design Credits Extended Det. D ILA Contro!�GA Controq Extended Det. E ISiteDesign\LAControi� waived Credits / Detention Floodplain Mgmt. Detention Detention Floodplain Mgmt. Figure 4.2. Examples of structural controls used in series. Referring to Figure 4.2 by line letter: A. Two general application (GA) structural controls, stormwater ponds and stormwater wetlands, can be used to meet the unified Stormwater sizing criteria in a single facility. B. The other general application structural controls (bioretention, sand filters, infiltration trench and enhanced Swale) are typically used in combination with detention controls to meet the unified stormwater sizing criteria. The detention facilities are located downstream from the water quality controls either on-site or combined into a regional or neighborhood facility. C. Line C represents a special case where an environmentally sensitive large lot subdivision has been developed that can be designed so as to waive the water quality treatment requirement altogether. However, detention controls may still be required for downstream channel protection, overbank flood protection and extreme flood protection. D. Where a limited application (LA) structural control does not meet the 80% TSS removal criteria, another downstream structural control must be added. For example, areas with high pollutant loading potential may be fit or retrofit with devices adjacent to parking or service areas designed to remove petroleum hydrocarbons. These devices may also serve as pre-treatment devices removing the coarser fraction of sediment. One or more downstream structural controls is then used to meet the full 80% TSS removal goal, as well as water quantity control. E. Site design credits have been employed to partially reduce the water quality volume requirement. In this case, for a smaller site, a well designed and tested Limited Application structural control provides adequate TSS removal while a dry detention pond handles the overbank flooding criteria. For this location, direct discharge to a large stream and local downstream floodplain management practices have Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) eliminated the need for channel protection volume and extreme flood protection structural controls on site. The combinations of structural stormwater controls are limited only by the need to employ measures of proven effectiveness and meet local regulatory and physical site requirements. Figure 4.3 illustrates the application of the treatment train concept for a large shopping mall site. -------------------- --- -------- - --------- ----- I � , 1 •'Ir ''4 1 1 •1. � •1, 1 1 �• 1 100 Yar Act" i oaf nuns It I alts wlt•41 � - t Salt tolfra"t, 1 1 Z yearb4 aad�• ' : ',-' , dtt.ultiort � � _ l 1 1 Permanent L IT11,LM IV61't.5 1 , faid_iwa d" -M d6 i 1 1 1 ' 1 1 1 I Figure 4.3. Example treatment train — commercial development (Source: NIPC, 2000). In this case, runoff from rooftops and parking lots drains to a depressed parking lot, perimeter grass channels, and bioretention areas. Slotted curbs are used at the entrances to these swales to better distribute the flow and to settle out the very coarse particles at the parking lot edge for removal. Runoff is then conveyed to a wet ED pond for additional pollutant removal and channel protection. Overbank and extreme flood protection are provided through parking lot detention. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 4.4.3 Calculation of Pollutant Removal for Structural Controls in Series For two or more structural stormwater controls used in combination, it is important to have an estimate of the pollutant removal efficiency of the treatment train. Pollutant removal rates for structural controls in series are not additive. For pollutants in particulate form, the actual removal rate (expressed in terms of percentage of pollution removed) varies directly with the pollution concentration and sediment size distribution of runoff entering a facility. For example, a stormwater pond facility will have a much higher pollutant removal percentage for very turbid runoff than for clearer water. When two stormwater ponds are placed in series, the second pond will treat an incoming particulate pollutant load differently from the first pond. The upstream pond captures the easily removed larger sediment sizes, passing on an outflow with a lower concentration of TSS but with a higher proportion of finer particle sizes. Hence, the removal capability of the second pond for TSS is considerably less than the first pond. Recent findings suggest that the second pond in series can provide as little as half the removal efficiency of the upstream pond. To estimate the pollutant removal rate of structural controls in series, a method is used in which the removal efficiency of a downstream structural control is reduced to account for the pollutant removal of the upstream control(s). The following steps are used to determine the pollutant removal: • For each drainage area, list the structural controls in order, upstream to downstream, along with their expected average pollutant removal rates from Table 4.3 for the pollutants of concern. For any general application structural control located downstream from another general application control or a limited application structural control that has TSS removal rates equivalent to 80%, the designer should use 50% of the normal pollutant removal rate for the second control in series. For a general application structural control located downstream from a limited application structural control that cannot achieve the 80% TSS reduction goal the designer should use 75% of the normal pollutant removal rate for the second control in series. For example, if a general application structural control has an 80% TSS removal rate, then a 40% (0.5 x 80%) TSS removal rate would be assumed for this control if it were placed downstream from another general application control in the treatment train. If it were placed downstream from a limited application structural control that cannot achieve the 80% TSS reduction goal a 60% (0.75 x 80%) TSS removal rate would be assumed. This rule should always be used with caution depending on the actual pollutant of concern and with allowance for differences among structural control pollutant removal rates for different pollutants. Actual data from similar situations should be used where available. • For cases where a limited application control is located upstream from a general application control in the treatment train, the downstream general application structural control is given full credit for removal of pollutants. • Apply the following equation for calculation of approximate total accumulated pollution removal for controls in series: o Final Pollutant Removal = (Total load * Controll removal rate) + (Remaining load * Contro12 removal rate) + removal for other Controls in series. Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Example TSS is the pollutant of concern and a commercial gravity (oil/grit) separator is inserted that has a 40% sediment removal rate. A stormwater pond is designed at the site outlet. What is the total TSS removal rate? The following information is given: Control 1 (Commercial Device) = 40% TSS removal Control 2 (Stormwater Pond 1) = 70% TSS removal (use 1.0 x design removal rate) 4.4.4 Routing with W% Removed When off-line structural controls such as bioretention areas, sand filters and infiltration trenches capture and remove some portion of the water quality volume (WQv), downstream structural controls do not have to account for the removed volume during design. That is, the volume removal may be subtracted from the total volume that would otherwise need to be routed through the downstream structural controls. From a calculation standpoint this would amount to removing the initial WQv (or removal portion) from the beginning of the runoff hydrograph - thus creating a "notch" in the runoff hydrograph. Since most commercially available hydrologic modeling package do not accommodate this, the following method has been created to facilitate removal from the runoff hydrograph of approximately the WQv: • Enter the horizontal axis on Figure 4.4 with the impervious percentage of the watershed and read upward to the predominant Hydrologic Soil Group (HSG). • Read left to the factor. • Multiply the curve number for the sub -watershed that includes the water quality basin by this factor - this provides a smaller curve number. The difference in curve number will generate a runoff hydrograph that has a volume less than the original volume by an amount approximately equal to the WQv. This method should be used only for bioretention areas, filter facilities and infiltration trenches where the drawdown time is >- 24 hours. Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 04 1%§ 1 h Wh. MMMMM-M-Mm Figure 4.4. Curve number adjustment factor. Example A site design employs an infiltration trench for the WQ, and has a curve number of 72, is B -type soil, and has an SECTION 4.5. STORMWATER QUALITY BMP LONG TERM MAINTENANCE Each water quality BMP installed on a site requires regular maintenance to ensure that it functions properly. A BMP -specific maintenance agreement for each development site is required. The maintenance agreement consists of the following: 1. An Inspection and Maintenance Agreement signed by the developer or BMP owner. 2. A long term maintenance plan written by the engineer or site designer that includes a description of the stormwater system and its components, inspection priorities and schedule for each component, and BMP schematics for each BMP. The plan should also include requirements for the proper disposal of any materials removed from the BMP during maintenance; and Chapter 4. — Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 3. A drawing of easements on a plat or a system location map to enable the City to locate BMPs as needed. The maintenance agreement and its attachments must be submitted for review by the City with the site plans. After the plans and the agreement are approved, the property owner shall record the maintenance agreement and its attachments with the register of deeds. The property owner, under the maintenance agreement, shall be responsible for inspecting and maintaining the BMPs and for turning in inspection reports annually to show that the facilities have been inspected and maintained. SECTION 4.6. REFERENCES Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Knox County Tennessee, 2008. Stormwater Management Manual, Volume 2. Metropolitan Nashville - Davidson County, 2012. Stormwater Management Manual, Volume 1 Regulations, Nashville, TN. http://www.nashville.gov/stormwater/regs/SwMgt-ManualVo101_2009.asp Chapter 4. - Water Quality You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 5. LOW IMPACT DEVELOPMENT SECTION 5.1. INTRODUCTION 5.1.1 Background and Purpose This chapter provides guidance on (1) implementing the principles of LID, (2) implementing the design, installation, and maintenance of LID -based Green Stormwater Practices (GSPs) for new development and re -development projects, and (3) applying the principles of the Runoff Reduction Method (RRM) to calculate runoff volume reduction and make runoff computation adjustments. This chapter should be used as a resource for incorporating LID elements in any project, whether or not the volume reduction goals can be met. The use of LID for new development and redevelopment projects within the City of Fayetteville is encouraged. The City also encourages the use of LID stormwater management strategies in street design and construction. The Master Street Plan provides information for incorporating LID into the City's typical street sections. Any LID elements incorporated into the street typical section and drainage design should be designed in accordance with the information provided in this chapter and the Master Street Plan. Runoff Reduction Method (RRM) The RRM is based on the approach that runoff volume reduction equals pollutant reduction with respect to stormwater runoff. This method assigns a rating in terms of percent rainfall capture to every post - development land surface. In Fayetteville the percent rainfall capture goal is 80% volume capture. To meet this capture goal the designer must apply site layout techniques and use a combination of infiltration, evapotranspiration, harvest and/or rainfall reuse practices, to capture and treat 80% of the rainfall volume based on a 1 -inch storm of moderate intensity. The steps outlined below highlight the process of Runoff Reduction design. • Step 1: Reduce runoff through land use, site layout and ground cover decisions with Intrinsic Green Stormwater Practices (GSPs). • Step 2: Direct runoff from impervious areas to pervious areas. • Step 3: Apply on-site structural GSPs to capture the remaining volume. It should be noted that the RRM is not required within the City of Fayetteville, however the use of this method and the design techniques and best management practices described in this chapter are encouraged. The RRM can be used to replace the TRM approach described in Chapter 4, or the two approaches may be used in combination. The runoff reduction goal for LID presented in this chapter is accomplished through volume removal. For developments implementing a LID site design, the project design team should use the guidance contained within this chapter to provide as much volume reduction as is feasible, with the goal of reaching 80% runoff reduction. If the 80% goal is not feasible for the project, the project will still be considered provided that all the other design requirements of the Drainage Criteria Manual, all applicable ordinances, and the Master Street Plan are met. Any LID features that are incorporated into the site design should be designed in `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) accordance with this chapter, and any volume reduction achieved by using LID should be accounted for in the hydrologic and hydraulic calculations required by the Drainage Criteria Manual. The project will benefit from any achieved reduction in runoff volume by reducing the post -development runoff; the corresponding credit is the reduced detention requirements for the site. 5.1.2 Overview: LID and Stormwater Management LID works to control stormwater runoff volume by attempting to mimic a site's natural hydrology through the use of design techniques that promote infiltration, filtration, storage, and evapotranspiration. LID uses GSPs that slow runoff, spread the flow of stormwater, and allow it to soak into the ground, thereby reducing the volume of runoff from the post -developed site. GSPs can be used in conjunction with traditional techniques to develop a comprehensive stormwater management plan. To implement LID, the designer should select a combination of GSPs that take into consideration existing hydrology on the site, complement traditional design techniques and provide volume reduction to help meet stormwater management goals. Traditional stormwater management does not typically account for the increase in the total volume of runoff that occurs from increases in impervious area. It also doesn't typically address pollutants that can occur from post -development land use and the associated increased impervious area. Applying the LID techniques or GSPs presented in this chapter to a site design can reduce the volume of stormwater discharged from a site as well as the quantity of pollutants discharged from impervious surfaces. The calculation method used for LID projects in Fayetteville is called the Runoff Reduction Method (RRM). The RRM quantifies the volume of runoff associated with particular land surfaces and GSPs by assigning a rating for rainfall capture for each land surface and a corresponding Runoff Reduction Credit (RR Credit) for each GSP. By quantifying the runoff volume for each land cover and design technique, the designer can compare the amount of runoff captured using various combinations of surface cover and design techniques and select the site design approach that achieves the desired volume reduction. Table 5.1. presents a comparison of the stormwater management goals for traditional stormwater design and LID design. This table emphasizes that LID works to complement, not replace, traditional flood management techniques, as both are equally important to a comprehensive stormwater management plan. Traditional Focus on large infrequent storms Prevent flooding by mitigating peak flow rates 5.1.3 Chapter Components Low Im GSPs Focus on small frequent storms Reduce total stormwater runoff volume Promote infiltration and Rroundwater recha Improve and protect water Section 5.2 provides guidance for the site planning and design of LID projects. It examines the specific considerations and processes used to select intrinsic, structural, and non-structural GSPs as part of the site design process. The planning and site design section provides guidance for using natural properties of a site Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) to manage stormwater using and preserving the pre -development characteristics of the site. General considerations for selection of the appropriate structural and non-structural GSPs are also detailed. Section 5.3 presents the Runoff Reduction Methodology. The Runoff Reduction Method (RRM) serves as the basis for Fayetteville's approach to LID using volume removal. Through this method every land surface can now have an assigned rating in terms of percent rainfall capture. Even impervious surfaces capture a small amount of water and therefore do not generate 100% runoff. Thus, understanding and calculating key aspects of a site's land condition in relation to volume removal is important to this process demonstrated with step-by-step equations and methodology. Section 5.4 presents intrinsic and structural/non-structural GSPs. Each GSP has a design specification included in either Appendix A or Appendix B. The design specifications include detailed design guidance for each GSP allowing the designer to plan and appropriately select the GSP or combination of GSPs that will help achieve the volume reduction goals presented in Section 3. Additional specifications are presented regarding the performance of infiltration tests that are required for certain media -based GSPs, and soil mix designs for use in various GSPs. 5.1.4 How Does this Chapter Relate to the LID Ordinance, the Fayetteville Drainage Criteria Manual, and other Ordinances? In 2009, the City passed a Low Impact Development Ordinance (Chapter 179: Low Impact Development of Title XV Unified Development Code). The design concepts presented in Chapter 5 are in support of the LID Ordinance. Implementing LID and using GSPs in accordance with this chapter, as specified in the LID Ordinance, is voluntary. A site development project will be required to meet all stormwater management requirements outlined and described throughout the Drainage Criteria Manual whether or not LID is implemented. Implementing LID in accordance with guidelines in this chapter may, however, assist in meeting the requirements of the DCM by reducing the post -development runoff volume and flow. The following ordinances and documents should also be considered through the process of implementing LID in Fayetteville. • Chapter 161- Zoning Regulations • Chapter 166 - Development • Chapter 167 - Tree Preservation & Protection • Chapter 168 - Flood Damage Prevention Code • Chapter 169 - Physical Alteration of Land • Chapter 170 - Stormwater Management, Drainage, & Erosion • Chapter 171- Streets and Sidewalks • Chapter 172 - Parking & Loading • Chapter 173 - Building Regulations • Chapter 177 - Landscape Regulations • Chapter 179 - Low Impact Development `t—Nis Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • City Plan 2030 • City of Fayetteville Landscape Manual • Streamside Protection BMP Manual The LID design techniques presented in this chapter or other similar techniques may be required in order to meet future MS4 NPDES stormwater permit requirements for post construction stormwater quality, increasing the importance that designers and owners understand LID design concepts and procedures presented herein. 5.1.5 How to Use this Chapter? Engineers, Planners, Developers, and City staff should use this chapter for direction on the process for designing, implementing, constructing, and maintaining LID projects. To obtain credit in the form of reduced detention, LID features shall be designed and constructed in accordance with this guidance. The process for implementing LID on a development project is shown in Figure 5.1. `t—Nis Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) -Conduct a comprehensive site analysis and inventory. Include consideration of any additional federal or state requirements, such as wetlands permitting requirements. -Refer to Section 5.2: Planning and Site Design. Step 2 - Apply Intrinsic Green Stormwater Practices -Reduce runoff by making the best use of existing natural features with proper site planning and design processes. • Refer to Section 5.2.2 : Applying Intrinsic Green Stormwater Practices. Step 3 - Select Structural & Nonstructural Green Stormwater Practices -Select preferred GSPs for use on development or redevelopment site. -Refer to Section 5.3 : The Runoff Reduction Method and Section 5.4: Green Stormwater Practices. MethodStep 4 - Apply Runoff Reduction Principles -Evaluate the site to determine the runoff reduction level achieved through GSP selection. • Refer to Section 5.3: The Runoff Reduction Method. Step 5 — Achieve 80% Runoff Reduction -If necessary, apply additional or different GSPs to the development or redevelopment site to reach the 80% runoff reduction goal. -Refer to Section 5.3: The Runoff Reduction Method and Section 5.4: Green Stormwater Practices. Figure 5.1. LID implementation process. SECTION 5.2. PLANNING AND SITE DESIGN 5.2.1 Introduction and Design Principles Site Design The correct pairing of land uses and Green Stormwater Practices Objectives an important first step in site planning. Land use and project type . Achieve Multiple the basis for selection of GSPs detailed on the criteria objectives (GSPs) is should be specification sheets in Appendix A and Appendix B. The Runoff conserve NaturalI Reduction Method detailed in Section 5.3 is a three step process used to " -L select intrinsic, non-structural, and structural GSPs, and to account for stormwater volume capture on a site. This section describes the specific considerations and processes used to select intrinsic, non-structural, and structural GSPs as a part of site design. This section also provides guidance for using the natural properties Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) and existing conditions of a site to optimize the management of stormwater using LID, and provides general information to aid selection of the most appropriate structural and non-structural GSPs. There are several important design goals and principles involved in incorporating GSPs. Some of these basic concepts are listed below. Achieve Multiple Objectives Stormwater management should be comprehensive and designed to achieve multiple stormwater objectives such as: managing peak flow and total volume; improving water quality control; maintaining or improving the pre -development hydrologic regime; and maintaining water temperature. In some cases, this requires multiple structural techniques; however, one objective of GSPs is to allow for less complex management systems to achieve multiple objectives through implementation of site planning as a precursor to the design process. Conserve Natural Features And Resources The conservation of natural features such as floodplains, higher permeability soils, and vegetation helps to retain predevelopment hydrologic functions, thus reducing runoff volumes. Impacts to natural features should be minimized by reducing the extent of construction impacts and minimize development practices that are adverse to predevelopment hydrology functions. Conservation techniques include the following: • Build upon the least permeable soils and limit construction activities to previously disturbed soils; • Avoid mass clearing and grading, and limit the clearing and grading of land to the minimum needed to construct the development and associated infrastructure; • Avoid disturbance of vegetation and highly erodible soils on slopes and near surface waters; • Leave undisturbed stream buffers along both sides of natural streams; as currently required by the City of Fayetteville Streamside Ordinance; • Preserve sensitive environmental areas; historically undisturbed vegetation; and native trees; also currently required in the City of Fayetteville; • Conform to watershed; conservation; and open space plans; • Design development to fit the site terrain and build roadways along site contours wherever possible; • Use cluster development to preserve higher permeability soils, natural streams, and natural slopes; and • Develop on previously developed sites (redevelopment or infill). Minimize Soil Compaction Soil compaction disturbs native soil structure, reduces soil porosity and permeability, affecting infiltration rates, and limits root growth and re-establishment of pre-existing vegetation. While soil compaction is necessary within a structure footprint to provide structurally sound foundations, areas away from `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) foundations are often excessively compacted by traffic during construction. Minimizing soil compaction can be achieved by the following methods: • Reduce disturbance through design and construction staging practices, • Limit areas of access for heavy equipment, • Avoid extensive and unnecessary clearing and stockpiling of topsoil, • Maintain existing topsoil and/or use quality topsoil during construction, • Rapid establishment of vegetative cover in bare but otherwise undisturbed areas to minimize compaction by rainfall, and • Avoid working or driving on wet soil. Manage Stormwater Close to the Source Redirecting runoff back into the ground, close to the point of origin, provides both environmental and economic benefits. Techniques to manage stormwater runoff close to the source include: • The use of GSPs to infiltrate stormwater into the ground instead of concentrating and collecting of flow and routing it offsite, and • Disconnection of impervious surfaces wherever feasible. Reduce and Disconnect Impervious Surfaces Reducing and disconnecting impervious surfaces increases the rainfall that infiltrates into the ground. Impervious areas may be reduced by maximizing landscaping and using pervious pavements. In addition, the amount of impervious areas with direct hydraulic connections to impervious conveyances (e.g., driveways, walkways, culverts, streets, or storm drains) should be minimized. The following measures are applicable: • Install green roofs; • Direct roof downspouts to vegetated areas, bioretention, cisterns, or planter boxes, and route runoff into vegetated swales instead of gutters; • Use porous pavements, where permitted; • Install shared driveways that connect two or more homes, where permitted, or install residential driveways with center vegetated strips; • Allow for shared parking in commercial areas; • Maximize usable space, not through large building footprints but through taller buildings with more floors; and • Minimize impervious footprints. `t—Nis Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 5.2.2 Applying Intrinsic Green Stormwater Practices The first tool in addressing stormwater management in new development and redevelopment involves making the best use Intrinsic Green of the Stormwater Practices existing natural features with proper site planning and design processes b applying Intrinsic GSPs. The Runoff Reduction Method, as Minimize soil described y Compaction in Section 5.3, provides a quantitative and qualitative approach . Minimize Total to allow capture credit for the majority of natural land cover types and I areas preserved using intrinsic GSPs. Intrinsic site design practices or intrinsic GSPs include evaluation not only of a site, but also where the stormwater is falling on a site and how to manage that rainfall before routing to a structural GSP. The goal is to minimize impervious cover and mass site grading and to maximize the retention of forest and vegetative cover, natural areas and undisturbed soils, especially those most conducive to landscape -scale infiltration. Through these methods, the amount of runoff and pollutants generated from both large-scale development projects and individual lot development can be reduced. Intrinsic GSPs are very site specific and should be carefully examined for applicability on different types of sites and for different proposed development. The goals of intrinsic GSPs are as follows: • Manage stormwater (quantity and quality) as close to the point of origin as possible and minimize collection and conveyance; • Prevent stormwater impacts rather than mitigating them downstream; • Use simple, non-structural methods for stormwater management that are less costly and require less maintenance than structural controls; • Create a multifunctional landscape; • Use hydrology as a framework for site design; and • Protect in-situ soils. The goal of this first step in the design process is to reduce the anticipated environmental impact "footprint" of the development and to maintain or improve the natural ability of the site to capture runoff while retaining and enhancing the owner/developer's purpose and vision for the site. Intrinsic site design concepts can reduce the cost of infrastructure while maintaining or even increasing the value of the property. Additional information for Intrinsic GSPs can be found in the specification sheets in Appendix A. The following benefits apply to almost all the Intrinsic GSPs (Southeastern Michigan Council of Government (SEMCOG), 2008). • Reduced land clearing costs, • Reduced costs for total infrastructure, • Reduced total stormwater management costs, • Enhanced community and individual lot aesthetics, and • Improved overall marketability and property values. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Intrinsic GSPs are both water quantity and water quality management tools and can reduce the size and cost of required structural GSPs. The site design approach can result in a more natural and cost-effective stormwater management system that better mimics the natural hydrologic conditions of the site, has a lower maintenance burden, and is more sustainable. 5.2.2.1 Initial Intrinsic GSP Site Considerations The process of selecting intrinsic GSPs begins after initial evaluation of the site and consideration of additional federal or state requirements and City ordinances. The goal of this process is to generate an initial concept design for the intrinsic GSPs that can then be evaluated for water quality and quantity volumes and further used to plan any additional structural GSPs that can be added to meet water quality goals. Using initial information collected during the site evaluation, the intrinsic practices can be conceptually designed to address the following considerations: Evaluating the Existing Site By taking a thorough inventory of the site's initial layout and resources, some intrinsic GSPs may be immediately eliminated from consideration. The designer should answer the following questions: • What features currently exist onsite that have the potential to be preserved as conservation/sensitive areas, natural flow paths, and areas of no soil compaction? • Are there riparian buffer areas that can be protected? • Are there any areas that could be preserved from disturbance? Proposed Site Use The proposed use of the site should be evaluated to correctly select the intrinsic GSPs that will most likely best fit the development. The following questions should be answered: • What type of development is to be placed on the site and what intrinsic GSPs are typically associated with this type of development? • Is cluster development an option? • Are there chances to reduce impervious cover and utilize stormwater disconnection? 5.2.2.2 Intrinsic GSP Selection Process There are several strategies to aid in selection of intrinsic GSPs to achieve runoff reduction. For each category the Intrinsic GSPs are listed adjacent to the category description. Some of the intrinsic GSPs bridge multiple categories. For additional information for each of the Intrinsic GSPs, see Appendix A. Conservation of Natural Features and Resources Identify and preserve the natural features and resources that can be used to Conservation protect water resources, reduce stormwater runoff, provide runoff storage, Intrinsic GSPs: reduce flooding, prevent soil erosion, promote infiltration, and remove stormwater pollutants. These features include: `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Areas of undisturbed vegetation • Floodplains and riparian areas • Ridgetops and steep slopes • Natural drainage pathways • Intermittent and perennial streams • Aquifers and recharge areas • Wetlands • Soils • Other natural features or critical areas Perform a delineation of natural features and a comprehensive site analysis and inventory before site layout design is performed. Approaches should: • Preserve undisturbed natural areas and riparian buffers. • Avoid floodplains and steep slopes. • Minimize siting on highly permeable or highly erodible soils. Low Impact Site Design Strategies After conservation areas are delineated, development of site design should include planning to avoid future downstream stormwater impacts from the development. Planning techniques should: • Fit the design to the terrain. Site Design Intrinsic • Reduce the limits of clearing and grading. GQPQ- • Locate development in less hydrologically sensitive areas. • Utilize open space development and/or nontraditional lot designs for residential areas. • Consider creative development design. Reduction of Impervious Cover Methods include: • Reduce roadway lengths. • Reduce roadway widths. • Reduce building footprint(s). • Reduce parking footprint(s). • Reduce setbacks and frontages. • Install fewer or alternative cul-de-sacs. Intrinsic GSPs: } # Chapter 5 – Low Impact Development `t— You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Use Natural Features for Stormwater Management Careful site design can reduce the need and size of structural conveyance systems and controls through use of natural site features and drainage systems. Methods of incorporating natural features into an overall stormwater management site plan may include: • Use buffers and undisturbed areas. Intrinsic GSPs: • Use natural drainageways instead of storm sewer systems. • Use vegetated swales instead of curb and gutter. • Drain runoff to pervious areas. 5.2.2.3 Implementing a Stormwater Sensitive Site Design Taken from the methodology above, Table 5.2 provides a review checklist for site design. The questions are organized by GSP categories. Table 5.2. Questionnaire for designer on implementing stormwater better site design practices. Natural Area Conservation • Is natural vegetation preserved on-site? Tree Conservation • Are original trees preserved on-site? Stream Buffers Conservation of Are stream buffer requirements properly enforced with at least the minimum area Natural Features left undisturbed? Floodplains • Are buildings to be located out of the 100 -year floodplain? Steep Slopes and Limiting Soils • Have buildings on steep slopes and slopes with highly erodible soils been minimized or restricted? Fitting Site Designs to the Terrain • Has developer worked to best fit design concepts to the site topography and to protect key site resources? Clearing and Grading • If multi -phased project, has developer limited the amount of cleared land to what is needed on a phase -by -phase basis? Is the Development Located in a Less Sensitive Area? Open Space Development Low Impact Site Designs • Was a cluster development considered, if an option? • Is a significant percentage of the open space managed in an undisturbed, natural condition? • Is an association planned that can effectively manage open space? Nontraditional Lot Designs • Are nontraditional lot designs and shapes included in the development? Creative Development Design • Are Planned Unit Developments (PUD's) included in the development? `t—Nis Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Additional guidance on implementing better site design is provided in "Low Impact Development. A Design Manual for Urban Areas" published in 2010 by the Community Design Center at the University of Arkansas (http: //uacdc.uark.edu/books.php). `t— Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Roadway Length • Are the most efficient site and street layouts used consistent with Fayetteville Master Street Plan to reduce overall street length? Roadway Width • Are the minimum pavement widths used based on Master Street Plan LID sections? Building Footprint • Are taller buildings and structures utilized, if permitted, to reduce the development's overall impervious footprint? Parking Footprint Reduction of Impervious Cover • Is the minimum applicable parking ratio used for the planned development? If mass transit is provided nearby, are parking ratios reduced? • Are the minimum stall width and length used for a standard parking space? • Do at least 30% of the spaces at larger commercial parking lots have smaller dimensions for compact cars? • Are shared parking arrangements used? • Is parking within structured decks or ramps rather than surface parking lots utilized? • Are porous surfaces used for overflow parking areas? • Are bioretention islands and other structural control practices used within landscaped areas or setbacks? Setbacks and Frontages • Are minimum front, rear, and side setbacks used for residential lots in accordance with cluster development? • Is the minimum frontage distance used for residential lots? Site Layout Alternative Cul-de-sacs Requirement • Are cul-de-sacs designed for the minimum allowed radius? • If allowed, are landscaped islands utilized within cul-de-sacs? • Are alternative turnarounds such as "hammerheads" used on short streets in low density residential neighborhoods? Using Buffers and Undisturbed Areas • Are level spreaders used to promote sheet flow of runoff across buffers and natural areas? Utilization of Natural Using Natural Drainageways Features Where possible, are natural systems used in place storm sewer systems? Using Vegetated Swales for Stormwater Management 0 Are vegetated swales used instead of curb and gutter, where possible? Rooftop Runoff • Is rooftop runoff designed to permanently discharge to pervious yard areas? • Where possible, is temporary ponding of runoff on lawns or rooftops implemented? Additional guidance on implementing better site design is provided in "Low Impact Development. A Design Manual for Urban Areas" published in 2010 by the Community Design Center at the University of Arkansas (http: //uacdc.uark.edu/books.php). `t— Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 5.2.3 Green Stormwater Practices Selection Criteria Steps 2 and 3 of the Runoff Reduction Method (Section 5.3) the use of structural and non-structural GSPs for a site. are general guidelines and limiting features to aid GSP selection. cases, limiting factors to GSP use may be overcome through design. In all cases, selected GSPs, along with supporting criteria computations, and any compromises or design features, shall be to the City to ensure proper evaluation and review. Factors for GSP involve Selection Following • Close to Source • Maximize Dual Use In some • Site Features innovative • Contributing Drainage and Area presented Structural or non-structural GSP selection should be based on the functional goal of the practice. The decision making process used to select a GSP must balance the goals of the proposed facility against site constraints and the limiting characteristics of the GSPs. A successful design process requires balancing the technical and nontechnical factors and is summarized in Figure 5.2. Aesthetic/Habitat related issues Maintenance Issues Construction consideration Applicability by land use Runoff quantity and runoff quality needs Close to source Maximize dual use Site factors Figure 5.2. Green stormwater practice selection factors. Site feasibility factors to consider include: • Proximity to runoff source, • Maximization of dual use, Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Topographic and/or geologic constraints, • Contributing drainage area size, and • Applicability by land use. The GSP factors to consider include: • Runoff quality and quantity, • Costs, • Construction considerations, • Maintenance, and • Aesthetics. When selecting the most appropriate GSP for a site, a treatment train or a set of GSPs in series may be necessary to achieve the reduction goals on a site for which one GSP is not sufficient. Treatment trains can have many combinations and used for all types of sites, as dictated by the site layout and proposed GSPs. An example is provided in Figure 5.3. The approach for including GSPs in series is located in Section 5.3. `t—Nis Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) I � I , '• I , 1 •'Ir ''4 1 1 •1. � •1, 1 1 �• 1 100 ymr ActaTtlda i oaf rains I alts wlt•41 � - � Salt %;01 12"t, 1 •' 1.; I 1 ' Z year w�aad�• ' : ',-' . dtt.ultiort � � _ l 1 1 Permanent L IVGI't.5 ,; 1 1: 771, F id_iwa -dnnm Snulr i 1 ------------------------------------------------------ 1 1 ' 1 1 1 1 Figure 5.3. Treatment train example - commercial development (Source: NIPC, 2000). 5.2.3.1 Site Feasibility Factors Source Considerations Manage stormwater runoff as close to the source, or origin, as possible. Implementing this factor will vary by site and by the proposed development. GSPs should be selected and placed so that runoff from treatment areas (impervious surfaces) flows directly into GSPs via sheet flow or is piped a short distance and then discharged into a GSP. Managing stormwater close to the source means treating the stormwater on site and minimizing piping to a centralized end -of -pipe treatment system. `t—Nis Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Maximize Dual Use Integrate stormwater management into already disturbed areas where possible (e.g., stormwater infiltration systems beneath parking areas, play fields on infiltration basins). Site Factors Each site should be inventoried for certain characteristics (e.g., soil type, depth to water table, slopes) that are a part of GSP selection and design. The GSP specification sheets highlight these site factors which are discussed in more detail in Appendix B. The following list describes some of the site factors that may need to be considered in Fayetteville. 1. Karst topography: These areas present very difficult challenges since any GSP which impounds water may cause underlying caverns or sink holes to expand and open at the surface. The use of liners may help the GSP hold the runoff as intended; however, the conveyance to the BMP, as well as the conveyance from the GSP to the receiving channel, must also be considered since the overall volume of runoff may be increasing or directed to areas previously not impacted by runoff. The presence of karst topography may allow a direct path for the stormwater runoff to enter the water table with little or no filtering of pollutants. Design in areas suspected to include karst topography should be supported by a karst survey and, if warranted, further geotechnical investigation. A Karst Area Sensitivity Map of Washington County (The Nature Conservancy, 2007) is available for review at httl2://nwarpc.org/ndf/GIS-Imagery/KASM WASHINGTON CO.pdf. 2. High water table: A high water table can impact the proper function of a GSP. Infiltration GSPs are restricted since a high water table will prevent the percolation of stormwater into the subsoils. A high water table may also cause dry detention GSPs to evolve into wet facilities. 3. Bedrock: The presence of bedrock near the surface can significantly impact a development project. The excavation costs can increase considerably. 4. Proximity to structures and steep slopes. One of the goals of stormwater facilities is to provide groundwater recharge. This tends to saturate the adjacent ground during and for a period of time after a storm event. Building foundations, basements, and other structures may be impacted by the wet/dry cycle of the surrounding soils. Saturating the soils on or adjacent to steep slopes (6 to 10 percent or greater) can cause a failure of the slope and adjacent structures. Areas of high pollutant potential are defined as land uses that generate higher concentrations of a particular pollutant or pollutants, such as sediment, hydro -carbons, trace metals, or toxicants, than are found in typical stormwater runoff. The use of GSPs is limited on sites considered to be have high pollution potential. Due to the potential for groundwater contamination, infiltration facilities are not recommended in such areas. The use of impoundment type structures in such areas should be qualified by an adequate vertical separation from the seasonal groundwater table (4 feet separation is desirable, and a 2 foot separation minimum); alternately, an impermeable liner may be used to prevent infiltration. Table 5.3 includes a list of typical areas with high pollution potential. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 5.3. Areas of high pollution potential. The following land uses and activities are typical areas of concern: Vehicle salvage yards and recycling facilities Marinas (service and maintenance) Vehicle fueling stations Outdoor liquid container storage Vehicle service and maintenance facilities Outdoor loading/unloading facilities Vehicle and equipment cleaning facilities Public works storage areas Fleet storage areas (bus, truck, etc.) Facilities that generate or store hazardous materials Industrial sites (for SIC codes reference Arkansas Department of Environmental Quality (ADEQ)) Commercial container nursery Applicability by Land Use Some land uses lend themselves to certain GSPs. Low density residential development lacks large congregate parking areas conducive to pervious pavement with infiltration, though pervious pavement might be appropriate for street side parking areas or roads. Conversely, rain barrels are especially good for residential use, but vegetated roofs are unlikely to be used on single-family homes. 5.2.3.2 Practice Feasibility Factors Not all structural GSPs are appropriate for varying types of sites and development. The selection process for the large array of structural GSPs can be complex, due to the number of factors. Tables 5.4 and 5.5 below provide a summary of the stormwater quality and quantity functions, cost, relative difficulty to construct, relative maintenance intensity, and performance level for each GSP, to aid in the selection process. The following factors should be considered when selecting GSPs: Water Quality and Quantity Goals The City of Fayetteville currently has a goal of at least 80% removal of TSS from flows that exceed predevelopment levels where practicable. The City also has water quantity standards for design. The post - development peak rate of surface discharge must not exceed the existing discharge for the 10 -year, 25 -year and the 100 -year, 24-hour storms. Additional channel protection requirements also apply. Refer to Chapter 1 for the Minimum Standard requirements for stormwater management. Cost GSP costs include both construction and long-term maintenance activities. Costs are often related to the size and nature of the development. "BMP and LID Whole Life Cost Models: Version 2.0", WERF (2009), and "Rapid Assessment of the Cost -Effectiveness of Low Impact Development for CSO Control", Montalto et al. (2007) are two resources that may aid in initial GSP selection decisions based on cost. Construction The GSP specifications in Appendices A and B include general construction guidelines that provide instruction on proper installation practices and materials. These guidelines shall be followed for projects designed to meet water quality goals within the City of Fayetteville. Maintenance A list of maintenance requirements must be included with GSP design, and considered when selecting a GSP. Some GSPs require greater maintenance to function properly. Vegetated GSPs require various types of landscape care. Structural GSPs such as pervious pavement require periodic maintenance such as `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) vacuuming, while infiltration basins, trenches, and grass channels are likely to require less maintenance. Some BMPs, especially those with plantings, may naturally improve in performance over time as vegetation grows and matures. In any case, general maintenance requirements are discussed for each GSP specification in Appendices A and B. Aesthetics/Habitat Landscape enhancement is an important goal in the City of Fayetteville. GSPs can fulfill the dual purpose of stormwater management and landscape feature. Bioretention and urban bioretention, water quality swales and filter strips, vegetated roofs, and many other GSPs should be integrated into landscape design when used, and can create value in addition to solving stormwater problems. Table 5.4. GSP selection factors. Structural Control Volume Selection Criteria Water Quality Bioretention • • Tree Planters/Urban Bioretention O • Permeable Pavement • O Infiltration • • Water Quality Swales (Dry) O • Dry Ponds O O Downspout Disconnection O O Grass Channels O O Sheet Flow • O Urban Reforestation • • Rain Tanks/Cisterns O O Green Roofs O O • Effective. O Moderately Effective. O Less Effective. `t—Nis Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • High/Intensive. O Moderate/Moderately Intensive. O Low/Less Intensive. SECTION 5.3. THE RUNOFF REDUCTION METHOD 5.3.1 Introduction 5.3.1.1 Background The Runoff Reduction Method (RRM) serves as the basis for the City of Fayetteville's approach to LID. The RRM derivation can be found in original references (Chesapeake Stormwater Network, not dated, Center for Watershed Protection, 2008). Volume removal is the focus of this approach, as volume reduction equals pollution reduction with respect to stormwater runoff. Through this method every post -development land surface is assigned a rating in terms of percent rainfall capture. Where practicable, runoff that is not captured through careful land use shall be managed using Green Stormwater Practices (GSPs). The RRM also provides a way to achieve removal of Total Suspended Solids (TSS) and some other pollutants. This is accomplished by removing the volume of water that contains TSS and other pollutants. For the purposes of the RRM, it is assumed that 100% of the TSS is removed from volumes that are infiltrated, evapotranspired, or reused for purposes such as landscape irrigation or grey water reuse. Any pollutant removal is accomplished via settling, filtering, adsorption, and/or biological uptake. The proportion of rainfall to be captured on the proposed post -development site shall be computed by calculating the weighted volumetric runoff coefficient (Rv) for the site using the assigned Rv values for each land use and Hydrologic Soil Group (HSG). For the purposes of this calculation, there are only three post - development land uses to chose from: forest or open space, disturbed soils, or impervious area. Rv is the proportion of the total precipitation that runs off a specific land use area. Rv is equal to the post - development runoff depth divided by the target rainfall depth at a given site (one inch in this case). For example, if the weighted Rv for the developed site is 0.20, then 80% of the rainfall has been captured. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table S.S. GSP cost selection factors. Structural Control Cost Selection Criteria Construction Maintenance Bioretention O • O Tree Planters/Urban Bioretention O O O Permeable Pavement O • • Infiltration O O O Water Quality Swales (Dry) O O O Dry Ponds • • O Downspout Disconnection O O O Grass Channels O O O Sheet Flow O O O Urban Reforestation O O O Rain Tanks/Cisterns O O O Green Roofs • • O • High/Intensive. O Moderate/Moderately Intensive. O Low/Less Intensive. SECTION 5.3. THE RUNOFF REDUCTION METHOD 5.3.1 Introduction 5.3.1.1 Background The Runoff Reduction Method (RRM) serves as the basis for the City of Fayetteville's approach to LID. The RRM derivation can be found in original references (Chesapeake Stormwater Network, not dated, Center for Watershed Protection, 2008). Volume removal is the focus of this approach, as volume reduction equals pollution reduction with respect to stormwater runoff. Through this method every post -development land surface is assigned a rating in terms of percent rainfall capture. Where practicable, runoff that is not captured through careful land use shall be managed using Green Stormwater Practices (GSPs). The RRM also provides a way to achieve removal of Total Suspended Solids (TSS) and some other pollutants. This is accomplished by removing the volume of water that contains TSS and other pollutants. For the purposes of the RRM, it is assumed that 100% of the TSS is removed from volumes that are infiltrated, evapotranspired, or reused for purposes such as landscape irrigation or grey water reuse. Any pollutant removal is accomplished via settling, filtering, adsorption, and/or biological uptake. The proportion of rainfall to be captured on the proposed post -development site shall be computed by calculating the weighted volumetric runoff coefficient (Rv) for the site using the assigned Rv values for each land use and Hydrologic Soil Group (HSG). For the purposes of this calculation, there are only three post - development land uses to chose from: forest or open space, disturbed soils, or impervious area. Rv is the proportion of the total precipitation that runs off a specific land use area. Rv is equal to the post - development runoff depth divided by the target rainfall depth at a given site (one inch in this case). For example, if the weighted Rv for the developed site is 0.20, then 80% of the rainfall has been captured. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Once the Rv is determined, GSPs shall be applied, if needed, to reduce the runoff volume. The weighted Rv shall be reduced by treating areas of the site with GSPs described in Appendices A and B that have been assigned specific values corresponding to the proportion of runoff volume reduction. Where practicable, GSPs shall be implemented together with careful land use to achieve a weighted Rv for the post -developed site of 0.20, which will accomplish the capture of 80% of the runoff volume. 5.3.1.2 Objectives The basis for the RRM is a percent rainfall volume capture goal. In Fayetteville the goal is 80% volume capture as explained in the Chapter 5 Executive Summary. To meet the 80% capture goal, where practicable, the designer shall lay out the site such that 80% of the rainfall is captured and treated on site through a combination of infiltration, evapotranspiration, harvest and/or use. This objective is accomplished through site layout and GSP design. Whether or not the 80% goal is achieved, if the LID approach is desired to be used to attain credit for runoff reduction, the designer shall calculate the Rv and an adjusted Curve Number (CNadi) to use in the runoff calculations for the site. The credit for using the RRM to calculate a CNadi is that the post -development flows may be reduced accordingly, thereby reducing stormwater detention requirements. 5.3.1.3 Conceptual Design Steps in the Runoff Reduction Method Conceptually, the RRM follows the steps shown in the flowchart in Figure 5.4 and briefly described below: Done Yes Done Yes Notu[�alPa� —� a No e ire 0 it, Yes irij flete MW ar'0444141 WOO- y� �6 Y No Figure 5.4. Runoff reduction design process. Step 1: Reduce Runoff Through Land Use and Ground Cover Decisions (Intrinsic GSPs). This step focuses on the existing and proposed land cover and how much of the rainfall it receives and removes from runoff. Design activities in Step linclude impervious area minimization, reduced soil disturbance, forest preservation, etc. The goal is to minimize impervious cover and mass site grading and `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) to maximize the retention of forest and vegetative cover, natural areas and undisturbed soils, especially those most conducive to landscape -scale infiltration. Information describing the methods and design criteria for Intrinsic GSPs can be found in Appendix A, Intrinsic Green Stormwater Practice Specifications. RRM calculations for Step 1 shall be based on the proposed post -developed site and cover and include the computation of volumetric runoff coefficients (Rv) for land use and existing Hydrologic Soil Group (HSG) combinations, including impervious cover. Step 2: A12121y Environmental Site Design Practices (Non -Structural GSPs). If the target rainfall volume capture (Rv <_ 0.2) has not been attained in Step 1, then non-structural GSPs provided in Appendix B can be implemented during the early phases of site layout to reduce the Rv. In this step, the designer enhances the ability of the existing land cover to reduce runoff volume through the planned and engineered use of such GSPs as disconnection of impervious areas (e.g., rooftops to sheet flow), pervious pavers, planned reforestation, etc. Each of these practices is assigned an ability to reduce one -inch of rainfall in a storm of moderate intensity; and this assignment is captured in the Runoff Removal Credit (RR Credit) in Table 5.7. Step 3: A12121y Structural GSPs. If the target volume capture (Rv <_ 0.2) has not been attained in Step 2, structural GSPs provided in Appendix B can be implemented to reduce the Rv. In this step, the designer evaluates combinations of engineered practices such as infiltration, bioretention, green roofs, stormwater planters, rainwater harvesting, etc. The designer applies the RR Credit for the area to be treated by each GSP to incrementally reduce the weighted Rv. • At the end of Step 3, the designer will have computed the weighted Rv and can determine the percentage of rainfall captured. The designer will have met the 80% volume capture goal if the weighted Rv is 0.20 or less. The following sections describe how to calculate Rv and the associated variables. 5.3.2 Technical Design Procedure 5.3.2.1 STEP 1: Land Use Rv Values As stated above, the volumetric runoff coefficient (Rv) is the ratio of the runoff depth divided by the target rainfall depth (one -inch in this case). If 45% of rainfall runs off the post -developed site, the Rv value equals 0.45. Unlike a Rational Method C Factor, for example, Rv is not a constant individual storm -based value, but is rainfall intensity and duration dependant. Rv values could be developed for individual storm intensities, seasons, or even annually. It should be noted that Rv is not equivalent to Curve Number. Site layout shall be designed to include Intrinsic GSPs where practicable to reduce impervious area and disturbed soils. Once the general design concept is developed, the site layout shall be characterized into areas of either undisturbed soil, disturbed soils, or impervious area with respect to each. For each Hydrologic Soil Group (HSG), land use area, the volumetric runoff coefficients listed in Table 5.6 can be applied to the site. A weighted Rv value shall be calculated to determine the proportion of runoff leaving the site. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 5.6 shows the Rv values assigned for the City of Fayetteville's post -development conditions. The values were derived by comparison with other sources, review of Fayetteville's land use and rainfall conditions, and simulation modeling of various land use categories and soil types. Table 5.6. Site cover runoff coefficients. Soil Condition Volumetric Runoff Coefficient (Rv) Impervious Cover 0.95 Hydrologic Soil Group A B C D Forest Cover/Open Spacel 0.02 0.03 0.04 0.05 Disturbed Soils' 0.15 0.18 0.20 0.23 1. Forest — undisturbed and non - compacted soils, protected forest, or reforested land. 2. Disturbed Soils — amended soils consisting of managed turf, graded for yards, or other landscaped areas 3. To be mowed/managed. Compacted fill qualifies as HSG D type soils only, whereas natural soils disturbed by light grading only shall be considered to be in the next higher HSG. Note: Areas where earthwork other than light grading has been performed (i.e., proofrolled, received structural/non-structural fill, compacted) shall be considered HSG group D, and the Rv value of 0.23 shall be used for that area, regardless of whether topsoil and sod are or have been applied. The area -weighted estimate of the total site Rv value is calculated using Equation 5.1. Weighted Rv = Rvi (Rvi % of site) + Rv2 (Rv2 % of site) Figure S.S. Site example with land uses. EXAMPLE 5.1. Intrinsic GSPs Eq. 5.1 As shown in Figure 5.5, a site is designed using Intrinsic GSPs and 50% of the site is impervious, 20% is undisturbed forest, and 30% is landscaped turf grass with underlying HSG B soils, the Rv value would be: Site Weighted Rv = 0.50*0.95 + 0.20* 0.03 + 0.30*0.18 = 0.54 That is, 54% of the rainfall on the site runs off. In order to achieve a higher percentage of runoff removal, i.e., to meet the 80% goal, additional GSPs must be planned and implemented. Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 5.3.2.2 STEPS 2 AND 3: Green Stormwater Practice Rv Values Steps 2 and 3 of the RRM describe how planning and design of additional Green Stormwater Practices (GSPs) shall be performed in cases where the goal is to reduce the total site Rv to 0.20 or less. The twelve GSPs that are acceptable for use in Fayetteville to attain additional volume reduction credit are listed in Table 5.7. Each GSP can be designed to either a Level 1 or Level 2 runoff reduction capability. The Level 1 designs have slightly less stringent design guidelines and therefore do not provide the same level of runoff reduction of a Level 2 design. Level 2 designs provide significant runoff reduction capability; however, very specific design requirements must be met to achieve the Level 2 treatment. Refer to Appendix B - GSP Specifications for design guidance for Green Stormwater Practices. Note that the first six GSPs themselves occupy site land area. Because of their ability to absorb the rain that falls on them they are assigned the corresponding Forest Cover Rv values from Table 5.6. Other GSPs, where applicable, are assigned the Disturbed Soils land cover Rv values from Table 5.6. Table 5.7. Green stormwater practices runoff reduction credit percentages. Rainfall Volume Removed (Captured) Level 1 Level 2 Structural Bioretention (GSP-01) 60 90 Urban Bioretention (GSP-02) 60 N/A Permeable Pavement (GSP-03) 45 75 Infiltration Trench (GSP-04) 50 90 Water Quality Swale (GSP-OS) 40 60 Extended Detention (GSP-06) 0 20 Grass Channel (GSP-08) 10/20 20/40 Rain Tanks/Cisterns (GSP-11) Design dependent Green Roof (GSP-12) 80 90 Non -Structural Disconnection — downspout (GSP-07) 25 50 Disconnection — sheet flow (GSP-09) 50 75 Reforestation (A, B, C, D soils) (GSP-10) 96 94 1 92 90 98 1 97 1 96 95 To calculate the R, value for a contributing drainage area flowing through a GSP use Equation 5.2. RvredUCed = CDA Rv(1 - RR Credit) Eq. 5.2 where CDA Rv equals the Contributing Drainage Area volumetric runoff coefficient for the drainage area being treated. CDA Rv should be weighted using Equation 5.1 if the drainage area has multiple land uses. If the drainage area contains only one land use, CDA Rv is the Rv for that land use. `t—Nis Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) EXAMPLE 5.2. Calculating Rv Reduced. Part of the proposed site is impervious and has a grass landscaped area draining to it. The CDA Rv = 0.85. This entire area is to be treated by a Level 2 bioretention structure (90% RR Credit). The reduced weighted volumetric runoff coefficient Rvreduced would be calculated as follows: Rvreduced = 0.85 *(1-0.90) = 0.09 Thus the bioretention facility meeting the Level 2 design criteria would cause the drainage area to exceed the goal of 80% volume reduction or better (an Rv of 0.20 or less) as the Rv is 0.09. 5.3.2.3 Rv Values for GSPs in Series The volume removal rate for controls in series may be computed by extending Equation 5.2, if appropriate GSPs are used as described herein. The upstream control has the benefit of initially addressing runoff from all storms, while the second control in the series must handle the overflow from the first, that will consist of a subset of fewer and larger storms. Therefore the ability to capture instantaneous volumes and store them for later removal is key for downstream controls. In addition to cisterns, only the first six controls in Table 5.7 can be used as the second GSP in a series volume removal calculation: bioretention, urban bioretention, permeable pavement, infiltration trench, water quality swale, and extended detention. The following equation shall be used for calculation of the total Rv factor for GSPs in series: Rv Series = CDA Rv(1 - RR, Credit) (1 - RR2 Credit) Eq. 5.3 where CDA Rv is the first GSP in the series, for example below (Rv = 0.95 for the impervious area), RR, Credit and RR2 Credit are respective the percent volume reduction credits for the first and second GSPs in the series from Table 5.7. Credit will be granted for, at most, two controls used in a series. Any more than that will not be counted towards the runoff reduction. EXAMPLE 5.3. GSPs in Series Runoff from an impervious area (Rv = 0.95) is treated with sheet flow disconnection then enters a bioretention facility. The following calculation gives the final Rv for that impervious area. The sheet flow GSP is Level 2 (RR Credit 75%) while bioretention design is Level 1 (RR Credit 60%). Total Rv credit = 0.95*(1-0.75)*(1-0.6) = 0.095 That is, 95% of the rainfall runs off the impervious area and enters the sheet flow area. Of that runoff, 75% is captured in the sheet flow area. The remainder (the larger storms) enters the bioretention facility and 60% of that is captured by that GSP, allowing 9.5% of the rainfall to overflow the facility. Hence, this CDA has a 90.5% rainfall removal rate - well above the 80% (Rv of 0.20 or less) goal. `t -Nis Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 5.3.2.4 Sizing of Media -Based GSPs For the sizing of media -based GSPs such as bioretention, tree planters, permeable pavement, infiltration trenches, water quality swales, and others it is assumed that the runoff from a one -inch storm is instantaneously contained within the control, and that the control is completely drained prior to the storm event, with no moisture susceptible to gravity drainage. These assumptions result in a design that conservatively approximates an 80% removal of runoff volume (Rv = 0.20) for native soil infiltration rates. To accommodate removal, underdrains are required for parent material infiltration rates less than 0.5 in/hr or for GSP designs where no infiltration test is performed to support calculations. As such the following guidance is provided for sizing these types of facilities. Details for each type are provided in the respective specification section in Appendices A and B. Details for sizing cisterns are also located in the GSP-11 specification in Appendix B. An in-situ infiltration test procedure is provided in Appendix C. Table 5.8 provides volume -based specifications for the engineered soil -based and gravel media prepared in accordance with the GSP designs in this manual. Average total porosity of non -compacted soil and gravel can range from 0.25 to 0.50 (Freeze and Cherry, 1979). Field capacity of the soil is the amount of moisture typically held in the soil/gravel after any excess water from rain events has drained and varies greatly between soil -based media and gravel. Effective porosity is defined as the difference between total porosity and field capacity. Values to be used for design are reported in Table 5.8. 1. Soil -Based Media GSPs - bioretention, water quality swales and tree planter boxes. 2. Gravel GSPs - design alternatives for soil -based GSPs, storage layers for permeable pavement, and infiltration trenches. All media -based GSPs shall be sized to provide storage volume for the complete runoff from one inch of rain over the contributing drainage area (CDA) in order to achieve the listed runoff reduction credit for the GSP. A GSP may be sized using less than one inch of rainfall over the contributing drainage area; however, Equation 5.12 in Section 5.3.2.6 shall be used to calculate the percentage of rainfall volume the GSP captures. All of the other design requirements detailed in Appendix B for the GSPs shall be followed. All media storage GSPs shall be sized using the following equations: where: 43560 ft21 ft l Tv = P(CDA)(Rv) ( 1 ac ) (12 in) = n(D)(SA) Eq. 5.4 Tv GSP treatment volume in cubic feet. CDA The drainage area in acres. P 1 inch. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 5.8. Parameter volume-basedTable Media Value Total Porosity, n Soil -Based Media' 0.40 Gravel' 0.3S Ponding 1.0 (void ratio) 1. Soil -Based Media GSPs - bioretention, water quality swales and tree planter boxes. 2. Gravel GSPs - design alternatives for soil -based GSPs, storage layers for permeable pavement, and infiltration trenches. All media -based GSPs shall be sized to provide storage volume for the complete runoff from one inch of rain over the contributing drainage area (CDA) in order to achieve the listed runoff reduction credit for the GSP. A GSP may be sized using less than one inch of rainfall over the contributing drainage area; however, Equation 5.12 in Section 5.3.2.6 shall be used to calculate the percentage of rainfall volume the GSP captures. All of the other design requirements detailed in Appendix B for the GSPs shall be followed. All media storage GSPs shall be sized using the following equations: where: 43560 ft21 ft l Tv = P(CDA)(Rv) ( 1 ac ) (12 in) = n(D)(SA) Eq. 5.4 Tv GSP treatment volume in cubic feet. CDA The drainage area in acres. P 1 inch. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Rv Runoff coefficient for the CDA. SA Surface area in square feet of the GSP. D Media depth of GSP in feet. n Total Porosity. D Depth, Dw if more than one media type is required. See Equation S.S. The equivalent storage depth for media -based GSPs with multiple layers of media must be calculated using the following equation: Weighted Storage Depth = Dw = n1(D1) + n2 (D2) + Eq. 5.5 Where nl and D1 are for the first layer, etc. Note that the Rv value is for the total area draining to the control. So if a filter strip is included in the area then a weighted Rv should be calculated but not a credit reduced Rv. EXAMPLE 5.4. Media -Based GSP A 1.5 acre parking lot is to drain to a Level 1 Bioretention GSP with the following media composition: 2 ft soil media and 0.5 ft of ponding. Then by application of Equations 5.4 and 5.5, solving for SA: Tv = 1`1.5*0.95*(43560/12) = 5173 cu ft = SA*Dw = SA (0.5 ft (1.0) + ((0.40) x 2 ft)) = SA (1.3 ft) SA of GSP = 3,979 sq ft 5.3.2.5 Calculation of Curve Numbers with Volume Removed The removal of volume by GSPs changes the runoff depth entering downstream flood control facilities. The resulting decrease is accounted for by calculating runoff based on an "adjusted SCS curve number" using (CNadj) which is less than the actual curve number (CN). The reduced runoff allows reduced detention storage. Standard SCS rainfall -runoff Equations 5.6 and 5.7 provide a way to calculate a total runoff if the rainfall and curve number are known, as: _ (P -(0.2+S))2 and S = 1000 _ 10 Q (P+(0.8+S)) CN Eqs. 5.6 and 5.7 Where P is the rainfall depth for the 24-hour design storm (Table 5.9), Q is the total runoff depth for that storm, S is potential maximum soil moisture retention, and CN is runoff curve number. Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 5.9. Fayetteville 24-hour rainfall depths. Return Period (Years) Rainfall Depth (Inches) 2 4.10 5 5.26 10 6.10 25 7.16 50 7.96 100 8.8 The adjusted total runoff depth entering the flood control facility downstream of a GSP is calculated as the difference between total runoff in depth and the depth captured by the GSP, as shown below. Qadj = Q — Qremoved Qremoved is defined in Equation 5.10. Eq. 5.8 The depth of captured rainfall (Qremoved)over the CDA is determined by first calculating the available volume that the GSP can capture (Vcap) as shown in Equation 5.9: V,ap = (Dp)(SA) + (n1)(D1)(SA) + (n2)(D2)(SA) Eq. 5.9 Where depths are in feet: Vcap Volume captured by the GSP, ft3 Dp Depth of ponding (if applicable), ft SA Surface area of the GSP, ftz nl Total porosity of first soil media or gravel layer D1 Depth of first soil media or gravel layer, ft The depth of total runoff, in inches, removed from site by the GSP, is calculated by Equation 5.10: (ucap)(12) Qremoved = 43560(A) Where A is site area in acres. Eq. 5.10 Equation 5.11 provides a method to calculate the modified curve number once the Qadj is found. CNadj = 1000 1/2 10+5P+10Qadj-10(Qadj2+1.25QadjP) The steps in calculating an adjusted Curve Number (CNadj) are: F7 Eq. 5.11 Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Step 1. Calculate Total Runoff for Storm (Q) Choose the design return period(s), and using appropriate return period rainfall as P, calculate an initial Q using Equations 5.6 and 5.7, with the calculated pre-GSP site curve number. Step 2. Calculate GSP Capture Volume (Vcap) Compute the captured volume in the GSP control using the dimensions of the media or proven cistern volume assuming a 72 -hour dry period since the last cistern filling event. Step 3. Calculate Depth of Total Runoff Removed from Site (Qremovea) Convert the captured volume into inches over the site or relevant CDA (Equation 5.10). Step 4. Calculate Adjusted Total Runoff (Qaai) As shown in Equation 5.8, subtract Qremoved from Q computed in Step 1. Step 5. Calculate Adjusted Curve Number (CNaaj) Using Qaai and the appropriate design storm rainfall depth P, calculate CNadj from Equation 5.11. Step 6. Use CNadj in calculations for the appropriate return period(s) in question. The following example illustrates this procedure. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) EXAMPLE S. S. Compute Adjusted Curve Number A 1.5 -acre parking lot (CN=98) is to drain into a site detention pond for the 2 -year through 100 -year storm. To account for a bioretention basin through which the parking lot drainage is directed prior to overflows entering the detention pond, an adjusted curve number (CN) should be calculated for the parking lot. The following example shows the calculation of a CNadj for the 100 -year storm event for the situation described. The CNadj for the other storms should be calculated to complete the routing calculations for the detention pond. Step 1. Using Equations 5.6 and 5.7 for the 100 -year storm (P = 8.8), the calculated runoff depth (Q) = 8.56 in. M (8.8 - (0.2 * 0.20))2 (8.8 + (0.8 * 0.20)) 1000 = 8.56 where S = 98 - 10 = 0.20 Step 2. Using the bioretention facility sized from the previous example the available captured volume (Vcap) = 5,173 cu ft. V,ap = (0.5)(3,979) + (0.4)(2)(3,979) = 5,173 cu ft Step 3. Over 1.5 acres the depth of runoff removed (Qremoved) = 0.95 in. Step 4. Qadj = 7.61 in. (5,173)(12) - Qremoved = 43,560(1.5) - 0.95 in. Qadj = 8.56 - 0.95 = 7.61 in. Step S. Using Qadj and the 100 -year P in Equation 5.11 we obtain the adjusted curve number of 87 (rounded to the next whole number). We can check our work by substituting this CN back into Equation 5.7 and applying Equation 5.6 to obtain the Q of step 4. 1000 CNadj =10 + 5(8.8) + 10(7.61) - 10[(7.61)2 + 1.25(7.61)(8.8)]1/2 - 90 The use of underdrains with media -based GSPs is required for soils that have infiltration rates of less than 0.5 inches/hour or if no in-situ infiltration test is provided to support calculations. When underdrains are used, the curve numbers shall not be adjusted but the portion of discharge captured by the GSP may be hydrologically routed through the GSP and underdrain system for the purpose of computing required detention storage. 5.3.2.6 Calculation of Rainfall Removal Based on Capture Depth If a GSP is sized to capture less than 80% rainfall removal (1 inch of rainfall), then the calculation of actual percentage of removal must be provided for review to the City Engineer together with the calculated RR credit. Equation 5.12 shall be used to calculate the percentage of rainfall capture. Figure 5.2 shows the relationship between capture depth and percent rainfall removal for the City of Fayetteville. For example, if a GSP captures 0.5 inches of runoff the capture is 56%. The rainfall capture is calculated as follows: Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Rainfall Capture = —0.0223D4 + 0.2262D3 — 0.8496D2 + 1.4316D + 0.0161 Eq. 5.12 Capture Depth vs. Percent Rainfall Removal 1W% T--- ------T---T 911% BE]% 7U% 4 0.5 1 1.5 1.5 Capture DWh D (inches) Figure 5.6. Capture depth vs. rainfall capture. SECTION 5.4. GREEN STORMWATER PRACTICES 5.4.1 Overview Green Stormwater Practices (GSPs) are intended to mimic the natural hydrologic condition and promote infiltration, filtration, storage/reuse, and evapotranspiration of stormwater runoff. The GSPs detailed in this manual include: bioretention, urban bioretention, permeable pavements, infiltration devices, water quality swales, extended detention ponds, downspout disconnection, grass channels, sheet flow, reforestation, rain tanks/cisterns, and green roofs. As detailed, GSPs are designed to meet multiple stormwater management objectives, including reductions in runoff volume, peak flow rate reductions, and water quality protection. Multiple small, localized controls may be combined into a "treatment train" to provide comprehensive stormwater management. The methodology for this is provided in Section 5.3. The GSPs in this section have been designed to be integrated into many common urban land uses on both public and private property, and may be constructed individually, or as part of larger construction projects. Decentralized management strategies are encouraged to be tailored to individual sites, which can eliminate the need for large-scale, capital -intensive centralized controls, and may improve the water quality in Fayetteville's streams. GSPs and the Transect: The Transect is a system of classifying urban environments from rural to urban according to intensity of development and land use character. The goal of Transect -based planning and design is to serve a variety of preferences for urban environments delivered in neighborhoods that are laid out and designed in an internally consistent manner. These zones may range in size from a few acres to `t—Nis Chapter 5 — Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) square miles. Lower density Transect zones (rural, suburban, and general urban) can more easily absorb stormwater runoff using natural green solutions - urban forests, disconnected impervious areas, swales, and larger bio -retention areas. As the density increases to the more urban Transects (urban center, urban core, special urban) the options to use Green Infrastructure is reduced to those elements that have a small footprint or none at all. In these cases volume -based stormwater designs make use of infiltration trenches, porous pavement, cisterns, urban bioretention, green roofs, etc. 5.4.2 Implementing GSPs The GSPs referenced throughout this chapter are located in the appendices specified below. Additional guidance on GSP selection is provided herein. Appendix A - Intrinsic Green Stormwater Practice Specifications includes intrinsic GSPs. These GSPs should be considered in the initial stages of site design to allow planning that incorporates the best suited practices for intrinsic volume control and water quality benefits to the site. Specification sheets for each intrinsic practice include the description, stormwater functions and calculations, maintenance, cost, and construction guidance, if applicable. Appendix B - Structural and Non -Structural Green Stormwater Practice Specifications includes twelve of the most common GSPs. A summary of these are shown in Tables 5.10 and 5.11 below. These tables are included to facilitate selection of the most appropriate practices for a given situation. Specification sheets for each practice provide a brief introduction to the practice, details on performance, suitability, limitations, and maintenance requirements. In addition, each practice is assigned a percentage of volume removal based on the design level selected and the particular GSP's ability to manage the first inch of runoff volume from a storm. The total runoff reduction percentage goal for a site is 80%, as explained in Section 5.3. 5.10. Effectiveness GSP of GSPs in meeting Volume stormwater management Peak Discharge objectives.Table Water Quality Bioretention • • • Tree Planters/Urban Bioretention O O • Permeable Pavement • • o Infiltration • • • Water Quality Swale o o • Extended Detention Basins O • O Downspout Disconnection O O O Grass Channels O O O Sheet Flow • • o Reforestation • • • Rain Tanks/Cisterns' O O O Green Roofs i O i • i • 1. A single cistern typically provides greater volume reduction than a single rain tank. • High effectiveness. O Medium effectiveness. O Low effectiveness. Rankings are qualitative. "High effectiveness" means that one of the primary functions of the GSP is to meet the objective. "Medium effectiveness" means that a GSP can partially meet the objective but should be used in `t—Nis Chapter 5 - Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) conjunction with other BMPs. "Low effectiveness" means that the contribution of the GSP to the objective is a byproduct of its other functions, and another decentralized control should be used if that objective is important. • Well suited for land use applications or relatively high dedicated land area required. O Average suitability for land use applications or relatively moderate dedicated land area required. O Relatively low dedicated land area required. N Not typically applicable for land use. SECTION 5.5. REFERENCES Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual Volume 1: Stormwater Policy Guidebook. Atlanta, GA. Available online at: http://www.georgiastormwater.com/ Center for Watershed Protection (CWP), 2008. Technical Memorandum: The Runoff Reduction Method. Chesapeake Stormwater Network (CSN), no date. Technical Support for the Bay -Wide Runoff Reduction Method, Ver. 2.0. CSN Tech. Bull. No. 4. Freeze and Cherry, 1979. Groundwater. Montalto, F., C. Behr, K. Alfredo, M. Wolf, M. Arye and M. Walsh, 2007. Rapid Assessment of the Cost - Effectiveness of Low Impact Development for CSO Control. Landscape and Urban Planning: 82: 117-131. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 5.11. Green stormwater practice lands use and land area selection matrix. Criteria Land Use GSP Single Multi- City Land Family Family Parks/Open ROW/ Utility Area Schools Comm'I Indust. Resid. Resid. Space Roadside Easements Required Bioretention • • N • • • • N Tree Planters/Urban O • N N • • • N O Bioretention Permeable • • o • • • • N O Pavement Infiltration • • N • • • O N O Water Quality • • N N • • • O O Swale Extended • • • N • o • • o Detention Basins Downspout • N • • • N N O Disconnection Grass Channels • • N • • O • • O Sheet Flow • • N • • o • • o Reforestation o N o 0 0 • • N O/• Rain Tanks/Cisterns • O O • • N N O O Green Roofs 1 • • • N • N N N O • Well suited for land use applications or relatively high dedicated land area required. O Average suitability for land use applications or relatively moderate dedicated land area required. O Relatively low dedicated land area required. N Not typically applicable for land use. SECTION 5.5. REFERENCES Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual Volume 1: Stormwater Policy Guidebook. Atlanta, GA. Available online at: http://www.georgiastormwater.com/ Center for Watershed Protection (CWP), 2008. Technical Memorandum: The Runoff Reduction Method. Chesapeake Stormwater Network (CSN), no date. Technical Support for the Bay -Wide Runoff Reduction Method, Ver. 2.0. CSN Tech. Bull. No. 4. Freeze and Cherry, 1979. Groundwater. Montalto, F., C. Behr, K. Alfredo, M. Wolf, M. Arye and M. Walsh, 2007. Rapid Assessment of the Cost - Effectiveness of Low Impact Development for CSO Control. Landscape and Urban Planning: 82: 117-131. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Southeastern Michigan Council of Government (SEMCOG), 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementors and Reviewers. Detroit, MI. Available online at: http://library.semcog.org/InmagicGenie/DocumentFolder/LIDManualWeb.pdf University of Arkansas Community Design Center (UACDC), 2010. Low Impact Development: A Design Manual for Urban Areas. USDA National Resource Conservation Service, 1969. Soil Survey, Washington County, Arkansas. US Department of Commerce Weather Bureau, 1961. Technical Paper No. 40, Rainfall Frequency Atlas of the United States. Virginia Department of Conservation and Recreation (VA DCR), 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA. Located online at: bttp://www.dcr.virginia.gov/stormwater management/ston-nwat.shtml#vswmhnbk Water and Environment Research Foundation (WERF), 2009. BMP and LID Whole Life Cost Models: Version 2.0. Water and Environment Research Foundation: Alexandria, VA. `t—Nis Chapter 5 – Low Impact Development You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 6. STORM DRAINAGE SYSTEM DESIGN SECTION 6.1. STORMWATER DRAINAGE DESIGN OVERVIEW 6.1.1 Stormwater Drainage System Design 6.1.1.1 Drainage System Components In the City of Fayetteville, the drainage system may be classified as the minor system and the major system. Three considerations largely shape the design of both these systems: flooding, public safety and water quality. The minor drainage system is designed to remove stormwater from areas such as streets and sidewalks for public safety reasons. The minor drainage system consists of inlets, street and roadway gutters, roadside ditches, small channels and swales, and small underground pipe systems which collect stormwater runoff and transport it to structural control facilities, pervious areas and/or the major drainage system (i.e., natural waterways, large man-made conduits, and large water impoundments). Paths taken by runoff from very large storms are called major systems. The major system (designed for the less frequent storm up to the 100 -year event) consists of natural waterways, large man-made conduits, and large water impoundments. In addition, the major system includes some less obvious drainageways such as overland relief swales and infrequent temporary ponding areas. The major system includes not only the trunk line system that receives the water from the minor system, but also the natural backup system which functions in case of overflow from or failure of the minor system. Overland drainage must be designed to protect houses, buildings or other property from flooding. The major/minor concept may be described as a 'system within a system' for it comprises two distinct but conjunctive drainage networks. The major and minor systems are closely interrelated, and their design needs to be done in tandem and in conjunction with the design of structural stormwater controls and the overall stormwater management concept and plan (Section 6.2). This chapter provides design criteria and guidance on drainage system components, including street and roadway gutters, inlets and storm drain pipe systems (Section 6.2); culverts and bridges (Section 6.3); vegetated and lined open channels (Section 6.4); and energy dissipation devices for outlet protection (Section 6.5). The rest of this section covers important considerations to keep in mind in the planning and design of stormwater drainage facilities. 6.1.1.2 Checklist for Drainage Planning and Design The following is a general procedure for drainage system design on a development site. 1. Analyze topography a. Check drainage pattern. b. Check on-site topography for surface runoff and storage, and infiltration Determine runoff pattern; high points, ridges, valleys, streams, and swales. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) ii. Overlay the grading plan, delineate watershed areas; calculate square footage (acreage), points of concentration, low points, etc. c. Check potential drainage outlets and methods On-site (structural control, receiving water) ii. Off-site capacity (highway, storm drain, receiving water, regional control) iii. Natural drainage system capacity (swales) iv. Existing drainage system capacity (drain pipe) 2. Analyze other site conditions. a. Land use and physical obstructions such as walks, drives, parking, patios, landscape edging, fencing, grassed area, landscaped area, tree roots, etc. b. Soil type determines the amount of water that can be absorbed by the soil. Areas of fill will be considered Hydrologic Soil Group D. c. Vegetative cover will control the erosion potential of drainage paths. Analyze areas for probable location of drainage structures and facilities. ii. Identify the type and size of drainage system components that are required. Design the drainage system and integrate with the overall stormwater management system and plan. If it is determined that offsite drainage improvements are required, then cost sharing will be in accordance with City ordinances. If the City is unable to contribute its share of the offsite costs, the developer shall have the option of (a) building the offsite improvements at his or her own expense, (b) providing retention to match pre -development downstream discharges, or (c) delaying the project until the City is able to share in the offsite costs. 6.1.2 Design Considerations 6.1.2.1 General Drainage Design Considerations and Requirements • Stormwater systems should be planned and designed to generally conform to natural drainage patterns and discharge to natural drainage paths within a drainage basin. These natural drainage paths should be modified as necessary to contain and safely convey the peak flows generated by the development. Runoff must be discharged in a manner that will minimize adverse impacts on downstream properties or stormwater systems. In general, runoff from development sites within a drainage basin should be discharged at the existing natural drainage outlet or outlets. Existing drainage problems or flooding at or adjacent to the project site should be identified by the downstream assessment to be performed as part of the drainage report. Offsite improvements may be required at the discretion of the City Engineer. • It is important to ensure that the combined minor and major system can handle flows in excess of the design capacity to minimize the likelihood of nuisance flooding or damage to private properties. If failure of minor systems and/or major structures occurs during these periods, the risk to life and `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) property could be significantly increased. If there are already existing drainage problems or historical flooding issues, downstream assessments should be performed as provided in Chapter 2, or as required by the City Engineer. Design storm requirements for various components of the minor and major drainage systems are provided below, in the applicable sections. The full build -out conditions shall be used to calculate flows for the appropriate design storm frequencies. Reasonable assumptions must be made for off- site flows. • The 100 -year design storm event shall be used as the check storm to estimate runoff for routing to evaluate effects on the facilities, adjacent property, floodplain encroachment and downstream areas. 6.1.2.2 Inlets and Drains Inlets should be located where they will not compromise safety or aesthetics or allow standing water in areas of vehicular or pedestrian traffic, but they should take advantage of natural depression storage where possible. 6.1.2.3 Storm Drain Pipe Systems (Storm Sewers) Includes storm drainage systems and pipe network that convey runoff in streets, parking lots (public or private), public right of ways and drainage easements or where permitted by the city engineer. • 10 -year design storm (for pipe design) • 10 -year design storm (for on -grade inlet) • 10 -year design storm (for sumped inlet) Ensure that storm pipe systems will safely convey flows that are in excess of pipe design flows without damaging structures or flooding major roadways. The 100 -year storm shall not be conveyed through driveway cuts or across private property, but shall remain within the ROW and/or a drainage easement. 6.1.2.4 Open Channels Open channels include all channels, swales, etc. • Ten-year design storm, evaluate and provide that 100 -year design storm is contained within public right of ways and/or drainage easements. Use backwater from receiving channel for the same design event for checks (i.e., 10 -year and 100 -year respectively). • Meander bends, if used, must be designed to accommodate increased shear stress. • Check that design flow velocity does not exceed the maximum velocity for channel lining proposed. Channels may be designed with multiple stages (e.g., a low flow channel section containing the 2 -year to 5 -year flows, and a high flow section that contains the design discharge) to improve stability and better mimic natural channel dimensions. Where flow easements can be obtained and structures kept clear, overbank areas shall be designed as part of a conveyance system wherein floodplain areas are designed for storage and/or conveyance of larger storms. The 100 -year design storm shall be calculated and demonstrated to be contained within public right of ways and/or drainage easements. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.1.2.5 Energy Dissipaters Design energy dissipation as needed to return flows to non-erosive velocities at and immediately downstream of the termination of project improvements. Includes all outlet protection facilities. • 10 -year design storm, evaluate for 100 -year design storm SECTION 6.2. MINOR DRAINAGE SYSTEM DESIGN 6.2.1 Introduction Minor stormwater drainage systems quickly remove runoff from areas such as streets and sidewalks for public safety purposes. The minor drainage system consists of inlets, grates, parking lots, street gutters, roadside ditches, small channels and swales, and underground pipe systems which collect stormwater runoff and transport it to structural control facilities, pervious areas and/or the major drainage system (i.e., natural waterways, large man-made conduits, and large water impoundments). This section provides criteria and guidance for the design of minor drainage system components including: • Street and roadway gutters, parking lots • Stormwater inlets • Storm drain pipe systems Ditch, channel and swale design criteria and guidance are covered in Section 6.4, Open Channel Design. Procedures for performing gutter flow calculations are based on a modification of Manning's Equation. Inlet capacity calculations for grate, curb and combination inlets are based on information contained in HEC -12 (USDOT, FHWA, 1984). Storm drain system design may be based on either the use of the Rational Formula for gutters and inlets, subject to the area limitations provided in Chapter 3, or the SCS or TR -55 methodologies. 6.2.1.1 General Criteria Criteria for Public Streets and Parking Lots Inlets shall be installed at low points and at such intervals to provide the appropriate clear traffic lane per street classification in each direction and for maximum ponding depths based upon peak discharges from the 10 -year design storm and the 100 -year check storm. Minimum lane clearance and maximum ponding depth requirements are provided in Table 6.1. All computations for the 10 -year and 100 -year, 24-hour storm shall be provided. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.1. Flow spread limits & .. d parking 100 -year (10 -year design storm). check Maximum Maximum Roadway Minimum clear space ponding ponding Classification depth depth Principal and Two 12 -feet traffic lanes, one in each direction, 0.5 feet 1.0 feet Arterial Streets independent of curb and gutter One 12 -feet traffic lane within 6 feet of roadway Collector Streets 0.5 feet 1.0 feet centerline Local and One 8 -feet traffic lane within 4 feet of roadway 0.5 feet 1.5 feet Residential Street centerline Parking Lots One 8 -feet traffic lane to points of egress 0.5 feet 1.0 feet Gutter design constraints include: • Per City standards, street and gutter cross slope shall match. • Minimum road and gutter cross slope (Sx)= 0.005 ft/ft. • Minimum longitudinal slope (S)= 0.005 ft/ft. • Standard gutter width = 1.0 ft. 6.2.2 Bypass Flow Bypass flow occurs when storm sewer inlets do not capture 100% of the flow upstream of their location. A variety of factors, including gutter flow rate, longitudinal slope and inlet type/geometry, play a role in the capture efficiency of an individual inlet. Flow bypassing each inlet must be included in the total gutter flow to the next inlet downstream. A bypass of 10 to 20% per inlet may result in a more economical drainage system. Refer to Sections 6.2.5 - 6.2.8 for inlet design. 6.2.3 Symbols and Definitions Use the symbols listed in Table 6.2 to provide consistency within this section as well as throughout this Manual. These symbols were selected because of their wide use. In some cases, the same symbol is used in existing publications for more than one definition. Where this occurs in this section, the symbol will be defined where it occurs in the text or equations. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.2.4 Street and Roadway Gutters Effective drainage of street and roadway pavements is essential to pavement longevity and traffic safety. Surface drainage is a function of transverse and longitudinal pavement slope, pavement roughness, inlet spacing, inlet capacity, and adequate subsurface drainage. The design of these elements is dependent on storm frequency and the allowable spread of stormwater on the pavement surface. Reference the Manning's "n" values in Table 6.9, in Section 6.3 for appropriate values. 6.2.4.1 Design Procedure (Step 1): Determine maximum allowable flow before an inlet is required, based upon the street classification and physical parameters of the proposed design. Flatter grades upstream of the proposed inlet must also be considered to ensure higher discharges do not occur elsewhere in the system. (Step 2): Identify required sump locations based on maximum allowable discharges and site conditions. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Symbol Table 6.2. Symbols and definitions. Definition Units A Gutter depression in A Area of cross section ftz d or D Depth of gutter flow at the curb line ft D Diameter of pipe ft Eo Ratio of frontal flow to total gutter flow Qw/Q - g Acceleration due to gravity (32.2 ft/S2) ft/S2 h Height of curb opening inlet ft H Head loss ft K Loss coefficient - L or LT Length of curb opening inlet ft L Pipe length ft n Roughness coefficient in the modified Manning's formula for triangular gutter flow P Perimeter of grate opening, neglecting bars and side against curb ft Q Rate of discharge in gutter cfs Qb Rate of bypass flow cfs Q; Intercepted flow cfs Q5 Gutter capacity above the depressed section cfs S or SX Cross Slope - Traverse slope ft/ft S or SL Longitudinal slope of pavement ft/ft Sf Friction slope ft/ft S'w Depression section slope ft/ft T Top width of water surface (spread on pavement) ft TS Spread above depressed section ft V Velocity of flow ft/s W Width of depression for curb opening inlets ft Z T/d, reciprocal of the cross slope - 6.2.4 Street and Roadway Gutters Effective drainage of street and roadway pavements is essential to pavement longevity and traffic safety. Surface drainage is a function of transverse and longitudinal pavement slope, pavement roughness, inlet spacing, inlet capacity, and adequate subsurface drainage. The design of these elements is dependent on storm frequency and the allowable spread of stormwater on the pavement surface. Reference the Manning's "n" values in Table 6.9, in Section 6.3 for appropriate values. 6.2.4.1 Design Procedure (Step 1): Determine maximum allowable flow before an inlet is required, based upon the street classification and physical parameters of the proposed design. Flatter grades upstream of the proposed inlet must also be considered to ensure higher discharges do not occur elsewhere in the system. (Step 2): Identify required sump locations based on maximum allowable discharges and site conditions. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) (Step 3): Determine actual discharges (for the design and check storms) for each inlet by delineating the drainage area, determining the rational coefficient, and calculating the time of concentration. (Step 4): Determine inlet capacity and gutter capacity in each direction, repeat process as needed based upon capacity and maximum allowable spread. Condition 1: Compute spread, given gutter flow. Establish longitudinal slope (S), cross slope (SX), gutter flow (Q), and Manning's n. Input parameters to calculate gutter spread. Condition 2: Compute gutter flow, given spread. Establish longitudinal slope (S), cross slope (SX), spread (T), and Manning's n. Input parameters to calculate gutter flow. Below is a sample output file based on Hydraflow Express computer software, applied for Condition 2. 6.3. reportTable Sample gutter Gutter Channel Section Gutter Section Data Highlighted Parameters Cross SI, Sx (ft/ft)* 0.030 Depth (ft) 0.18 Cross SI, Sw (ft/ft)* 0.030 Q (cfs) 2.769 Gutter Width (ft) 1.00 Velocity (ft/s) 5.13 Invert Elevation (ft) 100.00 Spread Width (ft) 6.00 Slope (%) 5.20 N -Value 0.013 Calculations Compute by: Known Depth Known Depth (ft) 0.18 *Gutter slope and road cross slope are 0.5% (minimum) per City standards. 6.2.5 Stormwater Inlets Inlets are drainage structures used to collect surface water through grate or curb openings and convey it to storm drains or direct outlet to culverts. Inlets used for the drainage of roadway surfaces can be divided into three major classes: • Curb Opening Inlets - These inlets are vertical openings in the curb covered by a top slab. • Grate Inlets - These drop inlets consist of an opening in the gutter covered by one or more grates. • Combination Inlets - These inlets usually consist of both a curb opening inlet and a grate inlet placed in a side-by-side configuration, but the curb opening must be located in part upstream of the grate. Inlets may be classified as being on a continuous grade or in a sump. The term "continuous grade" or "on grade" refers to an inlet located on the street with a continuous slope past the inlet with water entering from one direction. The "sump" condition exists when street grade is less than 1% or the inlet is located at a low point allowing water to enter from both directions. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Where significant ponding can occur, in locations such as underpasses and in sag vertical curves in depressed sections, it is good engineering practice to place flanking inlets, or extensions, on each side of the inlet at the low point in the sag. The flanking inlets should be placed so that they will limit spread on low gradient approaches to the level point and act in relief of the inlet at the low point if it should become clogged or if the design spread is exceeded. Curb inlet design is discussed in Section 6.2.5, grate inlet design in Section 6.2.6, and combination inlets in Section 6.2.7. 6.2.6 Curb Inlet Design 6.2.6.1 Curb Inlets on Grade Curb opening inlets are effective in the drainage of roadway pavements and in parking lots where flow depth at the curb is sufficient for the inlet to perform efficiently. Curb openings are relatively free of clogging tendencies and offer little interference to traffic operation. Street inlets shall be depressed 4 inches with a 12 -feet transition upstream and 4 -foot transition downstream. Where stormwater flow approaches an arterial street or tee intersection, an inlet is required. Inlet dimensional requirements: clear throat opening shall be 6 inches in height and 4 -feet minimum length. For all throat extensions, clear dimensions shall be 6 inches in height and 3 feet, 6 inches in length. City of Fayetteville standard drawings and details shall be used. Inlets with extensions shall have a maximum clear opening dimension of 18 -feet. This length represents a 4 -foot inlet opening with two 8 -feet extensions (four openings with clear opening dimensions of 3 feet, 6 inch in length). If additional length is needed to accommodate City spread and ponding depth requirements, additional inlets shall be added upstream. No clogging factor is required to be applied for curb inlets on grade. 6.2.6.2 Curb Inlets in Sump For the design of a curb -opening inlet in a sump location, the inlet operates as a weir to depths equal to the curb opening height (6 -inch standard) and as an orifice at depths greater than 1.4 times the opening height. At depths between 1.0 and 1.4 times the opening height, flow is in a transition stage. A 20% clogging factor shall be applied for curb inlets in sump. 6.2.6.3 Design Steps (Step 1) Determine the following inputs and constraints: Cross slope = Sx (ft/ft) Longitudinal slope = S (ft/ft) Gutter flow rate = Q (cfs) Manning's n = n Maximum Spread of water on pavement = T (ft) (Step 2) Assume an initial inlet geometry and apply clogging factor (where applicable): Apply inlet dimensional requirements for the City of Fayetteville. The gutter shall be depressed 4 inches. No clogging factor for inlets on grade, 20% clogging factor for inlets in a sump. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) (Step 3) Compute inlet ponding depth and spread of water on pavement (T, ft) for design and check storms. Check gutter spread of approach each direction. Repeat process as needed by adding or removing throat extensions or additional inlets upstream based upon capacity and maximum allowable spread. Table 6.4 below depicts input and output for a sample inlet, based on Hydraflow Express computer software. reportTable 6.4. Inlet Sample Inlet Report Output Q Q Q Curb Inlet Gutter Inlet Byp Inlet Q=CIA Jun c Ht L So W Sw Sx Depth Spread Dpt Sprd Dep carry capt Byp Line ID f cfs (cfs) (CIS) (CIS) 1 e in ft (ft/ft) ft ft/ft f/ft) ft ft ft ft in No Inlet 4.02 0.00 4.02 0.00 Curb 2.0 8.80* Sag 1.00 0.030 0.030 0.013 0.30 9.96 0.63 9.96 4.0 Off A Inlet 4.76 0.49 5.25 0.00 Curb 2.0 8.80* Sag 1.00 0.030 0.030 0.013 0.36 11.9 0.69 11.9 4.0 Off B Inlet 1.77 0.00 1.75 0.17 Curb 2.0 11.00 0.052 1.00 0.030 0.030 0.013 0.14 4.73 0.36 0.99 C Inl et 1.25 0.00 1.22 0.32 Curb 2.0 7.50 0.032 1.00 0.030 0.030 0.013 0.14 4.57 0.35 0.96 D *L adjusted based on City requirement of 20% clogging factor for inlets in a sump. 6.2.7 Grate Inlet Design 6.2.7.1 Grate Inlets on Grade Grate inlets (when approved by the City Engineer) must be bicycle safe and adequately support traffic with appropriate frames provided. Grates shall only be considered where drop inlets cannot function. A 20% clogging factor shall be used for grate inlets on grade. The capacity of an inlet depends upon its geometry and the cross slope, longitudinal slope, total gutter flow, depth of flow and pavement roughness. The depth of water next to the curb is the major factor in the interception capacity of both gutter inlets and curb opening inlets. At low velocities, all of the water flowing in the section of gutter occupied by the grate, called frontal flow, is intercepted by grate inlets, and a small portion of the flow along the length of the grate, termed side flow, is intercepted. On steep slopes, only a portion of the frontal flow will be intercepted if the velocity is high or the grate is short and splash -over occurs. Bicycle -safe grates shall be used without exception. The curved vane grate or the tilt bar grate may be considered, for both their hydraulic capacity and bicycle safety features. They also handle debris better than other grate inlets but the vanes of the grate must be turned in the proper direction at installation. Where debris is a problem, consideration should be given to debris handling efficiency rankings of grate inlets from laboratory tests in which an attempt was made to qualitatively simulate field conditions. 6.2.7.2 Grate Inlets in Sag A 50% clogging factor shall be used, and grate width shall not extend beyond gutter (1 foot). A grate inlet in a sag operates as a weir up to a certain depth, depending on the bar configuration and size of the grate, and as an orifice at greater depths. For a standard gutter inlet grate, weir operation continues to a depth of about 0.4 feet above the top of grate and when depth of water exceeds about 1.4 foot, the grate begins to operate as `t -Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) an orifice. Between depths of about 0.4 foot and about 1.4 foot, a transition from weir to orifice flow occurs. Transition from weir to orifice flow results in interception capacity less than that computed by either the weir or the orifice equation. Typical design software will account for this transition zone during computations. The capacity of grate inlets (on -grade or in sag) operating as a weir is: Qi = CPd"' Eq. 6.1 Where: P = perimeter of grate excluding bar widths and the side against the curb, feet C = 3.0 (weir coefficent) d = depth of water above grate, feet and as an orifice is: Qi = CA(2gd)"' Where: C = 0.67 orifice coefficient Eq. 6.2 A = clear opening area (do not include area of bars) of the grate, feet' g = 32.2 ft/s' The tendency of grate inlets to clog completely warrants consideration of a combination inlet, or curb - opening inlet in a sag where ponding can occur, and flanking inlets on the low gradient approaches. Grates shall not be used in sump conditions unless flanked by deep inlets as a combination inlet. 6.2.8 Combination Inlets 6.2.8.1 Combination Inlets On Grade On a continuous grade, the capacity of an unclogged combination inlet with the curb opening located adjacent to the grate is approximately equal to the capacity of the grate inlet alone. Thus capacity is computed by neglecting the curb opening inlet for the design procedures. A combination inlet on grade is required on gutter slopes of 7% or greater. For steep slopes, the grate should be located at the uphill side of the extension, not at the box. 6.2.8.2 Combination Inlets In Sump All debris carried by stormwater runoff that is not intercepted by upstream inlets will be concentrated at the inlet located at the low point, or sump. Because this will increase the probability of clogging for grated inlets, it is generally appropriate to estimate the capacity of a combination inlet at a sump by neglecting the grate inlet capacity. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.2.9 Storm Drain Pipe Systems 6.2.9.1 Introduction Storm drain pipe systems, also known as storm sewers, are pipe conveyances used in the minor stormwater drainage system for transporting runoff from roadway and other inlets to outfalls at structural stormwater controls and receiving waters. Pipe drain systems are suitable mainly for medium to high-density residential and commercial/industrial development where the use of natural drainageways and/or vegetated open channels is not feasible. An example of storm sewer output data to include in a final drainage report is provided as Exhibit 1 to Appendix H of this manual. 6.2.9.2 Requirements and General Design Procedure All computations and hydraulic profiles for the 10 -year and 100 -year, 24 hour storms shall be provided. The design of storm drain systems generally follows these steps: (Step 1) Determine inlet location and spacing. (Step 2) Determine drainage areas and compute runoff. (Step 3) Prepare a tentative plan layout of the storm sewer drainage system including: • Location of storm drains • Direction of flow • Location of manholes • Location of existing facilities such as water, gas, or underground cables (Step 4) After the tentative locations of inlets, drain pipes, and outfalls (including tailwater elevations) have been determined and the inlets sized, compute of the rate of discharge to be carried by each storm drain pipe and determine the size and gradient of pipe required to convey this discharge. This is done by proceeding in steps from upstream of a line to downstream to the point at which the line connects with other lines or the outfall, whichever is applicable. The discharge for a run is calculated, the pipe serving that discharge is sized, and the process is repeated for the next run downstream. (Step 5) Examine assumptions to determine if adjustments are needed to the final design. It should be recognized that the rate of discharge to be carried by any particular section of storm drain pipe is not typically the sum of the inlet design discharge rates of all inlets above that section of pipe, but is typically somewhat less than this total due to attenuation of peaks caused by variations in the timing of the peak discharges. As the time of concentration grows larger, the appropriate rainfall intensity to be used in the design grows smaller. 6.2.9.3 Design Criteria Storm drain pipe systems should conform to the following criteria: `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Storm drain pipes shall be sized on the assumption that they will flow full or practically full under the design discharge but will not be placed under pressure head. Pipes shall be designed to have crowns matched and shall not discharge into a smaller pipe. The Manning Formula is recommended for capacity calculations. The hydraulic grade line for the system should be contained in the pipe and below the crown for the 10 -year storm. • The minimum circular diameter for any public storm drain pipe is 18 inches. • The maximum hydraulic gradient shall not produce a velocity that exceeds 20 feet/s for the design storm. • The minimum desirable physical slope shall be 0.5% or the slope that will produce a minimum velocity of 2.5 ft/s for the design storm when the storm sewer is flowing full, whichever is greater. • Any storm drain pipe located in a right of way or drainage easement shall be reinforced concrete pipe (RCP) unless approved by the City Engineer. • The water surface elevation shall be at least 1 foot below ground elevation for the design flow, the top of the pipe, or the gutter flow line, whichever is lowest. Where required, adjustments shall be made in the system to reduce the elevation of the hydraulic grade line to meet this requirement. • The 100 -year storm shall be used as the check storm. Combined capacity of the street and minor systems must be equal to or greater than the peak rate of flow for the 100 -year storm. Apply maximum ponding depths from Table 6.1. 6.2.9.4 Capacity Calculations Formulas for Gravity and Pressure Flow The most widely used formula for determining the hydraulic capacity of storm drain pipes for gravity and pressure flows is the Manning's Formula, expressed by the following equation: V = [1.486 R2/3Si/21/n Where: V = mean velocity of flow, ft/s Eq. 6.3 R = the hydraulic radius, ft - defined as the area of flow divided by the wetted flow surface or wetted perimeter (A/WP) = the slope of hydraulic grade line, ft/ft n = Manning's roughness coefficient (see table 6.9 for values) In terms of discharge, the above formula becomes: Q = [1.486 AR2/3Si/21/n Eq. 6.4 Where: Q = rate of flow, cfs A = cross sectional area of flow, ft2 `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) For pipes flowing full, the above equations become: V = [0.590 D2/3Si/21/n Eq. 6.5 Q = [0.463 D8/3S1/21/n Eq. 6.6 Where: D = diameter of pipe, ft The Manning's equation can be written to determine friction losses for storm drain pipes as: Hf= [2.87 n2V2L]/[S4/31 Hf= [29 n2V2L]/[(R4/3) (2g)] Where: Hf = total head loss due to friction, ft n = Manning's roughness coefficient D = diameter of pipe, ft L = length of pipe, ft V = mean velocity, ft/s R = hydraulic radius, ft g = acceleration of gravity = 32.2 ft/S2 6.2.9.5 Hydraulic Grade Lines Eq. 6.7 Eq. 6.8 All head losses in a storm sewer system are considered in computing the hydraulic grade line to determine the water surface elevations, under design conditions, in the various inlets, catch basins, manholes, junction boxes, etc. Hydraulic control is a set water surface elevations from which the hydraulic calculations are begun. All hydraulic controls along the alignment are established. If the control is at a main line upstream inlet (inlet control), the hydraulic grade line is the water surface elevation minus the entrance loss minus the difference in velocity head. If the control is at the outlet, the water surface is the outlet pipe hydraulic grade line. 6.2.9.6 Junctions and Manholes Manholes, junction boxes or maintenance access ports will be required for public storm drain systems whenever there is a change in size, direction, elevation, grade, or where there is a junction of two or more sewers. The maximum spacing between manholes and manhole diameter for various pipe sizes shall be in accordance with Table 6.5. Wye connections can be used up to and including 24 inches x 24 inches. Wye connections larger than 24 inches x 24 inches may be used with the approval of the City Engineer. Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.5. Manhole spacing.Table sizes and Manhole Sizes Storm Sewer Diameter Manhole Diameter 15 inches to 18 inches 4 feet 24 inches to 42 inches 5 feet 48 inches to 54 inches 6 feet 60 inches and larger To be approved by City Manhole Spacing Storm Sewer Diameter Maximum Allowable Spacing 15 inches to 36 inches 400 feet 42 inches and larger 500 feet 6.2.9.7 Minimum Grade All storm drains should be designed such that velocities of flow will not be less than 2.5 feet/s at design flow or lower, with a minimum slope of 0.5%. For very flat flow lines the general practice is to design components so that flow velocities will increase progressively throughout the length of the pipe system. Upper reaches of a storm drain system should have flatter slopes than slopes of lower reaches. Progressively increasing slopes keep solids moving toward the outlet and deter settling of particles. The minimum slopes are calculated by the modified Manning's formula: S = [(nV)2]/[2.208R4/3] Where: S = the slope of the hydraulic grade line, ft/ft n = Manning's roughness coefficient V = mean velocity of flow, ft/s R = hydraulic radius, ft (area divided by wetted perimeter) SECTION 6.3. CULVERT and BRIDGE DESIGN Criteria for Culverts Eq. 6.9 Culvert design shall be based upon peak discharges for the appropriate design storm based on roadway type. Requirements are provided in Table 6.6. All computations, hydraulic profile, and energy transition to channel shall be provided for the design event and the 100 -year check storm. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.6. Culvert and bridge sizing requirements based on roadway type.' Minimum Minimum Roadway Classification Design Storm Event Freeboard' Freeboard' (Culvert) (Bridge) Principal and Minor 50 -year (2 %-annual-chance) 2 feet 1 foot Arterial Streets Collector Streets 25 -year (4 %-annual-chance) 2 feet 1 foot Local and Residential Streets 10 -year (10 %-annual-chance) 2 feet 1 foot (all others) Additional limitations apply to culverts or bridges along City of Fayetteville Protected Streams and within FEMA Regulatory Flood Hazard Areas 2. Freeboard for culverts shall be from top of low point in road. Freeboard for bridges shall be measured from low chord. 1. Route the 100 -year frequency storm through all culverts to be sure building structures (e.g., houses, commercial buildings) are not flooded or increased damage does not occur to the roadway or adjacent property for this design event. The flow shall be safely conveyed through drainage easements and/or the ROW. Use appropriate tailwater conditions, assuming the 100 -year event in receiving waters. 6.3.1 Overview A culvert is a short conduit that conveys stormwater runoff under an embankment, usually a roadway or driveway. The primary purpose of a culvert is to convey surface water. In addition to the hydraulic function, a culvert must also support the embankment and/or roadway, and protect traffic and adjacent property owners from flood hazards to the extent practicable. 6.3.2 Protected Streams New stream crossings including driveways, roadways, trails, or railroads, are allowed on City of Fayetteville Protected Streams when the City Engineer determines there is no practical and feasible alternative. The Protected Streams map, located on the City of Fayetteville website, may be accessed at: http://www.accessfayetteville.ore/government/strategic planning/documents/general documents/Streamside Protection Map 2 01 11.pdf The following criteria apply: • Minimize or reduce stream crossings through proper planning, • Minimize the amount of excavation and filling, • Maintain the dimension, pattern, and profile of the stream. • Minimize scour, erosion and flooding. Methods to minimize stream crossing impacts include: • Construct stream crossings during periods of low flow. • Locate crossings where streambed and banks are composed of firm, cohesive soils to minimize erosion. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Design crossings to reduce the possibility of obstructions such as debris and silt blockages through the minimization of channel obstructions. • Bridges and bottomless arches, wide enough to span the stream and allow for some dry ground or an artificial ledge beneath the bridge on one or both sides are preferred and should be used whenever possible. • Bridge soffits should be a minimum of 1 foot above the height of adjacent banks --high enough to allow wildlife passage. • Exceptionally wide stream crossings may be allowed to utilize piers in the channel under the discretion of the City Engineer. • Maintain a natural substrate underneath the bridge. If concrete is necessary to prevent scour, then it is recommended to cover the concrete with a natural substrate. • All disturbed areas shall be revegetated immediately upon completion of the work. The use of culverts on protected streams should be avoided. If culverts must be used, the following installation guidelines should be followed: • Provide water depths and velocities (at low flows) matching natural areas upstream and downstream of the crossing. • Create no drop-offs or plunge pools and no constriction of the channel. The practices listed may be subject to additional regulation per UDC Chapter 168 Flood Damage Prevention Code, Chapter 169 Physical Alteration of Land, and Chapter 170 Stormwater Management, Drainage and Erosion Control. 6.3.3 Symbols and Definitions To provide consistency within this section as well as throughout this Manual the symbols listed in Table 6.7 will be used. These symbols were selected because of their wide use. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.3.4 Design Criteria 6.3.4.1 Velocity Limitations Consider minimum and maximum velocities when designing a culvert. The maximum allowable velocity for reinforced concrete pipe is 20 ft/s. To ensure self-cleaning during partial depth flow, a minimum velocity of 2.5 ft/s for the 2 -year flow, when the culvert is flowing partially full, is required. 6.3.4.2 Length and Slope The maximum culvert slope using concrete pipe shall be 10%, while the minimum slope for standard construction procedures shall be 0.4% when possible. Maximum drop in a drainage structure or junction box is 10 feet. 6.3.4.3 Headwater Limitations Headwater is the water above the culvert invert at the entrance end of the culvert. The maximum allowable headwater elevation is that elevation above which damage may be caused to adjacent property and/or the roadway and is determined from an evaluation of land use upstream of the culvert and the proposed or existing roadway elevation. It is this allowable headwater depth that is the primary basis for sizing a culvert. The following criteria relates to headwater: The allowable headwater is the depth of water that can be ponded at the upstream end of the culvert during the design flood, `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.7 Symbols and definitions. Symbol Definition Units A Area of cross section of flow ftz B Barrel width ft Cd Overtopping discharge coefficient - D Culvert diameter or barrel depth inches or ft d Depth of flow ft dc Critical depth of flow ft d„ Uniform depth of flow ft g Acceleration of gravity ft/s Hf Depth of pool or head, above the face section of invert ft ho Height of hydraulic grade line above outlet invert ft HW Headwater depth above invert of culvert (depth from inlet invert to upstream total energy grade line) ft ke Inlet loss coefficient - L Length of culvert ft N Number of barrels - Q Rate of discharge cfs S Slope of culvert ft/f TW Tailwater depth above invert of culvert ft V Mean velocity of flow ft/s VC Critical velocity ft/s 6.3.4 Design Criteria 6.3.4.1 Velocity Limitations Consider minimum and maximum velocities when designing a culvert. The maximum allowable velocity for reinforced concrete pipe is 20 ft/s. To ensure self-cleaning during partial depth flow, a minimum velocity of 2.5 ft/s for the 2 -year flow, when the culvert is flowing partially full, is required. 6.3.4.2 Length and Slope The maximum culvert slope using concrete pipe shall be 10%, while the minimum slope for standard construction procedures shall be 0.4% when possible. Maximum drop in a drainage structure or junction box is 10 feet. 6.3.4.3 Headwater Limitations Headwater is the water above the culvert invert at the entrance end of the culvert. The maximum allowable headwater elevation is that elevation above which damage may be caused to adjacent property and/or the roadway and is determined from an evaluation of land use upstream of the culvert and the proposed or existing roadway elevation. It is this allowable headwater depth that is the primary basis for sizing a culvert. The following criteria relates to headwater: The allowable headwater is the depth of water that can be ponded at the upstream end of the culvert during the design flood, `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Headwater shall have minimal impact on upstream property, • Maximum headwater depth for design storm shall be 2 feet lower than top of road or curb, • Ponding depth shall be no greater than the elevation where flow diverts around the culvert, • For drainage facilities with cross-sectional area equal to or less than 30 feetz, HW/D should be equal to or less than 1.5, • For drainage facilities with cross-sectional area greater than 30 ftz, HW/D should be equal to or less than 1.2, • The headwater should be checked against the 100 -year flood (base flood) elevation to ensure compliance with floodplain management criteria. • The culvert should be sized to maintain flood -free conditions on major thoroughfares with 18 -inch freeboard from the low point of the road, • Identify the maximum acceptable outlet velocity, based on receiving channel conditions and Tables 6.12 and 6.13, • Either set the headwater to produce acceptable velocities, or use stabilization or energy dissipation where acceptable velocities are exceeded, • The constraint that gives the lowest allowable headwater elevation establishes the criteria for the hydraulic calculations. • Bridges require 1 -feet freeboard from the low chord. 6.3.4.4 Tailwater Considerations The hydraulic conditions downstream of the culvert site must be evaluated to determine tailwater depth for a range of discharge or the appropriate design storm and check storm. At times there may be a need for calculating backwater curves to establish the tailwater conditions. The following conditions must be considered: • If the culvert outlet is operating with a free outfall, the critical depth and equivalent hydraulic gradeline shall be determined. • For culverts that discharge to an open channel, the stage -discharge curve for the channel must be determined. The water surface elevation in the open channel for the relevant design storm event for the culvert should be evaluated as part of culvert capacity computations. See Section 6.4, Open Channel Design. • If an upstream culvert outlet is located near a downstream culvert inlet, the headwater elevation of the downstream culvert may establish the design tailwater depth for the upstream culvert. • If the culvert discharges to a lake, pond, or other major water body, the expected high water elevation for the design storm of the particular water body may establish the culvert tailwater. `t— Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.3.4.5 Culvert Inlets Hydraulic efficiency and cost can be significantly affected by inlet conditions. The inlet coefficient Ke, is a measure of the hydraulic efficiency of the inlet, with lower values indicating greater efficiency. Recommended inlet coefficients are given in Table 6.8. 6.3.4.6 Inlets with Headwalls Concrete headwalls or equivalent end treatments are required for all culverts installed in public right of ways or drainage easements. If high headwater depths are to be encountered, or the approach velocity in the channel will cause scour, provide a channel apron at the toe of the headwall. Extend the apron at least one pipe diameter upstream from the entrance. The top of apron elevation shall not protrude above the normal streambed elevation. 6.3.4.7 Wingwalls and Aprons Wingwalls are required where the side slopes of the channel adjacent to the entrance are unstable or where the culvert is skewed to the normal channel flow. Aprons shall be applied where required to prevent scour. 6.3.4.8 Material Selection If material other than reinforced concrete pipe (RCP) is to be used in roadway areas including under curbs, it shall be approved by the City Engineer. Galvanized CMP is not accepted. Coated corrugated metal pipe (CMP) and high density polyethylene pipe (HDPE) may be used in non -roadway areas. Coated CMP and HDPE flared end sections are prohibited within the right of way and drainage easements. All pipe shall be installed to manufacturer's recommendations including bedding, backfill and compaction. Bedding, backfill and compaction details must be included in the construction plans. 6.3.4.9 Culvert Skews Culvert skews shall not exceed 45 degrees as measured from a line perpendicular to the roadway centerline without approval by the City Engineer. 6.3.4.10 Culvert Sizes The minimum allowable circular pipe diameter shall be 15 inches for culverts. 6.3.4.11 Outlet Protection See Section 6.5 for information on the design of outlet protection. Outlet protection shall be provided for the 10 -year storm event. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 1. The Ke values for corrugated metal pipes are also recommended for HDPE pipes. * Note: End Section conforming to fill slope, made of either metal or concrete, are the sections commonly available from manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Source: HDS No. 5, 1985. `t— Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.8. Inlet coefficients. Type of Structure and Design of Entrance Coefficient Kel Pipe, Concrete Projecting from fill, socket end (grove -end) 0.2 Projecting from fill, square cut end 0.5 Headwall or headwall and wingwalls Socket end of pipe (groove -end) 0.2 Square -edge 0.5 Rounded radius = 1 12 D 0.2 Mitered to conform to fill slope 0.7 *End -Section conforming to fill slope 0.5 Beveled edges, 33.70 or 45 0 bevels 0.2 Side- or slope -tapered inlet 0.2 Pipe, or Pipe -Arch, Corrugated Metall Projecting from fill (no headwall) 0.9 Headwall or headwall and wingwalls square -edge 0.5 Mitered to fill slope, paved or unpaved slope 0.7 *End -Section conforming to fill slope 0.5 Beveled edges, 33.7 0 or 45 0 bevels 0.2 Side- or slope -tapered inlet 0.2 Box, Reinforced Concrete Headwall parallel to embankment no win walls Square -edged on 3 edges 0.5 Rounded on 3 edges to radius of 1 12 D or beveled edges on 3 sides 0.2 Wingwalls at 300 to 75- to barrel Square -edged at crown 0.4 Crown edge rounded to radius of [1/12(D) or beveled top edge 0.2 Wingwalls at 100 or 25- to barrel Square -edged at crown 0.5 Wingwalls parallel (extension of sides) Square -edged at crown 0.7 Side- or slope -tapered inlet 0.2 1. The Ke values for corrugated metal pipes are also recommended for HDPE pipes. * Note: End Section conforming to fill slope, made of either metal or concrete, are the sections commonly available from manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Source: HDS No. 5, 1985. `t— Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.9. Manning's n values. Type of structure, material and joint descriptionl.2 Manning's n Gutter or Pavement Concrete gutter, smooth 0.013 Concrete gutter, rough 0.016 Concrete gutter & asphalt pavement 0.014 Concrete gutter & pavement, float finish 0.014 Concrete gutter & pavement, broom finish 0.016 Pipe, Concrete 0.013 Concrete Box 0.013 Corrugated Metal Pipes and Annular Corrugations3 2 2/3- by 1/2 -inch corrugations (unpaved / paved) 0.024/0.012 6- by 1 -inch corrugation (unpaved / paved) 0.025/0.012 3- by 1 -inch corrugations (unpaved / paved) 0.027/0.012 6 -by 2 -inch structural plate 0.033 9 -by 2-1/2 inch structural plate 0.035 Pipes, Helical 24 -inch plate width 0.012 Spiral Rib Metal Pipe 3/4 by 3/4 inch recesses at 12 inch spacing, good joint 0.013 High Density Polyethylene (HDPE) Corrugated Smooth Liner 0.012 1. Source: HDS No. 5 (1985) 2. Estimates are by or based on the Federal Highway Administration, Source: USDOT, FHWA, HDS No. 3 (1961). 3. Source: Modern Sewer Design (1999) Note: For further information concerning Manning n values for selected conduits consult Hydraulic Design of Highway Culverts, Federal Highway Administration, HDS No. 5, page 163 (HDS-5). 6.3.5 Design Procedures 6.3.5.1 Types of Flow Control There are two types of flow conditions for culverts that are based upon the location of the control section and the critical flow depth - inlet and outlet: Inlet Control Flow Condition Water Surface Water Surface HW —_— _ TW d, [Control Section] , HW — Headwater TW — Tailwater d, — Critical Depth H — Losses Through Culvert Outlet Control Flow Condition Water Surface Water Surface HW Control Section Downstream A. Submerged Water Surface H -- H B. Unsubmerged dc [Control Section] Figure 6.1. Culvert flow conditions (Adapted from: HDS-5, 1985). Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Inlet Control - Inlet control occurs when the culvert barrel is capable of conveying more flow than the inlet, such as when the slope is steep. In this condition the control section of a culvert is just inside the entrance. Critical depth occurs at or near this location, and the flow regime immediately downstream is supercritical. Outlet Control - Outlet control flow occurs when the culvert barrel is not capable of conveying as much flow as the inlet opening will accept. The control section for outlet control flow in a culvert is located at the barrel exit or further downstream. Either subcritical or pressure flow exists in the culvert barrel under these conditions. Proper culvert design and analysis requires checking for both inlet and outlet control to determine which will govern particular culvert designs. For more information on inlet and outlet control, see HDS-5. 6.3.5.2 Procedures A list of acceptable software is provided in Appendix H, Stormwater Software. Additional software may be accepted for use by the City Engineer provided it is shown to be equivalent to approved softwares. 6.3.5.3 Design Procedure The following design procedure requires the use of computer design software. (Step 1) List design input data: Q = discharge (cfs) L = culvert length (ft) S = culvert slope (ft/ft) TW= tailwater depth (ft) V = velocity for trial diameter (ft/s) Ke= inlet loss coefficient Material type HW = allowable headwater depth for design storm (ft) (Step 2) Determine trial culvert open area by assuming a trial velocity 3 to 5 ft/s and computing the culvert area, A = Q/V. Determine the culvert shape, open size (diameter or span and rise), and number of barrels. (Step 3) Find the actual HW for the trial size culvert for both inlet and outlet control. • For inlet control, enter inlet control data into software with D and Q and find HW/D for the proper entrance type. • Compute HW and, if too large or too small, try another culvert size before computing HW for outlet control. • For outlet control enter the outlet control data into software with the culvert length, entrance loss coefficient, and trial culvert diameter. • Compute the headwater elevation HW from the equation: HW=H+ho - LS Eq. 6.10 Where: ho = Y2 (critical depth + D), or tailwater depth, whichever is greater Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) L = culvert length S = culvert slope (Step 4) Compare the computed headwaters and use the higher HW to determine if the culvert is under inlet or outlet control. • If inlet control governs, then the design is complete and no further analysis is required. If outlet control governs and the HW is unacceptable, select a larger trial size and repeat steps. Unless material or entrance conditions change, the inlet control conditions for the larger pipe need not be re -checked. (Step 5) Calculate exit velocity and, if erosion problems are expected, modify culvert size to reduce or eliminate erosion problems. If not achievable, refer to Section 6.5 for appropriate energy dissipation designs. Below is a sample output file using Hydraflow Express computer software. 1. HDS-S (coefficients K, M, c, Y are based on edge configurations). 6.3.5.4 Performance Curves - Roadway Overtopping A performance curve for any culvert can be obtained by repeating the steps outlined above for a range of discharges that are of interest for that particular culvert design. These curves are applicable through a range of headwater, velocities, and scour depths versus discharges for a length and type of culvert. To complete the culvert design, roadway overtopping should be analyzed. A performance curve showing the culvert flow as well as the flow across the roadway is a useful analysis tool. Rather than using a trial and `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.10. Sample culvert output file. Sample Culvert Culvert Data: Calculations: Invert Elevation Down (ft) 100.00 Qmin (cfs) 13.00 Pipe Length (ft) 80.00 Qmax (cfs) 13.00 Slope (%) 0.60 Tailwater Elevation (ft) (dc+D)/2 Invert Elevation Up (ft) 100.48 Rise (in) 18.0 Highlighted: Shape Circular Qtotal (cfs) 13.00 Span (in) 18.0 Qpipe (cfs) 13.00 No. Barrels 1 Qovertop (cfs) 0.00 n -Value 0.013 Velocity Down (ft/s) 7.50 Culvert Type Circular Concrete Velocity Up (ft/s) 7.36 Culvert Entrance Square edge w/headwall (C) HGL Down (ft) 101.43 Coefficients' K,M,c,Y,ke 0.0098,2,0.0398,0.67,0.5 HGL Up (ft) 102.60 Hw Elevation (ft) 103.63 Embankment Data: Hw/D (ft) 2.10 Top Elevation (ft) 105.00 Flow Regime Inlet Control Top Width (ft) 50.00 Crest Width (ft) 30.00 1. HDS-S (coefficients K, M, c, Y are based on edge configurations). 6.3.5.4 Performance Curves - Roadway Overtopping A performance curve for any culvert can be obtained by repeating the steps outlined above for a range of discharges that are of interest for that particular culvert design. These curves are applicable through a range of headwater, velocities, and scour depths versus discharges for a length and type of culvert. To complete the culvert design, roadway overtopping should be analyzed. A performance curve showing the culvert flow as well as the flow across the roadway is a useful analysis tool. Rather than using a trial and `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) error procedure to determine the flow division between the overtopping flow and the culvert flow, an overall performance curve can be developed. The overall performance curve can be determined as follows: (Step 1) Select a range of flow rates and determine the corresponding headwater elevations for the culvert flow alone. The flow rates should fall above and below the design discharge and cover the entire flow range of interest. Both inlet and outlet control headwaters should be calculated. (Step 2) Combine the inlet and outlet control performance curves to define a single performance curve for the culvert. (Step 3) When the culvert headwater elevations exceed the roadway crest elevation, overtopping will begin. Calculate the equivalent upstream water surface depth above the roadway (crest of weir) for each selected flow rate. Use these water surface depths and Equation 6.11 to calculate flow rates across the roadway. Q = EdL(HW)1.5 Eq. 6.11 Where: Q = overtopping flow rate (ft3/s) Cd = overtopping discharge coefficient L = length of roadway (ft) HW = upstream depth, measured from the roadway crest to the water surface upstream of the weir drawdown (feet) Note: Overtopping discharge coefficients may be obtained from a reference source or computed by the design software based on material and geometric configuration. Confirm appropriateness of discharge coefficients used. For more information, see HDS-5. (Step 4) Add the culvert flow and the roadway overtopping flow at the corresponding headwater elevations to obtain the overall culvert performance curve. 6.3.5.5 Multibarrel Installations For multibarrel installations exceeding a 3:1 width to depth ratio, the side bevels become excessively large when proportioned on the basis of the total clear width. For these structures, it is recommended that the side bevel be sized in proportion to the total clear width, B, or three times the height, whichever is smaller. The top bevel dimension should always be based on the culvert height. The shape of the upstream edge of the intermediate walls of multibarrel installations is not as important to the hydraulic performance of a culvert as the edge condition of the top and sides. Therefore, the edges of these walls may be square, rounded with a radius of one-half their thickness, chamfered, or beveled. The intermediate walls may also project from the face and slope downward to the channel bottom to help direct debris through the culvert. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Multibarrel pipe culverts should be designed as a series of single barrel installations since each pipe requires a separate bevel. SECTION 6.4. OPEN CHANNEL DESIGN Criteria for Open Channels Open channel design parameters shall be: • 10 -year storm frequency shall be used, • Maintain minimum freeboard of 1 foot (for v.,,, less than 8 ft/s), • Maintain minimum freeboard of 2 feet (for v.,,,z greater than 8 ft/s) - see Section 6.4.4.3 for flow depth exceeding 5 feet, • Maximum allowable channel velocities for earthen materials are provided in Table 6.12, • Maximum allowable channel velocities for vegetative linings are provided in Table 6.13, • Maximum allowable velocity for rigid -lined channel shall not exceed 20 feet/s, • Minimum bend radius is 25 feet or bottom width multiplied by 10 (whichever is greater). The 100 -year storm shall be used for the check storm. Discharges from the 100 -year event shall be safely conveyed through drainage easement or right-of-way. 6.4.1 Overview 6.4.1.1 Introduction Open channel systems are an integral part of stormwater drainage design, particularly for development sites utilizing better site design practices and open channel structural controls. Open channels include drainage ditches, grass channels, dry and wet swales, riprap channels and concrete -lined channels. This section provides an overview of open channel design criteria and methods. 6.4.1.2 Considerations for Use of Open Channels Open channels in major drainage systems have significant advantages in regard to cost and capacity. Disadvantages include increased right-of-way requirements, maintenance costs and habitat for insects. Open channels may be used in lieu of storm sewer systems to convey storm runoff where: • Sufficient right-of-way is available, • Sufficient cover for storm sewers is not available, • It is important to maintain compatibility with existing or proposed developments, and • Economy of construction can be shown without excessive long-term public maintenance expenditures. Intermittent alternating reaches of opened and closed systems should be avoided. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) The ideal channel is that carved by natural drainage processes over a long period of time. The benefits of such a channel are lower velocities and more stable channel bottom and banks, channel and overbank storage reducing peak flows, decreased maintenance associated with stability, and retention of desirable green belt area. Generally speaking, the natural channel or a man-made channel that most nearly conforms to the character of the natural channel is the most desirable. In many areas experiencing development, the runoff has been so minimal that natural channels do not exist. However, a small trickle path nearly always exists that provides an excellent basis for location and construction of channels to reduce development costs and minimize drainage problems. Channel stability is a well recognized problem in urban hydrology because of general increases in low flows and peak storm discharges. A natural channel with increased capacity demands due to development should be augmented with necessary measures to avoid future bottom scour and bank cutting in accordance with Minimum Standard No. 2. Sufficient right-of-way or permanent drainage easements shall be provided adjacent to open channels to allow entry of city maintenance vehicles. Typically these easements are 10 -feet minimum or as approved by the City Engineer. Drainage easements may not be in the same location as utility easements. Where easements overlap, adequate dimensions shall be provided for both maintenance vehicle entry and utility structures. 6.4.2 Open Channel Types The three main classifications of open channel types according to channel linings are vegetated, flexible and rigid. Vegetated linings include grass -lined, grass with mulch, sod and lapped sod, and wetland channels. Riprap and some forms of flexible man-made linings or gabions are examples of flexible linings, while rigid linings are generally concrete or rigid block. Flexible and rigid linings will be allowed only with express permission from the City Engineer. Vegetative Linings - Vegetation, where practical, is the preferred lining for manmade channels. It stabilizes the channel body and bed, reduces erosion on the channel surface, and provides habitat and water quality benefits (see Chapters 4 and 5 for more details on using enhanced swales and grass channels for water quality purposes). Conditions under which vegetation may not be acceptable include but are not limited to: • High velocities • Standing or continuously flowing water • Lack of maintenance needed to prevent growth of taller or woody vegetation • Lack of nutrients and inadequate topsoil • Lack of access for maintenance, including the back of residential lots where fences are likely • Excessive shade `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Proper seeding, mulching and soil preparation are required during construction to assure establishment of healthy vegetation. Also, erosion control matting or other geofabrics may be required to be placed along the base and / or side slopes of these channels to allow establishment of vegetation. Flexible Linings - Rock riprap, including rubble, is the most common type of flexible lining for channels. It presents a rough surface that can dissipate energy. These linings are usually less expensive than rigid linings. However, they may require the use of a filter fabric depending on the erosive characteristics of the underlying soils, and the growth of grass and weeds may present maintenance problems. Silty sand or silty loam soils typically require the use of a filter fabric. Rigid Linings - Rigid linings are generally constructed of articulated block or concrete and used where high flow capacity is required. Higher velocities, however, create the potential for scour at channel lining transitions and channel headcutting. 6.4.3 Symbols and Definitions To provide consistency within this section as well as throughout this Manual, the symbols listed in Table 6.11 will be used. These symbols were selected because of their wide use. In some cases, the same symbol is used in existing publications for more than one definition. Where this occurs in this section, the symbol will be defined where it occurs in the text or equations. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.4.4 Design Criteria 6.4.4.1 General Criteria Open channels shall be designed to the following criteria: • In all cases for open channels, the Design Engineer shall calculate the 100 -year flow and show the 100 -year flow boundary and water surface elevation on the grading plan. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Symbol Table 6.11. Symbols and definitions. Definition Units a Energy coefficient - A Cross-sectional area ft2 b Bottom width ft Cg Specific weight correction factor - D or d Depth of flow ft d Stone diameter ft delta d Superelevation of the water surface profile ft d## Diameter of stone for which some percentage, by weight, of the gradation is finer ft E Specific energy ft Fr Froude Number - g Acceleration of gravity 32.2 ft/s2 hloss Head loss ft K Channelconveyance - ke Eddy head loss coefficient ft KT Trapezoidal open channel conveyance factor - L Length of channel ft L Length of downstream protection ft n Manning's roughness coefficient - P Wetted perimeter ft Q Discharge rate cfs R Hydraulic radius of flow ft Rc Mean radius of the bend ft S Slope ft/ft SWs Specific weight of stone lbs/ft3 T Top width of water surface ft V or v Velocity of flow ft/s w Stone weight lbs Yc Critical depth ft Yn Normal depth ft z Critical flow section factor - 6.4.4 Design Criteria 6.4.4.1 General Criteria Open channels shall be designed to the following criteria: • In all cases for open channels, the Design Engineer shall calculate the 100 -year flow and show the 100 -year flow boundary and water surface elevation on the grading plan. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Channel or adjacent public drainage easement, floodway, etc., shall be capable of carrying the 100 -year storm. Adjacent public drainage easements shall contain the width of flow of channel, floodway, floodplain, etc., plus an additional 15 feet each side of the defined design of flood pool. For example, if the channel, floodway, or floodplain width is 50 feet wide, the drainage easement width at the same point will be 80 feet. • Channel side slopes shall be designed to have a maximum slope of 3:1 to allow for maintenance, unless otherwise justified. Roadside ditches should have a maximum side slope of 3:1. • Trapezoidal or parabolic cross sections are preferred. • For channels with vegetative lining, design stability shall be determined using Manning's n based upon poor vegetation conditions and for design capacity better conditions should be used. Channel velocities shall not exceed the maximum permissible velocities given in Table 6.13. • If a stream channel must be relocated, the cross-sectional shape, meander, pattern, roughness, sediment transport capacity, and slope should conform to the existing conditions to the extent practicable. Some means of energy dissipation may be necessary when existing conditions cannot be duplicated. • Streambank stabilization should be provided, when appropriate, as a result of any stream disturbance such as encroachment and should include both upstream and downstream banks as well as the local site. Disturbance of streambanks may be performed only in accordance with the City Streamside Protection Ordinance (Chapter 168.12 of Unified City Code). 6.4.4.2 Velocity Limitations The final design of artificial open channels should be consistent with the velocity limitations for the selected channel lining. Maximum velocity values for earthen materials categories are presented in Table 6.12. Seeding and mulch should only be used when the design value does not exceed the allowable value for bare soil. Velocity limitations for vegetative linings are reported in Table 6.13. Erosion Control Matting may be used if designed and constructed in accordance with manufacturer's specifications subject to the limitations provided in this manual. 6.12. Maximum velocities Material comparingTable for Maximum Velocity (ft/s) Sand 2.0 Silt IS Firm Loam IS Fine Gravel S.0 Stiff Clay S.0 Graded Loam or Silt to Cobbles S.0 Coarse Gravel 6.0 Shales and Hard Pans 6.0 Erosion control matting Source: AASHTO Model Drainage Manual, 1991. * Based on manufacturer specifications and subject to approval by City Engineer. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.13. Maximum velocities for vegetative channel linings. Vegetation Type g Yp Slope Range N, Maximum Velocity' (ft/s) Bermuda grass 0-10 5 Bahia 4 Tall fescue grass mixtures3 0-10 4 Kentucky bluegrass 0-5 6 Buffalo grass 0-10 5 >10 4 Grass mixture 0-51 4 5-10 3 Annuals4 0-5 3 Sod 4 Staked sod 5 1 Do not use on slopes steeper than 10% except for side -slope in combination channel. 2 Use velocities exceeding 5 ft/s only where good stands can be maintained. 3 Mixtures of Tall Fescue, Bahia, and/or Bermuda. 4 Annuals - use on mild slopes or as temporary protection until permanent covers are established. Source: Manual for Erosion and Sediment Control in Georgia, 1996. 6.4.4.3 Channel Cross Section Requirements The channel shape may be almost any type suitable to the location and to the environmental conditions. The shape may be able to be chosen to suit open space and recreational needs and to create additional benefits. 1. Bend Radius: The minimum bend radius required for open channels is 25 feet or 10 times the bottom width, whichever is larger. 2. Freeboard: Freeboard shall be based on velocities associated with the design storm and shall be a minimum of 1 foot for channel velocities up to 8 ft/s and 2 feet for velocities exceeding 8 ft/s at the design storm. For deep flows with high velocities, greater freeboard shall be required, calculated in accordance with the following formula: Freeboard (ft) = 1.0 + 0.025 vDl/3 Eq. 6.12 Where: v = velocity of flow (ft/s) D = depth of flow (ft) `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Additional freeboard shall be computed for a channel with a sharp curve less than the minimum bend radius, as: H = vz ((T + b)/2gR,) Eq. 6.13 Where: H = additional height on outside edge of channel (ft) v = velocity of flow (ft/s) T = top width of water surface (ft) b = bottom width of channel (ft) g = acceleration of gravity (32.2 ft/S2) Rc = mean radius of bend (ft) 3. Connections: Connections at the junction of two or more open channels shall be smooth. Pipe and box culvert or sewers entering an open channel shall not project into the normal channel section, and shall discharge into the receiving at an angle that directs flow downstream. 6.4.4.4 Channel Drops Sloped drops shall have roughened faces and shall be no steeper than 2:1. They shall be adequately protected from scour and shall not cause an upstream water surface drop that will result in high velocities upstream. The design shall include protection against side cutting just downstream from the drop, which is a common problem. 6.4.4.5 Baffle Chutes Baffle chutes are used to dissipate the energy in the flow at a larger drop. They require no tailwater to be effective. They are partially useful where the water surface upstream is held at a higher elevation to provide head for filling a side storage pond during peak flows. Baffle chutes may be used in channels where water is to be lowered from one level to another. The baffle piers prevent undue acceleration of the flow as it passes down the chute. The baffled apron shall be designed for the full discharge design flow and shall be protected from scouring at the lower end. A stilling basin shall be added where appropriate based on velocities. 6.4.4.6 Computation and Software Computer programs that utilize the Manning's equation shall be used for open channel design. Computer programs such as Hydraflow Express may be used for Uniform Flow conditions; however for more complex reaches, a backwater model such as HEC -RAS should be used. The general information to be provided in an open channel design is: • Design data (i.e., location, area, runoff coefficients, typical section, slope, etc.). `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Appropriate discharge volume and applicable design standards, design geometry required based on operational characteristics - freeboard, velocity, minimum standard capacity and site requirements. Flow regime - subcritical or supercritical - shall be reported and taken into consideration as part of design. Below is a sample output file using Hydraflow Express computer software. 6.4.5 Manning's n Values Recommended Manning's n values for artificial channel linings are given in Table 6.15. For natural channels, earthen channels, and various types of vegetation, Manning's n values should be estimated using experienced judgment and based on the information in Table 6.16. Additional details are provided in the Guide for Selecting Mannings Roughness Coefficients for Natural Channels and Flood Plains, FHWA-TS 84-204,1984. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.14. reportTable Channel Channel Section Channel Section Data: Highlighted: Bottom Width (ft) 2.00 Depth (ft) 0.80 Side Slopes (z:1) 3.00, 3.00 Q (cfs) 13.00 Total Depth (ft) 2.00 Area (sq ft) 3.S2 Invert Elevation (ft) 100.00 Velocity (ft/s) 3.69 Slope (%) 1.00 Wetted Perimeter (ft) 7.06 N -Value 0.025 Critical Depth, Yc (ft) 0.77 Top Width (ft) 6.80 Calculations: EGL (ft) 1.01 Compute by: Known Q Known Q (cfs) 13.00 6.4.5 Manning's n Values Recommended Manning's n values for artificial channel linings are given in Table 6.15. For natural channels, earthen channels, and various types of vegetation, Manning's n values should be estimated using experienced judgment and based on the information in Table 6.16. Additional details are provided in the Guide for Selecting Mannings Roughness Coefficients for Natural Channels and Flood Plains, FHWA-TS 84-204,1984. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.15. Manning's Category roughness coefficients Lining Type for artificial 0-0.5 ft lined channels. Depth Ranges 0.5-2.0 ft >2.0 ft Rigid Concrete 0.015 0.013 0.013 Grouted Riprap 0.04 0.03 0.028 Stone Masonry 0.042 0.032 0.03 Soil Cement 0.025 0.022 0.02 Asphalt 0.018 0.016 0.016 Unlined Bare Soil 0.023 0.02 0.02 Rock Cut 0.045 0.035 0.025 Temporary* Woven Paper Net 0.016 0.015 0.015 Jute Net 0.028 0.022 0.019 Fiberglass Roving 0.028 0.022 0.019 Straw with Net 0.065 0.033 0.025 Curled Wood Mat 0.066 0.035 0.028 Synthetic Mat 0.036 0.025 0.021 Gravel Riprap 1 -inch Dsa 0.044 0.033 0.03 2 -inch Dsa 0.066 0.041 0.034 Rock Riprap 6 -inch Dso 0.104 0.069 0.035 12 -inch Dsa ---- 0.078 0.04 Note: Values listed are representative values for the respective depth ranges. Manning's roughness coefficients, n, vary with the flow depth. *Some "temporary" linings become permanent when buried. Source: HEC -15, 1988. Table 6.16 Uniform flow values of roughness coefficient n. Type of Channel and Description Minimum Normal Maximum EXCAVATED OR DREDGED a. Earth, straight and uniform 0.016 0.018 0.020 1. Clean, recently completed 0.018 0.022 0.025 2. Clean, after weathering 0.022 0.025 0.030 3. Gravel, uniform section, clean 0.022 0.027 0.033 b. Earth, winding and sluggish 1. No vegetation 0.023 0.025 0.030 2. Grass, some weeds 0.025 0.030 0.033 3. Dense weeds/plants in deep channels 0.030 0.035 0.040 4. Earth bottom and rubble sides 0.025 0.030 0.035 5. Stony bottom and weedy sides 0.025 0.035 0.045 6. Cobble bottom and clean sides 0.030 0.040 0.050 c. Dragline-excavated or dredged 1. No vegetation 0.025 0.028 0.033 2. Light brush on banks 0.035 0.050 0.060 d. Rock cuts 1. Smooth and uniform 0.025 0.035 0.040 2. Jagged and irregular 0.035 0.040 0.050 `t -Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Table 6.16 Uniform flow values of roughness coefficient n. Type of Channel and Description Minimum Normal Maximum e. Channels not maintained, weeds and brush uncut 1. Dense weeds, high as flow depth 0.050 0.080 0.120 2. Clean bottom, brush on sides 0.040 0.050 0.080 3. Same, highest stage of flow 0.045 0.070 0.110 4. Dense brush, high stage 0.080 0.100 0.140 NATURAL STREAMS Minor streams (top width at flood stage < 100 ft) a. Streams on Plain 1. Clean, straight, full stage 0.025 0.030 0.033 2. Same as above, but some stones and weeds 0.030 0.035 0.040 3. Clean, winding, some pools and shoals 0.033 0.040 0.045 4. Clean, winding, but some weeds and some stones 0.035 0.045 0.050 5. Same as 4, lower stages, more ineffective slopes and sections 0.040 0.048 0.055 6. Same as 4, but more stones 0.045 0.050 0.060 7. Sluggish reaches, weedy, deep pools 0.050 0.070 0.080 8. Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush 0.075 0.100 0.150 b. Mountain streams, no vegetation in channel, banks usually steep, trees and brush along banks submerged at high stages 1. Bottom: gravels, cobbles, few boulders 0.030 0.040 0.050 2. Bottom: cobbles with large boulders 0.040 0.050 0.070 FLOODPLAINS a. Pasture, no brush 1. Short grass 0.025 0.030 0.035 2. High grass 0.030 0.035 0.050 b. Cultivated area 1. No crop 0.020 0.030 0.040 2. Mature row crops 0.025 0.035 0.045 3. Mature field crops 0.030 0.040 0.050 c. Brush 1. Scattered brush, heavy weeds 0.035 0.050 0.070 2. Light brush and trees in winter 0.035 0.050 0.060 3. Light brush and trees, in summer 0.040 0.060 0.080 4. Medium to dense brush, in winter 0.045 0.070 0.110 5. Medium to dense brush, in summer 0.070 0.100 0.160 d. Trees 1. Dense willows, summer, straight 0.110 0.150 0.200 2. Cleared land, tree stumps, no sprouts 0.030 0.040 0.050 3. Same as above, but with heavy growth of sprouts 0.050 0.060 0.080 4. Heavy stand of timber, a few down trees, little undergrowth, flood stage below branches 0.080 0.100 0.120 5. Same as above, but with flood stage reaching branches 0.100 0.120 0.160 MAJOR STREAMS (top width at flood stage > 100 ft). The n value is less than that for minor streams of similar description, because banks offer less effective resistance. a. Regular section with no boulders or brush 0.025 --- 0.060 b. Irregular and rough section 0.035 --- 0.100 Source: HEC -15, 1988. `t- Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.4.6 Uniform Flow Calculations 6.4.6.1 Channel Discharge — Manning's Equation Manning's Equation, presented in three forms below, shall be used for evaluating uniform flow conditions in open channels: v = (1.49/n) R2/3 Sl/z Q = (1.49/n) A R2/3 Sl/z S = [Q./(1.49 A R2/3)]2 Where: v = average channel velocity (ft/s) Q = discharge rate for design conditions (cfs) n = Manning's roughness coefficient A = cross-sectional area (ft') R = hydraulic radius A/P (ft) P = wetted perimeter (ft) S = slope of the energy grade line (ft/ft) Eq. 6.14 Eq. 6.15 Eq. 6.16 If the channel is uniform in resistance and gravity forces are in exact balance, the water surface will be parallel to the bottom of the channel. This is the condition of uniform flow. Open channel flow in urban drainage systems is complicated by bridge openings, curbs, and structures. Typically backwater computations will be required for channel design work; however, a check sould also be performed for velocity based on headwater controlled conditions. A water surface profile shall be computed for all channels and shown on all final drawings. Computation of the water surface profile should utilize standard backwater methods or acceptable computer routines (Appendix H, Stormwater Software), taking into consideration all losses due to the changes in velocity, drops, bridge openings, and other obstructions. Where practical, unlined channels should have sufficient gradient, depending upon the type of soil, to provide velocities that will be self-cleaning but will not cause erosion. Lined channels, drop structures, check dams, or concrete spillways may be required to control erosion that results from the high velocities of large volumes of water. Unless approved otherwise by the City Engineer, channel velocities in man-made channels shall not exceed those specified in Tables 6.12 and 6.13. Where velocities exceed specified velocities, riprap, pavement, or other approved erosion protection measures shall be required. As minimum protection to reduce erosion, all open channel slopes shall be seeded or sodded as soon after grading as possible. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.4.7 Vegetative Design Requirements Final design of temporary and vegetative channel linings involves the use of Tables 6.12 and 6.13 for both stability and design capacity. 6.4.8 Riprap Design Where the use of riprap is allowed by the City Engineer, riprap sizing shall be determined based on maximum anticipated channel velocities. Adequate erosion protection shall be provided for the design configurations. For example, if riprap will extend into a stream with higher water surface elevations and/or velocities, i.e., at a pipe outfall going into a creek, then the riprap must be sized to resist the forces of the higher flow in the creek. When rock riprap is used, the need for an underlying filter material must be evaluated. The filter material may be either a granular blanket or plastic filter cloth. See Figure 6.2 for riprap sizing criteria. The design velocity should be based on the higher velocity from the 10 -year design event or 100 -year check storm event, including velocities in receiving stream, if applicable. Extend a vertical line from the x-axis of the figure at the appropriate velocity until the curve is intersected, then extend a horizontal line to intersect the y-axis at the corresponding Dso, or median stone diameter for which no more than 50% of the stone by weight is smaller. Figure TS14C-5 Rock size based on Isbash curve S 0 w 0 za d A 60 40 20 10 12 14 16 is 20 Velocity {ftls) (210 -VI -NEE, August 2007) Figure 6.2. Riprap sizing curve. 15,000 10,000 r, 5,000 G 1,000 0 a. 500 ° 250 100 50 Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.4.9 Gradually Varied Flow— Backwater Modeling and Data Requirements The most common occurrence of gradually varied flow in storm drainage is the backwater created by culverts, storm sewer inlets, inline storage, or channel constrictions. For these conditions, the flow depth will exceed normal depth in the channel and the water surface profile should be computed using backwater techniques. Many computer programs are available for computation of backwater curves. For further information on acceptable software to use, refer to Appendix H, Stormwater Software. For the use of step -backwater computations for the purpose of site design, detailed topographic data of an accuracy to support 1 -foot or smaller contour intervals is required for existing and as -built models. In other areas (for downstream assessments, for example), the most recent publicly available data will generally be accepted. SECTION 6.5. ENERGY DISSIPATION DESIGN Criteria for Energy Dissipation 1. Energy dissipation is required at outlets and along transitions to existing channels where velocities are high to reduce the potential for erosion and slow the water. Evaluate the downstream channel stability and outlet velocities based on Tables 6.12 and 6.13 as appropriate, including that of the receiving stream, and provide appropriate erosion protection if channel degradation is expected to occur. 2. Additional downstream channel assessment should be conducted on a case by case basis as determined by the City Engineer. 6.5.1 Overview 6.5.1.1 Introduction The outlets of pipes and lined channels are points of critical erosion potential. Stormwater that is transported through man-made conveyance systems at design capacity generally reaches a velocity that exceeds the capacity of the receiving channel or area to resist erosion. To prevent scour at stormwater outlets, protect the outlet structure and minimize the potential for downstream erosion, a flow transition structure is needed to absorb the initial impact of flow and reduce the speed of the flow to a non-erosive velocity. General guidance for design of outlet protection is provided in Sections 6.5.3 (Baffled Outlets) and 6.5.4 (Outfall Protection). Additional guidance is provided in Appendix G, Outlet Structures. 6.5.1.2 General Criteria Erosion problems at culvert, pipe and engineered channel outlets are common. Determination of the flow conditions, scour potential, and channel erosion resistance shall be standard procedure for all designs. • Energy dissipators shall be employed whenever the velocity of flows leaving a stormwater management facility exceeds the erosive velocity of the downstream area channel system. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) • Energy dissipator designs will vary based on discharge specifics and tailwater conditions. Outlet structures should provide uniform redistribution or spreading of the flow without excessive separation and turbulence. 6.5.1.3 Recommended Energy Dissipators For many designs, the use of a baffled outlet provides sufficient protection at a reasonable cost. This section focuses on the design on these measures. The reader is referred to the Federal Highway Administration Hydraulic Engineering Circular No. 14 entitled, Hydraulic Design of Energy Dissipators for Culverts and Channels, for the design procedures of other energy dissipators. 6.5.2 Symbols and Definitions To provide consistency within this section as well as throughout this Manual, the symbols listed in Table 6.17 will be used. These symbols were selected because of their wide use. In some cases, the same symbol is used in existing publications for more than one definition. Where this occurs in this section, the symbol will be defined where it occurs in the text or equations. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) SymbolsTable 6.17. definitions Symbol Definition Units A Cross-sectional area ftz D Height of box culvert ft dso, dio Size of riprap ft dam, Culvert width ft Fr Froude Number - g Acceleration of gravity ft/S2 hs I Depth of dissipator pool ft L Length ft La Riprap apron length ft LB Overall length of basin ft LS Length of dissipator pool ft PI Plasticity index - Q Rate of discharge cfs S„ Saturated shear strength lbs/inz t Time of scour min. tc Critical tractive shear stress lbs/inz TW Tailwater depth ft VL Velocity L ft from brink ft/s Vo Normal velocity at brink ft/s Vo Outlet mean velocity ft/s VS Volume of dissipator pool ftz Wo Diameter or width of culvert ft WS Width of dissipator pool ft Ye Hydraulic depth at brink ft Y. Normal flow depth at brink ft `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6.5.3 Baffled Outlets 6.5.3.1 Description The baffled outlet (also known as the Impact Basin - USBR Type VI) is a boxlike structure with a vertical hanging baffle and an end sill, as shown in Figure 6.3. Energy is dissipated primarily through the impact of the water striking the baffle and, to a lesser extent, through the resulting turbulence. This type of outlet protection has been used with outlet velocities up to 50 feet per second. Tailwater depth is not required for adequate energy dissipation, but a tailwater will help smooth the outlet flow. 6.5.3.2 Design Procedure The following design procedure is based on physical modeling studies summarized from the U.S. Department of Interior (1978). The dimensions of a baffled outlet as shown in Figure 6.3 should be calculated as follows: (Step 1) Determine input parameters, including: h = Energy head to be dissipated (ft), can be approximated as the difference between channel invert elevations at the inlet and outlet Q = Design discharge (cfs) v = Theoretical velocity (ft/s = 2gh) A = Q/v = Flow area (ftz) d = AM= Representative flow depth entering the basin (ft), assumes square jet Fr = v/(gd)0.5 = Froude number, dimensionless (Step 2) Calculate the minimum basin width, W, in ft, using the following equation. W/d = 2.88Fro.s66 or W = 2.88dFro.s66 Eq. 6.17 W = minimum basin width (ft) d = depth of incoming flow (ft) Fr = v/(gd)0.5 = Froude number, dimensionless The limits of the W/d ratio are from 3 to 10, which corresponds to Froude numbers 1 and 9. If the basin is much wider than W, flow will pass under the baffle and energy dissipation will not be effective. (Step 1) Calculate the other basin dimensions as shown in Figure 6.3, as a function of W. Construction drawings for selected widths are available from the U.S. Department of the Interior (1978). (Step 2) Calculate required protection for the transition from the baffled outlet to the natural channel based on the outlet width. A riprap apron should be added of width W, length W (or 5 -feet minimum), and depth f (W/6). The side slopes should be 1.5HAV, and median rock diameter should be at least W/20. (Step 3) Calculate the baffled outlet invert elevation based on expected tailwater. The maximum distance between expected tailwater elevation and the invert should be b + f or some flow `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) will go over the baffle with no energy dissipation. If the tailwater is known and fairly controlled, the baffled outlet invert should be a distance, b/2 + f, below the calculated tailwater elevation. If tailwater is uncontrolled, the baffled outlet invert should be a distance, f, below the downstream channel invert. (Step 4) Calculate the outlet pipe diameter entering the basin assuming a velocity of 12 feet/s flowing full. (Step 5) If the entrance pipe slopes steeply downward, the outlet pipe should be turned horizontal for at least 3 feet before entering the baffled outlet. (Step 6) If it is possible that both the upstream and downstream ends of the pipe will be submerged, provide an air vent approximately 1/6 the pipe diameter near the upstream end to prevent pressure fluctuations and possible surging flow conditions. 6.5.4 Outfall Protection A design procedure is provided in Figure 6.4. The sizing of dso of the rock shall be determined based on the maximum discharge velocity by use of the curve provided as Figure 6.2. The maximum stone diameter should be 1.5 times the median diameter. If the ground slope downstream of the apron is steep, channel erosion may occur. The apron should be extended as necessary until the slope is gentle enough to prevent further erosion based on velocities computed for the design and check storms. Velocities below the outlet shall be computed to confirm they are below erosive velocities for the receiving channel as provided in Tables 6.12 and 6.13. If not, additional energy dissipation shall be provided or the protective riprap apron extended to and across the receiving channel and protection provided to a minimum of 6 inches above the water surface elevation at the design storm for the discharging outlet. The protective apron extents in the receiving channel shall extend to a width three times that of the outlet width where it intersects the receiving channel. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) f/2 4 SECTION 'B -B' a Protection 5' f Min. as Required Fillet H = 3/4 (w) a= 1/2 (w) V L b = 3/8 (w) — �- c = 112 (w) i0i I SECTION 'A -A' I� L 7 i� II I� iLFL = 4!3 (w) e ir_ f=1/6(w) e = 1/12 (w) L Ql ROCK DIAMETER (D50 PLAN FOR PROTECTION = 20) Figure 6.3. Schematic of baffled outlet (Source: U.S. Dept. of the Interior, 1978). `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) FLARED END SECTION OVERFLOW ELEVATION o ` FILTER RECEIVING MATERIAL ELEVATION THICKNESS ('d') =1.5 x MAX. ROCK DIAMETER -6" (150mm) MIN —^n 1 r La = 4.5 x'D' MIN. 'D'= PIPE DIAMETER a,�nan�oa0 6 bQ'ao e �^ l — _ ROCK d50 - 50% SHALL BE LARGER THAN 6" (150mm) MIN. q.0 x'D' 'D' I DIA. AND SIZED BASED ON m�� MIN. u- MAXIMUM DISCHARGE VELOCITY �� IIn�1� �n�1 ff PLAN NOTES: 1. 'La' = LENGTH OF APRON. DISTANCE'La' SHALL BE OF SUFFICIENT LENGTH TO DISSIPATE ENERGY. 2. APRON SHALL BESET AT A ZERO GRADE AND ALIGNED STRAIGHT. 3. FILTER MATERIAL SHALL BE FILTER FABRIC OR 6"(150 mm) THICK MINIMUM GRADED GRAVEL LAYER. Figure 6.4. Storm drain outlet protection. SECTION 6.6. REFERENCES American Association of State Highway and Transportation Officials, 1981 and 1998. Model Drainage Manual. American Association of State Highway and Transportation Officials, 1982. Highway Drainage Guidelines. American Association of State Highway and Transportation Officials, 1998. Model Drainage Manual. American Iron and Steel Institute, 1999. Modern Sewer Design, 4th Edition. Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook, Atlanta, GA. Debo, Thomas N. and Andrew J. Reese, 1995. Municipal Storm Water Management. Lewis Publishers. i` ' # Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Department of Irrigation and Drainage Malaysia, River Engineering Division, 2000. Urban Stormwater Management Manual for Malaysia (Draft). Federal Highway Administration, 1983. Hydraulic Design of Energy Dissipators for Culverts and Channels. Hydraulic Engineering Circular No. 14. Federal Highway Administration, 1987. HY8 Culvert Analysis Microcomputer Program Applications Guide. Hydraulic Microcomputer Program HY8. Federal Highway Administration, 1971. Debris -Control Structures. Hydraulic Engineering Circular No. 9. Federal Highway Administration, 1978. Hydraulics of Bridge Waterways. Hydraulic Design Series No. 1. Federal Highway Administration, 1985. Hydraulic Design of Highway Culverts. Hydraulic Design Series No. S. Federal Highway Administration, 1996. Urban Drainage Design Manual. Hydraulic Engineering Circular No. 22. HYDRAIN Culvert Computer Program (HY8). Available from McTrans Software, University of Florida, 512 Weil Hall, Gainesville, Florida 32611. Natural Resource Conservation Service, 2007. National Engineering Handbook. Part 654. Prince George's County, MD, 1999. Low -Impact Development Design Strategies, An Integrated Design Approach. U. S. Department of Interior, 1983. Design of Small Canal Structures. U.S. Department of Transportation, Federal Highway Administration, 1984. Drainage of Highway Pavements. Hydraulic Engineering Circular No. 12. `t—Nis Chapter 6. Storm Drainage System Design You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 7. STORMWATER DETENTION SECTION 7.1. GENERAL 7.1.1 Introduction This section provides guidance on stormwater runoff storage for meeting stormwater management control requirements (i.e., water quality treatment, downstream channel protection, overbank flood protection, and extreme flood protection). Storage of stormwater runoff within a stormwater management system is essential to providing the extended detention of flows for water quality treatment and downstream channel protection, as well as for peak flow attenuation of larger flows for overbank and extreme flood protection. Runoff storage can be provided within an on-site system through the use of structural stormwater controls and/or nonstructural features and landscaped areas. Design guidance is provided in Appendix F, Water Quality Structural Controls. 7.1.2 Volume of Detention Volumes of detention shall be evaluated according to the following methods: 1. The Soil Conservation Service (SCS) method shall be used for design of detention basins. 2. If another method is used, the Owner's Engineer shall submit the proposed method of evaluation for the sizing of the retention basin or detention basin to the City Engineer. The method will be evaluated for professional acceptance, applicability, and reliability by the City Engineer. No detailed review will be rendered before the method of evaluation of the retention or detention basin is approved. 7.1.3 Design Criteria Stormwater detention systems shall be designed to meet the stormwater sizing criteria described in Chapter 2 and shall provide structural control as needed to meet the Minimum Standards. SECTION 7.2. DETENTION DESIGN PROCEDURES 7.2.1 Introduction The design procedures for all structural control storage facilities are the same whether or not they include a permanent pool of water. Where present, the permanent pool elevation is taken as the "bottom" of storage and is treated as if it were a solid basin bottom for routing purposes. The location of structural stormwater controls is very important as it relates to the effectiveness of these facilities to control downstream impacts. In addition, multiple storage facilities located in the same drainage basin will affect the timing of the runoff through the conveyance system, which could decrease or increase flood peaks in different downstream locations. Therefore, a downstream peak flow analysis should be performed as part of the storage facility design process (see Section 7.5). Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 7.2.2 Estimating Detention Volume To estimate the storage volume (Vs) required to meet the four minimum standards, the following procedure may be used. 1. Compute required water quality volume, WQ, as described in Chapter 2, Section 2.1.2 and amplified in Chapter 4. If a permanent pool is used, the water quality volume may be split evenly between the permanent pool and extended detention storage (to be released over 24 hours). 2. Compute existing (predevelopment) and proposed (post development) peak discharges (pre - detention) for the 1-, 2-, 5-, 10-, 25-, and 100 -year 24-hour storm events. The 2-, 5-, and 10 -year peak discharges may only be needed to perform checks to verify adequate design. 3. Compute elevations in the receiving channel at the outlet for the storm events listed in step 2. The elevation for a storm event shall be applied as the tailwater condition for detention outlet calculations for that same given event. 4. Compute the Channel Protection volume, CPv, or storage volume required for channel protection based on the runoff hydrograph from the 1 -year, 24-hour storm event as described in Chapter 2, Section 2.1.3. The Cps shall be released over a 40 -hour period (72 -hour maximum drain time). If the post -development discharge is less than 2 cfs, then Cps storage is not required, but the 1 -year, 24- hour storm post development discharge shall not exceed the 1 -year, 24-hour storm predevelopment discharge for the project area, so a corresponding storage volume shall be provided. 5. Use the SCS Method described in steps 6 through 9 to compute the Extreme Flood, Qf, volume required such that the post -development peak discharge rate does not exceed the predevelopment rate for the 100 -year, 24-hour return frequency storm event (QF). 6. As a starting point, assume the peak discharge qo from the proposed detention basin is equal to the corresponding predevelopment peak discharge determined in step 1 (qf), and assume the proposed inflow to the detention basin (q;) is equal to the post -development peak discharge determined in Step 2. 7. Compute qo/qi and, using Figure 7.1 below, determine Vs/V,. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 6 �1 .4 E 4 C � c � o v r_+5 .2 t .2 .3 .4 .6 .6 .7 Peaicoutflowdischarge qo Peak inflow discharge Qi } Figure 7.1. Approximate detention basin routing for Type III rainfall distribution. I 8. Use the runoff depth (watershed inches) that was determined from the SCS method while computing post development peak discharges q; in step 2 to compute the runoff volume (Vr). 9. Vr = *A. Eq. 7.1 12 10. where: 11. Vr = runoff volume (acre-feet) 12. Q; = runoff depth (watershed inches) for the given post development inflow event 13. A,, = drainage area (acres) ' - Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 14. Use the results from steps 7 and 8 to compute K. Vs = Vr V Vs Fn_ r where: VS = storage volume required (acre-feet) 15. Due to the requirement for extended detention of the WQv and CPQ, it may be appropriate to add the storage volume required to meet these standards to the storage volume required for the extreme flood protection standard to obtain an initial total storage volume for detention design. 16. The stage in the detention basin corresponding to VS must be equal to the stage used to generate qo. 17. Once the initial estimates of stage -volume data are developed, detention design procedures described below in Section 7.2.3 shall be followed. 7.2.3 Detention Basin Design Procedure A general procedure for the design of storage facilities is presented below. Step 1 Perform preliminary calculations to evaluate detention storage requirements for the hydrographs as described above in Section 7.2.2. Step 2 Determine the physical dimensions necessary to hold the estimated volume from Step 1. The maximum storage requirement calculated from Section 7.2.2 should be used. From the selected shape determine the maximum depth in the pond. Develop the stage -storage curve for the detention basin. Step 3 Select the desired type of outlet and size the outlet structures based on allowable discharges for the design storm events, beginning with outlet structure sizing for the smaller Water Quality and Channel Protection Volume events to the extreme flood event and taking into consideration the tailwater in the receiving stream. The estimated peak stage for each storm event will occur for the maximum associated volume from Step 2. The outlet structure(s) should be sized to convey the allowable discharge for the corresponding stage. Refer to Appendix G, Section 3 for detailed steps regarding outfall design. The outfall structure shall be designed with appropriate erosion prevention measures. Step 4 Perform routing calculations using inflow hydrographs from Step 1 to check the preliminary design using an approved storage routing computer model (See Appendix H for approved analysis and design software). Step 5 Evaluate whether the routed post -development peak discharges from the design storms exceed the existing pre -development peak discharges. If so, then revise the dimensions of the pond or outlet device geometry accordingly, and repeat Steps 2 through 4 until the post -development peak discharges do not exceed the existing pre -development peak discharges for the watershed. Step 6 Evaluate the downstream effects of detention outflows for the 25- and 100 -year 24-hour storm events to ensure that the routed hydrograph does not cause downstream flooding problems. The outflow hydrograph from the storage facility should be routed through the downstream channel system to a confluence point that reflects no appreciable increase in discharges compared to the pre -development discharges at that location, or to a point designated by the City (see Section 7.5). M �t— ,l Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Step 7 Evaluate the control structure outlet velocity for all storms and provide channel and bank stabilization if the outlet velocities from any of the design storms will cause erosion problems downstream. Outlet protection shall include checking velocities and ensuring adequate erosion prevention measures to beyond the confluence with the receiving stream channel. Riprap placement or energy dissipater devices may be required. Guidance for riprap sizing and extents of placement and outlet design is provided in Section 6.5 and Appendix G. Routing of hydrographs through storage facilities is critical to the proper design of these facilities. Although storage design procedures using inflow/outflow analysis without routing have been developed, their use is not accepted by the City of Fayetteville. Additional information regarding the design requirements for extended detention and associated outlet design is provided in Appendix F. Water quality requirements and the associated Water Quality Protection Volume (WQv) shall be addressed in the design. Details regarding these requirements and the approach that may be used to address them are provided in Chapter 4, Water Quality, and Appendix F. For this Manual, it is assumed that designers will be using one of the many computer programs available for storage routing and thus other procedures and example applications will not be given here. A list of approved software to perform storage routing calculations is provided in Appendix H, Stormwater Software. SECTION 7.3. METHODS OF DETENTION Detention storage may be categorized as inline or offline. The City of Fayetteville only allows inline storage if it can be demonstrated that offline storage is not practicable. Figure 7.2 illustrates inline versus offline storage. Stormwater Conveyance Flow Diversion ► Y Structure Storage Storage Facility Facility Inline Storage Off Line Storage Figure 7.2. Inline versus offline storage. 7.3.1 Structural Controls Appropriate for Detention The following sections list the structural control practices appropriate for detention that are approved for use in the City of Fayetteville. A brief description is provided in this section, with detailed design M �t— ,l Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) requirements and procedures provided in Appendices E and F. Regardless of the detention practice selected, mosquito control measures shall be taken. Avoid creating areas of shallow stagnant water and low dissolved oxygen which create mosquito habitat. To avoid creating habitat, pools of water should be at least 5 feet deep and residence time should be less than 72 hours (excepting permanent pools). 7.3.1.1 Stormwater Ponds Stormwater ponds (also referred to as retention ponds, wet ponds, or wet extended detention ponds) are constructed stormwater retention basins that have a permanent (dead storage) pool of water throughout the year. They are categorized in this Manual as water quality structural controls and can meet the intent of Minimum Standard #1, however; they also can provide detention storage to meet the other Minimum Standards. In a stormwater pond, a certain design volume of runoff from each rain event is detained and treated in the pool through gravitational settling and biological uptake until it is displaced by runoff from the next storm. The permanent pool also serves to protect deposited sediments from re -suspension. Above the permanent pool level, additional temporary storage (live storage) is provided for runoff quantity control. Stormwater ponds are among the most cost-effective and widely used stormwater practices. A well-designed and landscaped pond can be an aesthetic feature on a development site when planned and located properly. The most common of stormwater pond designs include the wet pond, the wet extended detention pond, and the micropool extended detention pond. In addition, multiple stormwater ponds can be placed in series or parallel to increase performance or meet site design constraints. Refer to Appendix F for more information on stormwater ponds, including example details and design requirements. 7.3.1.2 Stormwater Wetlands Stormwater wetlands (also referred to as constructed wetlands) are constructed shallow marsh systems that are designed to both treat urban stormwater and control runoff volumes. As stormwater runoff flows through the wetland facility, pollutant removal is achieved through settling and uptake by marsh vegetation. Stormwater wetlands are categorized as water quality structural controls to meet Minimum Standard #1, however; they also can provide detention storage to meet the other Minimum Standards. Wetlands are an effective stormwater practices in terms of water quality and offer aesthetic value and wildlife habitat. Stormwater wetlands require a continuous base flow or a high water table to support aquatic vegetation. There are several design variations of the stormwater wetland, each design differing in the relative amounts of shallow and deep water, and dry storage above the wetland. These include the shallow wetland, the extended detention shallow wetland, pond/wetland system and pocket wetland. Refer to Appendix F for more information on stormwater wetlands, including example details and design requirements. 7.3.1.3 Dry Detention / Dry ED Basins Dry detention and dry extended detention (ED) basins are surface facilities intended to provide for the temporary storage of stormwater runoff to meet Minimum Standards two through four. These facilities Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) temporarily detain stormwater runoff, releasing the flow over a period of time. They are designed to completely drain following a storm event and are normally dry between rain events. Both dry detention and dry ED basins provide limited pollutant removal benefits and are not intended for water quality treatment. Detention -only facilities should be used in a treatment train approach with other structural controls to provide water quality treatment. Refer to Appendix E for more information on dry detention and dry ED basins, including example details and design requirements. 7.3.1.4 Multi-purpose Detention Areas Multi-purpose detention areas are site areas primarily used for one or more specific activities that are also designed to provide for the temporary storage of stormwater runoff to reduce downstream water quantity impacts. Example of multi-purpose detention areas include: • Parking Lots • Sports Fields • Recessed Plazas Multi-purpose detention areas are normally dry between rain events, and by their nature must be usable for their primary function the majority of the time. As such, multi-purpose detention areas should be used for meeting Minimum Standards # 3 and 4, but not for water quality treatment or channel protection with extended detention (Minimum Standards # 1 and 2). Multi-purpose detention areas should be used in a treatment train approach with other structural controls to provide water quality treatment. Refer to Appendix E for more information on multi-purpose detention areas, including example details and design requirements. 7.3.1.5 Underground Detention Underground detention facilities such as vaults, pipes, tanks, and other subsurface structures are designed to temporarily store stormwater runoff for water quantity control. As with above ground detention ponds, underground detention facilities are designed to drain completely between runoff events, thereby providing storage capacity for subsequent events. Underground detention facilities are intended to control peak flows, limit downstream flooding, and provide some channel protection. However, they provide little, if any, pollutant removal and are susceptible to re -suspension of sediment during subsequent storms. Underground detention systems serve as an alternative to surface dry detention for stormwater quantity control, particularly for space -limited areas where there is not adequate land for a dry detention basin or multi-purpose detention area. Basic storage design and routing methods are the same as for detention basins except that the bypass for high flows is typically included. Underground detention facilities may only be used where the HGL of the existing storm sewer network is low enough to allow adequate drainage to meet City design requirements within 72 hours after any design storm event. Underground detention facilities are not generally intended for water quality treatment and, unless it is specifically accommodated in design, should be used in a treatment train approach with other structural controls to provide water quality treatment. Providing treatment prior to discharging to the Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) underground detention facility will help prevent the underground system from becoming clogged with trash or sediment and significantly reduces the maintenance requirements for the system. Refer to Appendix E for more information on underground detention, including example details and design requirements. SECTION 7.4. DETENTION DESIGN STANDARDS The following conditions and limitations shall be observed in selection and use of the method or type of detention. Appendices E and F also provide detailed requirements for the various types of detention allowed in Fayetteville. 7.4.1 General Detention facilities shall be located within the parcel limits of the project under consideration. No detention or ponding will be permitted within public road right-of-ways. Location of detention facilities immediately upstream or downstream of the project will be considered by special request if proper documentation is submitted with reference to practicality, feasibility, and proof of ownership or right -of -use of the area proposed. Pond bottom slopes must be a minimum of 1% (longitudinal and cross -slope) to ensure positive drainage to outlet works. Orifices shall be provided that limit outflows to be in accordance with design requirements and to not exceed pre -development discharges. 7.4.2 Dry Detention / Dry ED Basins Wet weather ponds or dry reservoirs shall be designed with proper safety, stability, and ease of maintenance facilities. Maximum side slopes for grass reservoirs shall not exceed 1 -foot vertical for 3 -feet horizontal (3:1) unless approved by the City Engineer. In no case shall the limits of maximum ponding elevation be closer than 20 feet horizontally from any building and less than 1 foot vertically below the lowest adjacent grade, where practicable. The entire reservoir area shall be stabilized with vegetation established prior to final approval or issuance of certificate of occupancy unless approved by the City Engineer. Any area susceptible to, or designed as, overflow by higher design intensity rainfall shall be stabilized with sod or other approved vegetative stabilization practice or paved depending upon the outflow velocity. Plan view and cross-sections with adequate details for any dry detention basins and forebays and dry ED basins shall be provided in the Plans. 7.4.3 Stormwater Ponds Stormwater ponds with fluctuating volume controls may be used as detention areas provided that the limits of maximum ponding elevations are no closer than 50 -feet horizontal from any building, are at least 2 feet below the lowest sill or floor elevation of any building, and at least 1 foot below lowest adjacent grade. Maximum side slopes for the fluctuating area of stormwater ponds shall be 1 -foot vertical to 3 -feet horizontal (3:1) unless provisions are included for safety, stability, and ease of maintenance. Safety railing or other safety measures such as a shallow shelf shall be provided for ponds located in residential areas. All stormwater ponds shall include a sediment forebay at the inflow to the basin to allow heavier sediments to drop out of suspension before runoff enters the permanent pool. Sediment forebays shall be located at each point where piping or other conveyances discharge into the stormwater pond. Forebays shall be located such that they are accessible by maintenance equipment. Forebays shall be designed with adequate depth (preferably 4 to 6 feet to dissipate turbulent inflow - lesser design depths may be justified with supporting velocity computations) and volume to dissipate the energy of incoming stormwater flows and allow coarse - Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) grained sediments and particulates to settle out of the runoff. The sediment forebay should be sized to accommodate 0.25 inches of runoff per contributing on-site impervious acre of drainage area and should allow flow to exit the forebay at non-erosive velocities from the 1 -year to 10 -year 24-hour storm events. The forebay may be included as part of the required volume for detention with permanent pools. The entire fluctuating area of the permanent reservoir shall be shall be stabilized with vegetation established prior to final approval or issuance of certificate of occupancy unless approved by the City Engineer. Also, calculations must be provided to ensure adequate "live storage" is provided for the difference between the post- and pre -developed 100 -year, 24-hour storm. Any area susceptible to or designed as overflow by higher design intensity rainfall (100 -year frequency) shall be sodded, stabilized with an approved vegetative stabilization practice, or paved, depending on the design velocities. An engineering analysis shall be furnished of any proposed earthen dam or embankment configuration, with appropriate geotechnical testing and computations. Earthen dam structures shall be designed by a Professional Engineer. Plan view and cross-sections with adequate details for any stormwater ponds shall be provided in the Plans. In karst sensitive areas, or areas with high pollutant discharge potential, pond liners shall be used. If permanent pool areas are desired, use of pond liners may be permitted where necessary based on soil infiltration characteristics. 7.4.4 Parking lots Detention is permitted in parking lots to a maximum depth of 6 inches. In no case should the maximum limits of ponding (including inlet ponding) be designed closer than 10 feet from a building unless waterproofing of the building and pedestrian accessibility and safety are properly documented and approved. The maximum ponding elevation shall be 1 foot or more below the lowest sill or floor elevation and shall be at least below the lowest adjacent grade of contiguous habitable and commercial structures. In floodplain areas, Floodplain Ordinance requirements must also be met. Plan view and cross-sections of the parking lot with adequate details shall be included in Plans. 7.4.5 Low Impact Development Practices Low impact development (LID) practices can help reduce the peak flow of stormwater leaving the site. If LID practices are used on the project, they should be used upstream of any proposed detention facility. This will potentially result in reducing the quantity of stormwater necessary to be detained. Refer to Chapter 5 of the Drainage Criteria Manual for detailed design requirements for LID practices and for the approach to adjust peak discharges, where appropriate, based on implementation of LID features. 7.4.6 Underground Detention Underground detention, if used, shall be designed in accordance with the recommendations provided in Appendix E, DSC -03: Underground Detention. 7.4.7 Wetlands Wetlands, if designed for detention, shall be in accordance with recommendations provided in Appendix F, WSC-02: Stormwater Wetlands. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 7.4.8 Other Methods If other methods of detention are proposed, proper documentation of hydrologic and hydraulic calculations, soil data, percolation, geological features, etc., will be needed for review and consideration. 7.4.9 Verification of Adequacy Project closeout submittals shall include documented verification of adequacy in accordance with Section 1.4.5 of this manual. 7.4.10 Outlet Works Detention facilities shall be provided with effective outlet works. Flows shall be limited to design storm events consistent with applicable Minimum Standards. See Appendix G for example details and design requirements for various outlet structures. Safety considerations shall be an integral part of the design of all outlet works. Plan view and sections of the structure with adequate details shall be included in Plans. Overflow openings are required for all ponds. The overflow opening shall be designed to accept the total peak runoff of the improved tributary area. 7.4.11 Discharge Systems Existing upstream detention structures may be accounted for in design. Field investigations and hydrologic analysis shall be performed to substantiate benefits. A field survey of the existing physical characteristics of both the outlet structure and ponding volume shall be performed. A comprehensive hydrologic analysis shall be performed that simulates the attenuation of the contributing area ponds. This should not be limited to a linear additive analysis, but rather should consist of a network of hydrographs that considers incremental timing of discharge and potential coincidence of outlet peaks. 7.4.12 Ownership of Stormwater Detention Ponds Ownership of stormwater detention ponds that are not dedicated by the City of Fayetteville shall be vested in the property owner. The City will not process the Final Plat if all of the drainage features are not complete. No alteration of the drainage system will be allowed without the approval of the City Engineer. 7.4.13 Easements Easements shall be provided on the plans for detention facilities. A minimum 20 -feet wide drainage easement shall be provided within the reservoir area, providing vehicular access to the facility, and connecting the tributary pipes and the discharge system along the most passable route, when the discharge system is part of the public drainage system. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 7.4.14 Maintenance Detention facilities, when required, are to be built in conjunction with storm sewer installation and/or grading. Since these facilities are intended to control increased runoff, they must be partially or fully operational soon after the clearing of the vegetation. During project construction, silt and debris shall be removed as needed from the detention area and control structure(s) after each storm event to maintain the storage capacity of the facility. Post -construction maintenance of detention facilities is divided into two components. The first is long-term maintenance that involves removal of sediment from the basin and outlet control structure. Maintenance to an outlet structure is minimal with proper initial design of permanent concrete or pipe structures. Studies indicate that in developing areas, basin cleaning by front-end loader or grader is estimated to be needed once every 5 to 10 years. Annual maintenance is the second component and is the responsibility of the developer or association throughout the construction phases and of the pond owner in perpetuity after acceptance of the final plat or filing of the last subdivision phase that substantially adds stormwater to a detention basin. These items include: 1. Minor dirt and mud removal, 2. Outlet cleaning, 3. Mowing, 4. Herbicide spraying (in strict conformance with the City's policies and procedures), 5. Litter control, and 6. Forebay cleaning (where applicable). The responsibility for maintenance of the detention facilities and single -lot development projects shall remain with the general contractor until final inspection of the development is performed and approved, and a legal occupancy permit is issued. After legal occupancy of the project, the maintenance of detention facilities shall be vested with the owner of the detention pond. SECTION 7.5. DOWNSTREAM HYDROLOGIC ASSESSMENT 7.5.1 Introduction The purpose of the overbank flood protection and extreme flood protection criteria is to protect downstream properties from increases in flood hazard due to upstream development. These criteria require the designer to control peak flow at the outlet of a site such that post -development peak discharge equals pre - development peak discharge. In certain cases this does not always provide effective water quantity control downstream from the site and may actually exacerbate flooding problems downstream. The reasons for this have to do with (1) the timing of the flow peaks, and (2) the total increase in volume of runoff. This section outlines the procedure for determining the impacts of post -development stormwater peak flows and volumes on downstream flows. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 7.5.2 Reasons for Downstream Problems Flow Timing If water quantity control (detention) structures are indiscriminately placed in a watershed and changes to the flow timing are not considered, the structural control may actually increase the peak discharge downstream. The reason for this may be seen in Figure 7.4. The peak flow from the site is reduced appropriately, but the timing of the flow is such that the combined detained peak flow (the larger dashed triangle) is actually higher than if no detention were required. In this case, the shifting of flows to a later time brought about by the detention pond actually makes the downstream flooding worse than if the post - development flows were not detained. Figure 7.4. Detention timing example. Increased Volume An important impact of new development is an increase in the total runoff volume of flow. Thus, even if the peak flow is effectively attenuated, the longer duration of higher flows due to the increased volume may combine with discharge from downstream tributaries to increase the downstream peak flows. Figure 7.5 illustrates this concept. The figure shows the pre- and post -development hydrographs from a development site (Tributary 1). The post -development runoff hydrograph meets the flood protection criteria (i.e., the post -development peak flow is equal to the pre -development peak flow at the outlet from the site). However, the post -development combined flow at the first downstream tributary (Tributary 2) is higher than pre -development combined flow. This is because the increased volume and timing of runoff from the developed site increases the combined flow and flooding downstream. In this case, the detention volume would have to have been increased to account for the downstream timing of the combined hydrographs to mitigate the impact of the increased runoff volume. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Combined Flow ----------------------------------------------------------------------------------------------------- Peak Flow Combined Flow Increase in Tributary 1 Tributary 2 Tributary 1 Tributary 2 Pre -Development Post Development Detained Flow Before Development After Development Figure 7.5. Effect of increased post -development runoff volume with detention on a downstream hydrograph. 7.5.3 The Ten -Percent Rule In this Manual the "ten percent" criterion has been adopted as the most flexible and effective approach for ensuring that stormwater quantity detention ponds perform the desired function of maintaining pre - development peak flows throughout the system downstream. The ten -percent rule recognizes the fact that a structural control providing detention has a "zone of influence" downstream where its effectiveness can be felt. Beyond this zone, the influence of the structural control becomes relatively small and insignificant compared to the runoff from the total drainage area at that point. Based on studies and master planning results for a large number of sites, that zone of influence is considered to be the point where the drainage area controlled by the detention or storage facility comprises 10% of the total drainage area. For example, if the structural control drains 10 acres, the zone of influence ends at the point where the total drainage area is 100 acres or greater. The City Engineer may assign additional locations for assessment based on locations of known downstream flooding, high erosion potential, downstream development and channel constrictions. Typical steps in the application of the ten -percent rule are: 1. Determine the target peak flow for the site for predevelopment conditions. 2. Using a topographic map, assess the anticipated lower limit of the zone of influence (10% point). 3. Using a hydrologic model, to the same level of detail as for site project design, determine the pre - development peak flows and timing of those peaks at each tributary junction beginning at the pond outlet and ending at the next tributary junction beyond the 10% point. The designer shall use Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) hydrologic models obtained from the City of Fayetteville or the data therefrom, if available, for the assessment of the downstream subareas. 4. Change the land use on the site to post -development and rerun the model. 5. Design the structural control facility such that the overbank and extreme flood protection post development peak discharges are not increased above pre -development discharges at the outlet and the determined tributary junctions. 7.5.4 Example Problem Figure 7.6 illustrates the concept of the ten -percent rule for two sites in a watershed. Site A • + / ,,• '10 acres Site 6AV • SO acres t. 40 acres ■■■. ■ ' 120 acres 190 acre s e s Figure 7.6. Example of the ten -percent rule. Discussion Site A is a development of 10 acres, all draining to a wet ED stormwater pond. The overbank flooding and extreme flood portions of the design are going to incorporate the ten -percent rule. Looking downstream at each tributary in turn, it is determined that the analysis should end at the tributary marked "80 acres." The 100 -acre (10%) point is in between the 80 -acre and 120 -acre tributary junction points. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) The assumption is that if there is no peak flow increase at the 80 -acre point then there will be no increase through the next stream reach downstream through the 10% point (100 acres) to the 120 -acre point. The designer constructs a HEC -HMS model of the 80 -acre area using single existing condition sub -watersheds for each tributary. For the site area, watershed conditions will be modeled to the level of detail used for design. Detention structures existing in other tributaries must be modeled. Since flooding is an issue downstream, the pond design is iterated with subsequent re -checks until the peak flow does not increase at junction points downstream to the 80 -acre point. Site B is located downstream at the point where the total drainage area is 190 acres. The site itself is only 6 acres. The first tributary junction downstream from the 10% point is the junction of the site outlet with the stream. The total 190 acres is modeled as one basin (with the site at the same level of detail as for design) with care taken to estimate the time of concentration for input into the model of the watershed. The model shows that the detention facility, as designed, will actually increase the peak flow in the stream. The detention facility design is subsequently modified to eliminate the increase in peak flow. SECTION 7.6. STORMWATER DETENTION ANALYSIS SOFTWARE The city encourages the use of software to model the stormwater management system. Refer to Appendix H for additional information regarding appropriate software for stormwater management analysis in Fayetteville. SECTION 7.7. REFERENCES Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 1: Chapter 6, Floodplain Management. Chapter 7 - Stormwater Detention You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) CHAPTER 8. CONSTRUCTION SITE STORMWATER MANAGEMENT SECTION 8.1. PERMITS AND PLANS 8.1.1 Stormwater Pollution Prevention Plan The site owner bears responsibility for implementation and preparation of the SWPPP and notification of all contractors and utility agencies on the site. The Arkansas Department of Environmental Quality (ADEQ) requires that all construction activities disturbing one acre or more shall have a SWPPP to support issuance of a construction stormwater general permit. Sites with disturbance of more than one acre and less than 5 acres have automatic coverage. • For the small construction sites (1 to 5 acres), no submittal of individual permit documents is required and there is no fee; however the SWPPP and the automatic Notice of Coverage (NOC) shall be posted at the site prior to commencing construction. • For larger construction sites (disturbance of 5 acres or more), SWPPP documents and a Notice of Intent (NOI) must be submitted to ADEQ with fee for review and approval. For more information on specific requirements, please visit the ADEQ Construction Stormwater Program website at: httR//www.adeq.state.ar.us/water/branch permits /general permits/stormwater/construction/construction.htm 8.1.2 Grading and Drainage Permits A grading permit will not be issued until the perimeter sediment controls and permit box have been installed and approved by the City Engineering Division, and a preconstruction conference has been held. 8.1.3 Phased Construction The area of disturbance onsite at any one time shall be limited to 20 acres. An additional 20 acres (a maximum of 40 acres of disturbance at any one time) may be stripped with the permission of the City Engineer in order to balance cut and fill onsite. No additional area may be open without the permission of the City Engineer until the previously disturbed areas have been temporarily or permanently stabilized. 8.1.4 Installation and Maintenance. 8.1.4.1 Stormwater Pollution Prevention Plans. Preparation and implementation of Stormwater Pollution Prevention Plans for construction activity shall comply with the following: BMPs shall be installed and maintained by qualified persons in accordance with applicable City of Fayetteville and State requirements. The owner or their representative shall maintain a copy of the SWPPP on site and shall be prepared to respond to unforeseen maintenance requirements of specific BMPs. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 8.1.5 Qualified Inspector A qualified inspector (provided by the owner/developer/builder) shall inspect disturbed areas of the construction site and areas used for storage of materials that are exposed to precipitation that have been finally stabilized, and locations where vehicles enter or exit the site. BMPs must be observed to ensure proper operation. Inspectors must inspect for evidence of, or the potential for, pollutants entering the stormwater conveyance system. Discharge locations must be inspected to determine whether BMPs are effective in preventing significant impacts to waters of the State, where accessible. Where discharge locations are inaccessible, nearby downstream locations must be inspected to the extent that such inspections are practicable. The inspections must be conducted at least once every seven (7) calendar days or at least once every 14 calendar days and within 24 hours of the end of a storm that is 0.5 inches or greater as measured at the site or generally reported in the vicinity of the site. A rain gauge must be maintained on- site. A report shall be prepared for each inspection summarizing the scope of the inspection; name(s), title(s) and qualifications of personnel making the inspection; the date of the inspection; amount of rainfall and days since last rain event, BMPs on-site; observations relating to whether BMPs are in working order and whether maintenance is required (when scheduled and completed); the locations and dates when major construction activities begin, occur, or cease; and the signature of the inspector. The reports shall be retained as part of the stormwater pollution prevention plan for at least three (3) years from the date the site is finally stabilized and shall be made available upon request to the City. 8.1.6 Modifications Changes to the SWPPP are often necessary and may be made during the construction phase. Based on inspections performed by the owner or by authorized City personnel, modifications to the SWPPP will be necessary if at any time the specified BMPs do not meet the objectives of the City of Fayetteville UDC 170.10, Stormwater Discharges From Construction Activities. In this case, the owner/developer/builder or authorized representative shall meet with authorized City personnel to determine the appropriate modifications. All modifications shall be completed within seven (7) days of the referenced inspection, except in circumstances necessitating more timely attention, and shall be recorded on the owner's copy of the SWPPP. 8.1.7 Stabilization A record of the dates when grading activities occur, when construction activities temporarily or permanently cease on a portion of the site except as provided within bulleted text below, and when stabilization measures are initiated shall be included in the erosion and sediment control plan. Stabilization measures shall be initiated as soon as practicable in portions of the site where construction activities have temporarily or permanently ceased, but in no case more than 14 days after the construction activity in that portion of the site has temporarily or permanently ceased. • Where the initiation of stabilization measures by the 14th day after construction activity temporarily or permanently ceases is precluded by snow cover, stabilization measures shall be initiated as soon as practicable. • Where construction activity will resume on a portion of the site within 21 days from when activities ceased, (e.g., the total time period that construction activity is temporarily ceased is less than �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 21 days) then stabilization measures do not have to be initiated on that portion of the site by the 14th day after construction activity temporarily ceased. Stabilization practices may include: temporary seeding, permanent seeding, mulching, geotextiles, sod stabilization, vegetative buffer strips, protection of trees, and preservation of mature vegetation and other appropriate measures. See Chapter 167 of the UDC for tree protection requirements. SECTION 8.2. EROSION, RUNOFF, AND SEDIMENT CONTROLS FOR CONSTRUCTION SITES 8.2.1 Erosion Control Control of erosion during construction requires an examination of the entire site to identify potential problem areas such as steep slopes, highly erodible soils, soil areas that could be unprotected for long periods or during peak rainy seasons, and natural drainageways. Assure erosion control in these critical areas. After a rain, the effectiveness of erosion control measures must be re-evaluated. Maintenance and cleaning of these facilities is also important. EPA NPDES Fact Sheets for the following twelve Erosion Control BMPs are provided under the Erosion Control section of Appendix J: Compost Blankets Gradient Terraces Seeding Soil Roughening Dust Control Mulching Sodding Temporary Slope Drain Geotextiles Riprap Soil Retention Temporary Stream Crossings Additional items with respect to City erosion control requirements are provided herein: • Existing and Natural vegetation. Every means shall be taken to conserve and protect existing vegetation. The potential for soil loss shall be minimized by retaining natural vegetation wherever possible. Development in the Hillside/Hilltop Overlay District should comply with the recommendations of the Hillside/Hilltop Best Management Practices Manual with regard to the retention of natural vegetation on Hillside/Hilltops. Establishing New Vegetation. Vegetation practices may be either temporary or permanent and, at a minimum, should comply with City of Fayetteville UDC Chapter 169.04, Minimal Erosion Control Requirements. They may be applied singularly or in combination with other practices. Cutting, filling, and grading soils with heavy equipment results in areas of exposed subsoils or mixtures of soil horizons. Conditions such as acidity, low fertility, compaction, and dryness or wetness often prevail and are unfavorable to plant growth and should be accounted for in the selection of plantings is required as specified for each BMP. Long slopes and steep grades shall not be created. Stormwater drainage structures where such conditions already exist are normally subjected to hydraulic forces requiring both special establishment techniques and grasses that have high resistance to scouring. Vegetation practices and structural techniques are available to provide both temporary and permanent protective cover on these difficult sites, where encountered. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Temporary Vegetation: Earth moving activities such as heavy cutting, filling, and grading are generally performed in several stages and are often interrupted by lengthy periods, during which the land lies idle and is subject to accelerated erosion especially during rainfall events. In addition, final land grading may be completed during a season not favorable for immediate establishment of permanent vegetation. In such conditions, rapid growing annual grasses shall be used to rapidly establish protective cover. This can later be worked into the soil for use as mulch when the site is prepared for establishment of permanent vegetation. • Permanent Vegetation: Final selection should be based on adaptation of the plants to the soils and climate, suitability for their specific use, ease of establishment, longevity or ability to reseed, maintenance requirements, aesthetics, and other special qualities. Additional information regarding plantings suitable for use in the area is provided in Appendix D. Maintenance must be the most important consideration in selecting plants for permanent stabilization. Plants that provide long-lived stabilization with the minimum amount of required maintenance should be selected. Where management potential is limited because of specialized circumstances, the best plants to choose are those that are well adapted to the site and to the specific purpose for which they are to be used. For example, grasses used for waterway stabilization must be able to withstand submergence and provide a dense cover to prevent scouring of the channel boundary. In playgrounds, grasses must lend themselves to close grooming and be able to withstand heavy trampling. In some places, such as southern -exposed cut -and -fill slopes, the plants must be adapted to full sunlight and drought conditions. In other places, plants must be able to tolerate shade or high moisture conditions. Some plants can be used for beautification as well as for soil stabilization. • Dust Control. Saturate ground or apply dust suppressors. Keeping dust down to tolerable limits on the construction site and haul roads is very important. • Flexible Down Drain. This is a temporary structure used to convey stormwater from one elevation to another without causing erosion. It is made of heavy-duty fabric or other material that can be removed when the permanent water disposal system is installed. Mulching: When final grading has not been completed, straw, wood chips, jute matting, or similar materials can be applied to provide temporary protection. Areas brought to final grade during midsummer or winter can be mulched immediately and overseeded at the proper season with a number of permanent grasses or legume species. Application of mulch to disturbed areas allows for more infiltration of water into the soil, reduces runoff, holds seed, fertilizer, and lime in place, retains soil moisture; helps maintain temperatures conducive to germination, and greatly retards erosion. Mulch is essential in establishing good stands of grasses and legumes in disturbed areas. It is important to stabilize or anchor mulch using such practices as an anchoring tool, biodegradable tackifier (hydromulch), netting, peg and twine, or slitting to prevent it from blowing or washing off the site. Use of mulch in combination with Green Stormwater Practices shall comply with the requirements established in Chapter 5 and Appendix D. • Riprap. This is a layer of loose rock placed over the soil surface to prevent erosion by surface flow or wave action. Riprap may be used, as appropriate, at storm drain outlets, channel bank and bottom protection, roadside ditch protection, drop structures, etc. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Storm Drain Outlet Protection. This practice involves putting paving or riprap on channel sections immediately below storm drain outlets. A storm drain outlet is designed to reduce the velocity of flow and prevent downstream channel erosion. It is also known as an energy dissipater. Temporary Storage, Shop and Staging Areas. Locate storage and shop yards where erosion and sediment hazards are slight. If this is not feasible, apply necessary paving and erosion control practices. 8.2.2 Runoff Control Protect streams from chemicals, fuels, lubricants, sewage, or other pollutants. Do not place or dispose of fill in floodplains or drainageways. Use temporary bridges with culverts to ford streams. Avoid developing borrow areas where pollution from this operation cannot be controlled. EPA NPDES Fact Sheets for the following four Runoff Control BMPs are provided under the Runoff Control section of Appendix J: Check Dams Grass -Lined Channels Permanent Slope Diversions Temporary Diverson Dikes Additional items with respect to City runoff control requirements are provided herein: Erosion Control for Open Channels. In designing channels for erosion control, the velocity for the 10 -year event shall be computed and compared to the erosive potential of the channel material, in accordance with procedures and allowable velocities provided in Chapter 6, Section 6.4. Where the allowable velocity for a turf channel or earthen channel will be exceeded, alternatives include: lining the channel with impervious material, using drop structures or other velocity and erosion control measures; placing gravel or riprap bottoms with riprap side slopes; and gabions (rock enclosed in wire baskets) especially for steeper slope applications. The open channel and swale design should be evaluated for the extreme flood runoff with respect to flow velocities and erosion potential. Antecedent flow conditions resulting from previous storms are an important consideration. Open channels and swales may suffer damage during major storms, even if properly designed. Such damage shall be repaired promptly to prevent further erosion. It is important that open channels be constructed in accordance with design plans. When side slopes of intermittent channels are sodded to the depth of the expected flow, they can immediately provide erosion control for runoff from minor storms. It is not practical to establish turf in a drainage channel by seeding and mulching unless jute mats, or other similar erosion control matting materials, are placed over the seedbed. Diversion Dike. This is a compacted earthen ridge constructed immediately above a cut or fill slope. Its purpose is to intercept storm runoff from upstream soil drainage areas and divert the water away from the exposed stabilized outlet. Perimeter Dike. This is a compacted earthen dike constructed along the perimeter of a disturbed area to divert sediment -laden stormwater to onsite trapping facilities. It is maintained until the disturbed area is permanently stabilized. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Interceptor Dike. This is a temporary ridge of compacted soil or, preferably, gravel constructed across disturbed rights-of-way. An interceptor dike reduces erosion by intercepting stormwater .and diverting it to stabilized outlets. • Level Spreader. This is a temporary structure that is constructed at zero grade across the slope where concentrated runoff may be intercepted and diverted onto a stabilized outlet. The concentrated flow or stormwater is converted to sheet flow at the outlet. Diversions. These are designed, graded channels with a supporting ridge on the lower side constructed across the slope. Their purpose is to intercept surface water. Diversion structures may be temporary or permanent and graded or level. They are useful above cut slopes, borrow areas, gully heads, and similar areas. They can be constructed across cut slopes to reduce slope plains into nonerosive segments and can be used to move runoff water away from critical construction sites. They may be used at the base of cut or fill slopes to carry sediment -laden flow to traps or basins. Diversions should be located so that the water will empty into established runoff areas, natural outlets, or prepared individual outlets. Individual outlets can be designed as grass or paved waterways, chutes, or buried pipes. 8.2.3 Sediment Control EPA NPDES Fact Sheets for the following eleven Sediment Control BMPs are provided under the Sediment Control Section of Appendix J: Compost Filter Berms Fiber Rolls Sediment Filters and Chambers Silt Fences Compost Filter Socks Filter Berms Storm Drain Inlet Protection Construction Entrance/Exits Sediment Basins and Rock Dams Sediment Traps Vegetated Buffers Additional items with respect to City sediment control requirements are provided herein: Control and prevention of soil erosion during and after construction is the most important element of siltation and sediment control. However, it is physically and economically impractical to entirely eliminate soil erosion. Therefore, provisions must be made to trap eroded material at specified points. Some measures to implement are as follows: • As inlet protection and on long slopes or runs, silt fence, straw wattles, or rock check dams shall be used to create temporary ponds that store runoff and allow suspended solids to settle. These temporary ponds may be retained as part of the permanent storage system after construction; however, they must be inspected / surveyed to ensure that the design capacity of the system was not compromised by siltation. • Inlet protection shall be maintained throughout construction and shall not be removed until vegetation is established. Such measures shall be periodically inspected in accordance with the requirement of the SWPPP and repaired / replaced when no longer functioning in accordance with design. Silted -in areas shall be mucked out after significant rainfall to restore capacity. • Egress points from construction sites should be controlled so that the sediment is not carried offsite by construction traffic. A temporary construction entrance shall be constructed at points where traffic will be entering from or leaving a construction site to public right-of-way, street, alley, �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) sidewalk, or parking area. Its purpose is to reduce or eliminate the transport of mud from the construction area onto the public right-of-way by motor vehicles or by runoff. An additional track -out area should be established where appropriate if traffic from heavy equipment is limited to areas not typically disturbed by passenger vehicles entering / leaving the construction site. Construction exits shall comply with City of Fayetteville UDC 170.10, Stormwater Discharges From Construction Activities, and corresponding erosion control at such exits and on public streets shall comply with City of Fayetteville UDC 169.04, Minimal Erosion Control Requirements. • Construction Entrance/Exits. A stabilized rock exit is required on construction sites. Rock exits must be at least 20 feet wide by 20 feet long (1 & 2 family residential) or 50 feet long (all other construction sites) by 6 -inch thick stabilized rock having a minimum average diameter of 3 inches. If there is an existing curb, loose material such as fill dirt or gravel shall not be used to ramp up to it from the street. Temporary wooden ramps in front of curbs are acceptable. • Sediment Basins and Rock Check Dams. A rock check dam is an auxiliary structure installed in combination with and as a part of a diversion, interceptor, or perimeter dike, or other structures designed to temporarily detain sediment -laden stormwater. The rock check dam provides a means of draining off and filtering the stormwater while retaining the sediment behind the structure. Sediment basins can be used to trap runoff waters and sediment from disturbed areas. The water is temporarily detained to allow sediment to drop out and be retained in the basin while the water is automatically released. Sediment basins usually consist of a dam or embankment, a pipe outlet, and an emergency spillway. They are usually situated in natural drainageways or at the low corner of the site. In situations where embankments may not be feasible, a basin excavated below the earth's surface may serve the same purpose. A special provision, however, must be made for draining such an impoundment. Sediment basins may be temporary or permanent. Temporary basins serve only during the construction stage and are eliminated when vegetation is established and the area is stabilized. Permanent structures are designed to fit into the overall plan for the permanent installation. Design shall conform to the requirements within this manual. State and local safety regulations must be observed regarding design, warning signs, and fencing of these structures. • Sediment Traps. A sediment trap is a structure of limited capacity designed to create a temporary pond around storm drain inlets or at points where silt -laden stormwater is discharged. It is used to trap sediment on construction sites, prevent storm drains from being blocked, and prevent sediment pollution of watercourses. • Silt Fences. This is a temporary barrier constructed across or at the toe of the slope. Its purpose is to intercept and detain sediment from areas one-half acre or smaller where only sheet erosion may be a problem. • Dewatering. All rainwater pumped out of sumps and depressions on construction sites should be clear and free of sediment, and must discharge to a sedimentation pond, sediment bag, or settling tank in such a manner as to not cause additional erosion problems. • Water Control Subsurface drains used to remove excess groundwater are sometimes required at the base of fill slopes or around building foundations. When heavy grading is done and natural water channels are filled, the subsurface drains may be used to prevent accumulation of groundwater. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Subsurface drains may be needed in vegetated channels to lower a high water table and to improve drainage conditions so vegetation can be established and maintained. 8.2.4 Good Housekeeping EPA NPDES Fact Sheets for the following Good Housekeeping BMPs are provided under the Good Housekeeping section of Appendix J: Concrete Washout Spill Prevention and Control Additional items with respect to City good housekeeping practices and requirements are provided herein: • Storage of Materials. Public streets and sidewalks shall not be used for temporary storage of any containers or construction materials, especially loose gravel and topsoil. In addition to on -street storage being a violation, all liability for any accidents and/or damages due to such storage will be the responsibility of the owner of the stored materials. • Excavation Material. Excavation material shall not be deposited in or so near streams and other stormwater drainage systems where it may be washed downstream by high water or runoff. All excavation material shall be stabilized immediately with erosion control measures. Debris, Mud, and Soil. Debris, mud and soil shall not be allowed on public streets but if any debris, mud, or soil from development sites reaches the public street it shall be immediately removed via sweeping or other methods of physical removal. Debris, mud, or soil in the street may not be washed off the street or washed into the storm drainage system. Storm drainage systems downstream of a development site should be protected from debris, mud, or soil in the event that debris, mud, or soil reaches the drainage system. Dirt and Top Soil Storage. Top soil shall be stockpiled and protected for later use on areas requiring landscaping. All storage piles of soil, dirt or other building materials (e.g. sand) shall be located more than 25 feet from a roadway, drainage channel or stream (from top of bank), wetland, and stormwater facility. The City Engineer may also require top soil stockpiles to be located up to 50 feet from a drainage channel or stream, as measured from the top of the bank to the stockpile, for established TMDL water bodies; streams listed on the State 303(d) list; an Extraordinary Resource Water, Ecologically Sensitive Waterbody, and/or Natural and Scenic Waterbody, as defined by Arkansas Pollution Control and Ecology Commission Regulation No. 2; and/or any other uses at the discretion of the City Engineer. Topsoil piles surfaces must be immediately stabilized with appropriate stabilization measures. Stabilization practices may include: temporary seeding (i.e. annual rye or other suitable grass), mulching, and other appropriate measures. Sediment control measures such as silt fence shall be provided immediately for stockpiles and remain in place until other stabilization is in place. Storm drain inlets must be protected from potential sedimentation from storage piles by silt fence or other appropriate barriers. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) Concrete Washout. No washing of concrete trucks or chutes is allowed except in specific concrete wash pits located onsite. Proper runoff and erosion controls must be in place to retain all concrete wash water. • Spill Prevention and Control. SECTION 8.3. UNDERGROUND UTILITY CONSTRUCTION - PLANNING AND IMPLEMENTATION The property owner or main contractor onsite will be responsible for restoring all erosion and sediment control systems and public infrastructure damaged or disturbed by underground private or franchise utility construction such as water and sewer service leads, telephone, gas, cable, etc. Erosion and sediment control systems must be immediately restored after each utility construction. Utility agencies shall develop and implement Best Management Practices (BMPs) to prevent the discharge of pollutants and release of sediments from utility construction sites. Disturbed areas shall be minimized, disturbed soil shall be managed and construction site entrances shall be managed to prevent sediment tracking. Excessive sediment tracked onto public streets shall be removed immediately. Prior to entering a construction site or subdivision development, utility agencies shall have obtained from the owner or developer a copy of any SWPPPs for the project. Any disturbance to BMPs resulting from utility construction shall be repaired in compliance with the SWPPP. The property owner or main contractor is responsible for restoration of any damage by private or franchise utility construction, in accordance with City of Fayetteville Ordinance UDC 170.10 (A)(10). SECTION 8.4. POST -CONSTRUCTION SITE STABILIZATION STANDARDS Revegetation of disturbed areas shall be performed as soon after the completion of construction activities as is practicable. The area of disturbance at any one time shall be limited to 20 acres. No additional area may be open without the permission of the City Engineer until the previously disturbed areas have been temporarily or permanently stabilized. Revegetation shall be required to meet the following performance standards prior to issuance of the Final Plat or Certificate of Occupancy: 1. Topsoil: A minimum of 4 inches of topsoil shall be required in areas to be revegetated. Any application of topsoil and seeding under the drip line of a tree should be minimized to 3 inches so as not to damage the root system of the tree. 2. Zero to 10% grade: Revegetation shall be a minimum of seeding and mulching. Said seeding shall provide complete and uniform coverage that minimizes erosion and runoff in no more than two growing seasons. 3. 10:1 up to 4:1 grade: Revegetation shall be a minimum of hydroseeding with mulch and fertilizer, or staked sod, or groundcover. Said planting shall provide complete and uniform coverage that minimizes erosion and runoff in no more than two growing seasons. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) 4. 4:1 to 3:1 grade: The slope shall be covered with landscape fabric and hydro -seeded with mulch and fertilizer or staked sod groundcover. Said planting shall provide complete and uniform coverage in no more than two growing seasons. 5. Steeper than 3:1 grade: The slope shall be stabilized with one or more of the following: a. Retaining walls, b. Cribbing with landscaping fabric, c. Terracing with groundcover, d. Riprap, e. Staked Sod (up to 2:1 slope), or f. If cribbing, terracing, or rip rap are used, the slope stability and erodibility characteristics must be equivalent to or better than its predevelopment state. 6. Hillside/Hilltop Overlay District: Revegetation of lands within the Hillside/ Hilltop Overlay District shall be planted immediately after the physical alteration of the land with complete and uniform ground cover. Sod, erosion fabric, herbaceous groundcover (in wooded areas), and/or a hydroseed with warm season grasses is required. Revegetation requirements shall be met prior to the issuance of the certificate of occupancy. Cut and Fill tie -back slopes shall be re -vegetated with appropriate tree species to achieve a minimum of 25% tree canopy at maturity. Land shall be revegetated and restored as close as practically possible to its original conditions so far as to minimize runoff and erosion are concerned. Previously forested areas shall follow the City's Landscape Manual for mitigation of forested areas. 1. Flexible Down Drain: This is a temporary structure used to convey stormwater from one elevation to another without causing erosion. It is made of heavy-duty fabric or other material that can be removed when the permanent water disposal system is installed. Water Control: 1. Subsurface Drains: Subsurface drains used to remove excess groundwater are sometimes required at the base of fill slopes or around building foundations. When heavy grading is done and natural water channels are filled, the subsurface drains may be used to prevent accumulation of groundwater. Subsurface drains may be needed in vegetated channels to lower a high water table and to improve drainage conditions so vegetation can be established and maintained. SECTION 8.5. REFERENCES Arkansas Department of Environmental Quality (ADEQ), 2012. Stormwater Pollution Prevention Plan. Arkansas Department of Environmental Quality (ADEQ), 2011. Construction Stormwater General Permit. City of Fayetteville, AR, 1995. Drainage Criteria Manual. City of Fayetteville, AR, various dates. City Ordinances UDC 169, 170. �t— ,l 8. Construction Site Stormwater Management You created this PDF from an application that is not licensed to print to novaPDF printer (http://www.novapdf.com) APPENDIX A INTRINSIC GSP SPECIFICATIONS Appendix -Intrinsic GSP Specifications Drainage Criteria Manual i MINIMIZE SOIL COMPACTION Description and Function: Minimizing soil compaction relates directly to reducing total site disturbance, site clearing, site earthwork, the need for soil restoration, and the size and extent of costly, engineered stormwater management systems. Maintaining native soils can significantly reduce the cost of landscaping vegetation (higher survival rate, less replanting) and landscaping maintenance. Fencing off an area pre -construction can help minimize unnecessary soil compaction. Soil is a physical matrix of weathered rock particles and organic matter that supports a complex biological community. This matrix has developed over a long time period and varies greatly within the northwest Arkansas region. Healthy soils which have not been compacted due to human activities perform numerous valuable stormwater functions, including: • Effectively cycling nutrients, • Minimizing runoff and erosion, • Maximizing water -holding capacity, • Reducing storm runoff surges, • Absorbing and filtering excess nutrients, sediments, and pollutants to protect surface and groundwater, • Providing a healthy root environment, • Creating habitat for microbes, plants, and animals, and • Reducing the resources needed to care for turf and landscape plantings. Undisturbed soil consists of pores that have water -carrying and holding capacity. When soils are overly compacted, the soil pore spaces and permeability can be drastically reduced. In fact, the runoff response of vegetated areas with highly compacted soils closely resembles that of impervious areas, especially during large storm events (Schueler, 2000). Applications Minimizing soil compaction can be performed at any land development site during the design phase. It is especially suited for developments where significant "pervious" areas (i.e., post -development lawns and other maintained landscapes) are being proposed. If existing soils have already been excessively compacted, soil amendment may be used (see Appendix C for soil amendment information). Design Considerations Early in a project's design phase, the designer should develop a soil management plan based on soil types and existing level of disturbance (if any), how runoff will flow off existing and proposed impervious areas, AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i trees and natural vegetation that can be preserved, and tests indicating soil depth and quality. The soil management plan should clearly show the following: 1. No disturbance areas. Soil and vegetation disturbance is not allowed in designated no disturbance areas. Protecting healthy, natural soils is the most effective strategy for preserving soil functions. Not only can the functions be maintained, but protected soil organisms are also available to colonize neighboring disturbed areas after construction. 2. Minimal disturbance areas. Limited construction disturbance occurs. These areas may allow some clearing, but no grading should be performed in these areas. If any clearing occurs, the area should be immediately stabilized, revegetated, and protected from construction traffic and related activity. Minimal disturbance areas do not include construction traffic areas. 3. Construction traffic areas. Construction traffic is allowed in these areas. If these areas are to be considered fully pervious following development, a soil restoration program will be required. 4. Topsoil stockpiling and storage areas. If these areas are needed, they should be protected and maintained. S. Topsoil quality and placement. Soil tests are necessary to determine if proposed topsoil meets minimum parameters. Critical parameters include: adequate depth (four inches minimum for turf, more for other vegetation), organic content minimum of 5%, and compaction that does not exceed that for native or in-place soils in adjacent undisturbed areas (Hanks and Lewandowski, 2003). To allow water to pass from one layer to the other, scarify then till the topsoil/subsoil contact consistent with Construction Guideline #4 to allow bonding when topsoil is reapplied to disturbed areas. Construction Guidelines 1. At the start of construction, no disturbance and minimal disturbance areas must be identified with signage and fenced as shown on the construction drawings. 2. No disturbance and minimal disturbance areas should be strictly enforced. 3. No disturbance and minimal disturbance areas should be protected from excessive sediment and stormwater loads while adjacent areas remain in a disturbed state. 4. Topsoil stockpiling and storage areas should be maintained and protected at all times. When topsoil is reapplied to disturbed areas it should be "bonded" with the subsoil. This can be done by spreading a thin layer of topsoil (2-3 inches), tilling it into the subsoil, and then applying the remaining topsoil. Topsoil should meet City of Fayetteville specifications/requirements. Stormwater Functions and Calculations Volume and peak rate reduction: Minimizing soil compaction can reduce the volume of runoff by maintaining soil functions related to stormwater infiltration and evapotranspiration. Designers that use this intrinsic GSP should apply the relevant volumetric runoff coefficients (Rv) listed in Section 3 of Chapter 5 for the protected area to calculate the Rv for the contributing drainage area and determine if additional stormwater controls should be applied to meet the stormwater management goals (i.e. 80% reduction). The following guidance shall be following when applying Rv values for this Intrinsic GSP. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i • No Disturbance Areas: No disturbance areas with post -development uses of forest or undisturbed open space should use Rv values for "forest cover" for the corresponding soil type. • Minimal Disturbance Areas: Minimal disturbance areas with post -development land use of landscaped area and lawn should use the Rv values for "disturbed soils", due to the potential for compaction of the soils. • Construction Traffic Areas: Non -impervious areas that were used as construction traffic areas should use the "disturbed soils" Rv value of 0.23 for Hydrologic Soil Group (HSG) Type D unless they have been amended. If soil has been amended in accordance with Appendix C, the Rv values for the respective soil type may be used. • Topsoil stockpiling and storage areas: Non impervious areas that were used as topsoil stockpiling and storage areas should use the "disturbed soils" Rv value of 0.23 for HSG Type D unless they have been amended. If soil has been amended in accordance with Appendix C, the Rv values for the area's respective soil type may be used. • Water quality improvement: Minimizing soil compaction improves water quality through infiltration, filtration, chemical and biological processes in the soil and a reduced need for fertilizers and pesticides after development. Maintenance Sites with minimal soil compaction during the development process will require considerably less maintenance than sites with more compaction. Landscape vegetation, either retained or re -planted, will likely be healthier, have a higher survival rate, require less irrigation and fertilizer, and have better aesthetics. Some maintenance activities such as frequent lawn mowing can cause considerable soil compaction after construction and should be kept to a minimum. Planting low -maintenance native vegetation (see vegetation list in Appendix D) is the best way to avoid damage due to maintenance. Areas designated as 'no disturbance areas' on private property should have an easement, deed restriction, or other legal measure imposed to prevent future disturbance or neglect. Cost Minimizing soil compaction generally results in significant construction cost savings. Design costs may increase due to a more time intensive design. Appendix - Intrinsic GSP Specifications Drainage Criteria Manual i REFERENCES: Hanks, D. and Lewandowski, A. Protecting Urban Soil Quality: Examples for Landscape Codes and Specifications. USDA-NRCS, 2003. Ocean County Soil Conservation District. Impact of Soil Disturbance during Construction on Bulk Density and Infiltration in Ocean County, New Jersey. 2001. www.ocscd.org/publications.shtml Schueler, T. "The Compaction of Urban Soils," Watershed Protection Techniques. Technical Note #107, 3(2): 661-665, January 2000. Southeast Michigan Council of Governments. (SEMCOG) 2008 Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. Southeast Michigan Council of Governments, Detroit, MI. Appendix -Intrinsic GSP Specifications Drainage Criteria Manual i MINIMIZE TOTAL DISTURBED AREA A key component of LID is to reduce the impacts during development activities such as site grading, removal of existing vegetation, and soil mantle disturbance. This can be achieved through developing a plan to contain disturbed areas. Description and Function Disturbance at a development site can occur through normal construction practices, such as grading, cutting, or filling. Minimizing the total disturbed area of the site requires the consideration of multiple Intrinsic GSPs, such as cluster development and identifying and protecting sensitive areas. These GSPs serve to protect area resources by reducing site grading and maintenance required for long-term operation of the site. Minimizing the total disturbed area of a site specifically focuses on how to minimize the grading and overall site disturbance, maximizing conservation of existing native plant communities and the existing soil mantle of a site. If invasive plant species are present in the existing vegetation, proper management of these areas may be required to maximize runoff reduction and evapotranspirative capacity. Minimize grading: Reduction in grading can be accomplished in several ways, including conforming site design to existing topography and land surface, and aligning roads to follow existing contours as much as possible. Minimize overall site disturbance: Site design criteria have evolved in municipalities to ensure that developments meet safety standards (i.e. sight distance and winter icing) as well as certain quality or appearance standards. Roadway design criteria should be flexible in order to optimize the fit for a given parcel and achieve optimal roadway alignment. The avoidance of environmentally sensitive resources, such as important woodlands, may be facilitated through flexible roadway layout. Applications Minimizing the total disturbed area of a site is best applied in lower density single-family developments, but can also be applied in residential developments of all types including commercial, office park, retail center, and institutional developments. Larger industrial park developments can also benefit from this GSP. However, as site size decreases and density and intensity of development increases, this GSP is uniformly more difficult to apply successfully. At some larger sites where Ultra Urban, Retrofit, or Highway/Road development is occurring, limited application may be feasible. Design Considerations During the initial conceptual design phase of a land development project, the following information should be provided; ideally through development of a Minimum Disturbance/ Minimum Maintenance Plan: 1. Identify and Avoid Special Value/Sensitive Areas: Delineate and avoid environmentally sensitive resources using existing data from appropriate agencies and based on field reconnaissance. 2. Minimize Disturbance at Site: Modify road alignments (grades, curvatures, etc.), lots, and building locations to minimize grading, and earthwork as necessary to maintain safety standards and AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i municipal code requirements. Minimal disturbance design should allow the layout to best fit the land form without significant earthwork, such as locating development in areas of the site that has been previously cleared, if possible. If cut/fill is required, the use of retaining walls is preferable to earthwork. Limits of grading and disturbance should be designated on plan documentation submitted to the City for review/approval and should be physically designated at the site during construction via flagging, fencing, etc. In addition, utilizing natural drainage features generally results in less disturbance and requires less re -vegetation. 3. Minimize Disturbance at Lot: To decrease disturbance, grading should be limited to roadways and building footprints. Maintain maximum setbacks from structures, drives, and walks. Guidelines for limits of disturbance are given in the U.S. Green Building Council's Leadership in Energy & Environmental Design Reference Guide (Version: LEED 2009 November 2008). LEED's guidance is 40 ft beyond the building perimeter, 10 ft beyond surface walkways, patios, surface parking, and utilities less than 12 inches in diameter, 15 ft beyond primary roadway curbs and main utility branch trenches, and 25 ft beyond constructed areas with permeable surfaces. The City may alter these maximum setbacks at its discretion. Stormwater Functions and Calculations Volume and Peak Rate: Any portion of a site that can be maintained in its pre -development state by using this GSP will not contribute increased stormwater runoff. It may not be necessary to route these undisturbed areas through stormwater management control structures. If it is necessary to route them to stormwater control structures due to the site layout, the runoff should be routed through vegetated swales or low berms that direct flow to natural drainageways. Water quality improvement Water quality is benefited substantially by minimizing the disturbed area. Maintenance Minimizing site disturbance will reduce required site maintenance in both the short- and long-term. Areas of the site left as intact native plant communities do not typically require replacement with hard surfaces or additional vegetation to retain function. On the other hand, artificial surfaces such as pavement or turf grass require varying levels of maintenance throughout the life of a development. Higher levels of disturbance will also typically require significant maintenance of erosion control measures during the active development of a parcel, thus adding to short-term development costs. While intact natural areas may require small amounts of occasional maintenance (typically through invasive species control) to maintain function, levels of maintenance required for hard surfaces or turf grass will remain static or, in most cases, increase over time. Avoiding disturbance to natural areas benefits the short term developer and the long-term owner by minimizing time and money needed to maintain artificial surfaces. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Cost The reduced costs of minimized grading and earthwork should benefit the developer. Cost issues include both reduced grading and related earthwork as well as costs involved with site preparation, fine grading, and seeding. REFERENCES Arendt, Randall G, 2001. Growing Greener: Conservation by Design. Pennsylvania Department of Conservation and Natural Resources, Governor's Office of Local Government Services. Center for Watershed Protection, 1998, Better Site Design: A Handbook for Changing Development Rules in Your Community. Ellicott City, MD. Coffman, Larry. 2000, Low Impact Development Design Strategies: An Integrated Design Approach. EPA 841 B 00 0023. Prince George's County, MD: Department of Environmental Resources, Programs and Planning, 2000. Delaware Department of Natural Resources and Environment Control, 1997. Conservation Design for Stormwater Management: A Design Approach to Reduce Stormwater Impacts from Land Development. Dover, DE Metropolitan Washington Council of Governments, 1995, Site Planning for Urban Stream Protection. Washington, DC. Minnesota Pollution Control Agency, 2006, Minnesota Stormwater Manual Version 1.1., Minneapolis, MN. www.pca.state.mn.us/water/Stormwater/stormwater-manual.html Natural Resources Defense Council, June 2006, Rooftops to Rivers: A Policy Guide for Decision Makers on How to Use Green Infrastructure to Address Water Quality and Volume Reduction for Communities with Combined Sewer Overflow Issues. Washington, DC: www.nrdc.org/water/poIlution/rooftops/contents.asp Pennsylvania Department of Environmental Protection. 2006, Pennsylvania Stormwater Best Practices Manual. Harrisburg PA:, December 2006. Southeast Michigan Council of Governments. (SEMCOG), 2008, Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers, Detroit, MI. Tyne, R., 2000, Bridging the Gap: Developers Can See Green, Economic Benefits of Sustainable Site Design and Low- Impact Development, Land Development, Spring 2000, pp. 27-31. U.S. Environmental Protection Agency, 1993, Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Ocean Waters. Washington, DC: report. 840 B 92 002. Section 6217 (g) U.S. Environmental Protection Agency, 2007, Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices. PA 841-F-07-006. Washington, DC AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i PROTECT NATURAL FLOW PATHWAYS A main component of LID is to identify, protect, and use natural drainage features, such as swales, depressions, and watercourses to help protect water quality. By incorporating natural drainage features, designers can reduce the need for structural drainage elements. Description and Function Many natural undeveloped sites have identifiable drainage features such as swales, depressions, and watercourses which effectively manage the stormwater generated on the site. By identifying, protecting, and using these features, a development can minimize stormwater impacts. Naturally vegetated drainage features tend to slow runoff and thereby reduce peak discharges, improve water quality through filtration, and allow some infiltration and evapotranspiration to occur. Protecting natural drainage features can provide for significant open space and wildlife habitat, improve site aesthetics and property values, and reduce the generation of stormwater runoff itself. If protected and used properly, natural drainage features generally require very little maintenance and can function effectively for many years. Site designs should use and/or improve natural drainage pathways whenever possible to reduce or eliminate the need for stormwater pipe networks. This can reduce costs, maintenance burdens, and site disturbance related to pipe installation. Natural drainage pathways should be protected from significantly increased runoff volumes and rates due to development. The design should prevent the erosion and degradation of natural drainage pathways through the use of upstream volume and rate control BMPs, if necessary. Level spreaders, erosion control matting, revegetation, outlet stabilization, and check dams can also be used to protect natural drainage features. Variations Natural drainage features can be modified to increase efficacy through the design and construction process. Examples include constructing slight earthen berms around natural depressions or other features to create additional storage, installing check dams within drainage pathways to slow runoff and promote infiltration, and planting additional native vegetation within swales and depressions. Applications As density and overall land disturbance decreases, this GSP can be used with a greater variety of land uses and development types. It is best used in residential development, particularly lower density single-family residential development. Where municipal ordinances already require a certain percentage of the total development area to remain as undeveloped open space, this open space requirement can be overlain onto natural flow pathways/drainage features, as well as floodplains, wetlands, and related riparian areas. After minimizing runoff as much as possible, reduced runoff quantities can then be distributed into this natural flow pathway system, on a broadly distributed basis, lot by lot. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Other land uses such as commercial and industrial developments tend to be associated with higher density development. This results in higher impervious coverage and greater site disturbance, making protecting and conserving natural flow pathways/drainage areas more difficult. Applications for both retrofit and highway/road uses are limited. In terms of retrofitting, some developed sites may have elements of natural flow pathways/drainage features intact, although many presettlement site features may have been altered and/or eliminated. Lower density developments may offer limited retrofit potential. Similarly, highway/road projects are likely to be limited due to overall available area and significant drainage feature disturbance. Design Considerations 1. Identify natural drainage features: Identifying and mapping natural drainage features is generally done as part of a comprehensive site analysis. This process is an integral first step of site design. Subtle site features such as swales, drainage pathways, and natural depressions should be delineated in addition to more commonly mapped hydrologic elements such as wetlands, perennial, intermittent and ephemeral streams, and waterbodies. t C [.*`� •• $Itemk Source: Delaware Department of Natural Resources and Environmental Control - Conservation Design for Stormwater Management 2. Use natural drainage features to guide site design: Instead of imposing a two-dimensional paper design on a particular site, designers can use natural drainage features to steer the site layout. Drainage features define contiguous open space and other undisturbed areas as well as road alignment and building placement. The design should minimize disturbance to natural drainage features. Drainage features that are to be protected should be clearly shown on all construction plans. Methods for protection, such as signage and fencing, should also be noted on applicable plans. 3. Use native vegetation:Natural drainage pathways should be planted with native vegetative buffers and the features themselves should include native vegetation where applicable. If drainage features have been previously disturbed, they can be restored with native vegetation and buffers. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Stormwater Function and Calculations Volume reduction and Peak rate Protecting natural flow pathways can reduce the volume of runoff in several ways. Reducing disturbance and maintaining a natural cover reduces the volume of runoff through infiltration and evapotranspiration. Using natural flow pathways further reduces runoff volumes through allowing increased infiltration to occur, especially during smaller storm events. Encouraging infiltration in natural depressions also reduces stormwater volumes. Employing strategies that direct non-erosive sheet flow onto naturally vegetated areas also promotes infiltration - even in areas with less permeable soils (Hydrologic Soils Groups C and D). Water quality improvement Protecting natural flow pathways improves water quality through filtration, infiltration, sedimentation, and thermal mitigation. Maintenance Natural drainage features that are properly protected and used as part of site development should require very little maintenance. However, periodic inspections are important. Inspections should assess erosion, bank stability, sediment/debris accumulation, and vegetative conditions, including the presence of invasive species. Problems should be corrected in a timely manner. Cost Protecting natural flow pathways generally results in construction cost savings by reducing infrastructure. Protecting these features results in less disturbance, clearing, and earthwork and requires less revegetation. Using natural flow pathways reduces the need and size of more costly engineered stormwater conveyance systems. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i REFERENCES Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in your Community. Ellicott City, MD, Coffman, Larry. 2000. Low Impact Development Design Strategies: An Integrated Design Approach. EPA 841 B 00 0023. Department of Environmental Resources, Programs, and Planning, Prince George's County, MD. Delaware Department of Natural Resources and Environment Control. 1997. Conservation Design for Stormwater Management: A Design Approach to Reduce Stormwater Impacts from Land Development. Dover DE. Pennsylvania Department of Environmental Protection. 2006. Pennsylvania Stormwater Best Practices Manual. Harrisburg, PA. Southeast Michigan Council of Governments (SEMCOG). 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. Detroit, MI. U.S. Environmental Protection Agency. 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Ocean Waters. 840 B 92 002. Section 6217 (g) Report. Washington, DC. Washington State Department of Ecology. 1992. Stormwater Program Guidance Manual for the Puget Sound Basin. Olympia, WA. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i PROTECT RIPARIAN BUFFERS Description Riparian buffer areas are important elements of local communities' green infrastructure. These areas are critical to the biological, chemical, and physical integrity of our waterways. Riparian buffer areas protect water quality by cooling water, stabilizing banks, mitigating flow rates, and providing for pollution and sediment removal by filtering overland sheet runoff before it enters the water. The Environmental Protection Agency defines buffer areas as, "areas of planted or preserved vegetation between developed land and surface water, [which] are effective at reducing sediment and nutrient loads." The City of Fayetteville's Streamside Protection Best Management Practices Manual establishes the requirements for protecting riparian buffers. Applications As with the "protect sensitive areas" Intrinsic GSP, protecting riparian buffer areas has great value and utility for virtually all types of development proposals and land uses. This GSP works best on larger sites but can be used on any site. Although riparian buffer programs could be used in the densest of settings, their application is likely to be limited in high density contexts. Creative design can maximize the potential of riparian buffers. Clustering and density bonuses are design methods available to increase the amount and connectedness of open space areas such as riparian buffers. Design Considerations Physical design Consider the following when establishing additional riparian buffer area widths and related specifications: • Existing or potential value of the resource to be protected, • Site, watershed, and buffer characteristics, • Intensity of adjacent land use, and • Specific water quality and/or habitat functions desired. (Chesapeake Bay Riparian Handbook). Riparian buffers can be divided into different zones that include various vegetation targets to enhance the quality of the body of water. Zone 1: Also termed the "waterside zone," begins at the edge of the top of the stream bank of the active channel and extends a minimum distance of 25 ft, measured horizontally on a line perpendicular to the water body. The waterside zone should extend an additional 20 ft from the top of the bank for slopes that exceed 15%. Undisturbed vegetated area aims to protect the physical and ecological integrity of the stream ecosystem. The vegetative target for the streamside zone is undisturbed native woody species with native plants forming canopy, understory, and duff layer. Where such forest does not grow naturally, then native vegetative cover appropriate for the area (such as grasses, forbs, or shrubs) is the vegetative target. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Zone 2: Also termed the "management zone," extends immediately from the outer edge of Zone 1 for a minimum distance of 25 ft. This managed area of native vegetation protects key components of the stream ecosystem and provides distance between upland development and the streamside zone. The vegetative target for the middle zone is either undisturbed or managed native woody species or, in its absence, native vegetative cover of shrubs, grasses, or forbs. Undisturbed forest, as in Zone 1, is encouraged strongly to protect future water quality and the stream ecosystem. treamside Zones: Cross section THE WATERSIDE ZONE EXTENDS 25 FEET FROM TOP OF BANK (T.0.B.) WHEN THE SLOPE IS LESS THAN 15%. THE BOUNDARY MOVES BACK AN ADDITIONAL 20 FEET FROM THE T.O.B. FOR SLOPE THAT EXCEEDS 95%. DRAW NG NOT TO SCALE. OP ry d TA.A. T.O.A. co w uJ Z w Z WATERSIDE ZONE: 25' z > MANAGEMENT WATERSIDE (15% SLOPE OR LESS) � ° m �ID ZONE : 25' ZONE : 25' WATERSIDE ZONE: 45'(BOUNDARYM04ES BACK AN ADDITIONAL 20' FOR SLOPE EXCEEDING 15%) STREAM SI DE ZONE BOUNDARIES WATERSIDE ZONE BOUNDARIES WITH WITH 15% SLOPE OR LESS SLOPE GREATER THAN 95% Source: Streamside Protection Best Management Practices Manual Stormwater Functions and Calculations Volume reduction and Peak rate - Protecting riparian buffers can reduce the volume of runoff in several ways. Reducing disturbance adjacent to waterways and maintaining a natural cover reduces the volume of runoff through infiltration and evapotranspiration. Water quality improvement Water quality is benefited substantially by avoiding negative impacts which otherwise would have resulted from impacts to riparian buffers (e.g., loss of water quality functions from riparian buffers, from wetland reduction, etc.). AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Cost The costs of protecting riparian areas relate to a reduction in land available for development. However, most riparian areas are located in wetlands or floodplains, restricting the amount of buildable area. REFERENCES Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in your Community. Ellicott City, MD. Cwikiel, Wilfred. 2003. Michigan Wetlands - Yours to Protect: A Citizen's Guide to Wetland Protection (third edition). Petoskey, MI: Tip of the Mitt Watershed Council. Huron River Watershed Council. 2008. Riparian Buffer Model Ordinance. Keller, Chency, et al. 1993. Avian Communities in Riparian Forests of Different Widths in Maryland and Delaware, Wetlands, Volume 13: (2): 137-144. Meehan, William, Editor. 1991. Influences of Forest and Rangeland Management on Salmonid Fishes and their Habitats, Special Publication 19. American Fisheries Society. Bethesda, MD. Metropolitan Washington Council of Governments. 1995. Riparian Strategies for Urban Stream Protection. Washington, DC. Nutrient Subcommittee of the Chesapeake Bay Program. 1995. Water Quality Functions of Riparian Buffer Systems in the Chesapeake Bay Watershed. EPA 903-R-95-004. Oakland County Planning and Economic Development Services. 2007. Planning for Green River Buffers: A Resource Guide for Maximizing Community Assets Related to Rivers. Southeast Michigan Council of Governments. (SEMCOG) 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. Detroit, MI. U.S. Department of Agriculture, Forest Service, Northeastern Area, State and Private Forestry. 1998. Chesapeake Bay Riparian Handbook: A Guide for Establishing and Maintaining Riparian Forest Buffers. U.S. Department of Agriculture, Forest Service, Northeastern Area, State and Private Forestry. 1991. Riparian Forest Buffers: Function and Design for Protection and Enhancement of Water Resources. U.S. Department of Agriculture, Forest Service, Southern Region. 1992. Stream Habitat Improvement Handbook. Tech Pub R8-TP16. U.S. Environmental Protection Agency. 1992. Notes on Riparian and Forestry Management, USEPA Nonpoint Source News Notes. Washington DC. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i PROTECT SENSITIVE AREAS Protecting sensitive and special value features is the process of identifying and avoiding certain natural features during development. This allows these features to be used for various benefits, including reducing stormwater runoff. Protecting sensitive areas can be implemented both at the site level and throughout the community. For prioritization purposes, natural resources and their functions may be weighted according to their functional value. Sensitive areas should be preserved in their natural state to the greatest extent possible and are not the appropriate place to locate stormwater infrastructure. Description and Function Protecting sensitive areas challenges the site planner to inventory and then, to the greatest extent possible, avoid resource sensitive areas at a site, including riparian buffers, wetlands, hydric soils, floodplains, steep slopes, woodlands, valuable habitat zones, and other sensitive resource areas. Development, directed away from sensitive areas, can be held constant, if BMPs such as cluster development are also applied. A major objective of LID is to accommodate development with fewer impacts to the site. If development avoids encroachment upon, disturbance of, and impact to those natural resources which are especially sensitive to land development impacts and/or have special functional value, then low impact development may be achieved. The first step in protecting sensitive areas is for the site planner to define, inventory, and map which resources are especially sensitive and/or have special value at a site proposed for development. Many sensitive areas exist within the City of Fayetteville. The following is a partial list of potential sensitive area resources. • Lakes and Streams • Natural Rivers • Wetlands or Wetland Indicator Areas • Flood Prone Areas, Special Flood Hazard Areas • Parks and Recreation Areas • Historic Sites • Historic Bridges • Wet Prairie • Conservation Easements • Karst areas or Recharge Zones AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Potential Applications Regardless of land use type, protecting sensitive areas is applicable across all types of land development projects, whether residential of varying densities or office park, retail center or industrial and institutional uses. As density and intensity of uses increases, ease of application of this BMP decreases. Design Considerations In the future, the City may develop an inventory of sensitive areas. However, sensitive areas should be identified regardless of whether they have been inventoried by the City. Sensitive areas are subject to applicable state and federal regulations. Potential requirements but the City may consider in the future are listed below. • Conservation easement - Given to land conservancy or maintained by homeowners association. • Requirements in the master deed and bylaws for protection and preservation. • Boundary markers at edges of lots to minimize encroachment. • Cooperative agreements for stewardship of sensitive areas between homeowners' associations and local conservation organizations. Stormwater Functions and Calculations Volume reduction and Peak rate - Designers that use this intrinsic GSP can use natural predevelopment hydrologic conditions within protected areas thereby reducing site runoff. Water Quality Improvement Water quality is benefited substantially by avoiding negative impacts which otherwise would have resulted from impacts to sensitive areas (e.g., loss of water quality functions from riparian buffers, from wetland reduction, etc.). Construction Guidelines Although protecting sensitive areas happens early in the site plan process, it is equally important that the developer and builder protect these areas during construction. The following guidelines describe good planning practices that will help ensure protection of a few common environmentally sensitive resources during construction. Water Resources • If vegetation needs to be reestablished, plant native species, or use hydroseed and mulch blankets immediately after site disturbance. • Use bioengineering techniques, where possible, to stabilize stream banks. Appendix - Intrinsic GSP Specifications Drainage Criteria Manual i • Block or protect storm drains in areas where construction debris, sediment, or runoff could pollute waterways. • During and after construction activities, sweep the streets to reduce sediment from entering the storm drain system. • Avoid hosing down construction equipment at the site unless the water is contained and does not get into the stormwater conveyance system. • Implement spill control and clean-up practices for leaks and spills from fueling, oil, or use of hazardous materials. Use dry clean-up methods (e.g., absorbents) if possible. Never allow a spill to enter the stormwater conveyance system. • Avoid mobile fueling of equipment. If mobile fueling is necessary, keep a spill kit on the fueling truck. • Properly dispose of solid waste and trash to prevent it from ending up in our lakes and streams. • When protecting riparian buffer areas, consider the three buffer zones in protection criteria. Wetlands • Avoid impacts to wetlands whenever possible. If impractical, determine if a wetland permit is needed from the state or local government. (If any permit requirements or wetland regulations conflict with these guidelines, comply with the permit or regulation). • Excavate only what is absolutely necessary to meet engineering requirements. Do not put excavated material in the wetland. (Excavated material could be used in other areas of the site to improve seeding success). • If construction activities need to occur within a wetland, activities should be timed, whenever possible, when the ground is firm and dry. Avoid early spring and fish -spawning periods. • Install flagging or fencing around wetlands to prevent encroachment. • Travel in wetlands should be avoided. Access roads should avoid wetlands whenever possible. Crossing a wetland should be at a single location and at the edge of the wetland, if possible. • Never allow a spill to enter area wetlands. Floodplains • Design the project to maintain natural drainage patterns and runoff rates if possible. • Maintain as much riparian vegetation as possible. If riparian vegetation is damaged or removed during construction, replace with native species. • Use bioengineering techniques to stabilize stream banks. • Keep construction activity away from wildlife crossings and corridors. • Stockpile materials outside of the floodplain and use erosion control techniques. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Woodlands • Protect trees on sites with severe design limitations, such as steep slopes and highly erodible soils. • Preserve trees along watercourses to prevent bank erosion, decreased stream temperatures, and to protect aquatic life. • Protect the critical root zone of trees during construction. This is the area directly beneath a tree's entire canopy. For every inch of diameter of the trunk, protect 1.5 ft of area away from the trunk. • Avoid trenching utilities through the tree's critical root zone. • Avoid piling excavated soil around any tree. • Replace trees removed during construction with native trees. • Conduct post -construction monitoring to ensure trees impacted by construction receive appropriate care. General construction considerations • Conduct a pre -construction meeting with local community officials, contractors, and subcontractors to discuss natural resource protection. Communicate agreed-upon goals to everyone working on the project. • Insert special requirements addressing sensitive natural areas into plans, specifications, and estimates provided to construction contractors. Note the kinds of activities that are not allowed in sensitive areas. • Confine construction and staging areas to the smallest necessary and clearly mark area boundaries. Confine all construction activity and storage of materials to designated areas. • Install construction flagging or fencing around sensitive areas to prevent encroachment. • Excavate only what is absolutely necessary to meet engineering requirements. Do not put excavated material in sensitive areas. (Excavated material could be used in other areas of the site to improve seeding success.) • Conduct onsite monitoring during construction to ensure sensitive areas are protected as planned. Conduct post -construction monitoring to ensure sensitive areas that were impacted by construction receive appropriate care. Maintenance Ownership of these natural areas will be assumed by homeowners' associations or simply the specific individual property homeowners where these resources are located. Specific maintenance activities will depend upon the type of vegetation present in the preserved natural area where woodlands require little to no maintenance and open lawn require higher maintenance. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Cost When development encroaches into sensitive areas, dealing with their special challenges invariably adds to development and construction costs. Sometimes these added costs are substantial, as in the case of working with wetlands or steep slopes. Sometimes costs emerge only in longer-term operation, like encroachment in floodplains. This can translate into added risk of building damage for future owners, as well as health and safety impacts, insurance costs, and downstream flooding. If all short- and long-term costs of impacting sensitive areas were quantified and tallied, total real costs of sensitive area encroachment would increase substantially. Conversely, protecting sensitive areas results not only in cost savings, but also in water quality benefits. At the same time, reduction in potential development areas resulting from protecting and conserving sensitive areas can have the effect of altering - even reducing - a proposed development program, thereby reducing development yield and profit. To address this, this Intrinsic GSP can be applied together with a cluster development approach if appropriate. REFERENCES Arendt, Randall G. 2001. Growing Greener: Conservation by Design. Pennsylvania Department of Conservation and Natural Resources, Natural Lands Trust, Governor's Office of Local Government Services. Coffman, Larry. 2000. Low Impact Development Design Strategies: An Integrated Design Approach. EPA 841 B 00 0023. Department of Environmental Resources, Programs and Planning, Prince George's County, MD. Delaware Department of Natural Resources and Environment Control. 1997. Conservation Design for Stormwater Management: A Design Approach to Reduce Stormwater Impacts from Land Development. Brandywine Conservancy Environmental Management Center. Minnesota Pollution Control Agency. 2006. Minnesota Stormwater Manual, Version 1.1. www.pca.state.mn.us/water/stormwater/stormwater-manual.html Natural Resources Defense Council. 2006. Rooftops to Rivers: A Policy Guide for Decision Makers on How to Use Green Infrastructure to Address Water Quality and Volume Reduction for Communities with Combined Sewer Overflow Issues. Washington, DC. www.nrdc.org/water/pollution/rooftops/contents.asp Southeast Michigan Council of Governments (SEMCOG). 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. Detroit, MI. Stormwater Program Guidance Manual for the Puget Sound Basin. 1992. Washington State Department of Ecology, Olympia, WA. U.S. Department of Agriculture -Soil Conservation Service. 1988. Effects of Conservation Practices on Water Quantity and Quality, Vols. 1-4., Washington, DC. U.S. Environmental Protection Agency. 2007. Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices. PA 841-F-07-006. Washington, DC. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i REDUCE IMPERVIOUS AREA Reducing impervious surfaces includes minimizing areas such as streets, parking lots, and driveways. By reducing the amount of paved surfaces, stormwater runoff is decreased while infiltration and evapotranspiration opportunities are increased. Description and Function Reducing street imperviousness performs valuable stormwater functions in contrast to conventional development in the following ways: • Increases infiltration, • Decreases runoff volumes, • Increases stormwater time of concentration, • Improves water quality by decreasing nonpoint source pollutant loading, and • Decreases the concentration and energy of stormwater. Imperviousness greatly influences stormwater runoff volume and quality by increasing the rapid transport of stormwater and collecting pollutants from atmospheric deposition, automobile leaks, and additional sources. Stream degradation has been observed at impervious levels as low as 10-20 percent watershed - wide (Center for Watershed Protection, 1995), when these areas are managed conventionally. Recent findings indicate that degradation is observed even at much lower levels of imperviousness (Villanova University 2007 Stormwater Management Symposium, Thomas Schueler, Director, Chesapeake Stormwater Network). Reducing imperviousness improves an area's hydrology, habitat structure, and water quality. Design Considerations Refer to the City of Fayetteville Master Street plan and applicable subdivision regulations for minimum street width and parking requirements. The runoff from rooftop imperviousness can be mitigated through the use of green roofs. Refer to GSP-12 specification in Appendix B. Other GSPs may also apply. Stormwater Functions and Calculations Quantifying impervious areas at a proposed development site, pre- to post -development continues to dominate stormwater calculations. Stormwater calculations, as discussed in Section 3 of Chapter 5, are sensitive to pervious areas and their contribution to total volume of runoff, increased peak rate of runoff, and increased generation of nonpoint source pollutants. A reduction in imperviousness achieved through reduced street widths and lengths and reduced paved parking areas automatically reduces both the volume and peak rate of runoff. As water quality is affected by runoff volume, reduction in imperviousness generally translates into a reduction in water quality management requirements. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i Maintenance A reduction in impervious area results in decreased maintenance. For example, whether publicly or privately maintained, reducing roadway or parking lot imperviousness typically translates into reduction in all forms of maintenance required, from basic roadway repair to winter maintenance and snow removal. Cost Significant cost reductions can be achieved through minimizing the amount of impervious area on the developed site. REFERENCES Center for Watershed Protection. 1994. "The Importance of Imperviousness", Watershed Protection Techniques, Vol.1, No.3. Ellicott City, MD. Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Maryland Department of the Environment. 2000. Maryland Stormwater Design Manual. Schueler, Tom. 1995. Site Planning for Urban Stream Protection. Center for Watershed Protection. Silver Spring, MD. SEMCOG. 2003. Land Use Tools and Techniques: A Handbook for Local Communities. Detroit, MI, SEMCOG. 2002. Opportunities for Water Resource Protection in Plans, Ordinances, and Programs. Detroit, MI. Southeast Michigan Council of Governments (SEMCOG). 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. Detroit, MI. AppendixA - Intrinsic GSP Specifications Drainage Criteria Manual i APPENDIX 6 GSP SPECIFICATIONS BIORETENTION Description: Bioretention cells are vegetated, shallow depressions. Captured runoff is treated by filtration through an engineered soil medium, and is then either infiltrated into the subsoil or exfiltrated through an underdrain. Raingardens are simply smaller bioretention cells and are further addressed in Section 1. Variations: Constructed without underdrain in soils with measured infiltration rates greater than 0.5 inch per hour, or with an underdrain over a gravel sump in less permeable soils. • Reduced runoff volume • • Reduced peak discharge rate • • Reduced TSS • Reduced pollutant loading • Reduced runoff temperature • • Groundwater recharge (if soils are sufficiently permeable) • Habitat creation • Enhanced site aesthetics • Reduced heat island effect Problems with installation can lead to failure Minimum 2 ft separation from groundwater is required — not suitable in areas with bedrock less than 4 ft below final grade Not suitable for areas with high pollutant loads or within 100 ft of residential / commercial septic system fields • Maximum contributing drainage area of 5 acres, 2.5 acres of which may be impervious • Slope of drainage area = 1— 5% or terraced to slow flow • Setback for 0 to 0.5 acre drainage area: 10 ft. with maximum pool > 0.5 ft. below lowest adjacent grade; 50 ft. if up -gradient (See Section 5, Setbacks) • Setback for 0.5 to 5 acre drainage area: 25 ft. with maximum pool > 0.5 ft. below lowest adjacent grade; 100 ft. if up -gradient (See Section 5, Setbacks) Selection Criteria: Level 1— 60% Runoff Reduction Credit Level 2 — 90% Runoff Reduction Credit Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Regular inspection of landscaping to maintain healthy vegetative cover • Annual removal of invasives • Irrigation when necessary during first growing season • Periodic trash removal • Annual inspection © Maintenance Burden L = Low M = Moderate H = High • Used in medians and right of way — accommodate utility specific horizontal and vertical setbacks • Stormwater can be conveyed by sheet flow or grass channels • Pretreatment is especially important in roadway applications where sediment loads may be high • Design as a series of cells running parallel to roadway • See GSP-02 Urban Bioretention for additional information Appendix B — GSP Specifications Drainage Criteria Manual i , SECTION 1: DESCRIPTION Individual bioretention areas can serve impervious drainage areas of 2.5 acres or less; though several cells may be designed adjacent to each other to accommodate larger areas. Surface runoff is directed into a shallow landscaped depression that incorporates many of the pollutant removal mechanisms that operate in forested ecosystems. The primary component of a bioretention practice is the filter bed, which has a mixture of sand, soil and organic material as the filtering media with a surface mulch layer. During storms, runoff temporarily ponds 6 inches above the cell surface and then rapidly filters through the bed. If the subsoil infiltration rate is less than 0.5 inches per hour, the filtered runoff is collected in an underdrain and returned to the storm drain system. The underdrain consists of a perforated pipe in a gravel layer installed along the bottom of the filter bed. Bioretention can also be designed to infiltrate runoff into native soils. This can be done at sites with permeable soils, a low groundwater table, and a low risk of groundwater contamination. This design features the use of a "partial exfiltration" system that promotes greater groundwater recharge. Underdrains are only installed beneath a portion of the filter bed, above a stone "sump" layer, or eliminated altogether, thereby increasing stormwater infiltration. SECTION 2: PERFORMANCE Bioretention creates a good environment for runoff reduction, filtration, biological uptake, and microbial activity, and provides high pollutant removal. Bioretention can become an attractive landscaping feature with high amenity value and community acceptance. The overall runoff reduction capabilities of bioretention are summarized in Table 1.1. Additional details are provided in Table 1.2, Section 5. Table 1.1: Annual Stormwater Function runoff volume reduction provided bbioretentionbasins. Level 1 Design Level 2 Design Annual Runoff Volume Reduction (RR) 60% 90% Sources: CWP and CSN (2009); CWP (2007) SECTION 3: DESIGN VARIATIONS The most important design consideration for the application of bioretention to development sites is the scale, as follows: Rain Gardens. These are small, distributed practices designed to treat runoff from small areas, such as individual rooftops, driveways and other on -lot features in single-family detached residential developments. Inflow is typically sheet flow, or can be concentrated flow with energy dissipation, when located at downspouts. Rain gardens do not count toward a Runoff Reduction credit unless operations and maintenance are addressed to ensure their longevity. Bioretention Basins. These are structures treating parking lots and/or commercial rooftops, usually in commercial or institutional areas. Inflow can be either sheetflow or concentrated flow. Bioretention basins Appendix 8 — GSP Specifications Drainage Criteria Manual i , may also be distributed throughout a residential subdivision, but they should be located in common areas and within drainage easements, to treat a combination of roadway and lot runoff. The major design goal for bioretention is to maximize runoff volume reduction and pollutant removal. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced design (Level 2) that maximizes pollutant and runoff reduction. If soil conditions require an underdrain, bioretention areas can still qualify for the Level 2 design if they contain a stone storage layer beneath the invert of the underdrain. Table 1.2 in Section 5 outlines the Level 1 and 2 bioretention design guidelines. SECTION 4: TYPICAL DETAILS Figure 1.1. A typical bioretention basin treating a commercial rooftop Figures 1.2 through 1.6 provide some typical details for several bioretention configurations. Additional details are provided in Appendix 1-13 of this design specification Appendix 8 - GSP Specifications Drainage Criteria Manual i , 4,. A 1Y !,h ..... rr .:sur y� Y1 �,Y� NESTED BIORETENTION ` BASIN TURF COVER J (ADDITIONAL PON D I NG AR EA) PLAN VIEW ADDITIONAL PONDING ADDITIONAL CONCENTRATED AREA PONDING AREA INFLOW AND PRETREATMENT BIORETENTION BED SURFACE AREA MULCH LAYER STABILIZED SURFACE INFLOW ELEVATION ;I IITil�liiiIEll lE—.,:lir�ii IiIIIli ii-iI ALL UNDERDRAINS TO BE DAYLIGHTED 11-IICIIIJIhIE STONESUMP `(OPEN GRADED) CONCENTRATED INFLOW PRETREATMENT AS REQUIRED DEPTH VARIES LEVEL 1 MIN. =24" LEVEL 2 MIN. = 36" 1 FT. I�I1III (LEVEL 2) Figure 1.2. Typical detail of Bioretention with additional surface ponding (Source: VADCR, 2010). Appendix B - GSP Specifications Drainage Criteria Manual I , GRASSED PRFTRFATMFNT FILTER STRIP SHI=1=i FLOW UH PRETREATMENT LILT BIORETENTION CELL CONCENTRATED FLOW WI i H I-OHEBAY IFLAN V W EX i ENDED L]FTFNTICN STOPAGE SECTION VIEW BIORETENTION IN SHELF OF EXTENDED DETENTION POND Figure 1.3. Typical detail of a bioretention basin within the upper shelf of an ED pond (Source: VADCR, 2010). Appendix 8 - GSP Specifications Drainage Criteria Manual I , Y,. LL .I ..'J'.'.. 1 jj f_.._i_l°i=�;::°;�L 1 J . GRASS FILTER FOR SHEET FLOW PRETREATMENT I N7.5 S�•iFET F��•,•, i .. 40 :� � �— ,.�. it I � r-, • i .. 15 I` ' ll.• i IJ.. 01ASS FILTER FOR SHEET FLOW Pr�ETPEATMENT II NTS Figure 1.4. Pretreatment option - grass filter for sheet flow (Source: VADCR, 2010). NTS Figure 1.5. Pretreatment option — gravel diaphragm for sheet flow from impervious or pervious (Source: VADCR, 2010). Appendix 8 - GSP Specifications Drainage Criteria Manual I , ... '1' CROP �.IIf.•-? ?'-r,-�. ,i. •_ _..� _ T -- FRE-G:'...rR:i - _. _ ..i -, -' - - `r :I. �7 .I I :iij,.. -' GRAVEL DLkP1HRAGft'. SHEET FLQVPHETHEAIMENr NTS Figure 1.5. Pretreatment option — gravel diaphragm for sheet flow from impervious or pervious (Source: VADCR, 2010). Appendix 8 - GSP Specifications Drainage Criteria Manual I , H SHE CL PAVEVENT LEVEL 3TREADER -77T F, .T,f. FIL-E _r LI[JIr I- if 11-ILL, F,' I-F-111-Ir I Frl,_-I�.FFF PRE U17 tj1_7E _T_IJE LCE: ATIC E,EPT.] OF [�EI 'I JEU C,,EFI E .1 IA F F4 7TFITr, r.,IF L-Ir, ,5ECTIQN A-A _.! ITTCU F;_i_".',' :ELL __:R SPALL JC -H LIJEI FL -.%-I[:TH I=r EL r.ji Tf, I,-.r A& 6L SECTION B-6 CONCENTRATED FLOW CURBCUT PRETREATMENT GRAVEL FLOW SPREADER NTS Figure 1.6. Pre -Treatment Option — Gravel Flow Spreader for Concentrated Flow Outside of ROW (Source: VADCR, 2010). Appendix B - GSP SpecifticationFs Drainage Criteria Manual I , SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS 1. Storage depth is based on the Void Ratio (Vr) of the soil media and gravel layers and their respective thicknesses, plus the surface ponding depth. Compute as described in Section 6.1. 2. A ponding depth of 6 inches is preferred. Ponding depths greater than 6 inches will require a specific planting plan to ensure appropriate plant selection (Section 6.8). 3. These are recommendations for simple, shallow building foundations or road subgrade. All designs shall be prepared / reviewed by appropriately licensed professionals. Also, special footing design or use of impermeable linings with bioretention may be used to justify a reduction of the setbacks noted above. The urban bioretention GSP-02 may also be used to reduce setbacks. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 1.2. Bioretention design criteria Level 1 Design (1111:60) Level 2 Design (RR:90) Building and road setbacks s (Section S): 0 to 0.5 acre CDA = 10 ft. if down -gradient from building/road; 50 ft. if up -gradient. 0.5 to 2.5 acre CDA = 25 ft. if down -gradient from building/road or level; 100 ft. if up -gradient. Refer to additional setback criteria in Section 5) Sizing (Section 6.1) Sizing (Section 6.1) Surface Area (sq. ft.) = (Tv —the volume reduced by an Surface Area (sq. ft.) = [(1.25)(Tv) —the volume reduced by an upstream BMP) / Storage Depth 1 upstream BMP] /Storage Depth 1 Recommended maximum contributing drainage area per cell = 2.5 acres; 5 acre total Maximum Ponding Depth = 6 to 12 inchesz' Maximum Ponding Elevation > 0.5 ft. below lowest adjacent grade of structure Filter Media Depth minimum = 24 inches; Filter Media Depth minimum = 36 inches; recommended maximum = 6 ft. recommended maximum = 6 ft. Media & Surface Cover (Section 6.6, Appendix C) = mixed onsite or supplied by vendor; the final composition shall be as specified in Appendix C. Sub -soil Testing (Section 6.2): not needed if an underdrain Sub -soil Testing (Section 6.2): one per 1,000 sq. ft. of filter is used; Min infiltration rate > 0.5 inch/hour in order to surface; Min infiltration rate > 0.5 inch/hour in order to remove the underdrain requirement. remove the underdrain requirement. Underdrain (Section 6.7) = PVC or Corrugated HDPE with Underdrain & Underground Storage Layer clean -outs OR, none, if soil infiltration requirements are (Section 6.7) = PVC or Corrugated HDPE with clean outs, met (Section 6.2) and a minimum 12 -inch stone sump below the invert; OR, none, if soil infiltration requirements are met (Section 6.2) Inflow: sheetflow, curb cuts, trench drains, concentrated flow, or the equivalent Geometry (Section 6.3): Geometry (Section 6.3): Length of shortest flow path/Overall length = 0.3; OR, other Length of shortest flow path/Overall length = 0.8; OR, other design methods used to prevent short-circuiting; a one -cell design methods used to prevent short-circuiting; a two -cell design (not including the pre-treatment cell). design (not including the pretreatment cell). Pre-treatment (Section 6.4): a pretreatment cell, Pre-treatment (Section 6.4): a pretreatment cell grass filter strip, gravel diaphragm, gravel flow spreader, or plus one of the following: a grass filter strip, gravel diaphragm, another approved (manufactured) pre-treatment structure. gravel flow spreader, or another approved (manufactured) pre- treatment structure. Conveyance & Overflow (Section 6.5) Conveyance & Overflow (Section 6.5) Planting Plan (Section 6.8): a planting template to include Planting Plan (Section 6.8): a planting template to include perennials, grasses, sedges or shrubs to achieve a surface area perennials, grasses, sedges, and shrubs to achieve a surface coverage of at least 75% within 2 years by using the area coverage of at least 75% within 2 years by using the recommended spacing in Table 1.3 Material Specifications recommended spacing in Table 1.3 Material Specifications. PLUS trees planted at 1 tree/400 s.f. Deeded Maintenance O&M Plan (Section 8) 1. Storage depth is based on the Void Ratio (Vr) of the soil media and gravel layers and their respective thicknesses, plus the surface ponding depth. Compute as described in Section 6.1. 2. A ponding depth of 6 inches is preferred. Ponding depths greater than 6 inches will require a specific planting plan to ensure appropriate plant selection (Section 6.8). 3. These are recommendations for simple, shallow building foundations or road subgrade. All designs shall be prepared / reviewed by appropriately licensed professionals. Also, special footing design or use of impermeable linings with bioretention may be used to justify a reduction of the setbacks noted above. The urban bioretention GSP-02 may also be used to reduce setbacks. Appendix 8 — GSP Specifications Drainage Criteria Manual i , 5.1. Physical Feasibility Bioretention can be applied in most soils or topography, since runoff simply percolates through an engineered soil bed and can be returned to the stormwater system if the infiltration rate of the underlying soils is low. Key constraints with bioretention include the following: Available Space. Planners and designers can assess the feasibility of using bioretention facilities based on a simple relationship between the contributing drainage area and the corresponding required surface area. The bioretention surface area will be approximately 3% to 10% of the contributing drainage area, depending on the imperviousness of the contributing drainage area (CDA), the subsoil infiltration rate, and the desired bioretention design level. Site Topography. Bioretention is best applied when the grade of contributing slopes is greater than 1% and less than 5%. Terracing or other inlet controls may be used to slow runoff velocities entering the facility. Available Hydraulic Head. Bioretention is fundamentally constrained by the invert elevation of the existing conveyance system to which the practice discharges (i.e., the bottom elevation needed to tie the underdrain from the bioretention area into the storm drain system.) In general, 3 ft of elevation above this invert is needed to create the hydraulic head needed to drive stormwater through a proposed bioretention filter bed. Less hydraulic head is needed if the underlying soils are permeable enough to dispense with the underdrain. Water Table. Bioretention shall always be separated from the water table to ensure that groundwater does not intersect the filter bed. Mixing can lead to possible groundwater contamination or failure of the bioretention facility. A separation distance of 2 ft is required between the bottom of the excavated bioretention area and the seasonally high ground water table. Utilities, Infrastructure, and Setbacks. Designers should ensure that future tree canopy growth in the bioretention area will not interfere with existing overhead utility lines. Interference with underground utilities should also be avoided, particularly water and sewer lines. Local utility design guidance should be consulted in order to determine the horizontal and vertical clearance required between stormwater infrastructure and other dry and wet utility lines. At a minimum, locate bioretention basins at least 10 ft from down -gradient wet utility lines. Dry utility lines such as electric, cable and telephone may cross under bioretention areas if they are double -cased, if approved by the utility where applicable. To avoid the risk of seepage, do not allow bioretention areas to be hydraulically connected to structure foundations or pavement / pavement subgrade. Setbacks to structures and roads vary, based on the scale of the bioretention design (see Table 1.2 above). At a minimum, bioretention basins shall be located a horizontal distance of 100 ft from any water supply well or septic system. Bioretention basins can be constructed closer to structures and roads if appropriately designed and constructed. Please see GSP-02 for additional information on ROW applications. Soils. Soil conditions do not constrain the use of bioretention, although they determine whether an underdrain is needed. Low permeability soils in Hydrologic Soil Group (HSG) C or D usually require an underdrain, whereas HSG A soils and most HSG B soils generally do not. Initially, soil infiltration rates can be estimated from NRCS soil data, but they must be confirmed by an on-site infiltration evaluation. Contributing Drainage Area. Bioretention cells work best with smaller contributing drainage areas, where it is easier to achieve flow distribution over the filter bed without experiencing erosive velocities and Appendix 8 - GSP Specifications Drainage Criteria Manual i , excessive ponding times. Typical drainage area size can range from 0.1 to 2.5 acres of impervious cover due to limitations on the ability of bioretention to effectively manage large volumes and peak rates of runoff. However, if hydraulic considerations are adequately addressed to manage the potentially large peak inflow of larger drainage areas (such as off-line or low -flow diversions, forebays, etc.), there may be case-by-case instances where the City Engineer may allow these recommended maximums to be adjusted. In such cases, the bioretention facility shall be located within the drainage area so as to capture the Treatment Volume (Tv) equally from the entire contributing area, and not fill the entire volume from the immediately adjacent area, thereby bypassing the runoff from the more remote portions of the site. Land Uses with Potential for High Pollutant Loading. Runoff within areas designated to have potential for high pollutant loading shall not be treated with infiltrating bioretention (i.e., constructed without an underdrain). An impermeable bottom liner and an underdrain system may be utilized, at the discretion of the City Engineer, when bioretention is used to receive and treat such runoff. Floodplains. Bioretention areas shall be constructed outside the limits of the 1 -percent annual -chance floodplain. No Irrigation orBaseflow. The planned bioretention area shall not receive baseflow, irrigation water, chlorinated wash -water or other such non-stormwater flows that are not stormwater runoff, except for irrigation as necessary for the survival of plantings within the bioretention area. 5.2. Potential Bioretention Applications Bioretention can be used wherever water can be conveyed to a surface area. Bioretention has been used at commercial, institutional and residential sites in spaces that are traditionally pervious and landscaped. It should be noted that special care must be taken to provide adequate pre-treatment for bioretention cells in space -constrained high traffic areas. Typical locations for bioretention include the following: Parking lot islands. The parking lot grading is designed for sheet flow towards linear landscaping areas and parking islands between rows of spaces. Curb -less pavement edges can be used to convey water into a depressed island landscaping area. Curb cuts can also be used for this purpose, but they are more prone to blockage, clogging and erosion. Parking lot edge. Small parking lots can be graded so that flows reach a curb -less pavement edge or curb cut before reaching catch basins or storm drain inlets. The turf at the edge of the parking lot functions as a filter strip to provide pre-treatment for the bioretention practice. The depression for bioretention is located in the pervious area adjacent to the parking lot. Right of Way or commercial setback. A linear configuration can be used to convey runoff in sheet flow from the roadway, or a grass channel or pipe may convey flows to the bioretention practice. See Section 5.1 for specific setback recommendations. Courtyards. Runoff collected in a storm drain system or roof leaders can be directed to courtyards or other pervious areas on site where bioretention can be installed. Unused pervious areas on a site. Storm flows can be redirected from a storm drain pipe to discharge into a bioretention area. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Dry Extended Detention (ED) basin. A bioretention cell can be located on an upper shelf of an extended detention basin, after the sediment forebay, in order to boost treatment. Depending on the ED basin design, the designer may choose to locate the bioretention cell in the bottom of the basin. However, the design must account for the potentially deeper ponding depths (greater than 6 or 12 inches) associated with extended detention. Retrofitting. Numerous options are available to retrofit bioretention in the urban landscape, as described in GSP-02, Urban Bioretention SECTION 6: DESIGN CRITERIA 6.1. Sizing of Bioretention Practices 6.1.1. Stormwater Quality Sizing of the surface area (SA) for bioretention practices is based on the computed Treatment Volume (Tv) of the contributing drainage area and the storage provided in the facility. The required surface area (in sq feet) is computed as the Treatment Volume (in cubic feet) divided by the equivalent storage depth (in feet). The equivalent storage depth is computed as the depth of soil media, gravel, or surface ponding (in feet) multiplied by the void ratio. The maximum allowable Void Ratios (Vr) are: Bioretention Soil Media (See Section 6.6) Vr = 0.40 Gravel Vr = 0.35 Surface Storage Vr = 1.0 The equivalent storage depth for Level 1 with, for example, 2 ft of soil media, a 12 -inch layer of gravel, and a 6 -inch surface ponding depth is therefore computed as: Equation 1.1. Bioretention Level 1 Design Storage Depth (DSD) Weighted Storage Depth = Dw = Vrl(D1) + Vr2(D2) + "' (0.4 x 2 ft) + (0.35x1 ft) + (1.0 x 0.5 ft) = 1.65ft Where Vri and D1 are for the first layer, etc. The equivalent storage depth for Level 2 with 3 ft of soil media, a 12 -inch gravel layer, and a 6 -inch surface ponding depth is computed as: Equation 1.2. Bioretention Level 2 Design Storage Depth Weighted Storage Depth = DW = Vrl(D1) + Vr2(D2) + "' (0.4 x 3 ft) + (0.35x1 ft) + (1.0 x 0.5 ft) = 2.05ft Appendix 8 - GSP Specifications Drainage Criteria Manual i , While this method is simplistic, simulation modeling has indicated that it yields a total storage volume somewhat equivalent to 80% total average annual rainfall volume removal for infiltration rates from 0.5 inches/hour through 1.2 inches/hour. Depending on the in-situ subsurface infiltration rate, surface area size decreases may be granted at the discretion of City Engineer. Figure 1.7. Typical Level 2 Bioretention Section with void ratios for volume computations. The Level 1 Bioretention Surface Area (SA), for the above example, Equation 1.1, is computed as: Equation 1.3. Bioretention Level 1 Design Surface Area SA (sq. ft.) _ (Tv - the volume reduced by any upstream BMP) /Dw And the Level Bioretention Surface Area is computed as: Equation 1.4. Bioretention Level Design Surface Area SA (sq. ft.) _ ((1.25 * T j - the volume reduced by any upstream BMP/ Dw Where: SA = Minimum surface area of bioretention filter (sq. ft.) TV = Treatment Volume (cu. ft.)_ [(1.0 in.)(Rv)(A)*43560/12] Du, = Weighted Storage Depth as per Equations 1.1 or 1.2 Where: A = Area in acres (NOTE: Rv = the composite runoff coefficient from the RR Method. A table of RV values and the equation for calculating a composite Rv is located in Section 3.0 of this chapter) For example, a Level 2 Bioretention Surface Area (using the media described above for a Level 2 Design) for a treated area A = 1 acre, with no upstream BMPs would require a minimum design surface area of: SA = [(1.25*1 * 0.8*1 *43560/12)]/2.05 = 1770 sq ft Appendix B - GSP Specifications Drainage Criteria Manual i , Equations 1.1 through 1.4 should be modified with the addition of any surface or subsurface storage components (e.g., additional area of surface ponding, subsurface storage chambers, etc.). 6.1.2. Stormwater Quantity Designers may be able to create additional surface storage by expanding the surface ponding footprint in order to accommodate a greater quantity credit for channel and/or flood protection, without necessarily increasing the soil media footprint. In other words, the engineered soil media would only underlay part of the surface area of the bioretention (as shown in Figure 1.7). In this regard, the ponding footprint can be increased as follows to allow for additional storage: • 50% surface area increase if the ponding depth is 6 inches or less. • 25% surface area increase if the ponding depth is between 6 and 12 inches. These values may be modified as additional data on the long term permeability of bioretention filters becomes available. The removal of volume by bioretention changes the runoff depth entering downstream flood control facilities. An approximate approach to accounting for this in reducing the size of peak flow detention facilities is to calculate an "effective SCS curve number" (CNadj) which is less than the actual curve number (CN) based on typical SCS parameters. CNadj can then be used in hydrologic calculations and in routing; however, the CN must be recomputed for each storm or rainfall volume analyzed. The method can also be used for other hydrologic methods in which a reduction in runoff volume is possible. 6.2. Soil Infiltration Rate Testing In order to determine if an underdrain will be needed, one must measure the infiltration rate of subsoils at the invert elevation of the bioretention area, as noted in the soil testing requirements for each scale of bioretention, in Design Table 1.2. The infiltration rate of subsoils must exceed 0.5 inch per hour for bioretention basins. On-site soil infiltration rate testing procedures are outlined in Appendix C. Soil testing is not needed for bioretention areas where an underdrain is used. 6.3. BMP Geometry Bioretention basins must be designed with internal flow path geometry such that the treatment mechanisms provided by the bioretention are not bypassed or short-circuited. Examples of short-circuiting include inlets or curb cuts that are very close to outlet structures (see Figure 1.8), or incoming flow that is diverted immediately to the underdrain through stone layers. Short-circuiting can be particularly problematic when there are multiple curb cuts or inlets. Appendix 8 - GSP Specifications Drainage Criteria Manual i , !!►a: ;:� Last cum V CIA Figure 1.8. Examples of short-circuiting at bioretention facilities (Source: VADCR, 2010). In order for these bioretention areas to have an acceptable internal geometry, the "travel time" from each inlet to the outlet should be maximized, and incoming flow must be distributed as evenly as possible across the filter surface area One important characteristic is the length of the shortest flow path compared to the overall length, as shown in Figure 1.9 below. In this figure, the ratio of the shortest flow path to the overall length is represented as: Equation 1.5. Ratio of Shortest Flow Path to Overall Length SFP / L Where: SFP = length of the shortest flow path L = length from the most distant inlet to the outlet Cuib Irids r L outlet N stluch3C8 SF P/ rr Figure 1.9. Diagram showing shortest flow path as part of BMP geometry (Source: VADCR, 2010). Appendix B - GSP Specifications Drainage Criteria Manual i , For Level 1 designs, the SFP/L ratio must be 0.3 or greater; the ratio must be 0.8 or greater for Level 2 designs. In some cases, due to site geometry, some inlets may not be able to meet these ratios. However, the drainage area served by such inlets should constitute no more than 20% of the contributing drainage area. Alternately, the designer may incorporate other design features that prevent short-circuiting, including features that help spread and distribute runoff as evenly as possible across the filter surface. Field experience has shown that improperly prepared and poorly compacted soil media immediately around a raised inlet/outlet structures is prone to scouring, erosion and, thus, short-circuiting of the treatment mechanism. For example, water can flow straight down through scour holes or sinkholes to the underdrain system (Hirschman et al., 2009). Appropriate design options shall be used and good construction practices implemented to prevent this type of scouring. The designer should ensure that incoming flow is spread as evenly as possible across the filter surface to maximize the treatment potential. See Figure 1.10. PROVIDE MIM MUM OF 1' DP FRESSOARD FROM OF TOP TO TOP OF BE RK STRUCTURE d1LIOT PASS 1Q -YEAR $TQRM WITHOUT OVERTOPPING BERM TRASH RAGK SEE NOTE RAMP SOIL LAYER UP TO LIP OF STRUCTURE OR PROVIDE STONE PROTECTK W AROUND STRUCTURE 't'v'q MOTE: AS AN ALTERNAT[VE TO THE TRASH RACK, AN OVERFLOW STRMWIRE MAY OF A DROP 1NLr7 W1 H A VAR W(A] E, Figure 1.10. Typical Detail of how to prevent bypass or short-circuiting around the overflow structure (Source: VADCR, 2010). 6.4. Pre-treatment Pre-treatment of runoff entering bioretention areas is necessary to trap coarse sediment particles before they reach and prematurely clog the filter bed. Pre-treatment measures must be designed to evenly spread runoff across the entire width of the bioretention area. Several pre-treatment measures are feasible, depending on the scale of the bioretention practice and whether it receives sheet flow, shallow concentrated flow or deeper concentrated flows. The following are appropriate pretreatment options: For Bioreten tion Basins: • Pre-treatment Cells (channel flow): Similar to a forebay, this cell is located at piped inlets or curb cuts leading to the bioretention area and consists of an energy dissipater sized for the expected rates Appendix 8 - GSP Specifications Drainage Criteria Manual I , of discharge. It has a storage volume equivalent to at least 15% of the total Treatment Volume (inclusive) with a 2:1 length -to -width ratio. The cell may be formed by a stone check dam or an earthen or rock berm. Pretreatment cells do not need underlying engineered soil media, in contrast to the main bioretention cell. • Grass Filter Strips (for sheet flow): Grass filter strips extend from the edge of pavement to the bottom of the bioretention basin at a 5:1 slope or flatter. Alternatively, provide a combined 5 ft of grass filter strip at a maximum 5% (20:1) slope and 3:1 or flatter side slopes on the bioretention basin. (See Figure 1.5) • Gravel or Stone Diaphragms (sheet flow). A gravel diaphragm located at the edge of the pavement shall be oriented perpendicular to the flow path to pre -treat lateral runoff, with a 2 to 4 inch drop. The stone must be sized to resist erosion based on to the expected rate of discharge. (See Figure 1.6) • Gravel or Stone Flow Spreaders (concentrated flow). The gravel flow spreader is located at curb cuts, downspouts, or other concentrated inflow points, and shall have a 2 to 4 inch elevation drop from a hard -edged surface into a gravel or stone diaphragm. The gravel shall extend the entire width of the opening and create a level stone weir at the bottom or treatment elevation of the basin. (See Figure 1.7) • Innovative or Proprietary Structure: An approved proprietary structure with demonstrated capability of reducing sediment and hydrocarbons may be used to provide pre-treatment. 6.5. Conveyance and Overflow For On-line Bioretention: An overflow structure shall be incorporated into on-line designs to safely convey larger storms through the bioretention area. The following criteria apply to overflow structures: • The overflow associated with the design storms of the 2-, 5-, 10-, 25-, and 50 -year frequencies referenced in the Drainage Criteria Manual shall be controlled so that velocities are non-erosive at the inlet and outlet locations (i.e., to prevent downstream erosion). • Common overflow systems within bioretention practices consist of an inlet structure, where the top of the structure is placed at the maximum water surface elevation of the bioretention area, which is typically 6 to 12 inches above the surface of the filter bed (6 inches is the preferred ponding depth). • The overflow capture device (typically a yard inlet) shall be scaled to the application - this may be a landscape grate inlet or a commercial -type structure. • The filter bed surface should generally be flat so the bioretention area fills up like a bathtub. Off-line Bioretention: Off-line designs are preferred (see Figure 1.11 for an example). One common approach is to create an alternate flow path at the inflow point into the structure such that when the maximum ponding depth is reached, the incoming flow is diverted past the facility. In this case, the higher flows do not pass over the filter bed and through the facility, and additional flow is able to enter as the ponding water filtrates through the soil media. Appendix B - GSP Specifications Drainage Criteria Manual i , A ` 5 r � suf. �a rfr rwr yff�► rur. .wr x2 Q'. r1 E SECTION A -A' ,PLAN VIEW 6 1,N TER CJFJT1 CEPTH TOP W10TF{ iELE'.'ATIGr I C: Cl PRCPCISMD eICRETEf J71C IJ SURJ-:�E E-EV.ATiOF. - Q C' SECTION B -B' CEP'Tr+ 4 FT MIN �ELE•�%-Tlgfl 15• TOP WIDTHAa I r t PROPO SED8 JRETE I•IT1O�.I UPFACE ELEVAT O:J O Cr SECTION C -C' 5' NATER C7V!•LI_1Y 7EPY-.• rEL=_JATION o 4 FT hlIN TOP WIDTH C G C BOTTOLI CF $WALE ELE,!ATION = 1 5 EC -3 mAT-nwc, P RC PC5ED a ORE TENTI Or. SURFACE ELEVATIOrJ - 0 B �f OF SWALE ELEVELE'�.:'IC'J C• Ira EC -3 tut -'TTI NG DO-TOP11 OF SW -.LE ELE:'rTlOhi 0.0 - EC -3 MATT -MG Figure 1.11. Typical details for off-line biofiltration (Source: VADCR, 2010). Another option is to utilize a low -flow diversion or flow splitter at the inlet to allow only the Treatment Volume to enter the facility. This may be achieved with a weir or curb opening sized for the target flow, in combination with a bypass channel. Using a weir or curb opening helps minimize clogging and reduces the maintenance frequency. (Further guidance on determining the Treatment Volume design peak flow rate will be necessary in order to ensure proper design of the diversion structure.) Appendix B - GSP Specifications Drainage Criteria Manual i , 6.6. Filter Media and Surface Cover • The filter media and surface cover are the two most important elements of a bioretention facility in terms of long-term performance. The following are key factors to consider in determining an acceptable soil media mixture. General Filter Media Composition. Two soil mix designs are provided in Appendix C at the end of this specification. These mix designs are based on regional availability of materials and the desired infiltration characteristics. The post -construction bioretention feature must provide adequate infiltration capacity, however, so pre- and post -construction infiltration testing based on the mix design should be performed using the available materials. Where possible, on-site soils should be used for economy provided infiltration characteristics can be met. In this case, additional pre - construction tests should include particle size, or gradation analyses of these soils in accordance with ASTM D-422. Cation Exchange Capacity (CEC). The CEC of a soil refers to the total amount of positively charged elements that a soil can hold; it is expressed in milliequivalents per 100 grams (meq/100g) of soil. Soils with CECs exceeding 10 meq/100g are preferred for pollutant removal. For agricultural purposes, these cations are basic cations of calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+) and the acidic cations of hydrogen (H+) and aluminum (A13+). The CEC of the soil is determined in part by the amount of clay and/or humus or organic matter present. Increasing the organic matter content of any soil will help to increase the CEC, since it also holds cations as does clay. • Infiltration Rate. The bioretention soil media shall have a minimum infiltration rate of 1 to 2 inches per hour (a proper soil mix will have an initial infiltration rate that is significantly higher). • Depth. The standard minimum filter bed depth ranges from 24 and 36 inches for Level 1 and Level 2 designs, respectively, (18 to 24 inches for rain gardens). If trees are included in the bioretention planting plan, tree planting holes in the filter bed must be hand -excavated and extend at least 4 ft deep to provide enough soil volume for the root structure of mature trees. Use turf, perennials or shrubs instead of trees to landscape shallower filter beds. • Mulch. A 3 to 4 inch layer of mulch on the surface of the filter bed enhances plant survival, suppresses weed growth, and pre -treats runoff before it reaches the filter media. Shredded, aged hardwood mulch or pine straw make very good surface cover, as they retains a significant amount of nitrogen and typically will not float away. • Alternative to Mulch Cover. In some situations, designers may consider alternative surface covers such as turf, native groundcover, erosion control matting (coir or jute matting), river stone, or pea gravel. The decision regarding the type of surface cover to use should be based on function, cost and maintenance. 6.7. Underdrain and Underground Storage Layer Level 1 designs require an underdrain surrounded by a jacket of 1 inch stone unless the minimum of 0.5 inch/hour infiltration rate is met. For some Level 2 designs on an underdrain will not be required (where soil infiltration rates meet minimum standards; see Section 6.2 and Section 3 design tables). For Level 2 designs with an underdrain, an underground storage layer of at 12 inches shall be incorporated below the Appendix 8 - GSP Specifications Drainage Criteria Manual i , invert of the underdrain. The depth of the storage layer will depend on the target treatment and storage volumes needed to meet water quality, channel protection, and/or flood protection criteria. However, the bottom of the storage layer must be at least 2 ft above the seasonally high water table. The storage layer shall consist of approved clean, washed stone or an approved infiltration module. All bioretention basins shall include observation wells. The observation wells shall be tied into any T's or Y's in the underdrain system, and shall extend upwards to be flush with the surface, with a vented cap. In addition, cleanout pipes shall be provided if the contributing drainage area exceeds 1 acre. 6.8. Bioretention Planting Plans A landscaping plan must be provided for each bioretention area. Minimum plan elements shall include the proposed bioretention template to be used, delineation of planting areas, the planting plan, including the size, the list of planting stock, sources of plant species and the planting sequence, including post -nursery care and initial maintenance requirements. The planting plan must be prepared by a licensed landscape architect in order to tailor the planting plan to the site-specific conditions. Native plant species are preferred over non-native species, but some ornamental species may be used for landscaping effect if they are not aggressive or invasive. Refer to Appendix 1-13 for a list of suitable species for bioretention plants. The planting template refers to the form and combination of native trees, shrubs, and perennial ground covers that maintain the appearance and function of the bioretention area. Planting templates may be of the following types: Ornamental planting. This option includes perennials, sedges, grasses and/or trees in a mass bed planting. This template is acceptable for commercial sites where visibility is important. This template requires maintenance much like traditional landscape beds. Forest -type planting. This option plants a variety of seedlings and saplings modeled from characteristics of a forest ecosystem. This template is appropriate for large bioretention areas located at wooded edges or where a wooded buffer is desired. Refer to Reforestation GSP-10 for appropriate planting densities. Meadow Planting. This is a lower maintenance approach that focuses on the herbaceous layer and may resemble a wildflower meadow or prairie(e.g., with Joe Pye weed, black-eyed susan, sedges, grasses, etc.). The goal is to establish a more natural look that may be appropriate if the facility is located in a lower maintenance area (e.g., further from buildings and parking lots). Shrubs and trees may be incorporated. Erosion control matting may be used in lieu of the conventional mulch layer. Turf. This option is typically restricted to on -lot micro-bioretention applications, such as a front yard rain garden. Grass species should be selected that have dense cover, are relatively slow growing, and require the least mowing and chemical inputs (e.g., fine fescue, tall fescue). Perennial garden. This option uses herbaceous plants and native grasses to create a garden effect with seasonal cover. It may be employed in both micro -scale and small scale bioretention applications. This option is attractive, but it requires more maintenance in the form of weeding. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Perennial garden with shrubs. This option provides greater vertical form by mixing native shrubs and perennials together in the bioretention area. This option is frequently used when the filter bed is too shallow to support tree roots. Shrubs shall have a minimum height of 30 inches. Tree, shrub and herbaceous plants. This is the traditional landscaping option for bioretention. It produces the most natural effect, and it is highly recommended for bioretention basin applications. The landscape goal is to simulate the structure and function of a native forest plant community. Turf and tree. This option is a lower maintenance version of the tree -shrub -herbaceous option 4, where the mulch layer is replaced by turf cover. Trees are planted within larger mulched islands to prevent damage during mowing operations. The choice of which planting template to use depends on the scale of bioretention, the context of the site in the urban environment, the filter depth, the desired landscape amenities, and the future owner's capability to maintain the landscape. In general, the vegetative goal is to cover up the filter surface with vegetation in a short amount of time. This means that the herbaceous layer is equally or more important than widely -spaced trees and shrubs. The following additional guidance is provided regarding developing an effective bioretention landscaping plan: • Plants shall be selected based on a specified zone of hydric tolerance and must be capable of surviving both wet and dry conditions. • "Hydrophytic" or "water tolerant" species shall be planted near the center, whereas upland species do better planted near the edge. • Woody vegetation shall not be located at points of inflow; trees shall not be planted directly above underdrains, but shall be located closer to the perimeter. • If trees are part of the planting plan, a tree density of approximately one tree to 580 - 900 sq ft (i.e., 24 - 30 ft on -center) is recommended. • Shrubs and herbaceous vegetation shall generally be planted in clusters and at higher densities than trees (10 ft on -center and 1 to 1.5 ft on -center, respectively). • Temporary or supplemental irrigation may be needed for the bioretention plantings in order for plant installers to provide a warranty regarding plant material survival. • Supplemental irrigation by a rain tank system is also recommended (See GSP-11 Rain Tanks/Cisterns). Designers shall also remember that planting holes for trees must be at least 4 ft deep to provide enough soil volume for the root structure of mature trees. This applies even if the remaining soil media layer is shallower than 4 ft. • If trees are used, plant shade -tolerant ground covers within the drip line. • Maintenance is an important consideration in selecting plant species. Plant selection differs if the area will be frequently mowed, pruned, and weeded, in contrast to a site which will receive minimum annual maintenance. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.9. Bioretention Material Specifications Table 1.3 outlines the standard material specifications used to construct bioretention areas Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 1.3. Bioretention material specifications. Material Specification Notes Filter Media Filter Media shall consist of material specified The volume of filter media based on 110% Composition in Appendix C of the plan volume, to account for settling or compaction. Filter Media CEC greater than 10 meq/100g Testing Mulch Layer Use mulch meeting requirements specified Provide a 3- to 4 -inch layer on the surface of above. the filter bed. Alternative Use river stone or pea gravel, coir and jute Provide a 2- to 3- inch layer of cover to Surface Cover matting. suppress weed growth. Use anon -woven geotextile fabric with a Apply only to the sides and above the Geotextile/Liner flow rate of > 110 gal./min./sq. ft. (e.g., underdrain. For potential high pollutant Geotex 351 or equivalent) areas and certain karst sites only, use an appropriate liner on bottom. Lay a 2- to 4 -inch layer of sand over a 2 in. layer of filter or choker stone (a finer stone Filter Layer intended to impede flow of sediment into voids in larger stone layer, typically #8 or #89 washed gravel), which is laid over the underdrain stone. Stone Jacket for 1 -in. stone shall be double -washed and 12 inches for the underdrain; Underdrain clean and free of all fines (e.g., #57 stone). 12 to 18 inches inches for the stone storage and/or Storage Layer layer, if needed Use 6 inch PVC or corrugated HDPE pipe Lay the perforated pipe under the length of with 3/8 -in. perforations at 6 in. on center; the bioretention cell, and install non- Underdrains, position each pipe on a 1% or 2% slope perforated pipe as needed to connect with Cleanouts, and located no more than 20 ft. from the next the storm drain system. Install T's and Y's as Observation Wells pipe. Inter -connect pipe network to ensure needed, depending on the underdrain drainage. configuration. Extend cleanout pipes to the surface with vented caps at the Ts and Ys. Shrubs: Minimum 3 gal or 18-24 inches ht at Establish plant materials as specified in the 4 ft o.c. (or 1plant/15 sq ft). landscaping plan and the recommended Grasses, sedges, perennials: Size and max. plant list. spacing as called for in Tables 1.4 and 1.5 Alternate plant specification: When using the Plant Materials herein, and Appendix D. Forest -type planting template, Reforestation Trees: Minimum size 2 in. caliper, maximum planting densities noted in GSP-10 may be spacing for Level 2 design of 1 tree/580 used to meet these plant material sq.ft. (NOTE: the 2 in. cal, minimum allows it specifications. to be counted toward landscape ordinance requirements) Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Shallow Bedrock and Groundwater Connectivity Many areas within Fayetteville have shallow bedrock (less than 5 ft below ground surface, which can constrain the application of deeper bioretention areas (particularly Level 2 designs). In such settings, the following design adaptations may be helpful: • A linear approach to bioretention, using multiple cells leading to the ditch system, helps conserve hydraulic head. • The minimum depth of the filter bed may be 18 to 24 inches. It is useful to limit surface ponding to 6 to 9 inches and avoid the need for additional depth by establishing a turf cover rather than using mulch. The shallower media depth and the turf cover generally comply with the Water Quality Swale specification, and therefore will be credited with a slightly lower pollutant removal (See Water Quality Swales GSP-05). • It is important to maintain at least a 0.5% slope in the underdrain to ensure positive drainage. • The underdrain shall be tied into the ditch or conveyance system. 7.2. Steep Terrain In steep terrain, land with a slope of up to 15% may drain to a bioretention area, as long as a two -cell design is used to dissipate erosive energy prior to filtering. The first cell, between the slope and the filter media, functions as a forebay to dissipate energy and settle any sediment that migrates downslope. Designers may also want to terrace a series of bioretention cells to manage runoff across or down a slope. The drop in slope between cells shall be limited to 1 foot and shall be armored with river stone or a suitable equivalent to resist erosion and piping. 7.3. Karst Terrain Karst regions are found in and around Fayetteville, which complicates both land development and stormwater design. For the most recent publicly available data, please refer to the Karst Area Sensitivity Map of Washington County (The Nature Conservancy, 2007) at: http_//www.nwarpc.org/pdf/GIS- Imagery/KASM WASHINGTON CO.pdf. In karst sensitive areas, a karst survey shall be performed by a qualified professional. While bioretention areas produce less deep ponding than conventional stormwater practices (e.g., ponds and wetlands), Level 2 infiltration -based GSPs are not recommended in any area with a moderate or high risk of sinkhole formation (Hyland, 2005). On the other hand, Level 1 designs with adequate vertical separation from shallow groundwater (3 ft) and possess an impermeable bottom liner and underdrain may be appropriate. In general, micro-bioretention and bioretention basins with contributing drainage areas not exceeding 20,000 sq ft are preferred (compared to bioretention with larger drainage areas), in order to reduce the potential for sinkhole formation. However, it may be advisable to increase setbacks from structures. Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 8: CONSTRUCTION 8.1. Construction Sequence Construction Stage Erosion and Sediment Controls. Small-scale bioretention areas should be fully protected from construction impacts by silt fence or construction fencing, particularly if they will rely on infiltration (i.e., have no underdrains.) Completed bioretention cells should remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. Bioretention basin locations may be used as small sediment traps or basins during construction. However, these must be accompanied by notes and graphic details on the appropriate plan specifying that (1) the maximum excavation depth at the construction stage must be at least 1 foot above the completed installation, and (2) the completed bioretention facility must contain an underdrain. The plans and specifications must also show the proper procedures for converting the temporary sediment control practice to a permanent bioretention facility, including dewatering, cleanout and stabilization. 8.2 Bioretention Installation The following is a typical construction sequence to properly install a bioretention basin. The construction sequence for a rain garden is more simplified. These steps may be modified to reflect different bioretention applications or expected site conditions: Step 1. Construction of the bioretention area may only begin after the entire contributing drainage area has been stabilized with vegetation. THIS IS THE MOST IMPORTANT FACTOR DETERMINING THE SUCCESS OR FAILURE OF THE BIORETENTION AREA. BIORETENTION AREAS WILL FAIL IF SEDIMENT IS ALLOWED TO FLOW INTO THEM. It may be necessary to block certain curb or other inlets while the bioretention area is being constructed. The proposed site shall be checked for existing utilities prior to any excavation. Step Z. The designer and the installer shall have a preconstruction meeting, checking the boundaries of the contributing drainage area and the actual inlet elevations to ensure they conform to original design. Since other contractors may be responsible for constructing portions of the site, it is quite common to find subtle differences in site grading, drainage and paving elevations that can produce hydraulically important differences for the proposed bioretention area. The designer should clearly communicate, in writing, any project changes agreed upon during the preconstruction meeting to the installer and to the City as required, together with an explanation of the impact of such changes. Step 3. Temporary erosion and sedimentation controls are needed during construction of the bioretention area to divert stormwater away from the bioretention area until it is completed. Special protection measures such as erosion control fabrics may be needed to protect vulnerable side slopes from erosion during the construction process. At a minimum, these measures shall comply with current ADEQ standards. Step 4. Any pre-treatment cells shall be excavated first and then sealed to trap sediments. Step 5. Excavators or backhoes shall work from the sides to excavate the bioretention area to its appropriate design depth and dimensions. Excavating equipment should have scoops with adequate reach and should not be inside the footprint of the bioretention area. Contractors may use a cell construction approach in larger Appendix 8 - GSP Specifications Drainage Criteria Manual i , bioretention basins, whereby the basin is split into 500 to 1,000 sq ft temporary cells with a 10-15 ft earth bridge in between, so that cells may be excavated from the side. Step 6. It may be necessary to rip the bottom soils to a depth of 6 to 12 inches to promote greater infiltration. Step 7. Place geotextile fabric on the sides of the bioretention area with a minimum 6 -inch overlap on the sides. If a stone storage layer will be used, place the appropriate depth of stone on the bottom, install the perforated underdrain pipe, pack stone to 3 inches above the underdrain pipe, and add approximately 3 inches of choker stone/pea gravel as a filter between the underdrain and the soil media layer. If no stone storage layer is used, start with 6 inches of stone on the bottom, and proceed with the layering as described herein. Step B. Deliver or prepare the soil media, and store it on an adjacent impervious area or plastic sheeting. Apply the media in 12 -inch lifts without additional compaction until the desired top elevation of the bioretention area is achieved. Wait a few days to check for settlement, and add additional media, as needed, to achieve the design elevation. Where outlet structures are used, approved structural fill must be placed around the structure in accordance with design details. Structural earthen fill shall be used and compacted to a minimum of 95% Standard Proctor and +/-2 % of optimum moisture content within 2 ft of the structure. If piping or clogging occurs due to sediment transport, adjacent fill must be removed and replaced in the piping or clogged areas and the feature properly reconstructed to address the problem. Step 9. Prepare planting holes for any trees and shrubs, install the vegetation, and water accordingly. Install any temporary irrigation. Step 10. Place the surface cover in both cells (mulch, river stone or turf), depending on the design. If coir or jute matting will be used in lieu of mulch, the matting will need to be installed prior to planting (Step 9), and holes or slits will have to be cut in the matting to install the plants. Step 11. Install the plant materials as shown in the landscaping plan and as per City of Fayetteville Standards, and water them during weeks of no rain for the first two months. This should be continued until the plants are fully established. PLANT SURVIVAL AND MAINTENANCE IS CRITICAL TO PROPER PERFORMANCE OF THE BIORETENTION SYSTEM. Step 12. Conduct the final construction inspection (see Section 9.2). Then log the GPS coordinates for each bioretention facility and submit them to the City Planning/Engineering Department. 8.3. Construction Inspection Construction inspections shall be performed by the design engineer and the landscape architect to ensure that the bioretention system is installed in accordance with the plans and specifications. Items to inspect during construction include but are not limited to the following: • Pretreatment systems (if applicable) • Subgrade (check to make sure subgrade has not been compacted) • Filter fabric installation Appendix 8 - GSP Specifications Drainage Criteria Manual i , • Underdrain system (if applicable) • Gravel media • Engineered soil media • Plant installation • Inlet and Outlet elevations • Overflow structures SECTION 9: MAINTENANCE 9.1. Maintenance The requirements for maintenance include the development of a Long Term Maintenance Plan (LTMP) by the design engineer. The LTMP contains a description of the stormwater system components and information on the inspection and maintenance activities. 9.2. First Year Maintenance Operations Successful establishment of bioretention areas requires that the following tasks be undertaken in the first year following installation: • Initial inspections. For the first 6 months following construction, the site shall be inspected at least twice after storm events that exceed 0.5 inch of rainfall. • Spot Reseeding. Inspectors shall look for bare or eroding areas in the contributing drainage area or around the bioretention area, and make sure they are immediately stabilized with grass cover. • Fertilization. One-time, spot fertilization may be needed for initial plantings. A better choice may be a slow release fertilizer that can be incorporated into the planting medium. • Watering. Watering is needed once a week during the first 2 months, and then as needed during first growing season (April -October), depending on rainfall. • Remove invasives and replace dead plants. Since up to 10% of the plant stock may die off in the first year, construction contracts shall include a care and replacement warranty to ensure that vegetation is properly established and survives during the first growing season following construction. The typical thresholds below which replacement is required are 85% survival of plant material and 100% survival of trees. Long-term survival shall be in accordance with City of Fayetteville Tree and Landscape requirements. 9.3. Maintenance Inspections It is highly recommended that a spring maintenance inspection and cleanup be conducted at each bioretention area. The following is a list of some of the key maintenance problems to look for: • Check to see if 75% to 90% cover (mulch plus vegetative cover) has been achieved in the bed, and measure the depth of the remaining mulch. Remove invasives. Appendix 8 - GSP Specifications Drainage Criteria Manual i , • Check for sediment buildup at curb cuts, gravel diaphragms or pavement edges that prevents flow from getting into the bed, and check for other signs of bypassing. • Check for any winter- or salt -killed vegetation, and replace it with hardier species. • Note presence of accumulated sand, sediment and trash in the pre-treatment cell or filter beds, and remove it. • Inspect bioretention side slopes and grass filter strips for evidence of any rill or gully erosion, and repair it. • Check the bioretention bed for evidence of mulch flotation, excessive ponding, dead plants or concentrated flows, and take appropriate remedial action. • Check inflow points for clogging, and remove any sediment. • Look for any bare soil or sediment sources in the contributing drainage area, and stabilize them immediately. • Check for clogged or slow -draining soil media, a crust formed on the top layer, inappropriate soil media, or other causes of insufficient filtering time, and restore proper filtration characteristics. 9.4 Routine and Non -Routine Maintenance Tasks Maintenance of bioretention areas shall be integrated into routine landscape maintenance tasks. If landscaping contractors will be expected to perform maintenance, their contracts shall contain specifics on unique bioretention landscaping needs, such as maintaining elevation differences needed for ponding, proper mulching, sediment and trash removal, and limited use of fertilizers and pesticides. A customized maintenance schedule must be prepared for each bioretention facility, since the maintenance tasks will differ depending on the scale of bioretention, the landscaping template chosen, and the type of surface cover. A generalized summary of common maintenance tasks and their frequency is provided in Table 1.4. The most common non -routine maintenance problem involves standing water. If water remains on the surface for more than 48 hours after a storm, adjustments to the grading may be needed or underdrain repairs may be needed. The surface of the filter bed shall also be checked for accumulated sediment or a fine crust that builds up after the first several storm events. There are several methods that can be used to rehabilitate the filter (try the easiest things first, as listed below): • Open the underdrain observation well or cleanout and pour in water to verify that the underdrains are functioning and not clogged or otherwise in need of repair. The purpose of this check is to see if there is standing water all the way down through the soil. If there is standing water on top, but not in the underdrain, then there is a clogged soil layer. If the underdrain and stand pipe indicates standing water, then the underdrain must be clogged and will need to be snaked. • Remove accumulated sediment and till 2 to 3 inches of sand into the upper 8 to 12 inches of soil. • Install sand wicks from 3 inches below the surface to the underdrain layer. This reduces the average concentration of fines in the media bed and promotes quicker drawdown times. Sand wicks can be installed by excavating or augering (using a tree auger or similar tool) down to the gravel storage zone to create vertical columns which are then filled with a clean open -graded coarse sand material Appendix 8 - GSP Specifications Drainage Criteria Manual i , (ASTM C-33 concrete sand or similar approved sand mix for bioretention media). A sufficient number of wick drains of sufficient dimension shall be installed to meet the design dewatering time for the facility. • Remove and replace some or all of the soil media and plants. Table 1.4. Suggested Annual Bioretention Maintenance Activities. Maintenance Tasks Frequency Mowing of grass filter strips and bioretention turf cover At least 4 times a year Spot weeding, erosion repair, trash removal, and mulch raking Twice during growing season Add reinforcement planting to maintain the desired vegetation density As needed Remove invasive plants using recommended control methods As needed Stabilize the contributing drainage area to prevent erosion As needed Spring inspection and cleanup Annually Supplement mulch to maintain a 3 inch minimum layer Annually Prune trees and shrubs Annually Remove sediment in pre-treatment cells and inflow points Once every 2 to 3 years Replace the mulch layer Every 3 years SECTION 10: AS -BUILT REQUIREMENTS After the bioretention area has been constructed, the developer must provide the city engineers an as -built certification of the bioretention area prepared by a registered Professional Engineer. The as -built certification verifies that the BMP was installed as designed and approved. The following components are vital to ensure that the bioretention area works properly and they must be addressed in the as -built certification: 1. Pretreatment, such as a grass filter strip, for coarser sediments must be provided to prevent premature clogging of the system. Design guidance for grass filter strips used as pretreatment is provided in herein. 2. Surrounding drainage areas must be stabilized to prevent sediment from clogging the filter media. 3. Correct ponding depths and infiltration rates must be maintained to prevent killing vegetation. 4. The landscape plan must be provided. A mechanism for overflow for storm events exceeding design capacity must be provided. SECTION 11: COMMUNITY & ENVIRONMENTAL CONCERNS Potential areas of high pollutant loading are operations or activities that are known to produce higher concentrations of stormwater pollutants and/or have a greater risk for spills, leaks or illicit discharges. Table 1.5 presents a list of potential land uses or operations that may be designated as such areas. It should be noted that the actual area may only occupy a portion of the entire proposed use, and that some "clean" Appendix 8 — GSP Specifications Drainage Criteria Manual i , areas (such as rooftops or landscaping) can be diverted away to another infiltration or runoff reduction practice. Development proposals should be carefully reviewed to determine if any future operation, on all or part of the site, will be designated as an area of high pollutant loading. Based on this designation, infiltration may be restricted or prohibited within such areas. For the restricted category, use of infiltration may be permitted for employee parking and rooftop drainage areas, and if low permeability linings are used with an underdrain. Note: For a full list of potential stormwater hotspots. Consult Schueler et al. (2004). ■ depends on facility. ✓ criterion applies. 1. For some facilities, infiltration practices will be permitted for certain areas such as employee parking and roof drainage. SECTION 12: RIGHT OF WAY CONSIDERATIONS Bioretention can be used in the right of way and is a preferred practice for constrained right of ways when designed as a series of individual on-line or off-line cells. In these situations, the final design closely resembles that of water quality swales. Stormwater can be conveyed to the bioretention area by sheet flow, curb cuts, or grass channels. For details on bioretention setback from streets, please refer to Master Streets Plan Section 12.2.2 Detail 2b. The base of the bioretention feature shall be positioned on a 2H:1V slope (or Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 1.5. Potential Potential Area of High Pollutant Loading Areas of High Pollutant Loading. Restricted Infiltration No Infiltration' Facilities w/NPDES Industrial permits ■ ■ Public works yard ✓ Railroads/ equipment storage ✓ Auto and metal recyclers/scrap yards ✓ ✓ Petroleum storage facilities ✓ Highway maintenance facilities ✓ Wastewater, solid waste and composting facilities ✓ Industrial machinery and equipment ✓ Trucks and trailers ✓ Airfields ✓ Aircraft maintenance areas ✓ Fleet storage areas ✓ Parking lots (40 or more parking spaces) ✓ Gas stations ✓ Highways (2500 ADT) ✓ Construction business (paving, heavy equipment storage and maintenance ✓ Retail/wholesale vehicle/ equipment dealers ✓ Convenience stores/fast food restaurants ✓ Vehicle maintenance facilities ✓ Car washes ✓ Nurseries and garden centers ✓ Golf courses ✓ Note: For a full list of potential stormwater hotspots. Consult Schueler et al. (2004). ■ depends on facility. ✓ criterion applies. 1. For some facilities, infiltration practices will be permitted for certain areas such as employee parking and roof drainage. SECTION 12: RIGHT OF WAY CONSIDERATIONS Bioretention can be used in the right of way and is a preferred practice for constrained right of ways when designed as a series of individual on-line or off-line cells. In these situations, the final design closely resembles that of water quality swales. Stormwater can be conveyed to the bioretention area by sheet flow, curb cuts, or grass channels. For details on bioretention setback from streets, please refer to Master Streets Plan Section 12.2.2 Detail 2b. The base of the bioretention feature shall be positioned on a 2H:1V slope (or Appendix 8 — GSP Specifications Drainage Criteria Manual i , gentler) from edge of pavement (or curb, if applicable). If installation is within 10 ft of pavement subgrade, also refer to Urban Bioretention Practice GSP-02 for additional details. SECTION 13: REFERENCES CWP, 2007, National Pollutant Removal Performance Database, Version 3.0. Center for Watershed Protection, Ellicott City, MD. Lake County, CH. Bioretention Guidance Manual. Available online at: http://www2.lakecountyohio.org/smd/Forms.htm LIDC, 2003. Bioretention Specification. The Low Impact Development Center, Inc, Beltsville, MD. Available at: http_//www.lowimpactdevelopment.org/epa03/biospec.htm. Hirschman, D., L. Woodworth and S. Drescher, 2009. Technical Report: Stormwater BMPs in Virginia sJames River Basin - An Assessment of Field Conditions and Programs. Center for Watershed Protection. Ellicott City, MD. Hunt, W.F. III and W.G. Lord, 2006. Bioretention Performance, Design, Construction, and Maintenance, North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG -588-5. North Carolina State University. Raleigh, NC. Hyland, S., 2005. Analysis of sinkhole susceptibility and karst distribution in the Northern Shenandoah Valley (Virginia): impacts for LID site suitability models. M.S. Thesis, Virginia Polytechnic Institute and State University. Blacksburg, VA. Maryland Department of the Environment, 2001. Maryland Stormwater Design Manual. Minnesota Stormwater Steering Committee (MSSC), 2005. The Minnesota Stormwater Manual, Minneapolis, MN. Prince George's Co., MD, Bioretention Manual. Schueler et al. 2007. Urban Stormwater Retrofit Practices. Manual in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD. Schueler, T., 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD. Sinclair Knight Merz and Boffa Miskell, 2008. North Shore City Bioretention Design Guidelines. Wellington, New Zealand. State of Virginia, 2010. BMP Specification No. 8 - Bioretention, Richmond, VA. VADCR, 2010. Stormwater Design Specification No. 9: Bioretention, version 1.7. Virginia Department of Conservation and Recreation, Richmond, VA. Wisconsin Department of Natural Resources, Stormwater Management Technical Standards. http:/ /www.dnr.state.wi.us/org/water/wm/nps /stormwater/techstds.htm#Post Appendix 8 - GSP Specifications Drainage Criteria Manual i , URBAN BIORETENTION Description: Urban Bioretention is similar to traditional bioretention practices, except that the bioretention is fit into concrete -sided containers within urban landscapes, such as planter boxes or tree box planters. Captured runoff is treated by filtration through an engineered soil medium, and is then either infiltrated into the subsoil or exfiltrated through an underdrain. Variations: • Stormwater planters - in landscaping areas between buildings and roadways or sidewalks • Green Street swales and planters - on street edge of sidewalk where street landscaping is normally installed • Proprietary planting cells. • Reduced runoff volume • Reduced peak discharge rate • Reduced TSS • Reduced pollutant loading • Reduced runoff temperature • Groundwater recharge (if soils are sufficiently permeable) • Habitat creation • Enhanced site aesthetics • Reduced heat island effect • Minimum 2 -ft vertical separation from groundwater is required - not suitable in areas with bedrock less than 4 ft below final grade • Not suitable for areas with high pollutant loads or within 100 ft of septic system fields • Maximum contributing drainage area of 2,500 sq ft • Min infiltration rate = 0.5 in. per hour in order to remove the underdrain requirement • Underdrain required if in Right of Way • Design to drain within 24 hours • Maximum running slope of 3% • Design to prevent building seepage in zero setback applications • Used along curbside in urban areas • Stormwater can be conveyed by sheet flow or curb cuts • Pretreatment is especially important in roadway applications where sediment loads may be high • Design as a series of cells running parallel to roadway. • Impermeable liner must be installed roadside to protect subgrade • Cannot create hazard or interfere with walkability • Setbacks must comply with city and utility -specific requirements Appendix B — GSP Specifications Drainage Criteria Manual i , Selection Criteria: Level 1- 60% Runoff Reduction Credit Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Regular maintenance of landscaping to maintain healthy vegetative cover, surface raking to maintain permeability • Annual removal of invasives • Irrigation when necessary during first growing season • Annual inspection • Periodic trash removal • Tree replacement needed at 5-10 year intervals ©Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION Urban bioretention practices are similar in function to regular bioretention practices (GSP-01) except they are adapted to fit into "containers" within urban landscapes. Typically, urban bioretention is installed within an urban streetscape or city street Right of Way (ROW), urban landscaping beds, tree planters, and plazas. Urban bioretention is not intended for large commercial areas, nor shall it be used to treat small sub -areas of a large drainage area such as a parking lot. Rather, urban bioretention is intended to be incorporated into small fragmented drainage areas such as shopping or pedestrian plazas within a larger urban development. Urban Bioretention within the ROW can only be used to treat water that falls in the ROW. Urban bioretention features hard edges, often with vertical concrete sides, as contrasted with the more gentle earthen slopes of regular bioretention. If these practices are outside of the ROW, they may be open - bottomed, to allow some infiltration of runoff into the sub -grade, but they generally are served by an underdrain. SECTION 2: PERFORMANCE The runoff reduction function of an urban bioretention area is described in Table 2.1. Table 2.1. Urban Bioretention Runoff ReductionProvided by Urban Bioretention Areas. Stormwater Function Level 1 Design Level 2 Design Annual Runoff Volume Reduction (RR) 60% Level 1 Design Only Sources: CWP and CSN (2009); CWP (2007). SECTION 3: DESIGN CONSIDERATIONS Each urban bioretention variant is planted with a mix of trees, shrubs, and grasses as appropriate for its size and landscaping context. Stormwater planters (also known as vegetative box filters or foundation planters) take advantage of limited space available for stormwater treatment by placing a soil filter in a container located above ground or at grade in landscaping areas between buildings and roadways, provided with waterproof liner protection (Figure 2.1). The small footprint of foundation planters is typically contained in a precast or cast -in-place concrete vault. Other materials may include molded polypropylene cells and precast modular block systems. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Stormwater planters must be outside the ROW if they are treating roof water or runoff from areas outside of the ROW as additional surface drainage shall not be directed into the ROW. Powce 0ItgoFP-Xd&0d, 4R Figure 2.1. Stormwater planters (Source: City of Portland, OR). Green Street swales and planters are installed in the sidewalk zone near the street where urban street trees are normally installed. The soil volume for the tree pit is increased and used as a stormwater storage area (Figure 2.2). Treatment is increased by using a series of connected tree planting areas together in a row. The surface of the enlarged planting area may be mulch, overlain with grates or conventional pavement (subject to approval of the Urban Forester). Large and shared rooting space and a reliable water supply increase the growth and survival rates in this otherwise harsh planting environment. Figure 2.2. Deaderick Street tree planters (Source: City of Nashville, TN). Appendix 8 - GSP Specifications Drainage Criteria Manual i , Design criteria for urban bioretention is detailed in Table 2.2 in Section S. SECTION 4: TYPICAL DETAILS Figures 2.3 through 2.9 provide some typical details for several bioretention configurations. Plant and tree selection tables are provided in Appendix D. Planting plans shall be prepared by a licensed Landscape Architect. Figure 2.3. Stormwater planter cross-section (Source: VADCR, 2010). Appendix 8 - GSP Specifications Drainage Criteria Manual I , BUILDING OV=RrLow I DnW l POUT OR (SET 2 IN. GELOIN OTHER TOP OP PLANTER) yx " timed` c GNVEYANGE SYSTEM _ SPLP.3H ROCK&SL K } I V. 12 IN. MA,X' f 3 IN- MN DEPTH •r __ PEACRAV=L ,t-r� k+y'Ak'Ftl�?Fii]C�F 1 :y 4 V}1._ _u=:,•: ; k: �, BUILDING 90 IN. SIN V/,SNEEDEL7} r 12 IN. WiAVEL -Y fiX8 IN. TO &81N. OR' } Y RV'PAQVP_! -QLJ 1 FOUNIDATIUM DRAINS r14S Rrc UIRr•} PLANTER I WALLS PERFORATED PI E TO RUN LENGTk OF PLANTER PIPE TO STOWDRA]N Oi 9 O'J I FALL F'OIrl r OFrAIL: TYF7 ;l URE4N EIGRETENTIG?4 NTS Figure 2.3. Stormwater planter cross-section (Source: VADCR, 2010). Appendix 8 - GSP Specifications Drainage Criteria Manual I , Q Ld a a 301 d 3a' W 4 W MAXtLP W h W L.1 Z W W J N W Ye af LJ v 4 a W W INLET 'LA 4TING ZO •d tl ° W W W d W a CD W W w W W W W W W W w W W S CURB 5 - GUTTER W W W W W W W W W W PLANTING ZONE A ) W I W W I W '1W W Yll W W W W W W W W L.1 Z W W J W W W af LJ W W Q 'LA 4TING ZO E A 3 W W W W W CD W W w W W W W W W W w W W S W W W W w W W W W PLANTING ZONE A 8'-0" MIN, 2% MAX SLOPE PEDESTRIAN ACCESS AREA p d 4 END SLOPE (MINIMUM 3:1) a e •a d ° a a•. o d CHECK DAM (OPTIONAL) " a e • � Q p d' p e. d a4 G a" ti d i 8 d 4 e q A Q d A 6 tl 4 4 7 � " d Q d Q 6 ? Q 4 Vd d. Q tl • Q p A-- END SLOPE e (MINIMUM 3:1) e. SIDEWALK Figure 2.4. Green streets swale plan view (Source: Portland, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , GSP-02: Urban Bioretention GSP02-6 4 dd d 7 a ° � a 4 Q 44 d- .nd" .. 4" NOTCH FOR SIDEWALK METAL INLET a w w DRAINAGE, AS NECESSARY 4- a CHANNEL & GRATE a w w 4 W 4 d W W Q 4 W Ld W a O B 4 w Z _- 4 a -.o 312 ° g W� 31 30' w w a q MAX d "d 4 w 44" d Q a W W� 4 CHECK DAM ° (OPTIONAL) ° W W w a 4" THICK d CONCRETE a W 14, 4 SPLASH PAD a AT INLET FF -4 a a dd e tl "a CONCRETE 4 3. O [ a ..4 4 " Z e q d" d d 4 a - 4 d d 4 Q "d a d' 4 4 4 a Q Q CURB 2'-6" 6" 3'-0" 6" SIDEWALK & PARKING MIN GUTTER EGRESS Figure 2.6. Green streets planter plan view with parking (Source: Portland, 2011) Appendix 8 - GSP Specifications Drainage Criteria Manual i , EXPOSED WALL FINISHED GRADE O OF f ' ■ F PLANTER CONCRETE OR 4" EYPOSED WALL APPROVED SURFACE [END SIDEWALK DRAINAGE NOTCH■ LOWER SIDEWALK, TO FACILITY ORMWATER 6' mil FACILITY CURB AND TO SOIL GUTTER AGGREGATE (WHEN REQUIRED) WATER PROOF WALL ;•�;��,- � A �..�•.=.fir. BENCH. � �v►CONSTRUCTION, IF NECESSARY r, NECESSARY EXISTING 1 UNDERDR&IN SECTION B -B PLANTER WITH PARKING OR WITHOUT PARKING Figure 2.7. Green streets planter section view with or without parking (Source: Portland, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS Table 2.2. Urban Bioretention Design Criteria Level 1 Design Only (RR: 60) Sizing (Refer to Section 6.1): Surface Area (sq ft) = Tv/Storage Depth' = ((1.0 inch)(Rv)(A)/12 — the volume reduced by an upstream BMP}/Storage Depth' Underdrain = Slotted corrugated HDPE with clean -outs (Refer to GSP-01, Section 6.7) Maximum Drainage Area = 2,500 sq ft Maximum Ponding Depth = 6 inches Filter media depth minimum = 30 inches; maximum = 48 inches; Media and Surface Cover (Refer to GSP-01, Section 6.6) Sub -soil testing (Refer to GSP-01, Section 6.2) Inflow = sheetflow, curb cuts, trench drains, roof drains, concentrated flow, or equivalent Building setbacks (Refer to Section 5) Long-term maintenance plan (Refer to GSP-01, Section 9) 1. Storage depth is based on the Void Ratios (V,) of the soil media and gravel layers and their respective thicknesses, plus the surface ponding depth. Compare as described in Section 6.1. In general, urban bioretention has the same constraints as regular bioretention (See GSP-01), along with a few additional constraints as noted below: Contributing Drainage Area. Urban bioretention is classified as a micro-bioretention practice and is therefore limited to 2,500 sq ft of drainage area to each unit. However, this is considered a general rule; larger drainage areas may be allowed with sufficient flow controls and other mechanisms to ensure proper function, safety, and community acceptance. The drainage areas in these urban settings are typically considered to be 100% impervious. While multiple units can be installed to maximize the treatment area in ultra -urban watersheds, urban bioretention is not intended to be used as treatment for large impervious areas (such as parking lots). Adequate Drainage. Urban bioretention practice elevations must allow the untreated stormwater runoff to be discharged at the surface of the filter bed and ultimately connect to the local storm drain system. Available Hydraulic Head. In general, 3 ft of elevation difference is needed between the downstream storm drain invert and finished grade of the urban bioretention practice. A positive hydraulic differential must be maintained through all design storm and runoff events. This is generally not a constraint, due to the standard depth of most storm drain systems. Utilities, Infrastructure, and Setbacks. Urban bioretention practices frequently compete for space with a variety of utilities. Since they are often located parallel to the road right-of-way, care shall be taken to accommodate utility -specific horizontal and vertical setbacks. However, conflicts with water and sewer laterals (e.g., house connections) may be unavoidable, and the construction sequence must be altered as necessary to avoid impacts to existing service. Use of a tree planter box adjacent to streets may follow the tree well detail provided in the Tree and Landscape Manual for the City of Fayetteville, though a structural Appendix 8 — GSP Specifications Drainage Criteria Manual i , planter base may be used to minimize conflicts with existing utility infrastructure. Additional Tree Box Filter details may be found in the University of Arkansas Community Design Center Low Impact Development Manual. Adjacent to buildings, an impermeable liner and an underdrain shall be used and properly installed. Otherwise, the urban bioretention practice shall be positioned a minimum of 10 ft from the building. For ground -level installations, maximum pool elevation shall be at least 0.5 ft below the lowest adjacent grade of the building. Designers shall also plan future tree canopy heights achieved in conjunction with urban bioretention practices to not conflict with existing overhead telephone, cable communications and power lines. Minimizing External impacts. Because urban bioretention practices are installed in highly urban settings, individual units may be subject to higher public visibility, greater trash loads, pedestrian use traffic, vandalism, and even vehicular loads. These practices shall be designed in ways that prevent, or at least minimize, such impacts. In addition, designers shall clearly recognize the need to perform frequent landscaping maintenance to remove trash, check for clogging, and maintain vigorous vegetation. The urban landscape context may feature naturalized landscaping or a more formal design. When urban bioretention is used in sidewalk areas of high foot traffic, designers shall not impede pedestrian movement or create a safety hazard and maintain the American with Disabilities Act (ADA) required path of travel. Designers may also install low fences (such as a low garden fence), grates or other measures to prevent damage from pedestrian short -cutting across the practices. SECTION 6: DESIGN CRITERIA Urban bioretention practices are similar in function to regular bioretention practices except they are adapted to fit into "containers" within urban landscapes. Therefore, special sizing accommodations are made to allow these practices to fit in very constrained areas where other surface practices may not be feasible. 6.1. Sizing of Urban Bioretention The required surface area of the urban bioretention filter is calculated by dividing the Treatment Volume by the Equivalent Storage Depth (Equation 2.2 below), in the same manner as it is calculated for traditional bioretention. The equivalent storage depth is computed as the depth of media, gravel, or surface ponding (in feet) multiplied by the accepted ratio. The maximum allowable Void Ratios (Vr) are: Bioretention Soil Media (GSP-01) Vr = 0.40 (sandy loam, loamy sand, or loam) Gravel Vr = 0.35 Surface Storage Vr = 1.0 The equivalent storage depth for an urban bioretention facility with, for example, a 6 -inch surface ponding depth, a 30 -inch soil media depth, and a 12 -inch gravel layer is therefore computed as: Equation 2.1. Urban Bioretention Equivalent Storage Depth (ESD) Weighted Storage Depth = Dw = Vrl(D1) + Vr2(D2) + "' (1.0 x 0.5ft) + (0.4 x 2.5ft) + (0.35 x 1.0ft) = 1.85ft Appendix 8 - GSP Specifications Drainage Criteria Manual i , Where Vr1 and D1 are for the first layer, etc. The Surface Area (SA) corresponding to this example may be computed as: Where: Where: Equation 2.2. Urban Bioretention Sizing SA (sq ft) = TV(cu ft) /Dw SA (sq ft) = T„ (cu ft) / ESD SA = Minimum surface area of the urban bioretention facility (sq ft) T„ = Required Treatment Volume (in cu ft) Equation 2.3. Treatment Volume T„ _ (1.0 inch)(Rv)(A)/12] Tv = Treatment Volume (cu ft) A = the contributing drainage area (sq ft) (NOTE: Rv = the composite runoff coefficient from the RR Method. A table of RV values and the equation for calculating a composite Rv is located in Section 5.3 of this chapter). Equations 2.1 and 2.2 should be modified with the addition of any surface or subsurface storage components (e.g., additional area of surface ponding, subsurface storage chambers, etc.). 6.2. General Design Criteria for Urban Bioretention Design of urban bioretention shall follow the general guidance presented in this design specification. The actual geometric design of urban bioretention is usually dictated by other landscape elements such as buildings, sidewalk widths, utility corridors, retaining walls, etc. Designers can divert fractions of the runoff volume from small impervious surfaces into micro-bioretention units that are integrated with the overall landscape design. Inlets and outlets should be located as far apart as possible. The following is additional design guidance that applies to all variations of urban bioretention: • The ground surface of the micro-bioretention cell shall slope a minimum of 1% towards the outlet, unless a stormwater planter is used. • The soil media depth shall be a minimum of 30 inches. • If large trees and shrubs are to be installed, soil media thickness shall be a minimum of 4 ft. • All urban bioretention practices shall be designed to fully drain within 24 hours. • Any grates used above urban bioretention areas must be removable to allow maintenance access, beADA compliant and, if for tree planters, be approved by the Urban Forester. Appendix 8 - GSP Specifications Drainage Criteria Manual i , • The inlet(s) to urban bioretention shall be stabilized using coarse aggregate stone (cobblestone), splash block, river stone or other acceptable energy dissipation measures. The following forms of inlet stabilization are recommended: o Stone energy dissipators. o Sheet flow over a depressed curb with a 3 -inch drop. o Curb cuts allowing runoff into the bioretention area. o Covered drains that convey flows under sidewalks from the curb or from downspouts (if the bioretention area is outside of the ROW). o Grates or trench drains that capture runoff from the sidewalk or plaza area. Pre-treatment options overlap with those of regular bioretention practices. However, the materials used may be chosen based on their aesthetic qualities in addition to their functional properties. For example, river rock may be used in lieu of rip rap. Other pretreatment options may include one of the following: A trash rack between the pre-treatment cell and the main filter bed. This will allow trash to be collected from one location. A trash rack across curb cuts. While this trash rack may clog occasionally, it keeps trash in the gutter, where it can be picked up by street sweeping equipment. Conform to City of Fayetteville requirements. • A pre-treatment area above ground, or with a manhole or grate directly over the pre-treatment area. Overflows can either be diverted from entering the bioretention cell or dealt with via an overflow inlet. Optional methods include the following: • Size curb openings to capture only the Treatment Volume and bypass higher flows through the existing gutter. • Use landscaping type inlets or standpipes with trash guards as overflow devices. • Use a pre-treatment chamber with a weir design that limits flow to the filter bed area. 6.3. Specific Design Issues for Stormwater Planters Since stormwater planters are often located near building foundations, waterproofing by using a watertight concrete shell or an impermeable liner that is designed and constructed to prevent seepage is required. 6.4. Specific Design Issues for Green Streets Swales and Planters The bottom of the soil media must be a minimum of 4 inches below the root ball of plants to be installed, 12 inches for shrubs and 24 inches for trees. • Green streets designs sometimes cover portions of the filter media with pervious pavers (if outside the ROW) or cantilevered sidewalks. In these situations, it is important that the filter media is connected beneath the surface so that stormwater and tree roots can share this space. Appendix 8 - GSP Specifications Drainage Criteria Manual i , • Installing a tree pit grate over filter bed media is one possible solution to prevent pedestrian traffic and trash accumulation. • Low, wrought iron fences can help restrict pedestrian traffic across the tree pit bed and serve as a protective barrier if there is a drop-off from the pavement to the micro-bioretention cell. • A removable grate capable of supporting typical H-20 loads may be used to allow the tree to grow through it. Tree grates shall be approved by the Urban Forester. • Each tree needs a minimum of 120 sq ft of shared root space. • Proprietary tree pit devices are acceptable, provided they conform to this specification, comply with the ordinance and are approved by the Urban Forester. 6.5. Planting and Landscaping Considerations The degree of landscape maintenance that can be provided will determine some of the planting choices for urban bioretention areas. The planting cells can be formal gardens or naturalized landscapes but shall comply with existing city ordinances. Landscaping in the ROW shall be designed to limit visual obstructions for pedestrian and vehicular traffic. For plant and tree species lists, refer to Appendix D, Table D.6. In areas where less maintenance will be provided and where trash accumulation in shrubbery or herbaceous plants is a concern, consider a "turf and trees" landscaping model. Spaces for herbaceous flowering plants can be included. This may be attractive at a community entrance location. Native trees or shrubs are preferred for urban bioretention areas, although some ornamental species may be used. As with regular bioretention, selected perennials, shrubs, and trees must be tolerant of drought, and inundation. The landscape designer shall also take into account that de-icing materials may accumulate in the bioretention areas in winter and could kill vegetation. Additionally, tree species selected shall be those that are known to survive well in the compacted soils and polluted air and water of an urban landscape. For additional information regarding recommended planting densities see GSP-01 Bioretention, Section 6.8: Bioretention Planting Plans for shrubs and GSP-10 Reforestation, Section 2: Design Criteria for trees. SECTION 7: MATERIAL SPECIFICATIONS Please consult GSP-01, Table 1.3 for the typical materials needed for filter media, stone, mulch, and other bioretention components. In urban planters, pea gravel or river stone may be a more appropriate and attractive mulch than shredded hardwood. The unique components for urban bioretention may include the inlet control device, a concrete box or other containing shell, impermeable liner, protective grates, and an underdrain that daylights to another stormwater practice or connects to the storm drain system. The underdrain shall: • Consist of slotted pipe greater than or equal to 4 inches in diameter, placed in a layer of washed (less than 1% passing a #200 sieve) crushed stone. • Have a minimum of 3 inches of gravel laid above and below the pipe. Appendix 8 - GSP Specifications Drainage Criteria Manual i , • Be laid at a minimum slope of 0.5 %. • Extend the length of the box filter from one wall to within 6 inches of the opposite wall, and may be either centered in the box or offset to one side. • Be separated from the soil media by non -woven, geotextile fabric or a 2 to 3 inch filter layer of 1/8 to 3/8 inch pea gravel. SECTION 8: CONSTRUCTION The construction sequence and inspection requirements for urban bioretention are generally the same as other bioretention practices. Consult the construction sequence and inspection guidance provided in GSP-01. In cases where urban bioretention is constructed in the road or ROW, the construction sequence may need to be adjusted to account for traffic control, pedestrian access and utility notification. Additionally, urban bioretention areas shall only be constructed after the drainage area to the facility is completely stabilized and the inlet areas completely protected from construction traffic. The specified growth media shall be placed and spread by hand, in order to avoid compaction and maintain the porosity of the media. The media shall be placed in 12 inch lifts with no machinery allowed directly on the media during or after construction. The media shall be overfilled above the proposed surface elevation, as needed, to allow for natural settling. Lifts may be lightly watered to encourage settling. After the final lift is placed, the media shall be raked (to level it), saturated, and allowed to settle for at least one week prior to installation of plant materials. SECTION 9: AS-BUILTS After urban bioretention has been constructed, the developer must provide the city engineer with an as -built certification prepared by a registered Professional Engineer. The as -built certification verifies that the BMP was installed as designed and approved. The following components are vital to ensure that the bioretention area works properly and they must be addressed in the as -built certification: 1. The proper media and gravel depths were installed per plan. Photographs taken during phases of construction shall be included to demonstrate. 2. Surrounding drainage areas must be stabilized to prevent sediment from clogging the filter media. 3. Correct ponding depths and infiltration rates must be maintained to prevent killing vegetation. 4. The landscape plan must be provided. Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 10: MAINTENANCE Routine operation and maintenance are essential to gain public acceptance of highly visible urban bioretention areas. Weeding, pruning, the removal and replacement of dead vegetation and trash removal shall be done as needed to maintain the aesthetics necessary for community acceptance. During drought conditions, it may be necessary to water the plants, as would be necessary for any landscaped area. The requirements for maintenance include the development of a Long Term Maintenance Plan (LTMP) by the design engineer. The LTMP contains a description of the stormwater system components and information on the inspection and maintenance activities. To ensure proper performance, installers shall check that stormwater infiltrates properly into the soil within 24 hours after a storm. If excessive surface ponding is observed, corrective measures include inspection for soil compaction and underdrain clogging. Consult the maintenance guidance outlined in GSP-01. SECTION 11: RIGHT OF WAY DESIGN CONSIDERATIONS Green Street swales and planters are applicable along roads. They can be used along curbside in urban areas with stormwater being conveyed by sheet flow or curb cuts. Green Street swales and planters can also be designed as a series of cells running parallel to roadway. Successful designs in other locations have also featured an impermeable membrane separating the road subgrade from the bioretention feature. Please consult GSP-01, Table 1.3 for material slopes, types, and sizing. Connections to storm drain shall be consistent with connection requirements in the City of Fayetteville Drainage Criteria Manual. Figure 2.8. Flow-through planter (Source: Portland Bureau of Environmental Services). Appendix 8 - GSP Specifications Drainage Criteria Manual I , STRUCTURAL WALL Figure 2.9. Infiltration planter (Source: Portland Bureau of Environmental Services). BUILDING IMPERVIOUS F� DOWNSPOUT SURFACE a =• -• OROTHER \ �ti a� CONVEYANCE OVERFLOW' �- SYSTEM WALL OPENING ` ! GROWING GROWING SPLASH ROCKS ,C\SIJ MEDIUM MEDIUM BLOCK FILTER FABRIC .b.•E �•• WATERPROOF FILTER FABRIC-- BUILDING (AS NEEDED) 'a^��:• ^'�''••'�°••�'�"P' EXISTING - SOIL PERFDRATE6 PIPE PIPE TO t.— length of planter APPROVED DESTINATION Figure 2.8. Flow-through planter (Source: Portland Bureau of Environmental Services). Appendix 8 - GSP Specifications Drainage Criteria Manual I , STRUCTURAL WALL Figure 2.9. Infiltration planter (Source: Portland Bureau of Environmental Services). Figure 2.10. Portland State University street planters (Photo: Martina Keefe). Figure 2.11. Deaderick Street planters. SECTION 12: REFERENCES Center for Watershed Protection, 2006, Urban Watershed Forestry Manual. Part2: Conserving and Planting Trees at Development Sites. Ellicott City, MD. Available online at: http://www.cwp.org/forestry/index.htm City of Portland Bureau of Environmental Services (Portland BES), 2004, Portland Stormwater Management Manual, Portland, OR. http_//www.portlandonline.com/bes/index.cfm?c=dfbcc City of Portland Bureau of Environmental Services (Portland BES), 2011, Stormwater Management Manual Typical Details, Portland, OR. http://www.portlandonline.com/bes/index.cfm?c=47963 Credit Valley Conservation, 2008, Credit River Storm water Management Manual. Mississauga, Ontario. Northern Virginia Regional Commission, 2007, Low Impact Development Supplement to the Northern Virginia BMP Handbook, Fairfax, Virginia Saxton, K.E., W.J. Rawls, J.S. Romberger, and R.I. Papendick, 1986, Estimating generalized soil -water characteristics from texture, Soil Sci. Soc. Am. J. 50(4):1031-1036. Schueler, T., D. Hirschman, M. Novotney and J. Zielinski, 2007, Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series, Center for Watershed Protection, Ellicott City, MD. University of Arkansas Community Design Center, 2010, Low Impact Development, a design manual for urban areas, Fayetteville, AR. Virginia Department of Conservation and Recreation. (VADCR), 2010, Stormwater Design Specification No. 9, Appendix 9-A: Urban Bioretention IStormwater Planters/ Expanded Tree Planters IStormwater Curb Extensions, version 1. 7, Richmond, VA. Appendix 8 — GSP Specifications Drainage Criteria Manual i , PERMEABLE PAVEMENT Description: Permeable pavements allow stormwater runoff to filter through voids in the pavement surface into an underlying stone reservoir, where it is temporarily stored and/or infiltrated. Porous paving systems have several design variants. The four major categories are: 1) pervious concrete and asphalt; 2) modular block systems; 3) grass pavers; and 4) gravel pavers. All have a similar structure, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer and a filter layer or fabric installed on the bottom • Runoff volume reduction • Cost Selection Criteria: LEVEL 1— 45% Runoff Reduction Credit LEVEL 2 — 75% Runoff Reduction Credit Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Turf pavers can require mowing, fertilization, and irrigation. Plowing is possible, but requires use of skids • Sand and salt should not be applied • Adjacent areas should be fully stabilized with vegetation to prevent sediment - laden runoff from clogging the surface • A vacuum -type sweeper or high-pressure hosing (for porous concrete) should be used for cleaning ©Maintenance Burden L = Low M = Moderate H = High • Can increases aesthetic value • Maintenance, particularly associated with cold climates • Provides water quality treatment • Limited to low traffic areas with limited structural loading • Potential issues with handicap access • Infiltration can be limited by underlying soil property • Not effective on steep slopes • Installation procedures have significant impact on performance and longevity p • • Best used in low traffic and low load bearing areas • Applicable to parking lots (particularly overflow areas), driveways (commercial), sidewalks (outside the Right of Way), emergency access roads, maintenance roads and trails, etc. • While some studies have indicated maintenance challenges in cold climate regions, others have indicated that performance is not limited as long as appropriate maintenance measures are taken Appendix B — GSP Specifications Drainage Criteria Manual SECTION 1: DESCRIPTION Permeable pavements are alternative paving surfaces that allow stormwater runoff to filter through voids in the pavement surface into an underlying stone reservoir, where it is temporarily stored and/or infiltrated. Permeable pavements consist of a surface pavement layer, an underlying stone aggregate reservoir layer and a filter layer or fabric installed on the bottom (See Figure 3.1 below). The thickness of the reservoir layer is determined by both a structural and hydrologic design analysis. The reservoir layer serves to retain stormwater and also supports the design traffic loads for the pavement. In low -infiltration soils, some or all of the filtered runoff is collected in an underdrain and returned to the storm drain system. If infiltration rates in the underlying soils permit, permeable pavement can be designed without an underdrain, to enable full infiltration of runoff. A combination of these methods can be used to infiltrate a portion of the filtered runoff. Figure 3.1. Cross section of typical permeable pavement (Source: Hunt & Collins, 2008). Permeable pavement is typically designed to treat stormwater that falls on the actual pavement surface area, but it may also be used to accept run-on from small adjacent impervious areas, such as impermeable driving lanes or rooftops. However, careful sediment control is needed for any run-on areas to avoid clogging of the down -gradient permeable pavement. Permeable pavement has been used at commercial, institutional, and residential sites in spaces that are traditionally impervious. Pervious pavement is well suited for parking lots, walking paths, sidewalks, playgrounds, plazas, tennis courts, and other similar uses. Permeable pavement promotes a high degree of runoff volume reduction, and it can also reduce the effective of impervious cover on a development site. The various types of permeable pavement are listed below along with brief descriptions and photos. Appendix 8 - GSP Specifications Drainage Criteria Manual Pervious Concrete: Pervious concrete (Figure 3.2) is produced by substantially reducing the number of fines in the concrete mix design in order to establish voids for drainage. Pervious concrete shall always be underlain by a stone subbase designed for stormwater management and shall never be placed directly onto a soil subbase. Pervious concrete has a coarser appearance than conventional concrete. Also, care must be taken during placement to avoid working the surface and creating an impervious layer. See Table 3.3 for typical design information. A typical cross section is shown in Figure 3.8. Figure 3.2. Pervious concrete walkways (Source: EPA 2009). Pervious Asphalt: Pervious Asphalt (Figure 3.3) consists of standard bituminous asphalt in which the fines have been screened and reduced, allowing water to pass through small voids. Pervious asphalt is placed directly on the stone bedding layer above an open graded subbase. See Table 3.4 for typical design information. A typical cross section is shown in Figure 3.9. Figure 3.3. Pervious asphalt parking and recreation area (Source: PennDEP 2006). Appendix 8 - GSP Specifications Drainage Criteria Manual Modular Block Paver Systems: Pervious Paver Blocks (Figure 3.4) consist of interlocking units (often concrete) that provide some portion of surface area that may be filled with a pervious material such as gravel. These units are often very attractive and are especially well suited to plazas, patios, small parking areas, etc. A number of manufactured products are available for use as permeable pavement. The designer is encouraged to evaluate the benefits of various products with respect to the specific application and the desired stormwater management goals. See Table 3.5 for typical design information. A typical cross section is shown in Figure 3.10. F 7 rl�,I ;,,,::. t.N� V. Figure 3.4. Pervious asphalt parking and recreation area (Source: Atlanta Regional Commission 2001). Grass Pavers: Plastic rings in a flexible grid system are placed on a bedding layer above a base of No. 57 stone. The rings or grid system are then filled with a sandy topsoil such as sandy loam and planted with vegetation. This pavement gives designers a turfgrass alternative to asphalt or concrete for such low -traffic areas as firelanes, overflow and event parking, golf cart paths, residential driveways, and maintenance and utility access lanes (see Figure 3.5) The support base and the walls of the rings prevent soil compaction and reduce rutting and erosion by supporting the weight of traffic and concentrated loads, while the large void spaces in the rings allow a strong root network to develop. The end result is a load-bearing surface covered with natural grass and which is typically around 90% pervious. See Table 3.6 for typical design information. A typical cross section is shown in Figure 3.11. Appendix B — GSP Specifications Drainage Criteria Manual Figure 3.5. Reinforced grass parking (Source: PennDEP 2006). Gravel pavers: This pavement option (Figures 3.6 and 3.7) is intended for high frequency, low speed traffic areas. The same ring structure as with the grass paver is used, but the voids in the rings are filled with gravel in order to provide greater load bearing support for unlimited traffic volumes and/or parking durations. Manufacturers provide specifications on the aggregate gradation that shall be used to generate the clean gravel fill for the rings, and a geotextile fabric is used to prevent the gravel infill from migrating to the soil subbase. See Table 3.7 for typical design information. A typical cross section is shown in Figure 3.12. Figure 3.6. Reinforced gravel walkway (Source: PennDEP 2006). Appendix 8 - GSP Specifications Drainage Criteria Manual Figure 3.7. Gravel parking with concrete drive lanes (Source: FTN Associates, Ltd). SECTION 2: PERFORMANCE The overall annual runoff reduction of permeable pavement is shown in Table 3.1. Sources: CWP and CSN (2008) and CWP (2007). The choice of what kind of permeable pavement to use is influenced by site-specific design factors and the intended future use of the permeable surface. SECTION 3: DESIGN TABLE The major design goal of Permeable Pavement is to maximize runoff reduction. To this end, designers may choose to use a baseline permeable pavement design (Level 1) or an enhanced design (Level 2) that increases runoff reduction. To qualify for Level 2, the design must meet all design criteria shown in the right hand column of Table 3.2. Appendix 8 — GSP Specifications Drainage Criteria Manual 3.2. Permeable pavementTable Level 1 Design Level 2 Design Tv1= (1)(Rv)(A) 3630 Tv1 = (1.1)(Rv)(A) 3630 Subgrade soil infiltration is less than 0.5 in./hr. Soil infiltration rate exceeds 0.5 in./hr. Maximum contributing drainage area is twice the The permeable material contributing area is its surface. permeable surface area. Underdrain not required; OR Underdrain required If an underdrain is used, a 12 -in stone sump must be provided below the underdrain invert; OR The Tv has at least a 48-hour drain time, as regulated by a control structure. CDA = The permeable pavement area CDA = The permeable pavement area "A = Area in acres Appendix 8 — GSP Specifications Drainage Criteria Manual i SECTION 4: TYPICAL DETAILS For each type of permeable pavement a design table and typical design detail are provided below. PERVIOUS CONCRETE (DEPTH AND MIX DESIGN SHALL BE DETERMINED BY PROFESSIONAL ENGINEER) BASE/RESERVIOR (ASTM D448 #57 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER T4 MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM D 448 #8 STONE) NON -WOVEN GEOTEXTILE FILTER FABRIC (OPTIONAL) (TYPICALLY NOT COMPACTED_ IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE, AN UNDERDRAIN SYSTEM 15 REQUIRED) 2" MINIMUM BEDDING COURSE ASTM D443 #'B STONE) SUBBASE/RESERVOIR (ASTM D448 #2 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. Figure 3.8. Typical permeable concrete pavement section (Source: FTN Associates, Ltd). Appendix 8 — GSP Specifications Drainage Criteria Manual pavementTable 3.3. Pervious concrete Design References: ACI 522R-06, Pervious Concrete ACI522.1-08, "Specification for Pervious Concrete Pavement" NRMCA, Pervious Concrete Pavements Typical Cross Section: 5 inches to 8 inches Pavement thickness: (Depth shall be determined by Professional Engineer) Bedding Layer(Choker Course): 2 inches No. 8 stone 3 inches to 4 inches No. 57 stone Reservoir Layer: 6 inches to 12 inches No. 2 stone (Depth shall be determined by Professional Engineer) 6 inches to 8 inches coarse sand (ASTM C33) on top of Filter Layer: 2 inches to 4 inches No. 8 stone Optional Geotextile Fabric: 4 -oz minimum Non -Woven filter fabric PERVIOUS CONCRETE (DEPTH AND MIX DESIGN SHALL BE DETERMINED BY PROFESSIONAL ENGINEER) BASE/RESERVIOR (ASTM D448 #57 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER T4 MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM D 448 #8 STONE) NON -WOVEN GEOTEXTILE FILTER FABRIC (OPTIONAL) (TYPICALLY NOT COMPACTED_ IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE, AN UNDERDRAIN SYSTEM 15 REQUIRED) 2" MINIMUM BEDDING COURSE ASTM D443 #'B STONE) SUBBASE/RESERVOIR (ASTM D448 #2 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. Figure 3.8. Typical permeable concrete pavement section (Source: FTN Associates, Ltd). Appendix 8 — GSP Specifications Drainage Criteria Manual Table 3.4. Pervious Asphalt Pavement Design References: National Asphalt Pavement Association (NAPA), "Porous Asphalt Pavements for Stormwater Management, Design, Construction, and Maintenance Guide" (Revised November 2008). Typical Cross Section: Pavement thickness: 2-1/2 inches to 4 inches (Depth shall be determined by Professional Engineer) Bedding Layer(Choker Course): 2 inches No. 8 stone 3 inches to 4 inches No. 57 stone Base/Reservoir Layer: 12 inches to 18 inches No. 2 stone BASE/RESERVIOR (Depth of layers shall be determined by Professional Engineer) Filter Layer: 6 inches to 8 inches coarse sand (ASTM C33) ,_.::.,::.:,�::.:;.-,.,;,..,:.:'•:-��.��..:'_:>`.-..,;•,;�,•:"•�= on top of 2 inches to 4 inches No. 8 stone Geotextile Fabric: 4 -oz minimum Non -Woven filter fabric PERVIOUS ASPHALT (DEPTH AND Ail% DESIGN SHALL BE DETERMINED BY PROFESSIONAL ENGINEER) 2" MINIMUM BEDDING COURSE BASE/RESERVIOR _ _ [ASTM 0448 #8 STONE) (ASTM 0448 #57 STONE). DEPTH VARIES ,_.::.,::.:,�::.:;.-,.,;,..,:.:'•:-��.��..:'_:>`.-..,;•,;�,•:"•�= BY DE51GN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET � WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. FILTER LAYER_ SUBBASE/RESERVDIR (ASTM C33 COARSE SAND OVER ASTM D 448 #8 STONE) _ _ - I I- I MI I II I -I I I I III I-VIIIE (ASTM D448 #2 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY NON -WOVEN GEOTEXTILE SUBGRADE PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS FILTER FABRIC (OPTIONAL) (TYPICALLY NOT COMPACTED. IF COMPACTION IS NECESSARY AND TRAFFIC LOADING - TO STABIUZE THE SUBGRADE, AN UNDERORAIN SYSTEM 1S REQUIRED) Figure 3.9. Typical permeable asphalt pavement section (Source: FTN Associates, Ltd). Appendix 8 - GSP Specifications Drainage Criteria Manual MODULAR BLOCK PAVING SYSTEM OR INTERLOCKING CONCRETE PAVERS BASE/RESERVIOR (ASTM D448 #57 STONE)_ DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM ❑ 448 #8 STONE) NON—WOVEN GEOTExTILE FILTER FABRIC (OPTI (op -no PERVIOUS JOINT MATERIAL (SAND OR GRAVEL) (TYPICALLY NOT COMPACTED. IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE, AN UNDERDRAIN SYSTEM IS REQUIRED) 2" MINIMUM BEDDING COURSE (ASTM D448 #8 STONE) SUBBASE/RESERVOIR (ASTM D448 #2 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. Figure 3. 10. Typical modular block pavement section (Source: FTN Associates, Ltd). Appendix 8 - GSP Specifications Drainage Criteria Manual Table 3.5 Modular Design References: Interlocking Concrete Pavement Institute (ICPI), "Permeable Interlocking Concrete Pavements', Fourth Edition Refer to Manufacturer specifications for proprietary modular block systems. Typical Cross Section: Pavement thickness: Varies by product Joint Material: Pervious material, Sand or No. 8 stone gravel Bedding Layer(Choker Course): 2 inches No. 8 stone or sand with geotextile fabric Base/Reservoir Layer: 3 inches to 4 inches No. 57 stone 12 inches to 18 inches No. 2 stone (Depth of layers shall be determined by Professional Engineer) Filter Layer: 6 inches to 8 inches coarse sand (ASTM C33) on top of 2 inches to 4 inches No. 8 stone Geotextile Fabric: 4 -oz minimum Non -Woven filter fabric MODULAR BLOCK PAVING SYSTEM OR INTERLOCKING CONCRETE PAVERS BASE/RESERVIOR (ASTM D448 #57 STONE)_ DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM ❑ 448 #8 STONE) NON—WOVEN GEOTExTILE FILTER FABRIC (OPTI (op -no PERVIOUS JOINT MATERIAL (SAND OR GRAVEL) (TYPICALLY NOT COMPACTED. IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE, AN UNDERDRAIN SYSTEM IS REQUIRED) 2" MINIMUM BEDDING COURSE (ASTM D448 #8 STONE) SUBBASE/RESERVOIR (ASTM D448 #2 STONE). DEPTH VARIES BY DESIGN AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. Figure 3. 10. Typical modular block pavement section (Source: FTN Associates, Ltd). Appendix 8 - GSP Specifications Drainage Criteria Manual pavementTable 3.6. Reinforced grass Design References: Refer to Manufacturer specifications for proprietary systems. Typical Cross Section: Pavement thickness: Varies by product Joint Material: Fill cells with sandy soil growing medium (60% minimum sand) Bedding Layer(Choker Course): 2 inches No. 8 stone Base/Reservoir Layer: 12 inches to 18 inches No. 57 or No. 67 stone (Depth of layers shall be determined by Professional Engineer) Filter Layer: 6 inches to 8 inches coarse sand (ASTM C33) on top of 2 inches to 4 inches No. 8 stone Geotextile Fabric: 4 -oz minimum Non -Woven filter fabric CRASS SURFACE (SOD OR SEEDED) PLASTIC RING OR GRID PAVER FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM D 448 #8 STONE) NON -WOVEN GEOTEXTiLE�1—1—1 11-1l1-111—�4 FILTER FABRIC 1=111=111—I II-111=.II _ (OPTIONAL) SUBGRADE (TYPICALLY NOT COMPACTED. IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE. AN UNDFRDRAIN SYSTEM IS REQUIRED) FILL CELLS WITH SANDY SOIL GROWING MEDIUM (60% MINIMUM SAND CONTENT) OR AS RECOMMENDED BY THE MANUFACTURER_ 2" BEDDING/CHOKER COURSE (ASTM D448 #8 STONE) BASE/RESERVOIR (ASTM D448 #57 OR #67 STONE). DEPTH VARIES AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE RECMIREMENTS AND TRAFFIC LOADING_ Figure 3.11. Typical grass paver section (Source: FTN Associates, Ltd). Appendix 8 — GSP Specifications Drainage Criteria Manual FILL CELLS NTH WASHED GRAVEL (ASTM #8 STONE) OR AS RECOMMENDED 9Y THE MANUFACTURER. PLASTIC RING OR GRID PAVER FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM D 448 #8 STONE) NOM -WOVEN GEOTEXPLE I-111-1II_ 1-11 FILTER FABRIC 7---i-i-i (OPTfONAL) SUBGRADE (TYPICALLY NOT COMPACTED. IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE, AN UNDERDRAIN SYSTEM IS REQUIRED) BASE/RESERVOIR (ASTM D448 #57 OR #67 STONE). DEPTH VARIES AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. Figure 3.12. Typical reinforced gravel pavement section (Source: FTN Associates, Ltd). Appendix 8 - GSP Specifications Drainage Criteria Manual I pavementTable 3.7. Reinforced gravel Design References: Refer to Manufacturer specifications for proprietary systems. Typical Cross Section: Pavement thickness: Varies by product Joint Material: Fill cells with washed gravel (No. 8 stone) Base/Reservoir Layer: 12 inches to 18 inches No. 57 or No. 67 stone (Depth of layers shall be determined by Professional Engineer) Filter Layer: 6 inches to 8 inches coarse sand (ASTM C33) on top of 2 inches to 4 inches No. 8 stone Geotextile Fabric: 4 -oz minimum Non -Woven filter fabric FILL CELLS NTH WASHED GRAVEL (ASTM #8 STONE) OR AS RECOMMENDED 9Y THE MANUFACTURER. PLASTIC RING OR GRID PAVER FILTER LAYER (ASTM C33 COARSE SAND OVER ASTM D 448 #8 STONE) NOM -WOVEN GEOTEXPLE I-111-1II_ 1-11 FILTER FABRIC 7---i-i-i (OPTfONAL) SUBGRADE (TYPICALLY NOT COMPACTED. IF COMPACTION IS NECESSARY TO STABILIZE THE SUBGRADE, AN UNDERDRAIN SYSTEM IS REQUIRED) BASE/RESERVOIR (ASTM D448 #57 OR #67 STONE). DEPTH VARIES AND SHALL BE DETERMINED BY PROFESSIONAL ENGINEER TO MEET WATER STORAGE REQUIREMENTS AND TRAFFIC LOADING. Figure 3.12. Typical reinforced gravel pavement section (Source: FTN Associates, Ltd). Appendix 8 - GSP Specifications Drainage Criteria Manual I Figure 3.13 shows a typical underdrain. Refer to Section 6. 1 of this specification to determine the flow within the underdrain. Refer to Section 6.8 of this specification for details regarding the design and location of underdrains. If the pavement surface is sloped, the pavement subbase shall be terraced in cells with underdrains that collect stormwater from each cell. The subbase slope shall not exceed 1%. Figure 3.8 shows a typical installation of a terraced subbase on a sloped pavement. PERVIOUS PAVEMENT SUBBASE/RESERVOIR -if -- - r7' r.0511 r1 •i� r. i6i iR �•L6i s r �:r•i� i r S�:r �L� . i�.:•:A � i�.i �S�i� i ii r i ii i`0 {r.:,fl ,•�.! fir. ��s�! �rs�!!'•�.rsl�.,-ir4�!.i+rs��.s�r. ��s�r ��i�!'s+.!� f!.��i �i �!. �.l'^i.►' �,! ��+rr-■!r ■!tom! �1■ r��r=+r��rr-�!►;�rr+!��rr ��i�,!ri��i�,!�i,�r*r�;�i� �� ,rri;�R•*!ri,�:+� �;�ir+r�i is■,,�r.',i+sN„', r `, r, is' iy- e ; r. i , it i, i. r$ i', r$ i. •, it i,- ry r ', rs i,,- �., 'r.:' i :�. !,r, i:• rr• r:• ir, r: iii!! rv, r; i:” i:, .r' r:, ..;� rr, ::, r:, Mi r. r r%• r :,:,� f .,:,• ..' r L� r•i+ r•i■er•.� r•i�er. L�.r•.A r :�.r•L� r :�.r•i� r :Y.r•.� r•.■.r• � r•.■ � �� .�. � r. r� i.,• r. rr, r.y r� r'., r� r.,• rr r, r. r►, rr r•: r. r, rr, r., rr, r., r, i., r,- iry �r ir,' i —WOVENFILTER LAYER ■ ON - - FILTER (OPTIONAL) PERFORATED UNDERDRAIN (6" TYPICAL) Figure 3.13. Typical underdrain detail (Source: FTN Associates, Ltd). TYPICAL OUTLET STRUCTURE (PROVIDE OVERFLOW INLET AT LEAST i" BELOW TOP OF STONE SOLID PIPE RESERVOIR LAYER TO PREVENT BACKWASHING PAVEMENT LAYER FLOW PAVEMENT CELLS SIZED TO CAPTURE DESIRED TREATMENT VOLUME II C ELL 1 II II II CELL 2 I II II 3 CELL 3 II II II II PAVEMENT II II II I I AII SLOPE II fl II II II II II II PERFORATED UNDERDRAIN J CONCRETE EDGE (WHERE APPLICABLE) Figure 3.14. Typical sloped pavement underdrain layout (Source: UDFCD 2010). Appendix 8 - GSP Specifications Drainage Criteria Manual SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS Since permeable pavement has a very high runoff reduction capability, it should generally be considered as an alternative to conventional pavement. Permeable pavement is subject to the same feasibility constraints as most infiltration practices, as described below. Available Space. A prime advantage of permeable pavement is that it does not normally require additional space at a new development or redevelopment site, which can be important for tight sites or areas where land prices are high. Soils. Soil conditions do not constrain the use of permeable pavement, although they do determine whether an underdrain is needed. Impermeable soils in Hydrologic Soil Groups (HSG) C or D usually require an underdrain, whereas HSG A and B soils often do not. In addition, permeable pavement above fill soils shall be designed with an impermeable liner and underdrain. If the proposed permeable pavement area is designed to infiltrate runoff without underdrains, it must have a minimum infiltration rate of 0.5 inches per hour. Initially, projected soil infiltration rates can be estimated from USDA-NRCS soil data, but they must be confirmed by on-site infiltration measurements. Designers shall evaluate existing soil properties during initial site layout, and seek to configure the site to conserve and protect the soils with the greatest recharge and infiltration rates. In particular, areas of HSG A or B soils shown on NRCS soil surveys should be considered as primary locations for all types of infiltration. External Drainage Area. Any external drainage area contributing runoff to permeable pavement shall generally not exceed twice the surface area of the permeable pavement (for Level 1 design), and it shall be as close to 100% impervious as possible. Some field experience has shown that an upgradient drainage area (even if it is impervious) can contribute particulates to the permeable pavement and should not lead to clogging (Hirschman, et al., 2009). Therefore, careful sediment source control and/or a pre-treatment strip or sump (e.g., stone or gravel) shall be used to control sediment run-on to the permeable pavement section. Pavement Slope. Steep slopes can reduce the stormwater storage capability of permeable pavement and may cause shifting of the pavement surface and base materials. Designers should consider using a terraced design for permeable pavement in sloped areas. The maximum pavement surface slope is 3%. However, the bottom of the subbase slope shall be as flat as possible (0% to 1.0% longitudinal slope) to enable even distribution and infiltration of stormwater. An underdrain system shall be employed to collect stormwater that does not infiltrate if the bottom of the subbase is sloped. Lateral slopes shall be 0%. Minimum Hydraulic Head. The elevation difference needed for permeable pavement to function properly is generally nominal, although 2 to 4 feet of head may be needed to drive flows through underdrains. Flat terrain may affect proper drainage of Level 1 permeable pavement designs, so underdrains shall have a minimum 0.5% slope. Minimum Depth to Water Table. A high groundwater table may cause ponding at the bottom of the permeable pavement system. Therefore, a minimum vertical distance of 2 feet above the bottom of the Appendix B - GSP Specifications Drainage Criteria Manual permeable pavement installation (i.e., the bottom invert of the reservoir layer) and the seasonal high water table. Setbacks. To prevent seepage, permeable pavement shall not be hydraulically connected to structure foundations. Setbacks to structures and roads vary, based on the scale of the permeable pavement installation (see Table 3.3 below). At a minimum, small- and large-scale pavement applications shall be located a minimum horizontal distance of 100 feet from any water supply well, 50 feet from septic systems, and at least 5 feet down -gradient from dry or wet utility lines. Informed Owner. The property owner should clearly understand the unique maintenance responsibilities inherent with permeable pavement, particularly for parking lot applications. The owner should be capable of performing periodic routine and long-term actions (e.g., vacuum sweeping) to maintain the pavement's hydrologic functions, and avoid future practices (e.g., winter sanding, seal coating or repaving) that diminish or eliminate them. High Loading Situations. Permeable pavement is not intended to treat sites with high sediment or trash/debris loads, since such loads will cause the practice to clog and fail. Groundwater Protection. Section 10 of the Bioretention specification (GSP-01) presents a list of potential areas with high pollutant loading that pose a risk of groundwater contamination. Infiltration of runoff from designated areas with high pollutant loading is highly restricted or prohibited. Limitations. Permeable pavement can be used as an alternative to most types of conventional pavement at residential, commercial and institutional developments; however, it is not currently approved for use within the Right of Way (ROW). Design Scales. Permeable pavement can be installed at the following three scales: 1. The smallest scale is termed Micro -Scale Pavements, which applies to converting impervious surfaces to permeable ones on small lots and redevelopment projects, where the installations may range from 250 to 1000 square feet in total area. Where redevelopment or retrofitting of existing impervious areas results in a larger foot -print of permeable pavers (small-scale or large- scale, as described below), the designer shall implement the Load Bearing, Observation Well, Underdrain, Soil Test, and Building Setback criteria associated with the applicable scale. 2. Small-scale pavement applications treat portions of a site between 1,000 and 10,000 square feet in area, and include areas that only occasionally receive heavy vehicular traffic. 3. Large scale pavement applications exceed 10,000 square feet in area and typically are installed within portions of a parking lot. Table 3.8 outlines the different design requirements for each of the three scales of permeable pavement installation. Appendix B - GSP Specifications Drainage Criteria Manual 1. Minimum setback required for features down -gradient of structure 2. Minimum setback required for features up -gradient of structure Regardless of the design scale of the permeable pavement installation, the designer shall carefully consider the expected traffic load at the proposed site and the consequent structural requirements of the pavement system. Sites with heavy traffic loads will require a thick aggregate base. In contrast, most micro -scale applications should have little or no traffic flow to contend with. SECTION 6: DESIGN CRITERIA 6.1. Sizing of Permeable Pavement Structural Design. If permeable pavement will be used in a parking lot or other setting that involves vehicles, the pavement section must be able to support the maximum anticipated traffic load. The structural design process will vary according to the type of pavement selected, and the manufacturer's specific recommendations should be consulted. The thickness of the permeable pavement and reservoir layer must be sized to support loading and to temporarily store the design storm volume (e.g., the water quality, channel protection, and/or flood control volumes). On most new development and redevelopment sites, the structural support requirements will dictate the depth of the underlying stone reservoir. The structural design of permeable pavements involves consideration of four main site elements: • Total traffic • In-situ soil strength characteristics based on design maximum water loads • Environmental elements, and Appendix 8 — GSP Specifications Drainage Criteria Manual Table 3.8. The Three Design Scales for Permeable Pavement Design Factor Micro -Scale Pavement Small -Scale Pavement Large -Scale Pavement Impervious Area 250 to 1000 sq ft 1000 to 10,000 sq ft More than 10,000 sq ft Treated Driveways, Sidewalk Network, Fire Parking Lots with more than Typical Walkways, Lanes 40 spaces Applications Courtyards Plazas Road Shoulders, Spill -Over Individual Sidewalks Parking, Plazas Low Speed Residential Streets Load Bearing Foot traffic Heavy vehicles (moving Light vehicles Capacity Light vehicles & parked) Reservoir Size Infiltrate or detain some or Infiltrate or detain the full Tv pavement area all of the Tv External Drainage Impervious cover up to twice Impervious cover up to twice Area? No pavement area with pavement area with Level 1 design. Level 1 design. Observation Well No No Yes Underdrain? Depends on soil Depends on soil Back-up underdrain Required Soil One per 5000 sq ft of Tests One per practice Two per practice proposed practice 5 ft 10 ft1 25 ft' Building Setbacks 25 ft 50 ftz 100 ft 1. Minimum setback required for features down -gradient of structure 2. Minimum setback required for features up -gradient of structure Regardless of the design scale of the permeable pavement installation, the designer shall carefully consider the expected traffic load at the proposed site and the consequent structural requirements of the pavement system. Sites with heavy traffic loads will require a thick aggregate base. In contrast, most micro -scale applications should have little or no traffic flow to contend with. SECTION 6: DESIGN CRITERIA 6.1. Sizing of Permeable Pavement Structural Design. If permeable pavement will be used in a parking lot or other setting that involves vehicles, the pavement section must be able to support the maximum anticipated traffic load. The structural design process will vary according to the type of pavement selected, and the manufacturer's specific recommendations should be consulted. The thickness of the permeable pavement and reservoir layer must be sized to support loading and to temporarily store the design storm volume (e.g., the water quality, channel protection, and/or flood control volumes). On most new development and redevelopment sites, the structural support requirements will dictate the depth of the underlying stone reservoir. The structural design of permeable pavements involves consideration of four main site elements: • Total traffic • In-situ soil strength characteristics based on design maximum water loads • Environmental elements, and Appendix 8 — GSP Specifications Drainage Criteria Manual • Bedding and Reservoir layer design The resulting structural requirements may include, but are not limited to, the thickness of the pavement, filter, and reservoir layer. Designers should note that if the underlying soils have a low California Bearing Ratio (CBR), they may need to be compacted to at least 95% of the Standard Proctor Density, which rules out their use for infiltration. Designers shall determine structural design requirements by consulting transportation design guidance sources, such as the following: • City of Fayetteville Roads Master Plan, Minimum Street Standards • AASHTO Guide for Design of Pavement Structures (1993), and • AASHTO Supplement to the Guide for Design of Pavement Structures (1998) Hydraulic Design. Permeable pavement is typically sized to store the complete water quality Treatment Volume (Tv) or another design storm volume in the reservoir layer. Modeling indicates that this simplified sizing rule approximates an 80% average annual rainfall volume removal for subsurface soil infiltration rates up to one inch per hour. More conservative values are established for this practice as experience has shown that clogging of the permeable material can be an issue long-term, especially with larger contributing areas carrying significant soil materials onto the permeable surface. The sub -surface soil infiltration rate typically will be less than the flow rate through the pavement, so that some underground reservoir storage will usually be required. Designers shall initially assume that there is no outflow through underdrains, using Equation 3.1 to determine the depth of the reservoir layer, assuming runoff fully infiltrates into the underlying soil: Equation 3.1. Depth of Reservoir Layer with no Underdrain dr — {(d, x R) + P — (l/2 x tf)) Vr Where: dp = The depth of the reservoir layer (ft) dc = The depth of runoff from the contributing drainage area (not including the permeable paving surface) for the Treatment Volume (Tv/Ac), or other design storm (feet) R = Ac /Ap = The ratio of the contributing drainage area (Ac, not including the permeable paving surface) to the permeable pavement surface area (Ap) [NOTE: With reference to Table 3.3, the maximum value for the Level 1 design is R = 2, (the external drainage area Ac is twice that of the permeable pavement area Ap; and for Level 2 design R = 0 (the drainage area is made up solely of permeable pavement Ap]. P = The rainfall depth for the Treatment Volume (Level 1 = 1 inch; Level 2 = 1.1 inch), or other design storm (feet) i = The field -verified infiltration rate for native soils (feet/day) tf = The time to fill the reservoir layer (day) typically 2 hours or 0.083 day Appendix 8 - GSP Specifications Drainage Criteria Manual Vr = The void ratio for the reservoir layer (0.35) The maximum allowable depth of the reservoir layer is constrained by the maximum allowable drainage time, which is calculated using Equation 3.2. Equation 3.2. Maximum Depth of Reservoir Layer C12 X td) dp-max - V, Where: dp_,,,ax = The maximum depth of the reservoir layer (feet) i = The field -verified infiltration rate for native soils (feet/day) td = The maximum allowable time to drain the reservoir layer, typically 1 to 2 days Vr = The void ratio for the reservoir layer (0.35) The following design assumptions apply to Equations 3.1 and 3.2: • The contributing drainage area (Ac) should not contain pervious areas. • For conservative design, the native soil infiltration rate or the field-tested soil infiltration rate is divided by a factor of safety of 2. The minimum acceptable native soil infiltration rate is 0.5 inches/hour. • The void ratio (Vr) of 0.35 shall be used in the equations for the stone reservoir. • Maximum drain time for the reservoir layer shall be not less than 24 hours and no more than 48 hours. If the depth of the reservoir layer is too great (i.e. dp exceeds dp-max), or the verified soil infiltration rate is less than 0.5 inches per hour, then the design method typically changes to account for underdrains. The storage volume in the pavements must account for the underlying infiltration rate and outflow through the underdrain. In this case, the design storm shall be routed through the pavement to accurately determine the required reservoir depth. Alternatively, the designer may use Equations 3.3 through 3.5 to approximate the depth of the reservoir layer for designs using underdrains. Equation 3.3 can be used to approximate the outflow rate from the underdrain. The hydraulic conductivity, k, of gravel media is very high (-17,000 ft/day). However, the permeable pavement reservoir layer will drain more slowly as the storage volume decreases (i.e. the hydraulic head decreases). To account for this change, a conservative permeability coefficient of 100 ft/day can be used to approximate the average underdrain outflow rate. Equation 3.3. Outflow through Underdrain q,,=kXm Appendix 8 - GSP Specifications Drainage Criteria Manual Where: q„ = Outflow through the underdrain (per outlet pipe, assumed 6 -inch diameter) (feet/day) k = Hydraulic conductivity for the reservoir layer (ft/day - assume 100 feet/day) M = Underdrain pipe slope (feet/foot) Once the outflow rate through the underdrain has been approximated, Equation 3.4 is used to determine the depth of the reservoir layer needed to store the design storm. Equation 3.4. Depth of Reservoir Layer with Outflow through Underdrain {(d,xR)+P-(1/2xtf)-(q,,xtf)) dp = V, Where: dp = Depth of the reservoir layer (feet) d, = Depth of runoff from the contributing drainage area (not including the permeable pavement surface) for the Treatment Volume (Tv/Ac), or other design storm (feet) R = Ac/Ap = The ratio of the contributing drainage area (Aj (not including the permeable pavement surface) to the permeable pavement surface area (Ap) P = The rainfall depth for the Treatment Volume (Level 1 = 1 inch; Level 2 = 1.1 inch), or other design storm (feet) i = The field -verified infiltration rate for the native soils (feet/day) tf = The time to fill the reservoir layer (day) - typically 2 hours or 0.083 day Vr = The void ratio for the reservoir layer (0.35) q„ = Outflow through Underdrain (feet/day) The maximum allowable depth of the reservoir layer is constrained by the maximum allowable drain time, which is calculated using Equation 3.5. Equation 3.5. Maximum Depth of Reservoir Layer with Outflow through Underdrain dp-max = W/2 X td) — (qu X td)) V,. Where: dp_,,,aX = The maximum depth of the reservoir layer (feet) i = The field -verified infiltration rate for the native soils (feet/day) Vr = The void ratio for the reservoir layer (0.35) td = The time to drain the reservoir layer (day - typically 1 to 2 days) q„ = Outflow through Underdrain (feet/day) If the depth of the reservoir layer is still too great (i.e. dp exceeds dp-.,,x), the number of underdrains can be increased, which will increase the total outflow rate. Permeable pavement can also be designed to address, in whole or in part, the detention storage needed to comply with channel protection and/or flood control requirements. The designer can model various Appendix 8 - GSP Specifications Drainage Criteria Manual approaches by factoring in storage within the stone aggregate layer, expected infiltration, and any outlet structures used as part of the design. Routing calculations properly used can also provide a more accurate solution of the peak discharge and required storage volume. Once runoff passes through the surface of the permeable pavement system, designers shall calculate outflow pathways to handle subsurface flows. Subsurface flows can be regulated using underdrains, the volume of storage in the reservoir layer, the bed slope of the reservoir layer, a control structure at the outlet, and/or appropriately designed natural filter layers. 6.2. Soil Infiltration Rate Testing To design a permeable pavement system without an underdrain, the measured infiltration rate of subsoils must be 0.5 inch per hour or greater and the design must accommodate the 1 to 2 day reservoir drain time. On-site soil infiltration rate testing procedures are outlined in Appendix C. Alternately, a double -ring infiltrometer test may be used. Test quantities shall be based on Table 3.8. In most cases, a single soil test is sufficient for micro -scale and small-scale applications. At least one soil boring must be taken to confirm the underlying soil properties at the depth where infiltration is designed to occur (i.e., to ensure that depth to water table, depth to bedrock, or karst layer is defined). Soil infiltration testing shall be conducted above a confining layer, if found, within 4 feet of the bottom of a proposed permeable pavement system. 6.3. Type of Surface Pavement The type of pavement shall be selected based on a review of the factors described in Sections 1 and 2, the project objectives, and designed according to the product manufacturer's recommendations. 6.4. Internal Geometry and Drawdowns • Elevated Underdrain. To promote greater runoff reduction for permeable pavement located on lower filtration subgrade soils, an elevated underdrain shall be installed with a stone jacket that creates a 12- to 18 -inch deep storage layer below the underdrain invert. The void storage in this layer can help qualify a site to achieve Level 2 design. • Rapid Drawdown. When possible, permeable pavement shall be designed so that the target runoff reduction volume stays in the reservoir layer for at least 24 hours before being discharged through an underdrain. • Conservative Infiltration Rates. Designers shall always decrease the measured infiltration rate by a factor of 2 during design, to approximate long term infiltration rates. 6.5. Pretreatment Pretreatment for most permeable pavement applications is not necessary, since the surface acts as pretreatment to the reservoir layer below. Additional pretreatment may be appropriate if the pavement receives run-on from an adjacent pervious or impervious area. 6.6. Conveyance and Overflow Permeable pavement designs may include methods to convey larger storms (e.g., 50- and 10 -percent annual chance) to the storm drain system. The following is a list of methods that can be used to accomplish this: Appendix 8 - GSP Specifications Drainage Criteria Manual • Place a perforated pipe horizontally near the top of the reservoir layer to pass excess flows after water has filled the base. The placement and/or design shall be such that the incoming runoff is not captured (e.g., placing the perforations on the underside only). • Increase the thickness of the top of the reservoir layer by as much as 6 inches over initial design requirement (i.e., create freeboard). The design computations used to size the reservoir layer often assume that no freeboard is present. • Create underground detention within the reservoir layer of the permeable pavement system. Reservoir storage may be augmented by corrugated metal pipes, plastic or concrete arch structures, etc. • Route excess flows to another detention or conveyance system that is designed for the management of discharges due to larger storms. • Set the storm drain inlets flush with the elevation of the permeable pavement surface to effectively convey excess stormwater runoff past the system (typically in remote areas). The design shall also make allowances for outlet structures or overflows to prevent unacceptable ponding depths during larger rainfall events. 6.7. Reservoir layer The thickness of the reservoir layer is determined by runoff storage needs, the infiltration rate of in situ soils, structural requirements of the pavement sub -base, depth to water table and bedrock, and frost depth conditions. The engineer shall inspect subgrade for suitability. • The reservoir below the permeable pavement surface shall be composed of clean, washed stone aggregate and sized for both the storm event to be treated and the structural requirements of the expected traffic loading. • The storage layer may consist of clean washed No. 57 stone, although No. 2 stone is preferred because it provides additional storage and structural stability. • The bottom of the reservoir layer shall be horizontal to allow runoff to infiltrate evenly through the entire surface. 6.8. Underdrains The use of underdrains is recommended when there is a reasonable potential for infiltration rates to decrease over time, when underlying soils have an infiltration rate of less than 0.5 inch per hour, when shallow bedrock is present, or when subgrade soils must be compacted to achieve a desired Proctor density. Underdrains can also be used to manage extreme storm events to keep detained stormwater from backing up above the permeable pavement. • Underdrains will be required in the majority of locations within the City of Fayetteville since the majority of existing soils in Fayetteville are HSG type C or D. Appendix 8 - GSP Specifications Drainage Criteria Manual • Underdrains shall be 4 to 6 -inch perforated pipe and shall be placed to collect excess stormwater that cannot be infiltrated into the ground within the allowable reservoir drain time. • The number of underdrains required depends upon the size of the pavement area, the depth of the storage reservoir needed, the slope of pavement section, and the infiltration rate of the underlying soils. • Underdrains shall be placed within the reservoir and encased by 8 to 12 inches of clean, washed stone as measured from the outside of the underdrain to the outer edge of the encasing layer. • The underdrain outlet can be fitted with a flow -reduction orifice as a means of regulating the stormwater detention time. The minimum diameter of any orifice shall be 0.5 inch. Orifices must be inspected regularly to ensure the system is functioning properly. • Underdrains can also be installed and capped at the downstream outlet as an option for future use if maintenance observations indicate a reduction in the soil permeability. 6.9. Maintenance Reduction Features Maintenance is a crucial element to ensure the long-term performance of permeable pavement. The most frequently cited maintenance problem is surface clogging caused by organic matter and sediment, which can be reduced by the following measures: Periodic Vacuum Sweeping. The pavement surface is the first line of defense in trapping and eliminating sediment that may otherwise enter the stone base and soil subgrade. The rate of sediment deposition shall be monitored and vacuum sweeping done once or twice a year. This frequency shall be adjusted according to the intensity of use and deposition rate on the permeable pavement surface. At least one sweeping pass shall occur at the end of winter. Protecting the Bottom of the Reservoir Layer. There are two options to protect the bottom of the reservoir layer from intrusion by underlying soils. The first method involves covering the bottom with nonwoven, polypropylene geotextile that is permeable, although some practitioners recommend avoiding the use of filter fabric since it may become a future plane of clogging within the system. Permeable filter fabric is still recommended to protect the excavated sides of the reservoir layer, in order to prevent soil piping. The second design method is to form a natural filter barrier of choker stone and sand. In this case, underlying native soils shall be separated from the reservoir base/subgrade layer by one or more filter layers of sand and gravel based on gradation sizing of the subgrade and subsequent filter layers to prevent piping and undermining of the subgrade soils. Observation Well. An observation well, consisting of a well -anchored, perforated 2 to 4 inch (diameter) slotted PVC pipe that extends vertically to the bottom of the reservoir layer, shall be installed at the downstream end of all large-scale permeable pavement systems. The observation well shall be fitted with a lockable cap installed flush with the ground surface (or under the pavers) to facilitate periodic inspection and maintenance. The observation well is used to observe the rate of drawdown within the reservoir layer following storm events. • Overhead Landscaping. Plan and design landscaping to account for permeable pavement maintenance limitations. Large-scale permeable pavement applications shall be carefully planned to Appendix B - GSP Specifications Drainage Criteria Manual integrate this landscaping in a manner that maximizes runoff treatment and minimizes the risk that sediment, mulch, grass clippings, leaves, nuts, and fruits will clog the paving surface. 6.10. Material Specifications Permeable pavement material specifications vary according to the specific pavement product selected. Table 3.9 describes general material specifications for the component structures installed beneath the permeable pavement. Table 3.10 provides specifications for general categories of permeable pavements. Designers shall consult manufacturer's technical specifications for specific criteria and guidance. Appendix 8 — GSP Specifications Drainage Criteria Manual pavementTable 3.9. Material specifications for underneath the Material Specification Notes Pervious Concrete, Asphalt, and Interlocking pavers: 2 inches of No. 8 stone over 3 to 4 inches No. 57 ASTM D448 size No. 8 stone (e.g., 3/8 to 3/16 inch in size). Bedding Layer Reinforced Grass Pavers: 2 inches No. 8 Shall be double -washed and clean and free of all fines. stone Reinforced Gravel Pavers: 2 inches No. 8 stone (fill pavers) ASTM D448 size No. 57 stone (e.g., 11/2 to % inch in size); Pervious Concrete, Asphalt, and No. 2 Stone (e.g., 2-1/2 in. to 3/4 in. in size). Depth is Reservoir Layer Interlocking pavers: No. 2 stone based on the pavement structural and hydraulic Reinforced Grass Pavers: No. 57 Stone requirements. Shall be double -washed, clean, and free of all fines. Use 4- to 6 -inch diameter perforated HDPE or PVC (AASHTO M 252) pipe, with maximum 3/8 -inch perforations at 6 inches on center; each underdrain installed at a minimum 0.5% slope spaced 20 ft on centers ( equivalent corrugated HDPE may be used for smaller load-bearing applications). Underdrain Perforated pipe installed for the full length of the permeable pavement cell, and non -perforated pipe, as needed, is used to connect with the storm drain system in accordance with Fayetteville Drainage Manual. T's and Y's installed as needed, depending on the underdrain configuration. Extend cleanout pipes to the surface with vented caps at Ts and Ys. The underlying native soils shall be separated from the stone reservoir by a thin, 2 to 4 inches layer of The sand shall be placed between the stone reservoir and Filter Layer choker stone (e.g., No. 8) covered the choker stone, which shall be placed on top of the by a 6 to 8 inches layer of coarse underlying native soils. sand (e.g., ASTM C 33, 0.02-0.04 inches). Use a needled, non -woven, polypropylene geotextile with Grab Tensile Strength equal to or greater than 120 pounds (ASTM D4632), with a Mullen Burst Strength equal to or greater than 225 Filter Fabric pounds/square inch (ASTM D3786), with a Flow Rate greater than 125 gpm/sq ft (ASTM D4491), (optional) and an Apparent Opening Size (AOS) equivalent to a US # 70 or # 80 sieve (ASTM D4751). The geotextile AOS selection is based on the percent passing the No. 200 sieve in "A" Soil subgrade, using FHWA or AASHTO selection criteria. Appendix 8 — GSP Specifications Drainage Criteria Manual SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS The design adaptations described below permit permeable pavement to be used on a wider range of sites. However, it is important not to force this practice onto marginal sites. Other runoff reduction practices are often preferred alternatives for difficult sites. 7.1 Shallow Bedrock Underdrains must be used in locations in which bedrock is encountered less than 2 feet beneath the planned invert of the reservoir layer. Karst terrain situations may be reviewed on a case-by-case basis. If approved by the City Engineer, design shall conform with additional requirements in the above Table 3.10. SECTION 8: CONSTRUCTION Experience has shown that proper installation is absolutely critical to the effective operation of a permeable pavement system. Installation shall be overseen by a qualified engineer or engineer's representative. 8.1 Necessary Erosion & Sediment Controls • All permeable pavement areas shall be fully protected from sediment intrusion by silt fence or construction fencing, particularly if they are intended to infiltrate runoff. Appendix 8 — GSP Specifications Drainage Criteria Manual Table 3.10. Additional Permeable Pavement Specifications Material Specification Notes Permeable Surface open area: 5% to 15%. Must conform to ASTM C936 specifications. Interlocking Thickness: 3.125 inches for vehicles. Reservoir layer required to support the Compressive strength: 55 Mpa (8000 psi). Open Concrete Pavers void fill media: aggregate structural load. Open void content: 20% to 50%. Thickness: 3.5 inches. Must conform to ASTM C 1319 specifications. Concrete Grid Compressive strength: 35 Mpa(-5000 psi).. Reservoir layer required to support the Pavers Reservoir Laye Open void fill media: aggregate, topsoil and grass, structural load. coarse sand. Void content: depends on fill material. Plastic Reinforced Compressive strength: varies, depending on fill Reservoir layer required to support the structural Grid Pavers material. load. Open void fill media: aggregate, topsoil and grass, coarse sand. Impermeable Use a thirty mil (minimum) PVC Geomembrane liner covered by 8 to 12 oz./sq. yd. non -woven Liner geotextile (both sides), where Karst formations are present, only with approval of City Engineer) Use a perforated or slotted 2 to 4 in. vertical PVC pipe (AASHTO M 252) with a lockable cap, installed Observation Well flush with the surface. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS The design adaptations described below permit permeable pavement to be used on a wider range of sites. However, it is important not to force this practice onto marginal sites. Other runoff reduction practices are often preferred alternatives for difficult sites. 7.1 Shallow Bedrock Underdrains must be used in locations in which bedrock is encountered less than 2 feet beneath the planned invert of the reservoir layer. Karst terrain situations may be reviewed on a case-by-case basis. If approved by the City Engineer, design shall conform with additional requirements in the above Table 3.10. SECTION 8: CONSTRUCTION Experience has shown that proper installation is absolutely critical to the effective operation of a permeable pavement system. Installation shall be overseen by a qualified engineer or engineer's representative. 8.1 Necessary Erosion & Sediment Controls • All permeable pavement areas shall be fully protected from sediment intrusion by silt fence or construction fencing, particularly if they are intended to infiltrate runoff. Appendix 8 — GSP Specifications Drainage Criteria Manual • Permeable pavement areas shall remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. Permeable pavement areas shall be clearly marked on all construction documents and grading plans. To prevent soil compaction, heavy vehicular and foot traffic shall be kept out of permeable pavement areas during and immediately after construction. • During construction, care shall be taken to avoid tracking sediments onto any permeable pavement surface to avoid clogging. • Any area of the site intended ultimately to be a permeable pavement area shall not be used as the site of a temporary sediment basin. • Where locating a sediment basin on an area intended for permeable pavement is unavoidable, the invert of the sediment basin must be a minimum of 2 feet above the final design elevation of the bottom of the aggregate reservoir course. • All sediment deposits in the excavated area shall be carefully removed prior to completion of the excavation and installation of the subbase, base and surface materials. 8.2 Permeable Pavement Construction Sequence The following is a typical construction sequence to properly install permeable pavement: Step 1. Construction of the permeable pavement shall only (or) begin after the entire contributing drainage area has been stabilized. The proposed site shall be checked for existing utilities prior to any excavation. Do not install the system in rain or snow, and do not install frozen bedding materials. Step 2. As noted above, temporary erosion and sediment (E&S) controls are needed during installation to divert stormwater away from the permeable pavement area until it is completed. Special protection measures such as erosion control fabrics may be needed to protect vulnerable side slopes from erosion during the excavation process. The proposed permeable pavement area must be kept free from sediment during the entire construction process. Construction materials that are contaminated by sediments must be removed and replaced with clean materials. Step 3. Excavators or backhoes shall work from the sides to excavate the reservoir layer to its appropriate design depth and dimensions. For micro -scale and small-scale pavement applications, excavating equipment shall have arms with adequate extension so they do not have to work inside the footprint of the permeable pavement area (to avoid compaction). Contractors can utilize a cell construction approach, whereby the proposed permeable pavement area is split into 500 to 1000 sq ft temporary cells with a 10 to 15 foot earth bridge in between, so that cells can be excavated from the side. Excavated material shall be placed away from the open excavation so as to not jeopardize the stability of the side walls. Step 4. The native soils along the bottom and sides of the permeable pavement system shall be scarified or tilled to a depth of 3 to 4 inches prior to the placement of the filter layer or filter fabric. In large scale paving applications with weak soils, the soil subgrade may need to be compacted to 95% of the Standard Proctor Density to achieve the desired load-bearing capacity. (NOTE: This improves strength characteristics and significantly reduces the permeability of the subgrade, and it will typically require installation of an underdrain. The reduced infiltration characteristics must be addressed during hydrologic design.) Appendix 8 - GSP Specifications Drainage Criteria Manual Step 5. Filter fabric shall be installed on the bottom and the sides of the reservoir layer. Filter fabric strips shall overlap down-slope by a minimum of 2 feet, and be secured a minimum of 4 feet beyond the edge of the excavation. Where the filter layer extends beyond the edge of the pavement (to convey runoff to the reservoir layer), install an additional layer of filter fabric 1 foot below the surface to prevent sediments from entering into the reservoir layer. Excess filter fabric shall not be trimmed until the site is fully stabilized. Step 6. Provide a minimum of 2 inches of aggregate above and below the underdrains. The underdrains shall slope down towards the outlet at a grade of 0.5% or steeper. The up -gradient end of underdrains in the reservoir layer shall be capped. Where an underdrain pipe is connected to a structure, there shall be no perforations within 1 foot of the structure. Ensure that there are no perforations in clean -outs and observation wells within 1 foot of the surface. Step 7. Moisten and spread 6 -inch lifts of the appropriate clean, washed stone aggregate (usually No. 2 or No. 57 stone). Place at least 4 inches of additional aggregate above the underdrain, and then compact it using a vibratory roller in static mode until there is no visible movement of the aggregate. Do not crush the aggregate with the roller. Step B. Install the desired depth of the bedding layer, depending on the type of pavement, as follows: Pervious Concrete, Asphalt, and Interlocking Pavers: The bedding layer for porous asphalt pavement consists of 2 inches of clean, washed ASTM D 448 No.8 stone above 3 to 4 inches of ASTM D 448 No. 57 stone. The filter course must be leveled and pressed (choked) into the reservoir base with at least four (4) passes of a 10 -ton steel drum static roller. For interlocking pavers, the thickness of the bedding layer is to be based on the block manufacturer's recommendation or a minimum of 2 inches whichever is greater. • Reinforced Grass and Gravel Pavers: The bedding layer for reinforced grass and gravel pavers pavement consists of 2 inches of clean, washed ASTM D 448 No.8 stone. Step 9. Paving materials shall be installed in accordance with manufacturer or industry specifications for the particular type of pavement. • Installation of Porous Asphalt. The following has been excerpted from various documents, most notably Jackson (2007). o Install porous asphalt pavement similarly to regular asphalt pavement. The pavement shall be laid in a single lift over the filter course. The laying temperature shall be between 230oF and 2600F, with a minimum air temperature of 50oF, to ensure that the surface does not stiffen before compaction. o Complete compaction of the surface course when the surface is cool enough to resist a 10 -ton roller. One or two passes of the roller are required for proper compaction. More rolling could cause a reduction in the porosity of the pavement. o The mixing plant must provide certification of the aggregate mix, abrasion loss factor, and asphalt content in the mix. Test the asphalt mix for its resistance to stripping by water using ASTM 1664. Appendix 8 - GSP Specifications Drainage Criteria Manual If the estimated coating area is not above 95%, additional antistripping agents must be added to the mix. o Transport the mix to the site in a clean vehicle with smooth dump beds sprayed with a non - petroleum release agent. The mix shall be covered during transportation to control cooling. o Test the full permeability of the pavement surface by application of clean water at a rate of at least five gallons per minute over the entire surface. All water must infiltrate directly, without puddle formation or surface runoff. o Inspect the facility 18 to 30 hours after a significant rainfall (greater than 1/2 inch) or artificial flooding, to determine that the facility is draining properly. Installation of Pervious Concrete. The basic installation sequence for pervious concrete is outlined by the American Concrete Institute (2008). It is strongly recommended that concrete installers successfully complete a recognized pervious concrete installers training program, such as the Pervious Concrete Contractor Certification Program offered by the NRMCA. The basic installation procedure is as follows: o Drive the concrete truck as close to the project site as possible. o Water the underlying aggregate (reservoir layer) before the concrete is placed, so that the aggregate does not draw moisture from the freshly laid pervious concrete. o After the concrete is placed, approximately 3/8 to 1/2 inch is struck off, using a vibratory screed. This is to allow for compaction of the concrete pavement. o Compact the pavement with a steel pipe roller. Care shall be taken so that overcompaction does not occur. o Cut joints for the concrete to a depth of 1/4 inch. o The curing process is very important for pervious concrete. Cover the pavement with plastic sheeting within 20 minutes of the strike -off, and keep it covered for at least seven (7) days. Do not allow traffic on the pavement during this time period. Installation of Interlocking Pavers. The basic installation process is described in greater detail by Smith (2006). o Place edge restraints for open jointed pavement blocks before the bedding layer and pavement blocks are installed. Permeable interlocking concrete pavement (IP) systems require edge restraints to prevent vehicle loads from moving the paver blocks. Edge restraints may be standard City of Fayetteville curbs or gutter pans, or precast or cast -in-place reinforced concrete borders a minimum of 6 inches wide and 18 inches deep, constructed with Class A3 concrete. Edge restraints along the traffic side of a permeable pavement block system are recommended. o Place the No. 57 stone in a single lift. Level the filter course and compact it into the reservoir course beneath with at least four (4) passes of a 10 -ton steel drum static roller until there is no visible movement. The first two (2) passes are in vibratory mode, with the final two (2) passes in static mode. The filter aggregate shall be moist to facilitate movement into the reservoir course. o Place and screed the bedding course material (typically No. 8 stone). Appendix 8 - GSP Specifications Drainage Criteria Manual o Fill gaps at the edge of the paved areas with cut pavers or edge units. When cut pavers are needed, cut the pavers with a paver splitter or masonry saw. Cut pavers no smaller than one- third (1/3) of the full unit size. o Pavers may be placed by hand or with mechanical installers. Fill the joints and openings with stone. joint openings must be filled with No. 8 stone, although smaller stone may be used where needed to fill narrower joints. Remove excess stones from the paver surface. o Compact and seat the pavers into the bedding course with a minimum low -amplitude 5,000-lbf, 75- to 95 -Hz plate compactor. o Do not compact within 6 feet of the unrestrained edges of the pavers. o The system must be thoroughly swept by a mechanical sweeper or vacuumed immediately after construction to remove any sediment or excess aggregate. o Inspect the area for settlement. Any blocks that settle must be reset and re -inspected. o Inspect the facility 18 to 30 hours after a significant rainfall (0.5 inch or greater) or artificial flooding to determine whether the facility is draining properly. • Installation of Plastic Grid Reinforcement for Grass and Gravel Pavement. The installation process is determined by the manufacturer of the plastic grid reinforcement product being installed. 8.3. Construction Inspection Inspections before, during and after construction are needed to ensure that permeable pavement is built in accordance with these specifications. For a minimum, use detailed inspection checklists that require sign -offs by qualified individuals at critical stages of construction, to ensure that the contractor's interpretation of the plan is consistent with the designer's intent. Some common pitfalls can be avoided by careful construction supervision that focuses on the following key aspects of permeable pavement installation: • Store materials in a protected area to keep them free from mud, dirt, and other foreign materials. • The contributing drainage area shall be stabilized prior to directing water to the permeable pavement area. • Check the aggregate material to confirm that it is clean and washed, meets specifications and is installed to the correct depth. • Check elevations (e.g., the invert of the underdrain, inverts for the inflow and outflow points, etc.) and the surface slope. • Ensure that caps are placed on the upstream (but not the downstream) ends of the underdrains. • Make sure the permeable pavement surface is even, runoff evenly spreads across it, and the storage bed drains within 48 hours. • Inspect the pretreatment structures (if applicable) to make sure they are properly installed and working effectively. Appendix 8 - GSP Specifications Drainage Criteria Manual Once the final construction inspection has been completed, log the GPS coordinates for each facility and submit them to City of Fayetteville City Engineer. SECTION 9: AS -BUILT REQUIREMENTS During and after completion of the permeable pavement installation, an as -built inspection and certification must be prepared and provided to the City by a Professional Engineer. The as -built certification verifies that the BMP was installed as designed and approved. The following components must be addressed in the as - built certification: 1. The infiltration rate of the permeable pavement must be verified. 2. The infiltration rate test of the underlying soils shall be included if Level 2 is used without an underdrain. 3. Surrounding drainage areas must be stabilized to prevent sediment from clogging the pavement. SECTION 10: MAINTENANCE 10.1. Maintenance Plan It is recommended that a Long Term Maintenance Plan (LTMP) be developed by the design engineer. The LTMP contains a description of the stormwater system components and information on the required inspection and maintenance activities. The LTMP for permeable pavement shall also note which conventional parking lot maintenance tasks must be avoided (e.g., sanding, re -sealing, re -surfacing, power - washing). Signs shall be posted on larger parking lots to indicate their stormwater function and special maintenance requirements. 10.2. Maintenance Tasks It is difficult to prescribe the specific types or frequency of maintenance tasks that are needed to maintain the hydrologic function of permeable pavement systems over time. Most installations work reasonably well year after year with little or no maintenance, whereas some have problems. One preventive maintenance task for large-scale applications involves vacuum sweeping on a frequency consistent with the use and loadings encountered in the parking lot. An annual, dry -weather sweeping in the spring months shall be performed. The contract for sweeping shall specify that a vacuum sweeper be used that does not use water spray, since spraying may lead to subsurface clogging. Vacuum settings for large- scale interlocking paver applications shall be calibrated so they do not pick up the stones between pavement blocks. 10.3. Maintenance Inspections It is highly recommended that a spring maintenance inspection and cleanup be conducted at each permeable pavement site, particularly at large-scale applications. Appendix 8 - GSP Specifications Drainage Criteria Manual Maintenance of permeable pavement is driven by annual inspections that evaluate the condition and performance of the practice. The following are suggested annual maintenance inspection points for permeable pavements: • The drawdown rate shall be measured at the observation well for three (3) days following a storm event in excess of 0.5 inch in depth. If standing water is still observed in the well after three days, this is a clear sign that clogging in the reservoir or underdrain is a problem. If standing water is on the pavement surface, surface clogging is the issue. Inspect the surface of the permeable pavement for evidence of sediment deposition, organic debris, staining or ponding that may indicate surface clogging. If any signs of clogging are noted, schedule a vacuum sweeper (no brooms or water spray) to remove deposited material. Vacuum settings for large-scale interlocking paver applications shall be calibrated so they do not pick up stones between pavement blocks. Then, test sections by pouring water from a five gallon bucket to ensure they work. • Inspect the structural integrity of the pavement surface, looking for signs of surface deterioration, such as slumping, cracking, spalling or broken pavers. Replace or repair affected areas, as necessary. • Check inlets, pretreatment cells and any flow diversion structures for sediment buildup and structural damage. Note if any sediment needs to be removed. • Inspect the condition of the observation well and make sure it is still capped. • Generally inspect any contributing drainage area for any controllable sources of sediment or erosion. SECTION 11: COMMUNITY & ENVIRONMENTAL CONCERNS Compliance with the Americans with Disabilities Act (ADA). Interlocking concrete pavers are considered to be ADA compliant, if designers ensure that surface openings between pavers do not exceed 0.5 inch. However, some forms of interlocking pavers may not be suitable for handicapped parking spaces. Interlocking concrete pavers interspersed with other hardscape features (e.g., concrete walkways) can be used in creative designs to address ADA issues. Groundwater Protection. While well -drained soils enhance the ability of permeable pavement to reduce stormwater runoff volumes, they may also increase the risk that stormwater pollutants might migrate into groundwater aquifers. Designers shall avoid the use of infiltration -based permeable pavement in areas known to provide groundwater recharge to aquifers used for water supply. In these source water protection areas, designers shall include liners and underdrains in large-scale permeable pavement applications (i.e., when the proposed surface area exceeds 10,000 square feet). Potential Areas with High Pollutant Loading. Designers shall also certify that the proposed permeable pavement area will not accept any runoff from a designated area with high pollutant loading. Areas with potential for high pollutant loading are operations or activities that are known to produce higher concentrations of stormwater pollutants and/or have a greater risk of spills, leaks or illicit discharges. Examples include certain industrial activities, gas stations, public works areas, petroleum storage areas (for a complete list of these areas where infiltration may be restricted or prohibited, see Section 11 of GSP-01 Appendix 8 - GSP Specifications Drainage Criteria Manual Bioretention). Restricted infiltration means that a minimum of 50% of the total Tv must be treated by a filtering or bioretention practice prior to the permeable pavement system. For known severe areas with high pollutant loading the risk of groundwater contamination from spills, leaks or discharges is so great that infiltration of stormwater or snowmelt through permeable pavement is prohibited. Underground Injection Control Permits. The Safe Drinking Water Act regulates the infiltration of stormwater in certain situations pursuant to the Underground Injection Control (UIC) Program, which is administered by Arkansas Department of Environmental Quality (ADEQ). In general, the EPA (2008) has determined that permeable pavement installations are not classified as Class V injection wells, since they are always wider than they are deep. Air and Runoff Temperature. Permeable pavement appears to have some value in reducing summer runoff temperatures, which can be important in watersheds with sensitive cold -water fish populations. The temperature reduction effect is greatest when runoff is infiltrated into the sub -base, but some cooling may also occur in the reservoir layer, when underdrains are used. ICPI (2008) notes that the use of certain reflective colors for interlocking concrete pavers can also help moderate surface parking lot temperatures. Vehicle Safety. Permeable pavement is generally considered to be a safer surface than conventional pavement, according to research reported by Smith (2006), Jackson (2007) and ACI (2008). Permeable pavement has less risk of hydroplaning, more rapid ice melt and better traction than conventional pavement. SECTION 12: REFERENCES American Society for Testing and Materials (ASTM), 2003. Standard Classification for Sizes of Aggregate for Road and Bridge Construction, ASTM D448 -03a. West Conshohocken, PA. Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Chesapeake Stormwater Network (CSN), 2009. Technical Bulletin No. 1. Stormwater Design Guidelines for Karst Terrain in the Chesapeake Bay watershed. Version 2.0. Baltimore, MD. www.chesapeakestormwater.net City of Fayetteville, 2011. City of Fayetteville Streets Master Plan, City Plan 2030, Fayetteville, AR. EPA, 2009. BMP Fact Sheets, Pervious Concrete Pavementhttp://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=browse&Rbutton=d etail&bmp=137&minmeasure=5 EPA, 2009. BMP Fact Sheets„Pervious Asphalt Pavement http://cfpub.epa.gov/npdes/stormwater/menuofbmps /index.cfm?action=browse&Rbutton=detail&bmp =135&minmeasure=5 EPA, 2009. BMP Fact Sheets. Permeable Interlocking Concrete Pavement http_/ Icfpub.epa.gov/np des/stormwater/menuofbmps /index.cfm?action=browse&Rbutton=detail&bmp =136&minmeasure=5 Hathaway, J. and W. Hunt. 2007, Stormwater BMP Costs. Report to NC DEHNR. Department of Biological and Agricultural Engineering. North Carolina State University. Raleigh, NC. Appendix B - GSP Specifications Drainage Criteria Manual Hirschman, D., L. Woodworth and S. Drescher, 2009. Technical Report. Stormwater BMPs in Virginia's James River Basin: An Assessment of Field Conditions & Programs. Center for Watershed Protection. Ellicott City, MD. Hunt, W. and K. Collins, 2008. Permeable Pavement. Research Update and Design Implications, North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG -588-14. North Carolina State University. Raleigh, NC. Available online at: http://www.bae.ncsu.edu/stormwater/PublicationFiles/ Perm Pave2008.pdf. Interlocking Concrete Pavement Institute (ICPI), 2008. Permeable Interlocking Concrete Pavement. A Comparison Guide to Porous Asphalt and Pervious Concrete. Jackson, N., 2007. Design, Construction and Maintenance Guide for Porous Asphalt Pavements. National Asphalt Pavement Association. Information Series 131. Lanham, MD. www.hotmix.com Northern Virginia Regional Commission (NVRC). 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia Pennsylvania Department of Environmental Protection (PennDEP), 2006. Pennsylvania Stormwater Best Management Practices Manual. Available online at. http:/ /www.elibrary.dep.state.pa.us/dsweb/View/Collection-8305 Sustainable Infrastructure Alternative Paving Materials Subcommittee, Subcommittee Report, Portland, Oregon. Available online at: http://www.portlandonline.com/bes/index.cfm?c=34602& Schueler, T., C. Swann, T. Wright and S. Sprinkle, 2004. Pollution Source Control Practices. Manual No. 8 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD. Schueler et al 2007. Urban Stormwater Retrofit Practices. Manual No. 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD. Schueler, T., 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net Smith, D., 2006. Permeable Interlocking Concrete Pavement -selection design, construction and maintenance. Third Edition. Interlocking Concrete Pavement Institute. Herndon, VA. U.S EPA, 2008. June 13 2008 Memo. L. Boornaizian and S. Heare. "Clarification on which stormwater infiltration practices/technologies have the potential to be regulated as "Class V" wells by the Underground Injection Control Program." Water Permits Division and Drinking Water Protection Division. Washington, D.C. UDFCD, 2010. Urban Storm Drainage Criteria Manual Volume 3, Stormwater Best Management Practices, November 2010. Urban Drainage and Flood Control District. http://www.udfcd.org/downloads/down critmanual vollll.htm Virginia Department of Conservation and Recreation, 2011. Stormwater Design Specification No. 7. Permeable Pavement, version 1.8http://vwrrc.vt.edu/swc/NonProprietaryBMPs.html. Water Environment Research Federation, 2005. Performance and Whole -life Costs of Best Management Practices and Sustainable Urban Drainage Systems. Alexandria, VA. Appendix 8 - GSP Specifications Drainage Criteria Manual INFILTRATION DEVICES Description: Infiltration devices are trenches or basins that fill with stormwater runoff and allow the water to exfiltrate, i.e., exit the device by infiltrating into the soil. There are four major types of infiltration devices: infiltration trenches, infiltration basins, dry wells, and subsurface structures. • Infiltration Trenches: Excavated trench (3 to 8 ft depth) filled with stone media (1.5- to 2.5 -in diameter); pea gravel and sand filter layers • Infiltration Basins: Impoundments constructed over permeable soils that allow stormwater to infiltrate into the soil. • Dry Wells: Small excavated pit filled with stone or a small structure surrounded by stone. Infiltrates runoff from very small drainage areas. • Subsurface Structures: Underground systems that capture runoff and allow it to infiltrate. Common types include pre -cast concrete, plastic pits, chambers (manufactured pipes), perforated pipes, and galleys. • Pretreatment: A sediment forebay and grass channel, or equivalent upstream pretreatment, must be provided for each type of infiltration device • Soil infiltration: Rate of 0.5 in/hr or greater required • Elevated underdrains: May be incorporated into the design to collect excess stormwater in marginal soils. • Observation well: To monitor percolation • Provides for groundwater • Potential for groundwater recharge contamination • Good for small sites with • High clogging potential; shall porous soils not be used on sites with fine- • Cost effective particled soils (clays or silts) in • High community drainage area acceptance when • Cannot be used on sites that integrated into a have been identified to have development karst features • Geotechnical testing required, two borings per facility • Community -perceived concerns with mosquitoes and safety Selection Criteria: Level 1— 50% Runoff Reduction Credit Level 2 — 90% Runoff Reduction Credit Land Use Considerations: © Residential © Commercial ■ Industrial* *With proper pretreatment and City approval. Maintenance: • Remove sediment from pretreatment devices. • Remove debris from inlet and outlet structures. • Inspect twice annually and following large rain events. • Check that system drains within 72 hours following rain event. ©Maintenance Burden L = Low M = Moderate H = High See Page 2 for Right of Way Applications and Design Considerations. Appendix B — GSP Specifications Drainage Criteria Manual i , • Stormwater can be conveyed by sheet flow or grass channels • Pretreatment is especially important in roadway applications where sediment loads may be high • Impermeable liner must be installed roadside to protect subgrade • Cannot create hazard or interfere with walkability • Setbacks must comply with city and utility -specific requirements • Maximum Drainage Area: ■ Infiltration Trench: 2 acres Infiltration Basin: 10 acres Dry Well: 1 acre Subsurface Structure: 5 acres • Space Required —Varies depending on the depth of the facility. • Site Slope — No more than 6% slope (for pre -construction facility footprint) • Minimum Depth to Water Table — 2 ft recommended between the bottom of the infiltration trench and the elevation of the seasonally high water table. • Proximity to buildings, drinking water supplies, karst features, and other sensitive areas • Soil types • Shall provide positive overflow in most uses • Infiltration devices can be designed (optional) for extra storage to control peak flow discharge for channel and flood protection. The extra storage can be either surface storage or underground storage to accommodate large storm volumes. The CN used to design the detention should be adjusted as described in Section 5.3 of Chapter 5 of the DCM to account for runoff reduction by the infiltration device. SECTION 1: DESCRIPTION Infiltration practices use temporary surface or underground storage to allow incoming stormwater runoff to exfiltrate into underlying soils. Runoff first passes through multiple pretreatment mechanisms to trap sediment and organic matter before it reaches the practice. As the stormwater penetrates the underlying soil, chemical and physical adsorption processes remove pollutants. Infiltration practices have the greatest runoff reduction capability of any stormwater practice and are suitable for use in residential and other urban areas where measured soil infiltration exceeds 0.5 inches per hour (see Appendix C Infiltration Procedure for details). To prevent possible groundwater contamination, infiltration shall not be used at sites with potential for high pollutant loading (see Chapter 8 for list of such sites). SECTION 2: PERFORMANCE When used appropriately, infiltration practices have a very high runoff volume reduction capability, as shown in Table 4.1. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 4.1. Annual Runoff Volume Reduction Provided by Infiltration Devices. Stormwater Function Level 1 Design Level 2 Design Annual Runoff Volume Reduction (RR) 50% 90% Sources: CWP and CSN (2009); CWP (2007) SECTION 3: DESIGN TABLE The major design goal for Infiltration is to maximize runoff volume reduction and nutrient removal. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced design (Level 2) that maximizes nutrient and runoff reduction. To qualify for Level 2, the infiltration practice must meet all the design criteria shown in the right hand column of Table 4.2. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 4.2. Infiltration Level 1 Design (1111:50) Device Design Criteria. Level 2 Design (RR:90) Sizing: Tv _ [(Rv)(A)/12] — the volume reduced by an upstream BMP. Sizing: T„ _ [1.1(Rv)(A)/12] —the volume reduced by an upstream BMP. At least two forms of pre-treatment (See Table 4.6) At least three forms of pre-treatment (See Table 4.6) Soil infiltration rate 0.5 to 1 in/hr (see Section 6.1 and Appendix C); number of tests depends on the scale (Table 4.3) Soil infiltration rate 1.0 to 4.0 in/hr (see Section 6.1 and Appendix C); number of tests depends on the scale (Table 4.3) Minimum of 2 ft. between the bottom of the infiltration practice and the seasonal high water table or bedrock Tv infiltrates within 36 to 48 hours (Section 6.6) Building Setbacks — see Table 4.3 Infiltration practices are not allowed for site with high pollutant loads without City Engineer's approval Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 4: TYPICAL DETAILS Figures 4.1 through 4.6 below provide typical schematics for water quality swales. Parking Lot High -Flow Bypass OPTIONAL TOPSOIL AND SOD ON TOP OF PEA GRAVEL METAL CAP WITH LOCK OVERFLOW BERM NOTE: SEE SPECIFICATIONS FOR UNDERDRAIN DESIGN IF NEEDED. Appendix 8 - GSP Specifications Drainage Criteria Manual I , Flow Splitter Forebay Leval Spreader Age— --- .71t Grass :hannel Inlihration Trench Berm RUNOFF FILTERS THROUGH GRASS BUFFER STRIP: GRASS CHANNEL OR SEDIMENT FOREBAY PEA GHAVEL OR RIVER STONE PROTECTIVE LAYER OF FILTER FABRIC CLEAN, AGGREGATE WITH MAX. DIAMETER OF 3.5 IN. AND A MIN DIAMETER OF 1.5 IN SAND BED 6 - 8" DEEP (OR FABRIC EQUIVALENT) NOTE: RUNOFF EXFILTRATED THROUGH UNDISTURBED SUBSOILS WITH A MIN RATE OF 0.5 INCHES PER HOUR Figure 4.1. Infiltration Plan and Section (Source: VADCR, 2011). PEA GRA' OPTIONAL TOPSOIL AND SOD_ ON TOP OF PEA GRAVEL FILTER FABRIC CLEAN WASHED I� GRAVEL SEDIMENT - 8" SAND LAYER 2' MIN OVERLAP OF FILTER FABRIC PERFORATED OVERFLOW COLLECTION PIPE LARGE DIAMETER PERFORATED PIPE OR ARCH WITH EXTERNAL PRETREATMENT (AT THE INLETS, OR OTHER LOCATION PRIOR TO ENTERING THE STONE TRENCH) OR INTERNAL PRETREATMENT (FILTER FABRIC LINING INSIDE THE PIPE OR ARCH) Figure 4.2a. Infiltration Section with Supplemental Pipe Storage (Source: VADCR, 2011). PROVIDE PRETREATMENT FOR CONCENTRATED FLOW AS REQUIRED PROPOSED GRADE 2 FT MINIMUM OVERLAP OF FILTER FABRIC • a • • e OVERFLOW , v a v v DETENTION STORAGE WITH CONTROLLED RELEASE PERFORATED PIPE i CLEAN, AGGREGATE WITH MAK DIAMETER OF 3.5 IN_ AND A MIN DIAMETER OF OUTLET 1.5 IN NON -WOVEN q GEOTDCTILE 4} INFILTRATION STORAGE OPTIONAL STORAGE CHAMBERS VOLUME FOR ADDITIONAL VOLUME (PERFORATED PIPE, ARCH CHAMBER, OR EQUIVALEM) Figure 4.2b. Combined underground Detention (Channel and/or Flooding Protection) and Infiltration (Source: VADCR, 2011) Appendix B - GSP Specifications Drainage Criteria Manual I , OPTIONAL TOPSOIL AND SOD ON TOP OF PEA GRAVEL —\ METAL 11 11" LOCK 555% ► ' ■ : :►IIrI - I I I -I I I -I I I -I I I -I I I -I I I -I I F- PEA GRAVEL OR RIVER STONE F_►114\■ ■►► III— III—III— `Jmi I I I=CLEAN, AGGREGATE WITH MAK —III—III—III °— DIAMETER OF 3.5 IN. AND A MIN UNDISTURBED MATERIAL - °e°DIAMETER OF 1.5 IN =III=III= a=III=III=III=III- -_FILTER 6 - 8' DEEP 4 - 6N. — _- — — —NLQ6�1 (DR FABRIC EQUIVALENT) PERFORATED PIPE-III—I FOOT PLATE Figure 4.3. Observation Well (Source: VADCR, 2011). 4" P.V.C. SCREW CAP ON GRADE 4" P.V.C. NONPERFORATED i STANDPIPE USE 45` WYE AND FITTING OR EQUIVALENT DIRECTIONAL CLEANOUT TO CONNECT UNDERDRAIN TO STANDPIPE WATERTIGHT CAP ON TERMINAL END OF PIPE l FLOW M?mFdFiATFD SOH❑ 4d P.V.G. UNDERDRAIN Figure 4.4. Four -inch PVC Cleanout (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , OPTIONAL TOPSOIL AND SOD ON TDP OF PEA GRAVEL PEA GRAVEL OR RIVER STONE OUTLET TO STORM SEWER OR DAYLIGHT OUTFALL INFLOW PRETREATMENT AS REQUIRED CLEAN, AGGREGATE WITH MAX. DIAMETER OF 3.5 IN. AND A MIN DIAMETER OF 1.5 IN PROFILE SAND FILTER 6" DEEP (OR FABRIC EQUIVALENT) OVERFLOW SLOPES r OBSERVATION WELL CONSISTENT WITH----- PRETREATMENT ITH PRETREATMENT PEA GRATEL OR RIVER STONE ►► ��/� GEOTEXTILE`•1: ■IyICLEAN, AGGREGATE ►i� Ii:WITH -, .. A MIN DIAMETER OF 1.5 IN SAND FILTER 6 - 8" DEEP (OR FABRIC EQUIVALENT) SECTION Figure 4.5. Typical Infiltration Trench (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual 1, 20' Grass Filter Filter Slope < 5% Pavement Gravel e Replaceable Filter Fabric `Failure Plane" 4 ❑ \o Observation 0 00 Filter Iter Fabric Well ❑p Keyed In At Top \� O O ❑ no 0% Aggregate 1 Figure 4.6. Infiltration Trench Section (Source: VADCR, 2011). SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS Since infiltration practices have a very high runoff reduction capability, they should be considered when initially evaluating a site. Designers should evaluate the range of soil properties during initial site layout and seek to configure the site to conserve and protect the soils with the greatest recharge and infiltration rates. In particular, areas of Hydrologic Soil Group A or B soils shown on NRCS soil surveys shall be considered as primary locations for infiltration practices. At this point, designers should carefully identify and evaluate constraints on infiltration, as follows: Contributing Drainage Area. The maximum contributing drainage area (CDA) to an individual infiltration practice shall be less than 2 acres and as close to 100% impervious as possible. This specification covers three scales of infiltration practices (1) Micro -infiltration (250 to 2,500 sq ft of CDA), (2) small-scale infiltration (2,500 to 20,000 sq ft of CDA) and (3) conventional infiltration (20,000 to 100,000 sq ft of CDA). The design, pretreatment and maintenance requirements differ, depending on the scale at which infiltration is applied (see Table 4.3 below for a summary). Appendix 8 — GSP Specifications Drainage Criteria Manual 1, 1. Although permeable pavement is an infiltration practice, a more detailed specification is provided in GSP-03 Permeable Pavements. 2. Note that the building setback of 5 ft is intended for simple shallow foundations. The use of a dry well or French drain adjacent to an in -ground basement or finished floor area shall be carefully designed and coordinated with the design of the structure waterproofing system (foundation drains, etc.), or avoided altogether. Site Topography. Unless slope stability calculations demonstrate otherwise, infiltration practices shall be located a minimum horizontal distance of 200 ft from down -gradient slopes greater than 20%. The average slope of the contributing drainage areas shall be less than 15%. Practice Slope. The bottom of an infiltration practice shall be flat (i.e., 0% longitudinal slope) to enable even distribution and infiltration of stormwater, although a maximum longitudinal slope of 1% is permissible if an underdrain is employed. Lateral slopes shall be 0%. Minimum Hydraulic Head. The elevation difference needed to operate a micro -scale infiltration practice is nominal. However, 2 or more feet of head may be needed to drive small-scale and conventional infiltration practices. Minimum Depth to Water Table or Bedrock. A minimum vertical distance of 2 ft must be provided between the bottom of the infiltration practice and the seasonal high water table or bedrock layer. Appendix B — GSP Specifications Drainage Criteria Manual i , .•4 .3. Design Scales for Infiltration Practices. Design Factor Micro -Infiltration Small -Scale Infiltration Conventional Infiltration Impervious Area 250 to 2,500 sq ft 2,500 to 20,000 sq ft 20,000 to 100,000 sq ft Treated Dry Well Infiltration Trench Typical Practices' French Drain Infiltration Trench Infiltration Basin Paving Blocks Min. Infiltration 0.5 in/hr measured at the depth in which the infiltration will occur Rate Design Infiltration 50% of measured rate Rate Observation Well No Yes Yes Type of External (leaf screens, grass Vegetated filter strip or grass channel, Pretreatment filter strip, etc) forebay, etc. Pretreatment Cell (See Table 4.6) Depth Max. 3 -ft depth Max. 5 -ft depth Max. 6 -ft depth Head Required Nominal: 1 to 3 ft. Moderate: 1 to 5 ft. Moderate: 2 to 6 ft. Underdrain An elevated underdrain only None required Backup underdrain Requirements on marginal soils Required Soil Tests One per practice One (1) per 1,000 sq ft of surface area or One per 1,000 sq ft of max. two (2) per practice. surface area. Building Setbacks 5 ft down-gradientz 10 ft down -gradient 25 ft down -gradient 25 ft where up -gradient 50 ft where up -gradient 100 ft where up -gradient 1. Although permeable pavement is an infiltration practice, a more detailed specification is provided in GSP-03 Permeable Pavements. 2. Note that the building setback of 5 ft is intended for simple shallow foundations. The use of a dry well or French drain adjacent to an in -ground basement or finished floor area shall be carefully designed and coordinated with the design of the structure waterproofing system (foundation drains, etc.), or avoided altogether. Site Topography. Unless slope stability calculations demonstrate otherwise, infiltration practices shall be located a minimum horizontal distance of 200 ft from down -gradient slopes greater than 20%. The average slope of the contributing drainage areas shall be less than 15%. Practice Slope. The bottom of an infiltration practice shall be flat (i.e., 0% longitudinal slope) to enable even distribution and infiltration of stormwater, although a maximum longitudinal slope of 1% is permissible if an underdrain is employed. Lateral slopes shall be 0%. Minimum Hydraulic Head. The elevation difference needed to operate a micro -scale infiltration practice is nominal. However, 2 or more feet of head may be needed to drive small-scale and conventional infiltration practices. Minimum Depth to Water Table or Bedrock. A minimum vertical distance of 2 ft must be provided between the bottom of the infiltration practice and the seasonal high water table or bedrock layer. Appendix B — GSP Specifications Drainage Criteria Manual i , Soils. Native soils in proposed infiltration areas must have a minimum infiltration rate of 0.5 inch per hour (typically Hydrologic Soil Group A and B soils meet this criterion). Initially, soil infiltration rates can be estimated from NRCS soil data, but must be confirmed by an on-site infiltration evaluation (Appendix C). Use on Urban Soils/Redevelopment Sites. Sites that have been previously graded or disturbed do not retain their original soil permeability due to compaction. Therefore, such sites are not ideal candidates for infiltration practices, however, if the measured infiltration rate of the underlying soils is 0.5 inch/hour or greater then infiltration can be used. Infiltration practices shall never be situated above earthen fill. Soils with significant clay content that is of moderate to high plasticity may lose their exfiltration properties over time due to swelling from absorbed water. Dry Weather Flows. Infiltration practices shall not be used on sites receiving regular dry weather flows from sump pumps, irrigation nuisance water, and similar kinds of flows. Setbacks. Infiltration practices shall not be hydraulically connected to structure foundations or pavement, in order to reduce seepage complications. Setbacks from structures and roads vary based on the scale of infiltration (see Table 4.3 above). At a minimum, conventional and small-scale infiltration practices shall be located a minimum horizontal distance of 100 ft from any water supply well, 50 ft from septic systems, and at least 5 ft down -gradient from dry or wet utility lines. High Loading Situations. Infiltration practices are not intended to treat sites with high sediment or trash/debris loads, because such loads will cause the practice to clog and fail. Groundwater Protection. GSP-01, Bioretention, presents a list of areas with potential for high pollutant loading that could pose a risk of groundwater contamination if used. Infiltration of runoff from these areas is restricted or prohibited and the list shall be referenced before selection of practices. Site -Specific Considerations. Infiltration practices can be applied to most land uses that have measured soil infiltration rates that exceed 0.5 inches per hour. However, there is no single infiltration application that fits every development situation. The nature of the actual design application depends on four key design factors, described below: 1. The first factor is the Design Scale at which infiltration will be applied: • Micro -infiltration is intended for residential rooftop disconnection, rooftop rainwater harvesting systems, or other small scale application (250 to 2,500 sq ft of impervious area treated); Small-scale infiltration is intended for residential and/or small commercial applications that meet the feasibility criteria noted above; and • Conventional infiltration can be considered for most typical development and redevelopment applications and therefore has more rigorous site selection and feasibility criteria. Table 4.3 above compares the different design approaches and requirements associated with each infiltration scale. 2. The second key design factor relates to the mode (or method) of temporarily storing runoff prior to infiltration - either on the surface or in an underground trench. When storing runoff on the surface (e.g., an infiltration basin), the maximum depth shall be no greater than 1 ft. However, if pretreatment cells are used, a maximum depth of 2 ft is permissible. In the underground mode, runoff is stored in the voids of Appendix 8 - GSP Specifications Drainage Criteria Manual i , the stones, and infiltrates into the underlying soil matrix. Perforated corrugated metal pipe, plastic pipe, concrete arch pipe, or comparable materials can be used in conjunction with the stone to increase the available temporary underground storage. In some instances, a combination of filtration and infiltration cells can be installed in the floor of a dry extended detention (ED) pond. The third design factor relates to the degree of confidence that exfiltration can be maintained over time, given the measured infiltration rate for the subsoils at the practice location and the anticipated land uses. This determines whether an underdrain is needed, or whether an alternative practice, such as bioretention, is needed at the site (see Table 4.4 below). 4. The final factor is whether the infiltration practice will be designed as an on-line or off-line facility, as this determines the nature of conveyance and overflow mechanisms needed. Offline practices are sized to only accept some portion of the treatment volume, and employ a flow splitter to safely bypass large storms. On-line infiltration practices may be connected to underground perforated pipes to detain the peak storm event, or have suitable overflows to pass the storms without erosion. SECTION 6: DESIGN CRITERIA 6.1. Defining the Infiltration Rate Soil permeability is the single most important factor when evaluating infiltration practices. A field -verified minimum infiltration rate of at least 0.5 inches/hour is needed for the practice to work. Projected Infiltration Rate. For planning purposes, the projected infiltration rate for the site can be estimated using the NRCS soil textural triangle or engineering test results for the prevailing soil types shown on the local NRCS Soil Survey. This data is used solely to locate portions of the site where infiltration may be feasible and to pinpoint where actual on-site infiltration tests will be taken to confirm feasibility. Measured Infiltration Rate. On-site infiltration investigations shall be conducted to establish the actual infiltration capacity of underlying soils, using the methods presented in Appendix C. Design Infiltration Rate. Several studies have shown that ultimate infiltration rates decline by as much as 50% from initial rates, so designers should not attempt to use infiltration practices where infiltration rates barely meet minimum. To provide a factor of safety, the infiltration rate used in the design may be no greater than 50% of the measured rate. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table4.4on Measured Infiltration Rate Measured Infiltration Rate (inches/hour) Less than 0.5 0.5 to 1.0 1.0 to 4.0 More than 4.0 Use Infiltration without an Use Infiltration, Use Bioretention or underdrain or with a 12 -inch Use Infiltration, Recommended Bioretention, ora a Water Quality stone reservoir below the Bioretention, or a Design Water Quality Swale Swale with an underdrain invert. Alternately, use Dry Swale without an Solution without an underdrain. Bioretention with an elevated underdrain. underdrain. underdrain. 4. The final factor is whether the infiltration practice will be designed as an on-line or off-line facility, as this determines the nature of conveyance and overflow mechanisms needed. Offline practices are sized to only accept some portion of the treatment volume, and employ a flow splitter to safely bypass large storms. On-line infiltration practices may be connected to underground perforated pipes to detain the peak storm event, or have suitable overflows to pass the storms without erosion. SECTION 6: DESIGN CRITERIA 6.1. Defining the Infiltration Rate Soil permeability is the single most important factor when evaluating infiltration practices. A field -verified minimum infiltration rate of at least 0.5 inches/hour is needed for the practice to work. Projected Infiltration Rate. For planning purposes, the projected infiltration rate for the site can be estimated using the NRCS soil textural triangle or engineering test results for the prevailing soil types shown on the local NRCS Soil Survey. This data is used solely to locate portions of the site where infiltration may be feasible and to pinpoint where actual on-site infiltration tests will be taken to confirm feasibility. Measured Infiltration Rate. On-site infiltration investigations shall be conducted to establish the actual infiltration capacity of underlying soils, using the methods presented in Appendix C. Design Infiltration Rate. Several studies have shown that ultimate infiltration rates decline by as much as 50% from initial rates, so designers should not attempt to use infiltration practices where infiltration rates barely meet minimum. To provide a factor of safety, the infiltration rate used in the design may be no greater than 50% of the measured rate. Appendix 8 — GSP Specifications Drainage Criteria Manual i , 6.2. Sizing of Infiltration Facilities Several equations are needed to size infiltration practices. The first equations establish the maximum depth of the infiltration practice, depending on whether it is a surface basin (Equation 4.1)or underground reservoir (Equation 4.2). Where: Equation 4.1. Maximum Surface Basin Depth dm�=1/2fxtd Equation 4.2. Maximum Underground Reservoir Depth d..T% (1/2f x td/ r dmax = maximum depth of the infiltration practice (feet) f = measured infiltration rate (feet/day) td = maximum drawdown time (normally 1.5 to 2 days) (day) Vr = void ratio of the stone reservoir (assume 0.35) Designers shall compare these results to the maximum allowable depths in Table 4.5, and use whichever value is less for subsequent design. Mode of Entry Table 4.5. Maximum Depth for Infiltration Practices. Infiltration Depth, ft Micro Infiltration Small Scale Infiltration Conventional Infiltration Surface Basin 1.0 1.5 2.0 Underground Reservoir 3.0 5.0 varies Once the maximum depth is established, calculate the surface area needed for an infiltration practice using Equation 4.3 or Equation 4.4: Equation 4.3. Surface Basin Surface Area SA = TV d+/ fxtf Appendix 8 — GSP Specifications Drainage Criteria Manual 1, Equation 4.4. Underground Reservoir Surface Area SA = TV V,x(d+/f)xtf Where: SA = Surface area (square feet) TV = Design volume (e.g., portion of the treatment volume, in cubic feet) Vr = Void Ratio (assume 0.35) d = Infiltration depth (maximum depends on the scale of infiltration and the results of Equation 4.1 or 4.2 (feet) f = Measured infiltration rate (feet/day) tf= Time to fill the infiltration facility (days - typically 2 hours, or 0.083 days) If the designer chooses to infiltrate less than the full Treatment Volume (e.g., through the use of micro - infiltration or small-scale infiltration), the runoff reduction rates shown in Table 4.1 above must be directly prorated in the Runoff Reduction Method (RRM) calculations. To qualify for Level 2 runoff reduction rates, designers must provide a design based upon 110% of the site -adjusted Treatment Volume. 6.3. Soil Infiltration Rate Testing The acceptable methods for on-site soil infiltration rate testing procedures are outlined in Appendix C. Since soil infiltration properties can vary, the different scales of infiltration shall be tested according to the following recommendations: • Micro -infiltration: One test per facility • Small -Scale Infiltration: One per 1,000 sq ft of surface area, or a maximum of two tests per facility • Conventional Infiltration: One test per 1,000 sq ft of proposed infiltration bed 6.4. Pretreatment Features Every infiltration practice must include multiple pretreatment techniques, although the nature of pretreatment practices depends on the scale at which infiltration is applied. The number, volume and type of acceptable pretreatment techniques needed for the three scales of infiltration are provided in Table 4.6. Appendix 8 - GSP Specifications Drainage Criteria Manual i , A minimum of 50% of the runoff reduction volume must be pre-treated by a filtering or bioretention practice prior to infiltration if the site is an area with potential for high pollutant loading. This application in such areas requires approval from the City engineer. Note that conventional infiltration practices require pretreatment of at least 25% of the treatment volume, including a surface pretreatment cell capable of keeping sediment and vegetation out of the infiltration cell. All pretreatment practices shall be designed such that exit velocities are non-erosive for the two year design storm and evenly distribute flows across the width of the practice (e.g., using a level spreader). 6.5. Conveyance and Overflow The nature of the conveyance and overflow to an infiltration practice depends on the scale of infiltration and whether the facility is on-line or off-line (Table 4.7). Where possible, conventional infiltration practices shall be designed offline to avoid damage from the erosive velocities of larger design storms. Micro -scale and small-scale infiltration practices shall be designed to maintain non-erosive conditions for overland flows generated by the 2 -year and 10 -year design storms (typically 3.5 to 5.0 ft per second). Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 4.6. Required Pretreatment Elements for Infiltration Practices Scale of Infiltration Pretreatment) Micro Infiltration Small Scale Infiltration Conventional Infiltration 3 techniques; 25% Number and Volume of 2 external techniques; no 3 techniques; 15% minimum pretreatment Pretreatment Techniques minimum pretreatment minimum pretreatment volume required Employed volume required. volume required (inclusive); at least one (inclusive). separate pre-treatment cell. Leaf gutter screens Grass filter strip Sediment trap cell Acceptable Pretreatment Grass filter strip Grass channel Sand filter cell Techniques Upper sand layer Plunge pool Sump pit Washed bank run gravel Gravel diaphragm Grass filter strip Gravel diaphragm A minimum of 50% of the runoff reduction volume must be pre-treated by a filtering or bioretention practice prior to infiltration if the site is an area with potential for high pollutant loading. This application in such areas requires approval from the City engineer. Note that conventional infiltration practices require pretreatment of at least 25% of the treatment volume, including a surface pretreatment cell capable of keeping sediment and vegetation out of the infiltration cell. All pretreatment practices shall be designed such that exit velocities are non-erosive for the two year design storm and evenly distribute flows across the width of the practice (e.g., using a level spreader). 6.5. Conveyance and Overflow The nature of the conveyance and overflow to an infiltration practice depends on the scale of infiltration and whether the facility is on-line or off-line (Table 4.7). Where possible, conventional infiltration practices shall be designed offline to avoid damage from the erosive velocities of larger design storms. Micro -scale and small-scale infiltration practices shall be designed to maintain non-erosive conditions for overland flows generated by the 2 -year and 10 -year design storms (typically 3.5 to 5.0 ft per second). Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table Conveyance• Overflow Scale of Infiltration Conveyance and Overflow Micro Infiltration Small Scale Infiltration Conventional Infiltration Discharge to a non-erosive An overflow mechanism such as an elevated drop inlet or pervious overland flow flow splitter shall be used to redirect flows to a Online Design path designed to convey non-erosive down-slope overflow channel or stabilized the 2 -year design storm to water course designed to convey the 10 -year design the street or storm drain system. storm A flow splitter or overflow structure can be used for this Off-line Design Not Recommended purpose using the design guidance in Claytor and Schueler (1996) and ARC (2001). 6.6. Internal Geometry and Drawdowns Runoff Reduction Volume Sizing. The proper approach for designing infiltration practices is to avoid forcing a large volume to be infiltrated into a small area. Therefore, individual infiltration practices that are limited in size due to soil permeability and available space need not be sized to achieve the full Treatment Volume for the contributing drainage area, as long as other runoff reduction practices are applied at the site to meet the remainder of the Tv. The total runoff reduction volume must be documented using the Runoff Reduction Method spreadsheet or another locally approved methodology that achieves equivalent results. The minimum amount of runoff from a given drainage area that can be treated by individual infiltration practices is noted in Table 4.2 above. Infiltration Basin Restrictions. The maximum vertical depth to which runoff may be ponded over an infiltration area is 24 inches (i.e., infiltration basin). The side -slopes shall be no steeper than 4HAV, and if the basin serves a CDA greater than 20,000 sq ft, a surface pre-treatment cell must be provided (this may be sand filter or dry sediment basin). Rapid Drawdown. When possible, infiltration practices shall be sized so that the target runoff reduction volume infiltrates within 36 hours to 48 hours, to provide a factor of safety that prevents nuisance ponding conditions. Conservative Infiltration Rates. Designers shall use the design infiltration rate, rather than the measured infiltration rate, to approximate long term infiltration rates (see Section 6.1 above). Void Ratio. A maximum porosity value of 0.35 shall be used in the design of stone reservoirs, although a larger value may be used (based on actual storage / total volume computations) if perforated corrugated metal pipe, plastic pipe, concrete arch pipe, or comparable materials are installed within the reservoir. Appendix B — GSP Specifications Drainage Criteria Manual i , 6.7. Landscaping and Safety Infiltration trenches can be effectively integrated into the site plan and aesthetically designed with adjacent native landscaping or turf cover, subject to the following additional design considerations: • Infiltration practices shall be installed only after all up -gradient construction is completed AND pervious areas are stabilized with dense and healthy vegetation. • Vegetation associated with the infiltration practice buffers shall be regularly mowed and maintained to keep organic matter out of the infiltration device and maintain enough native vegetation to prevent soil erosion from occurring. • Infiltration practices do not typically pose major safety hazards after construction. However, if an infiltration practice will be excavated to a depth greater than 5 ft, OSHA health and safety guidelines shall be followed for safe construction practices. • Fencing of infiltration trenches is neither necessary nor desirable. Designers shshould evaluate the nature of future operations to determine if the proposed site will be designated as a site with potential high pollutant loading and comply with the appropriate restrictions or prohibitions applicable to infiltration. 6.8. Maintenance Reduction Features Maintenance is a crucial element that ensures the long-term performance of infiltration practices. The most frequently cited maintenance problem for infiltration practices is clogging of the stone by organic matter and sediment. The following design features can minimize the risk of clogging: Observation Well. Small-scale and conventional infiltration practices should include an observation well, consisting of an anchored 6 -inch diameter perforated PVC pipe fitted with a lockable cap installed flush with the ground surface, to facilitate periodic inspection and maintenance. No Filter Fabric on Bottom. Avoid installing geotextile filter fabric along the bottom of infiltration practices. Experience has shown that filter fabric is prone to clogging, and a layer of coarse washed stone (choker stone) is a more effective substitute. However, permeable filter fabric must be installed on the trench sides to prevent soil piping. Direct Maintenance Access. Access must be provided to allow personnel and heavy equipment to perform non -routine maintenance tasks, such as practice reconstruction or rehabilitation. While a turf cover is permissible for micro- and small-scale infiltration practices, the surface must never be covered by an impermeable material, such as asphalt or concrete. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.9. Infiltration Material Specifications The basic material specifications for infiltration practices are outlined in Table 4.8 below. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Karst Terrain Conventional infiltration practices shall not be used on sites that contain karst features due to concerns about sinkhole formation and groundwater contamination. Micro- or small-scale infiltration areas are permissible IF geotechnical studies indicate there is at least 4 ft of vertical separation between the bottom of the infiltration facilities and the underlying karst layer AND an impermeable liner and underdrain are used. In many cases, bioretention is a preferred stormwater management alternative to infiltration in karst areas. 7.2. Steep Terrain Forcing conventional infiltration practices in steep terrain can be problematic with respect to slope stability, excessive hydraulic gradients and sediment delivery. Unless slope stability calculations demonstrate otherwise, it is generally recommended that infiltration practices shall be located a minimum horizontal Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 4.8. Infiltration Material Specifications Material Specifications Notes Stone Clean, aggregate with a maximum diameter of 3.5 inch and a minimum diameter of 1.5 inch (No. 1 size coarse aggregate, ASTM D448) or equivalent. Install a vertical 6 -inch Schedule 40 PVC Install one per 50 ft length of the Observation Well perforated pipe, with a lockable cap and infiltration practice. anchor plate. Trench Bottom Install a 6 to 8 inch sand layer (ASTM C33) above a 2 inch minimum layer of choker stone (ASTM D 448, No. 8 stone) Install a 3 -inch layer of river stone or pea This provides an attractive surface cover Trench Surface Cover gravel. Turf is acceptable when there is that can suppress weed growth. subsurface inflow (e.g., a roof leader). Buffer Vegetation Keep adjacent vegetation from forming an overhead canopy above infiltration practices, in order to keep leaf litter, fruits and other vegetative material from clogging the stone. Filter Fabric (sides only) Use non -woven polyprene geotextile with a flow rate of > 110 gal/min/sq ft (e.g., Geotex 351 or equivalent) Choking Layer Install a 2 to 4 -in layer of choker stone (typically ASTM D448 No. 8 or No. 89 washed gravel) over the underdrain stone. Use 6 -in rigid schedule 40 PVC pipe, with Install non -perforated pipe with one or 3/8 -in perforations at 6 inches on center, more caps, as needed from the downspout Underdrain (where needed) with each underdrain, installed at a slope to a point 15 ft from the structure. Install of 1% for the length of the infiltration T's as needed for the underdrain practice. configuration. The stone shall be double -washed and Install a minimum 3 -inch layer of #57 Stone Jacket for Underdrain clean and free of all soil and fines. stone above the underdrain and a minimum 12 inch layer. beneath it. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Karst Terrain Conventional infiltration practices shall not be used on sites that contain karst features due to concerns about sinkhole formation and groundwater contamination. Micro- or small-scale infiltration areas are permissible IF geotechnical studies indicate there is at least 4 ft of vertical separation between the bottom of the infiltration facilities and the underlying karst layer AND an impermeable liner and underdrain are used. In many cases, bioretention is a preferred stormwater management alternative to infiltration in karst areas. 7.2. Steep Terrain Forcing conventional infiltration practices in steep terrain can be problematic with respect to slope stability, excessive hydraulic gradients and sediment delivery. Unless slope stability calculations demonstrate otherwise, it is generally recommended that infiltration practices shall be located a minimum horizontal Appendix 8 — GSP Specifications Drainage Criteria Manual i , distance of 200 ft from down -gradient slopes greater than 20%. Micro -scale and small-scale infiltration can work well, as long as up -gradient and down -gradient building setbacks are satisfied. 7.3. Linear Highway Sites Infiltration practices can work well for linear highway projects, where soils are suitable and can be protected from heavy disturbance and compaction during road construction operations. SECTION 8: CONSTRUCTION 8.1. Construction Sequence The following is a typical construction sequence to properly install infiltration practices. The sequence may need to be modified to reflect the scale of infiltration, site conditions, and whether or not an underdrain needs to be installed. Infiltration practices are particularly vulnerable to failure during the construction phase for two reasons. First, if the construction sequence is not followed correctly, construction sediment can clog the practice. In addition, heavy construction equipment and disturbance can result in compaction of the soil, which can then reduce the soil's infiltration rate. For this reason, a careful construction sequence needs to be followed. During site construction, the following steps are absolutely critical: • Avoid excessive compaction by preventing construction equipment and vehicles from traveling over the proposed location of the infiltration practice. • Keep the infiltration practice "off-line" until construction is complete. Prevent the entrance of sediments to the infiltration practice using silt fence, diversion berms or other means. In the erosion and sediment control plan, indicate the earliest time at which stormwater runoff may be directed to a conventional infiltration basin. This control plan must also indicate the specific methods to be used to temporarily keep runoff from the infiltration site. • Infiltration practice sites shall never serve as the sites for temporary sediment control devices (e.g., sediment traps, etc.) during construction. • Upland drainage areas need to be completely stabilized with a thick layer of vegetation prior to commencing excavation for an infiltration practice. Stabilization should be verified by the local erosion and sediment control inspector/program. The actual installation of an infiltration practice is done using the following steps: 1. Excavate the infiltration practice to design dimensions using a backhoe or excavator. The floor of the pit shall be completely level, but mechanized equipment shall be kept off the base of the excavation to prevent soil compaction. 2. Scarify the bottom of the infiltration practice, and spread 6 inches of sand on the bottom as a filter layer. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 3. Install filter fabric on the trench sides. Large tree roots shall be trimmed flush with the sides of infiltration trenches to prevent puncturing or tearing of the filter fabric during subsequent installation procedures. The geotextile layout width shall include sufficient material to compensate for perimeter irregularities in the trench and accommodate a minimum 6- inch overlap at the top of the trench. The filter fabric itself shall be extended 4 -inches beneath the sand layer on the bottom of the infiltration trench, but shall not be extended across the trench. Stones or other anchoring objects shall be placed on the fabric at the trench sides, to keep the trench open during windy periods. If voids are present between the fabric and the excavated sides of a trench, they shall be backfilled with sand or pea gravel, to ensure the fabric conforms smoothly to the sides of excavation. 4. Install the underdrain, if one is needed. S. Anchor the observation well(s), and add stone to the practice in 1 -ft lifts. 6. Use sod to establish a dense turf cover for at least 10 ft on each side of the infiltration practice, to reduce erosion and sloughing. If the vegetation is seeded instead, use primarily native grasses. 8.2. Construction Inspection Inspections should be performed during construction to ensure that the infiltration practice is built in accordance with the approved design and this guidance. Qualified individuals shall use detailed inspection checklists to include sign -offs at critical stages of construction, to ensure that the contractor's interpretation of the plan is consistent with the designer's intentions. A performance evaluation of the infiltration system shall be prepared and provided by the design engineer certifying that the system was installed correctly and is functioning as intended. The performance evaluation shall be submitted to the City prior to receiving the Certificate of Occupancy. SECTION 9: MAINTENANCE 9.1. Maintenance Plans It is recommended that a Long Term Maintenance Plan (LTMP) be developed by the design engineer. The LTMP contains a description of the stormwater system components and information on the required inspection and maintenance activities. When micro -scale or small-scale infiltration practices are installed on private residential lots, homeowners will need to (1) be educated about their routine maintenance needs, (2) understand the long-term maintenance plan, and (3) agree to a deed restriction, drainage easement or other enforceable mechanism to ensure that infiltrating areas are not converted or disturbed. The mechanism shall, if possible, grant authority for local agencies to access the property for inspection or corrective action. In addition, the GPS coordinates shall be logged for all infiltration practices, upon facility acceptance, and submitted to the City Engineer. 9.2. Maintenance Inspections Annual site inspections are critical to the performance and longevity of infiltration practices, particularly for small-scale and conventional infiltration practices. Maintenance of infiltration practices is driven by annual inspections that evaluate the condition and performance of the practices, including the following: Appendix 8 - GSP Specifications Drainage Criteria Manual i , • The drawdown rate shall be measured at the observation well for three days following a storm event in excess of 1/2 inches. If standing water is still observed in the well after three days, this is a clear sign that that clogging is a problem. • Check inlets, pre-treatment cells, and any flow diversion structures for sediment buildup and structural damage. Note if any sediment needs to be removed. • Inspect the condition of the observation well and make sure it is still capped. • Check that vegetation has not formed an overhead canopy that may drop leaf litter, fruits and other vegetative materials that could clog the infiltration device. • Evaluate the vegetative quality of the adjacent grass buffer and peform spot -reseeding if the cover density is less than 90%. • Generally inspect the upland CDA for any controllable sources of sediment or erosion. • Look for weedy growth on the stone surface that might indicate sediment deposition or clogging. • Inspect maintenance access to ensure it is free of woody vegetation, and check to see whether valves, manholes and/or locks can be opened and operated. • Inspect internal and external infiltration side slopes for evidence of sparse vegetative cover, erosion or slumping, and make necessary repairs immediately. Based on inspection results, specific maintenance tasks may be triggered. 9.3. Ongoing Maintenance Effective long-term operation of infiltration practices requires a dedicated and routine maintenance inspection schedule with clear guidelines and schedules, as shown in Table 4.9 below. Where possible, facility maintenance shall be integrated into routine landscaping maintenance tasks. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 4.9. Typical Maintenance Activities for Infiltration Practices Maintenance Activity Schedule • Replace pea gravel/topsoil and top surface filter fabric (when clogged). As needed • Mow vegetated filter strips as necessary and remove the clippings. • Ensure that the contributing drainage area, inlets, and facility surface are clear of debris. • Ensure that the contributing drainage area is stabilized. Quarterly • Remove sediment and oil/grease from pre-treatment devices, as well as from overflow structures. • Repair undercut and eroded areas at inflow and outflow structures • Check observation wells 3 days after a storm event in excess of 1/2 inch in depth. Standing water observed in the well after three days is a clear indication of clogging. Semi-annual inspection • Inspect pre-treatment devices and diversion structures for sediment build- up and structural damage. • Remove trees that start to grow in the vicinity of the infiltration facility. • Clean out accumulated sediments from the pre-treatment cell. Annually Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 10: AS -BUILT REQUIREMENTS After the infiltration device has been constructed, the developer must have an as -built certification of the infiltration system prepared by a registered Professional Engineer and submit this to the City Engineer. The as -built certification verifies that the BMP was installed as designed and approved. The following components must be addressed in the as -built certification: • The infiltration device cannot be located in a sinkhole area or in karst soils. • Infiltration rates must be verified. • Proper dimensions for the trench must be verified. • A mechanism for overflow for large storm events must be provided. SECTION 11: REFERENCES Center for Watershed Protection (CWP), 2003. New York State Stormwater Management Design Manual. Prepared for the New York State Department of Environmental Conservation. Albany, NY. Center for Watershed Protection (CWP), 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Ellicott City, MD. City of Delaware, Delaware Urban Runoff Management Approach. Maryland Department of Environment (MDE), 2000. Maryland Stormwater Design Manual. Baltimore, MD. New Jersey Stormwater Best Management Practices Manual. North Shore City, 2007. Infiltration Design Guidelines. Sinclair, Knight and Merz. Auckland, New Zealand Pennsylvania. Draft Stormwater Best Management Practices Manual. Available online at: http/ /www.dep.state.pa.us/dep/subj ect/advcoun/Stormwater/stormwatercomm.htm Southeast Michigan Council of Governments. (SEMCOG), 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementors and Reviewers. Southeast Michigan Council of Governments, Detroit, MI. Schueler, T., C. Swann, T. Wright and S. Sprinkle, 2004. Pollution Source Control Practices. Manual No. 8 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD. VADCR (Virginia Department of Conservation and Recreation), 2011. Stormwater Design Specification No. S: Infiltration, Version 1.9, Richmond, VA. Available at: http//vwrrc.vt.edu/swc/NonProprietaryBMPs.html. VADCR, 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA. Appendix B - GSP Specifications Drainage Criteria Manual i , WATER QUALITY SWALE Description: Vegetated open channels designed to capture and infiltrate stormwater runoff within a dry storage layer beneath the base of the channel. • Open trapezoidal or parabolic channel to store entire treatment volume, which is ultimately infiltrated • Filter bed of permeable, engineered soils • Underdrain system for impermeable soils • Level spreaders every 50 ft., if length exceeds 100 ft. Disadvantages/Limitations • Stormwater treatment • Higher maintenance than combined with conveyance curb and gutter • Less expensive than curb • Cannot be used on steep and gutter slopes • Reduces runoff velocity • High land requirement • Promotes infiltration • Requires 3 ft of head • Longitudinal slopes less than 2% • Bottom channel width of 2 to 8 ft • Underdrain required for subsoil infiltration rates less than 0.5 inch/hr. • Side slopes of 3:1 or flatter; 4:1 recommended • Must convey the 10 -year storm event with a minimum of 6 in. of freeboard See Page 2 for Right of Way Applications. Appendix B — GSP Specifications Drainage Criteria Manual i , Selection Criteria: Level 1— 40% Runoff Reduction Credit Level 2 — 60% Runoff Reduction Credit Land Use Considerations: © Residential © Commercial ■ Industrial* * With City Engineer's approval Maintenance: • Maintain grass height (if turf) • Remove sediment from forebay and channel • Remove accumulated trash and debris • Re-establish plants as needed ©Maintenance Burden L = Low M = Moderate H = High • Used in medians and right of way — accommodate utility specific horizontal and vertical setbacks • Stormwater can be conveyed by sheet flow or grass channels • Pretreatment is especially important in roadway applications where sediment loads may be high • Design as a series of cells running parallel to roadway • Cannot create hazard or interfere with walkability • Setbacks must comply with city and utility -specific requirements SECTION 1: DESCRIPTION Water quality swales are essentially bioretention cells that are shallower, configured as linear channels, and covered with turf or other planting material. The water quality swale is a soil filter system that temporarily stores and then filters the desired Treatment Volume (Tv). Water quality swales rely on a pre -mixed soil media filter below the channel that is similar to that used for bioretention. If soils are extremely permeable, runoff infiltrates into underlying soils. Otherwise, the runoff treated by the soil media flows into an underdrain that conveys treated runoff to the conveyance system further downstream. The underdrain system consists of a perforated pipe within a gravel layer on the bottom of the swale, beneath the soil media. Water quality swales may appear as simple grass channels with the same shape and turf cover, while others may have more elaborate landscaping. Swales can be planted with turf grass, tall meadow grasses, decorative herbaceous cover or trees. SECTION 2: PERFORMANCE The primary pollutant removal mechanisms operating in swales are settling, filtering infiltration and plant uptake. The overall runoff reduction capabilities of water quality swales are summarized in Table 5.1. Table SA: Annual Runoff Volume Reduction Provided . Stormwater Function Level 1 Design Level 2 Design Annual Runoff Volume Reduction (RR) 40% 60% Sources: CWP and CSN (2009); CWP (2007). SECTION 3: DESIGN TABLE Swales can be oriented to accept runoff from a single discharge point, or to accept runoff as lateral sheet flow along the swale's length. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 5.2. Water Quality Design Level 1 Design (RR:40) Level 2 Design (RR:60) Sizing: See Section 6.1 Sizing: See Section 6.1 Surface Area (sq. ft.) _ (Tv — the volume reduced by an Surface Area (sq. ft.) _ ((1.1)(Tv) — the volume reduced by upstream BMP) / Storage depth' an upstream BMP I/ Storage Depth' Effective swale slope <_ 2% Effective swale slope <_ 1 Media Depth minimum = 18 inches; Media Depth minimum = 24 in. Recommended maximum = 36 inches Recommended maximum = 36 in. Sub -soil testing (Section 6.2): not needed if an Sub -soil testing (Section 6.2): one per 200 linear underdrain is used; min. infiltration rate must be > feet of filter surface; min. infiltration rate must be > 0.5 inches/hour to remove the underdrain requirement; 0.5 in./hr. to remove the underdrain requirement Underdrain and Underground Storage Layer Underdrain (Section 6.7): Schedule 40 PVC or HDPE with (Section 6.7): Schedule 40 PVC or HDPE with clean outs, clean -outs and a minimum 12 -in. stone sump below the invert; OR none if the soil infiltration requirements are met (see Section 6.2) Media (Section 6.6): supplied by the vendor or mixed onsite2 Inflow: sheet or concentrated flow with appropriate pre-treatment Pre -Treatment (Section 6.4): a pretreatment cell, level spreader, or another approved (manufactured) grass filter strip, gravel diaphragm, or gravel flow pre-treatment structure. On-line design Off-line design or multiple treatment cells Planting Plan: turf grass, tall meadow grasses, decorative herbaceous cover, or trees' 1. The storage depth is the sum of the Void Ratio (Vr) of each soil media and gravel layer multiplied by its depth, plus the surface ponding depth (Refer to Section 6.1). 2 Refer to Appendix C for soil specifications and infiltration test information. 3 Refer to Appendix D for planting lists. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Figure 5.1. Typical Water Quality Swale in commercial/office setting (Source: VADCR, 2011). SECTION 4: TYPICAL DETAILS Figures 5.2 through 5.6 below provide typical schematics for water quality swales. Appendix 8 — GSP Specifications Drainage Criteria Manual i , UNDEFOR N OUTFALL OHKaK DAM (AS NECESSAiF Y) Th.1ENT AS REflLNIEQ �� 1911M1 L, TO IF J r .. � �� .... ...I:: _:.. L—.1 t8' hAN (r sal❑ +0 PERFORATM PVO UN EREIRMN f, VHIEKNEEUEZ j Nm T ❑EPTH PFA GRAVEL SECTION LEVEL 1 Figure 5.2. Typical Detail for Level 1 Water Quality Swale (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , DAM EF= CIETAIL {HFlA❑E UAGff&4T:KEk SOL ry K40t;br AM MW 3r DEPTH AEA G&WEL AND ABOVE CFKYmd OF LPKIERDRNN 4_,@' Gm%na- GLUP RFWYBF-ATMENT AsRRXMN�D I L21DENERAIN 68*Ehl NEEEEEq SECTM LOYEL 2 MTH-UNCERCRAN PPE-rFF-A—i NUIT AS PE -0 LKRM 79 c?qffnNo,anADE 12r" ?".24. MIN MIN 2r DEFTH PEA C-MVEL AND AECVE GROWN OF UNDER BFIAIN :g- I &LGqAVEL SUMP MWT OW STONE OPEN GqAD17-0 WA�iHEI)� SECTION LEVEL2WITHOUT'LINCERORAIN Figure 5.3. Typical Detail for Level 2 Water Quality Swale (Source: VADCR, 2011). Appendix B - GSP SpecifticationF s Drainage Criteria Manual I , APA(',ING PFR UESIGN GUIUELINES 11 i APL LOI UAL N U1 CI WEIR JA a 1 OR LESS I'HEFEHHEL: 33:1 MAX TURF COVER } B RFRAR arc ai.c a.r aur. awc �� m.r._ aux. arr ANU]gOHS "' .r .�. s. yP�F��•'9' auo. .rr arc ya rr ar arc arc arc arc a� Bi A b:1 UH LESS f'HE FEHHED 3:1 PLAN MAX SEE DETAIL 12' MAX 1 M 61 MIN. 1 3 RIA ROUNDFF) CCIRRI F CiRAVFI 1 C UL PLAULU W U'LLP AND J WEIR DETAIL UNDERLYMN BY NON WOVEN FILTER FABRIC. T(Df I OF C-IECK UANI 2-3 MINIMUM I COMPOST AIVIFNDFF7 S ]ILS FXISTIN• ; (;RAIDF t4 RI -INF ORGINCI PAR MIN. 18' ULL�YAI GPADI=; GRAVELS -ONE SFI AS41 APRC:•N SEG -ION A -A' FN[1NFFRFR SC)II f<•I17C PIIUVIUL 1;2 VVEEP HOLES NOTE CI IECK DAN` CONSTRUCTEC OF RAILROAD TIES OR PRESSURE TREATED (-Xi' OR TIMRFRS C HF(:K CLAM SPANS FNTIRF 'WIF)TH OF SWAT F AND IS ANCHORED INTO THE SWVALE A MINIMUM OF 2 FES ON EACH SIDE. CHECK DAM IS KEYED INT(-) TI {E [ ;ROUND AT .A 2-3 INCI I DEPTH AND UNDERLAIN DY FILTER FABRIC PE STD & 3PEC 3.19: RIP RAP V--3 --:H. 19'32 SVALL 3RAVEL 3P LASH PAC RRCVIDED AT DC'VJNSTREAL' SIDE OF CHECK DAMS Figure 5.4. Typical Detail for Water Quality Swale Check Dam (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , T-'= 3l .-F'F — P'..' ..':F -j GRASS FILTER FOR SHEET FLOW PRETREATMENT I �IFF =.l -,PF F H _' 7 1-- � SHEET FLC r --- GRASS-, GRASS FILTER FOR SHEET FLOW PRETREATMENT 11 SHEFT Fl 0 H rr. P,A .'Ep 2'�'CROP ':"UEMENI SUE,, F LT -E R F-E� LL -,rJLrJ- b - V L771 ZE tt- -,;L-)E A"I E' GRAVEL DIAPHRAGM SHEET FLOW PRETREATMENT MTS NTS Figure S.S. Pretreatment. Gravel Diaphragm for Sheet Flow from Impervious or Pervious (Source: VADCR, 2011). Appendix B - GSP SpecifticationF s Drainage Criteria Manual I , ki _rF -F _=ILE T -FrJTFR Ur !E F L E, ,.J-, FEF FF1,-,RE`EI•J-I•::1A ELL'.•R i-F!FR F•Fi:•�:71�C : Er'1L' i? - E''T��-.' J F!L'EF F.AERI: , J•. TF '.T..-•r.IF •%14 E- D=T•T�I E E L:E•�{..,i ]cLi 6-:,6L _r! L•E_:I -,rJ SFf:TI,`)N A A -.'.:Fc.:: IIF'H :•1JF�I. .. :':IF:TH ..; RE•- - IREI F. , -:Ef l-FI•:TE- II IR. ''r;; MPF :.Ii.=M-JR. i'6 r--THF.F.. ',F',MFT;i, ,,r jr. Fu- iJ F,,% -F rJ -'TF ".T - r IF-.!ZF=•!!F•FI=FTH T, -'FE 3EC1 EIA _-EC '=:Fl = C _.I :, r.:.-- ONE 8 LII 11'�I•.:,,=�::�F��.':I� IC �, e�. [IE:, 44 Er 1 ,Ir:EE ,' ME1 Ir CONCENTRATED FLOW CURBCUT PRETREATMENT - GRAVEL FLOW SPREADER NTS Figure 5.6. Pre -Treatment - Gravel Flow Spreader for Concentrated Flow (Source: VADCR, 2011). SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS Water quality swales can be implemented on a variety of development sites where density and topography permit their application. Some key feasibility issues for water quality swales include the following: Contributing Drainage Area. The maximum contributing drainage area to a water quality swale shall be 5 acres or less. When water quality swales treat larger drainage areas, the velocity of flow through the surface channel often becomes too great to treat runoff or prevent erosion in the channel. Similarly, the longitudinal flow of runoff through the soil, stone, and underdrain may cause hydraulic overloading at the downstream sections of the water quality Swale. An alternative is to provide a series of inlets or diversions that convey the treated water to an outlet location. Appendix B - GSP Specifications Drainage Criteria Manual I , Available Space. Water quality swale footprints can fit into relatively narrow corridors between utilities, roads, parking areas, or other site constraints. Water quality swales should be approximately 3% to 10% of the size of the contributing drainage area, depending on the amount of impervious cover. Site Topography. Water quality swales should be used on sites with longitudinal slopes of less than 4%, but preferably less than 2%. Check dams can be used to reduce the effective slope of the swale and lengthen the contact time to enhance filtering and/or infiltration. Steeper slopes adjacent to the swale may generate rapid runoff velocities into the Swale that may carry a high sediment loading (refer to pre-treatment criteria in Section 6.4). Available Hydraulic Head. A minimum amount of hydraulic head is needed to implement water quality swales, measured as the difference in elevation between the inflow point and the downstream storm drain invert. Water quality swales typically require 3 to 5 ft of hydraulic head since they have a filter bed and potentially an underdrain. Hydraulic Capacity. Level 1 water quality swales are an on-line practice and must be designed with enough capacity to (1) convey a portion of the runoff from the 100 -year design storms at non-erosive velocities, and (2) contain the 10 -year flow within the banks of the swale. This means that the swale's surface dimensions are more often determined by the need to pass the 10 -year storm events, which can be a constraint in the siting of water quality swales within existing right of way (e.g., constrained by sidewalks). Depth to Water Table. Designers shall ensure that the bottom of the water quality swale is at least 2 ft above the seasonally high groundwater table, to ensure that groundwater does not intersect the filter bed, since this could lead to groundwater contamination or practice failure. Soils. Soil conditions do not constrain the use of water quality swales, although they normally determine whether an underdrain is needed. Low -permeability soils with an infiltration rate of less than 0.5 inch per hour, such as many of those classified in Hydrologic Soil Groups (HSG) C and D, will require an underdrain. Designers must verify site-specific soil permeability at the proposed location using the infiltration test presented in Appendix C in order to eliminate the requirements for an underdrain. Setbacks from Utilities, Buildings and Roads. Designers shall consult local utility design guidance for the horizontal and vertical clearance between utilities and the swale configuration. Utilities can cross linear swales if they are specially protected (e.g., double -cased). Water and sewer lines may need to be placed under road pavements if water quality swales are placed adjacent to roadways. Water and sewer lines that parallel the swale shall be positioned at least 10 ft horizontally from the edge of the infiltration section of the water quality swale. Water and sewer lines that cross under the Swale shall have 18 inches of separation from the bottom of the infiltration portion of the swale and shall be encased or be installed with ductile iron pipe with mechanical joints. Storm drainage pipes can cross under water quality swales, as long as adequate cover for the pipes is provided and it functions adequately with the stormwater management system. Given their landscape position, water quality swales are not subject to normal building setbacks. The bottom elevation of swales shall be at least 1 ft below the invert of any adjacent road bed. Avoidance of Irrigation or Baseflow. Water quality swales shall be located so as to avoid inputs from springs, irrigation systems, chlorinated wash -water, or other dry weather flows. Appendix B - GSP Specifications Drainage Criteria Manual i , Areas with Potential for High Pollutant Loading. Runoff from land uses with potential for high pollutant loading should not be treated using infiltrating water quality swales. An impermeable liner with underdrain shall be used for filtration of runoff from these areas. Community Acceptance. The main concerns of adjacent residents are perceptions that swales will create nuisance conditions or will be hard to maintain. Common concerns include the continued ability to mow grass, landscape preferences, weeds, standing water, and mosquitoes. Water quality swales are actually a positive stormwater management alternative, because all these concerns can be fully addressed through the design process and proper on-going operation and routine maintenance. The ponding time is less than the time required for one mosquito breeding cycle, so well-maintained water quality swales should not create mosquito problems or be difficult to mow. SECTION 6: DESIGN CRITERIA 6.1. Sizing of Water Quality Conveyance and Water Quality Treatment Swales Sizing of the surface area (SA) for water quality swales is based on the computed Treatment Volume (Tv) of the contributing drainage area and the storage provided within the swale media and gravel layers and behind check dams. The required surface area (in square feet) is computed as the Treatment Volume (in cubic feet) divided by the equivalent storage depth (in feet). The equivalent storage depth is computed as the depth of the soil media, the gravel, and surface ponding (in feet) multiplied by the accepted void ratio. The maximum Void Ratios (Vr) acceptable are: Water Quality Swale Soil Media Vr= 0.40 Gravel Vr= 0.35 Surface Storage behind check dams Vr =1.0 For example, the equivalent storage depth for the Level 1 design (without considering surface ponding) is therefore computed as: Equation 5.1. Equivalent Storage Depth (ESD) - Level 1 (1.5 ft x 0.40+ (0.25 ft x 0.35) = 0.69 ft And the equivalent storage depth for the Level 2 design (without considering surface ponding) is computed as: Equation 5.2. Equivalent Storage Depth (ESD) - Level 2 (2.O ft x 0.40+ (1.0 ft. x 0.35) = 1.15 ft The effective storage depths will vary according to the actual void ratios, design depths of the soil media and gravel layer. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Note: When using Equations 3 or 4 below to calculate the required surface area of a water quality swale that includes surface ponding (with check dams), the storage depth calculation (Equation 1 or 2) shall be adjusted accordingly. The Level 1 Water Quality Swale Surface Area (SA) is computed as: Equation 5.3. Surface Area - Level 1 SA (sq ft) _ (Tv - the volume reduced by an upstream GSP) / ESD Level 1 And the Level 2 Water Quality Swale SA is computed as: Equation 5.4. Surface Area - Level 2 SA (sq ft) _ [(1.1 * Tv) - the volume reduced by an upstream GSP]/ ESD Level 2 NOTE: The volume reduced by upstream pretreatment practices is supplemented with the anticipated volume of storage created by check dams along the swale length. Where: SA = Minimum surface area of Water Quality Swale (sq ft) Tv = Treatment Volume (cu. ft.) _ [(1 inch) (Rv) (A)]*3630, A = Area in acres. The final water quality swale design geometry will be determined by dividing the SA by the swale length to compute the required width; or by dividing the SA by the desired width to compute the required length. 6.2. Soil Infiltration Rate Testing The second key sizing decision is to measure the infiltration rate of subsoils below the water quality swale area to determine if an underdrain will be needed. The infiltration rate of the subsoil must exceed 0.5 inch per hour to avoid installation of an underdrain. The acceptable methods for on-site soil infiltration rate testing are outlined in Appendix C. A soil test shall be conducted for every 200 linear feet of water quality swale. 6.3. Water Quality Swale Geometry Design guidance regarding the geometry and layout of water quality swales is provided below. Shape. A parabolic shape at the surface is preferred for water quality swales for aesthetic, maintenance and hydraulic reasons. However, the design may be simplified with a trapezoidal cross-section at the surface, as long as the soil filter bed boundaries are "keyed in" with vertical sides and lay in the flat bottom areas. Side Slopes. The side slopes of water quality swales shall be no steeper than 3H: 1V for maintenance considerations (i.e., mowing). Flatter slopes are encouraged where adequate space is available, to enhance pre-treatment of sheet flows entering the swale. Swales should have a bottom width of from 2 to 8 ft to ensure that an adequate surface area exists along the bottom of the swale for filtering. If a swale will be Appendix 8 - GSP Specifications Drainage Criteria Manual i , wider than 8 ft, the designer should incorporate berms, check dams, level spreaders or multi-level cross- sections to prevent braiding and erosion of the swale bottom. Swale Longitudinal Slope. The longitudinal slope of the swale should be moderately flat to permit the temporary ponding of the Treatment Volume within the channel. The recommended swale slope is less than or equal to 2% for a Level 1 design and less than or equal to 1% for a Level 2 design, though slopes up to 4% are acceptable if check dams are used. The minimum recommended slope for an on-line water quality swale is 0.5%. Refer to Table 5.3 for check dam spacing based on the swale longitudinal slope. Table 5.3. Typical Check D.m (CD) Spacing to Achieve Effective Swale Slope LEVEL 1 LEVEL 2 Spacing of 12 -inch Spacings of 12 -inch Swale Longitudinal High( max.) Check High (max.) Check Slope Dams ' to Create an Dams 2.3 to Create an Effective Slope of 2% Effective Slope of 0 to 1% 0.5% — 200 ft to — 1.0% — 100 ft to — 1.5% — 67 ft to 200 ft 2.0% — 50 ft to 100 ft 2.5% 200 ft 40 ft to 67 ft 3.0% 100 ft 33 ft to 50 ft 3.5% 67 ft 30 ft to 40 ft 4.0% 50 ft 25 ft to 33 ft 1. The spacing dimension is half of the above distances if a 6 -inch check dam is used. 2. Check dams require a stone energy dissipater at the downstream toe. 3. Check dams require weep holes at the channel invert. Swales with slopes less than 2% will require multiple weep holes - at least three in each check dam. Check dams. Check dams must be firmly anchored into the side -slopes to prevent outflanking and be stable during the 10 year storm design event. The height of the check dam relative to the normal channel elevation shall not exceed 12 inches. Each check dam shall have a minimum of one weep hole or a similar drainage feature so it can dewater after storms. Armoring may be needed behind the check dam to prevent erosion. The check dam must be designed to spread runoff evenly over the water quality swale's filter bed surface, through a centrally located depression with a length equal to the filter bed width. In the center of the check dam, the depressed weir length shall be checked for the depth of flow and sized for the appropriate design storm (Figure 5.3). Check dams shall be constructed of wood or stone. Soil Plugs. Soil plugs are small areas of compacted soil placed in front of and below the check dam to prevent water from flowing under the check dams. Soil plugs serve to help minimize the potential for blow-out of the soil media underneath the check dams, due to hydrostatic pressure from the upstream ponding. Soil plugs are appropriate for water quality swales with 12 -inch high check dams. Ponding Depth. Drop structures or check dams can be used to create ponding cells along the length of the swale. The maximum ponding depth in a swale shall not exceed 12 inches at the most downstream point. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Drawdown. Water quality swales shall be designed so the desired Treatment Volume is completely filtered within 24 hours or less. This drawdown time can be achieved by using the soil media mix specified in Section 6.6 and an underdrain along the bottom of the swale, or native soils with adequate permeability, as verified through testing (Section 6.2). Underdrain. Underdrains are provided in water quality swales to ensure that they drain properly after storms. The underdrain shall have a minimum diameter of 4 inches and be encased in a 6 -inch deep gravel bed. Two layers of stone shall be used. A choker stone layer, consisting of #8 or #78 stone at least 3 inches deep, shall be installed immediately below the filter media. Below the choker stone layer, the main underdrain layer shall be at least 12 inches deep and composed of clean washed #57 stone. The underdrain pipe shall be set at least 4 inches above the bottom of the stone layer. 6.4. Pre-treatment Several pre-treatment measures are feasible, depending on whether the specific location in the water quality swale system will be receiving sheet flow, shallow concentrated flow, or fully concentrated flow: Initial Sediment Forebay (channel flow). This grass cell is located at the upper end of the water quality Swale segment with a 2:1 length to width ratio and a storage volume equivalent to at least 15% of the total Treatment Volume. • Check dams (channel flow). These energy dissipation devices are acceptable as pre-treatment on small swales with drainage areas of less than 1 acre. • Tree Check dams (channel flow). These are street tree mounds that are placed within the bottom of a water quality swale up to an elevation of 9 to 12 inches above the channel invert. One side has a gravel or river stone bypass to allow storm runoff to percolate through. • Grass Filter Strip (sheet flow). Grass filter strips extend from the edge of the pavement to the bottom of the water quality swale at a 5:1 slope or flatter. Alternatively, provide a combined 5 feet of grass filter strip at a maximum 5% (20:1) slope and 3:1 or flatter side slopes on the water quality swale. (Figure 5.4) • Gravel Diaphragm (sheet flow). A gravel diaphragm located at the edge of the pavement shall be oriented perpendicular to the flow path to pre -treat lateral runoff, with a 2 to 4 inch drop. The stone must be sized according to the expected rate of discharge. (Figure 5.5) • Pea Gravel Flow Spreader (concentrated flow). The gravel flow spreader is located at curb cuts, downspouts, or other concentrated inflow points, and shall have a 2 to 4 inch elevation drop from a hard -edged surface into a gravel or stone diaphragm. The gravel shall extend the entire width of the opening and create a level stone weir at the bottom or treatment elevation of the swale. (Figure 5.6) 6.5. Conveyance and Overflow The bottom width and slope of a water quality swale shall be designed such that the velocity of flow from a 1 -inch rainfall will not exceed 3 feet per second. Check dams may be used to achieve the needed runoff reduction volume, as well as to reduce the flow. Check dams shall be spaced based on channel slope and ponding requirements, consistent with the criteria in Table 5.3. Appendix 8 - GSP Specifications Drainage Criteria Manual i , The swale shall also convey the 2- and 10 -year storms at non-erosive velocities with at least 6 inches of freeboard. The analysis shall evaluate the flow profile through the channel at normal depth, as well as the flow depth over top of the check dams. Water quality swales may be designed as off-line systems, with a flow splitter or diversion to divert runoff in excess of the design capacity to an adjacent conveyance system. Or, strategically placed overflow inlets may be placed along the length of the swale to periodically pick up water and reduce the hydraulic loading at the downstream limits. 6.6. Filter Media Water quality swales require replacement of native soils with a prepared soil media. The soil media provides adequate drainage, supports plant growth, and facilitates pollutant removal within the water quality Swale. At least 18 inches of soil media should be added above the choker stone layer to create an acceptable Level 1 filter. The mixture for the soil media is identical to that used for bioretention and details are provided in Appendix C under Engineered Soil Mix A. 6.7. Underdrain and Underground Storage Layer Some Level 2 water quality swale designs will not use an underdrain (where soil infiltration rates meet minimum standards (see Section 6.2 and the design table in Section 3). For Level 2 designs with an underdrain, an underground storage layer, consisting of a minimum 12 inches of stone, shall be incorporated below the invert of the underdrain. The depth of the storage layer will depend on the target treatment and storage volumes needed to meet water quality, channel protection, and/or flood protection criteria. However, the bottom of the storage layer must be at least 2 feet above the seasonally high groundwater table. The storage layer shall consist of clean, washed #57 stone or an approved infiltration module. A water quality swale should include observation wells with cleanout pipes along the length of the swale, if the contributing drainage area exceeds 1 acre. The wells shall be tied into any T's or Y's in the underdrain system, and shall extend upwards to be flush with surface, with a vented cap. 6.8. Landscaping and Planting Plan Designers should choose grasses, herbaceous plants or trees that can withstand both wet and dry periods and relatively high velocity flows for planting within the channel. Salt tolerant grass species should be chosen for water quality swales located along roads. Taller and denser grasses are preferable, although the species is less important than good stabilization and dense vegetative cover. Grass species shall have the following characteristics: a deep root system to resist scouring; a high stem density with well -branched top growth; water -tolerance; resistance to being flattened by runoff; and an ability to recover growth following inundation. Information in Appendix D is provided for selection of appropriate plantings. 6.9. Water quality Swale Material Specifications Appendix 8 - GSP Specifications Drainage Criteria Manual i , Table 5.4 outlines the standard material specifications for constructing water quality swales. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Karst Terrain Shallow water quality swales are an acceptable practice in karst regions. To prevent sinkhole formation and possible ground water contamination, water quality swales shall use impermeable liners and underdrains. Therefore, Level 2 water quality Swale designs that rely on infiltration are not recommended in any area with a moderate or high risk of sinkhole formation (CSN, 2009). 7.2. Steep Terrain In areas of steep terrain, water quality swales can be implemented with contributing slopes of up to 20% gradient, as long as a multiple cell design is used to dissipate erosive energy prior to filtering. This can be accomplished by terracing a series of water quality swale cells to manage runoff across or down a slope. The drop in elevation between cells shall be limited to 1 foot and the splash apron area armored with river stone or a suitable equivalent. A greater emphasis on properly engineered energy dissipaters and/or drop structures is warranted. Appendix 8 — GSP Specifications Drainage Criteria Manual i , QualityTable 5.4. Water Material Specification Notes The volume of filter media is based on Filter Media Engineered Soil Mix A provided in 110% of the product of the surface area Composition Appendix C and the media depth, to account for settling. Filter Media Testing Mix on-site or procure from an approved media vendor (refer to Appendix C for additional soil media information. Filter Fabric A non -woven polyprene geotextile with a flow rate of > 110 gal/min/sq ft (e.g., Geotex 351 or equivalent); Apply immediately above the underdrain only. Choking Layer A 3- inch layer of choker stone (typically ASTM D448 No. 8 or No. 89 or equivalent (Initial Filtering Layer) washed gravel) laid above the underdrain stone. Stone and/or Storage A 9 to 18 inch layer (depending on the desired depth of the storage layer) of ASTM D448 Layer No. 57 stone shall be double -washed and clean and free of all soil and fines. Underdrains, 6 -inch rigid schedule 40 PVC pipe, with Install perforated pipe for the full length Cleanouts, and 3/8 -inch perforations. For observation well, of the water quality swale. Observation Wells perforated sections shall only extend Use non -perforated pipe, as needed, to through stone/gravel layer. connect with the storm drain system. Vegetation Plant species as specified on the landscaping plan Use non-erosive material such as wood, gabions, riprap, or concrete. All check dams Check Dams shall be underlain with filter fabric, and include weep holes. Wood used for check dams shall consist of pressure -treated logs or timbers, or water-resistant tree species such as cedar, hemlock, swamp oak or locust. Erosion Control Fabric Where flow velocities dictate, use woven biodegradable erosion control fabric or mats that are durable enough to last at least 2 growing seasons. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Karst Terrain Shallow water quality swales are an acceptable practice in karst regions. To prevent sinkhole formation and possible ground water contamination, water quality swales shall use impermeable liners and underdrains. Therefore, Level 2 water quality Swale designs that rely on infiltration are not recommended in any area with a moderate or high risk of sinkhole formation (CSN, 2009). 7.2. Steep Terrain In areas of steep terrain, water quality swales can be implemented with contributing slopes of up to 20% gradient, as long as a multiple cell design is used to dissipate erosive energy prior to filtering. This can be accomplished by terracing a series of water quality swale cells to manage runoff across or down a slope. The drop in elevation between cells shall be limited to 1 foot and the splash apron area armored with river stone or a suitable equivalent. A greater emphasis on properly engineered energy dissipaters and/or drop structures is warranted. Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 8: CONSTRUCTION 8.1. Construction Sequence Construction Stage Erosion Protection and Sediment Controls. Water quality swales shall be fully protected by silt fence or construction fencing, particularly if they will provide an infiltration function (i.e., have no underdrains). Ideally, water quality swale areas should remain outside the limits of disturbance during construction to prevent soil compaction by heavy equipment. Water quality swale locations may be used for small sediment traps or basins during construction. However, these must be accompanied by notes and graphic details on the EPSC plan specifying that the maximum excavation depth of the sediment trap/basin at the construction stage must (1) be at least 1 foot above the depth of the post -construction water quality swale installation, (2) contain an underdrain, and (3) specify the use of proper procedures for conversion from a temporary practice to a permanent one, including de- watering, cleanout and stabilization. 8.2. Construction Sequence The following is a typical construction sequence to properly install a water quality swale, although the steps may be modified to adapt to different site conditions. Step 1. Protection during Site Construction. As noted above, water quality swales should remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. However, this is seldom practical given that swales are a key part of the drainage system at most sites. In these cases, temporary sediment controls such as dikes, silt fences and other similar measures should be integrated into the Swale design throughout the construction sequence. Specifically, barriers shall be installed at key check dam locations, erosion control fabric shall be used to protect the channel, and excavation shall be no deeper than 2 feet above the proposed invert of the bottom of the planned underdrain. Water quality swales that lack underdrains (and rely on filtration) must be fully protected by silt fence or construction fencing to prevent compaction by heavy equipment during construction. Step 2. Installation may only begin after the entire contributing drainage area has been stabilized by vegetation. The designer should check the boundaries of the contributing drainage area to ensure it conforms to original design. Additional EPSC may be needed during swale construction, particularly to divert stormwater from the water quality swale until the filter bed and side slopes are fully stabilized. Pre-treatment cells shall be excavated first to trap sediments before they reach the planned filter beds. Step 3. Excavators or backhoes shall work from the sides to excavate the water quality swale area to the appropriate design depth and dimensions. Excavating equipment shall be adequate reach so it does not sit inside the footprint of the water quality swale area. Step 4. The bottom of the water quality swale should be ripped, roto -tilled or otherwise scarified to promote greater infiltration. Step 5. Place an acceptable filter fabric on the underground (excavated) sides, but not the base, of the water quality swale with a minimum 6 inch overlap. Place filter bed and then place the stone needed for storage layer over the filter bed. Place the perforated underdrain pipe and check its slope. Add the remaining stone Appendix 8 - GSP Specifications Drainage Criteria Manual i , jacket, pack ASTM D 448 No. 57 stone to 3 inches above the top of the underdrain, then add 3 inches of pea gravel as a filter layer. Step 6. Add the soil media in 12 -inch lifts without compaction until the desired top elevation of the water quality swale is achieved. Wait a few days to check for settlement, and add additional media as needed. Step 7. Install check dams, driveway culverts and internal pre-treatment features, as specified in the plan. Step B. Prepare planting holes for specified trees and shrubs, install erosion control fabric where needed, spread seed or lay sod, and install any temporary irrigation. Step 9. Plant landscaping materials as shown in the landscaping plan, and water them weekly during the first 2 months. The construction contract should include a care and replacement warranty to ensure that vegetation is properly established and survives during the first growing season following construction. Step 10. Conduct a final construction inspection and develop a punchlist for facility acceptance. 8.3. Construction Inspection Inspections are needed during construction to ensure that the water quality swale is built in accordance with this guidance and project -specific specifications, and certified as -built plans will be required post - construction. Detailed inspection checklists should be used that include sign -offs by qualified individuals at critical stages of construction, to ensure that the contractor's interpretation of the plan is consistent with the designer's intent. Some common pitfalls can be avoided by careful construction supervision that focuses on the following key aspects of water quality swale installation. • Check the filter media to confirm that it meets specifications and is installed to the correct depth. • Check elevations such as the invert of the underdrain, inverts for the inflow and outflow points, and the ponding depth provided between the surface of the filter bed and the overflow structure. • Ensure that caps are placed on the upstream (but not the downstream) ends of the underdrains. • Make sure the desired coverage of turf or erosion control fabric has been achieved following construction, both on the filter beds and their contributing side -slopes. • Inspect check dams and pre-treatment structures to make sure they are properly installed and working effectively. • Check that outfall protection/energy dissipation measures at concentrated inflow and outflow points are stable. The real test of a water quality swale occurs after its first big storm. The post -storm inspection should focus on whether the desired sheet flow, shallow concentrated flows or fully concentrated flows assumed in the plan actually occur in the field. Also, inspectors should check that the water quality swale drains completely within 24 hour drawdown period. Minor adjustments are normally needed as a result of this post -storm inspection (e.g., spot reseeding, gully repair, add armoring at inlets or outfalls, and check dam realignment). Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 9: MAINTENANCE 9.1. Maintenance Plans It is recommended that a Long Term Maintenance Plan (LTMP) be developed by the design engineer. The LTMP contains a description of the stormwater system components and information on the required inspection and maintenance activities. The BMP owner must maintain and update the BMP operations and maintenance plan. 9.2. Maintenance Inspections Annual inspections are used to trigger maintenance operations such as sediment removal, spot revegetation and inlet stabilization. The following is a list of several key maintenance inspection points: • Add reinforcement planting to maintain 95% turf cover or vegetation density. Reseed or replant any dead vegetation. • Remove any accumulated sand or sediment deposits on the filter bed surface or in pre-treatment cells. • Inspect upstream and downstream of check dams for evidence of undercutting or erosion, and remove trash or blockages at weepholes. • Examine filter beds for evidence of braiding, erosion, excessive ponding or dead grass. • Check inflow points for clogging, and remove any sediment. • Inspect side slopes and grass filter strips for evidence of any rill or gully erosion, and repair as needed. • Look for any bare soil or sediment sources in the contributing drainage area, and stabilize immediately. Ideally, inspections should be conducted in the spring of each year. 9.3 Routine Maintenance and Operation Once established, water quality swales have minimal maintenance needs outside of the spring clean-up, regular mowing, and pruning and management of trees and shrubs. The surface of the filter bed can become clogged with fine sediment over time, but this can be alleviated through core aeration or deep tilling of the filter bed. Rake around existing vegetation and replant any vegetation that has not survived. Additional effort may be needed to repair check dams, stabilize inlet points and remove deposited sediment from pre- treatment cells. Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 10: AS -BUILT REQUIREMENTS After the water quality swale has been constructed, the developer must have an as -built certification of the swale prepared by a registered Professional Engineer and provide it to the City Engineer. The as -built certification verifies that the BMP was installed as designed and approved. The following components must be addressed in the as -built certification: • Appropriate underdrain system for water quality swales. • Correctly sized treatment volume. • Appropriate filter media and stone installed. • Adequate vegetation in place. • Overflow system in place for high flows. SECTION 11: REFERENCES Center for Watershed Protection (CWP). 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD. Chesapeake Stormwater Network (CSN). 2009. Technical Bulletin No. 1. Stormwater Guidance for Karst Terrain in the Chesapeake Bay Watershed, Version 2.0. Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD. Hirschman, D. and J. Kosco. 2008. Managing Stormwater in Your Community: A Guide for Building an Effective Post -Construction Program. EPA Publication 833-R-08-001, Tetra -Tech, Inc. and the Center for Watershed Protection. Ellicott City, MD. Maryland Department of Environment (MDE). 2000. Maryland Stormwater Design Manual. Baltimore, MD. Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban Stormwater RetrofitPpractices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD. Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net VADCR (Virginia Department of Conservation and Recreation). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA. VADCR. 2011. Stormwater Design Specification No. 10: Dry Swales. Version 1.9. Division of Soil and Water Conservation. Richmond, VA. Appendix 8 - GSP Specifications Drainage Criteria Manual i , EXTENDED DETENTION Description: Constructed stormwater detention basin that has a permanent pool (or micropool). Runoff from each rain event is captured and treated primarily through settling and biological uptake mechanisms. Variations: Wet extended detention, micropool extended detention, multiple pond system. • Permanent pool / micropool — prevents re -suspension of solids • Live storage above permanent pool — sized for a percentage of water quality volume and flow attenuation. • Forebay— settles out larger sediments in an area where sediment removal will be easier • Spillway system — spillway system(s) provides outlet for stormwater runoff when large storm events occur and maintains the permanent pool • Can be designed as a multi- • Potential for thermal functional BMP impacts downstream • Cost effective • Not recommended in • Can be designed as an karst terrain amenity within a • Community perceived development concerns with • Wildlife habitat potential mosquitoes and safety • High community acceptance when integrated into a development • Minimum contributing drainage area of 25 acres; 10 acres for micropool extended detention (Unless water balance calculations show support of permanent pool by a smaller drainage area) • Sediment forebay or equivalent pretreatment must be provided • Minimum length to width ratio = 3:1 • Maximum depth of permanent pool = 8 ft. • 3:1 side slopes or flatter around pond perimeter Appendix B — GSP Specifications Drainage Criteria Manual i , Runoff Reduction Removal Credit: 15% for design specified 0% if lined Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Remove debris from inlet and outlet structures • Maintain side slopes/remove invasive vegetation • Monitor sediment accumulation and remove periodically © Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION An Extended Detention (ED) Pond relies on delayed release (from 24 to 48 hours)of stormwater runoff after each rain event. An under -sized outlet structure restricts stormwater flow so it is stored within the basin, with drawdown of 24 -hours for the City of Fayetteville. The temporary ponding enables particulate pollutants to settle out and reduces the maximum peak discharge to the downstream channel, thereby reducing the effective shear stress on banks of the receiving stream. ED differs from stormwater detention, since it is designed to achieve a minimum drawdown time, rather than a maximum peak rate of flow (which is commonly used to design for peak discharge or flood control purposes and often detains flows for just a few minutes or hours). However, detention used for channel protection, may result in extended drawdown times. Therefore, designers are encouraged to evaluate the detention drawdown as compared to the ED requirements in order to meet both criteria. ED ponds rely on gravitational settling as their primary pollutant removal mechanism. Consequently, they generally provide fair -to -good removal for particulate pollutants, but low or negligible removal for soluble pollutants, such as nitrate and soluble phosphorus. The use of ED alone generally results in the lowest overall pollutant removal rate of any single stormwater treatment option. As a result, ED is normally combined with other practices to maximize pollutant removal rates. Figure 6.2 illustrates the concept of extended detention hydrologically. POST- A T = TIME OF EXTENDED DETENTION DEVELOPMENT F� A T POST DEVELOPMENT \xl � WITH DETENTION LU PREDEVELOPMENT TIME INTERVAL (T) Figure 6.2. Hydrologic Example of Extended Detention (Source: Cahill Associates). Appendix 8 - GSP Specifications Drainage Criteria Manual I , SECTION 2: PERFORMANCE Table 6.1. Annual Runoff Volume Reduction Provided by ED Ponds. Stormwater Function Specified Design Annual Runoff Volume Reduction (RR) 15% Sources: CWP and CSN (2009); CWP (2007). SECTION 3: DESIGN TABLE ED ponds must be designed with a Storage Volume equal to 1.25(Rv)(A). Table 6.2 lists the criteria for qualifying designs. See Section 6 for more detailed design guidelines. Table 6.2. Extended Detention (ED) Pond Criteria. Design Criteria Tv1= [(1.25) (Rv) (A)] *3630—the volume reduced by an upstream BMP A minimum of 40% of Tv in the permanent pool (forebay, micropool, or deep pool, or wetlands) Length/Width ratio OR flow path = 3:1 or more Length of the shortest flow path / overall length = 0.7 or more Average Tv ED time = 36 hours Maximum vertical Tv ED limit of 4 ft Trees and wetlands in the planting plan Includes additional cells or features (deep pools, wetlands, etc.) Refer to Section 5 CDA is greater than 10 acres 1 A= Area in Acres SECTION 4: TYPICAL DETAILS Figure 6.1 portrays a typical schematic for an ED pond. Appendix 8 — GSP Specifications Drainage Criteria Manual i , FCREBAY ARMORED SPILLWAY FLO',N PATH LANIDSCAPING INTERNAL BAFFLES ACi ATC.5AFETY BENCH __�4CCF�S aQA[) AODMONAL DEET' R_.OL OR %,VULAN-D CELLS E.MER NCY SALLWAY PLAN VIEW EXTENDED DETENTION POND MAX DES�SN NIMH WATER TV EDL FEVEL SAFETVACDUATIC: FREEBOARD EVE90ENCY BENCH I SRLLWAY CHANNEL PROTECTON VCLUME :..II....III:'."11_'..!11.: II`I I: I=I —i I I I I I; -I I I -I' D.S' To 1.L7171 - FpREgAY _I I'=INTERNAL S-A I—III'— „— I I —I —. _I_l_I _: I I—� —III_ —I I .— I :I I— I -1I 1— T F1 _7 PRINCIPAL I—_J-1I'�i 11.11 �I I II_ I I I. OUTLET:M: qn PC;OLI�' I� IIVJAY- " REVERSE SLOPE LOW J '�11 TIII-11 .11 7--1 —11'-1 I� I--_I—I —; II—_PLOD PIPE I I PROF€LE EXTENDED DFTF=NTIJN POND Figure 6.1. Typical Extended Detention Pond Details (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS The following feasibility issues need to be evaluated when ED ponds are considered as the final practice in a treatment train. Many of these issues will be influenced by the type of ED Pond being considered (refer to Design Applications at the end of this section). Space Required. A typical ED pond requires a footprint of 1% to 3% of its contributing drainage area, depending on the depth of the pond (i.e., the deeper the pond, the smaller footprint needed). Contributing Drainage Area. A minimum contributing drainage area of 10 acres is recommended for ED ponds, in order to sustain a permanent micropool to protect against clogging. Extended detention may still work with drainage areas less than 10 acres, but designers should be aware that these "pocket" ponds will typically (1) have very small orifices that will be prone to clogging, (2) experience fluctuating water levels, and (3) generate more significant maintenance problems. Available Hydraulic Head. The depth of an ED pond is usually determined by the amount of hydraulic head available at the site. The bottom elevation is normally the invert of the existing downstream conveyance system to which the ED pond discharges. Typically, a minimum of 6 to 10 ft of head is needed for an ED pond to function. Minimum Setbacks. Local ordinances and design criteria shall be consulted to determine minimum setbacks to property lines, structures, and wells. Generally, ED ponds shall be set back at least 10 ft from property lines, 25 ft from building foundations, 50 ft from septic system fields, and 100 ft from private wells. Depth -to -Water Table and Bedrock. ED ponds are not allowed if the water table or bedrock will be within 2 ft of the floor of the pond. Soils. The permeability of soils is seldom a design constraint for micropool ED ponds. Soil infiltration tests need to be conducted at proposed pond sites to estimate infiltration rates, which can be significant in Hydrologic Soil Group (HSG) A soils and some group B soils. Infiltration through the bottom of the pond is encouraged unless it will impair the integrity of the embankment. Geotechnical tests shall be conducted to determine the infiltration rates and other subsurface properties of the soils underlying the proposed ED pond. If the site is on karst topography, an alternative practice or combination of practices shall be employed at the site, if possible. See Technical bulletin No. 1 (CSN, 2009) for guidance on stormwater design in karst terrain. The Extended Detention Basin should be the option of last resort and, if used in karst, must have an low permeability clay or geosynthetic liner. Jurisdictional Streams. A Section 404 permit application will be required for impacts to ephemeral channels exhibiting an Ordinary High Water Mark (OHWM), intermittent channels, and perennial channels as the result of creation of ponds/detention areas along channels. Minor impacts would require a nationwide permit whereas impacts totaling more than 300 or 500' (depending on Nationwide permit used) would likely require an individual permit submittal to the United States Army Corps of Engineers (USACE). Appendix B - GSP Specifications Drainage Criteria Manual i , Design Applications Extended Detention is normally combined with other stormwater treatment options within the stormwater facility (e.g., wet ponds, and constructed wetlands) to enhance its performance and appearance. Other design variations are also possible where a portion of the runoff is directed to bioretention, infiltration, etc., that are within the overall footprint but housed in a separate cell, where the ponding depth of the Tv, channel protection storage, and/or flood protection storage is limited by the criteria of that particular practice. Figure 6.1 above illustrates several ED pond design variations. While ED ponds can provide for channel and flood protection, they will rarely provide adequate runoff volume reduction and pollutant removal to serve as a stand-alone compliance strategy. Therefore, designers should maximize the use of upland runoff reduction practices, (e.g., rooftop disconnections, small-scale infiltration, rainwater harvesting, bioretention, grass channels and water quality swales) that reduce runoff at its source (rather than merely treating the runoff at the terminus of the storm drain system). Upland runoff reduction practices can be used to satisfy most or all of the runoff reduction requirements at most sites. However, an ED pond may still be needed to provide any remaining channel protection requirements. Upland runoff reduction practices will greatly reduce the size, footprint and cost of the downstream ED pond. SECTION 6: DESIGN CRITERIA 6.1. Overall Sizing Designers can use a site -adjusted Rv (see Chapter 3.2 of Volume 5 for appropriate equations) to reflect the use of upland runoff reduction practices to compute the remaining treatment, channel protection, and/or flood protection volumes that must be treated by the ED pond, using the accepted applicable method. ED ponds are then designed to capture and treat the remaining runoff volume as necessary. Runoff treatment (Tv) credit may be taken for the entire water volume below the normal pool elevation of any micropools, forebays and wetland areas (minimum of 40%), as well as the temporary extended detention above the normal pool. 6.2. Treatment Volume Drawdown and Detention Design For more information on the design of outlet orifices and weirs and for achieving the target drawdown of the Treatment Volume design, refer to the Detention Design chapter of the Drainage Criteria Manual. Low flow orifices can be sized using the following equation. If a different equation is used or different type of low flow orifice is used, provide supporting calculations. Equation 6.1. Area of Low Flow Orifice 2A(H — Hof' a 3600CT(2g)os Appendix 8 - GSP Specifications Drainage Criteria Manual i , Where: a = Area of orifice (sq. ft.) A = Average surface area of the pond (sq. ft.) C = Orifice coefficient, 0.66 for thin, 0.80 for materials thicker than orifice diameter T = Drawdown time of pond (hr), must be greater than 24 hours g = Gravity (32.2 ft/sect) H = Elevation when pond is full to storage height (ft) Ho= Final elevation when pond is empty (ft) Refer to Table 6.2 for maximum ponding depths and other design criteria. 6.3. Required Geotechnical Testing Soil borings shall be taken below the proposed embankment, in the vicinity of the proposed outlet area, and in at least two locations within the proposed ED pond treatment area. Soil boring data is needed to (1) determine the physical characteristics of the excavated and subsurface soils, (2) evaluate adequacy for use as structural fill or spoil, (3) strength and performance characteristics for structural designs of the outlet works (e.g., bearing capacity and buoyancy) and the embankment, (4) investigate the depth to groundwater and bedrock and (5) evaluate potential infiltration losses (and the potential need for a liner). 6.4. Pretreatment Forebay Sediment forebays are considered to be an integral design feature to maintain the longevity of ED ponds. A forebay must be located at each major inlet to trap sediment and preserve the capacity of the main treatment cell. Other forms of pre-treatment for sheet flow and concentrated flow for minor inflow points shall be designed consistent with pretreatment criteria found in GSP-01: Bioretention. The following criteria apply to forebay design: • A major inlet is defined as an individual storm drain inlet pipe or open channel serving at least 10% of the ED pond's contributing drainage area. • The forebay consists of a separate cell, formed by an acceptable barrier. (e.g., an earthen berm, concrete weir, gabion baskets, etc.). • The forebay shall be at least 4 ft deep and must be equipped with a variable width aquatic bench for safety purposes. The aquatic benches shall be 4 to 6 ft wide at a depth of 18 inches below the water surface. • The total volume of all forebays shall be at least 15% of the total Treatment Volume. The relative size of individual forebays shall be proportional to the percentage of the total inflow to the pond. Similarly, any outlet protection associated with the end section or end wall shall be designed according to state or local design standards. • The forebay shall be designed in such a manner that it acts as a level spreader to distribute runoff evenly across the entire bottom surface area of the main treatment cell. • The bottom of the forebay may be hardened (e.g., concrete, asphalt, or grouted riprap) in order to make sediment removal easier. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.5. Conveyance and Overflow No Pilot Channels. Micropool ED ponds shall not have a low flow pilot channel, but instead must be constructed in a manner whereby flows are evenly distributed across the pond bottom, to promote the maximum infiltration possible. Internal Slope. The minimum longitudinal slope through the pond shall be 0.5% and the maximum slope shall be 1%. Primary Spillway. The primary spillway shall be designed with acceptable anti -flotation, anti -vortex, and trash rack devices. The spillway must generally be accessible from dry land. Non -Clogging Low Flow Orifice. ED Ponds with drainage areas of 10 acres or less, where small diameter pipes are typical, are prone to chronic clogging by organic debris and sediment. Orifices less than 3 inches in diameter may require extra attention during design to minimize the potential for clogging. Designers shall assess the potential for higher sediment and woody debris loads. The risk of clogging in outlet pipes with small orifices can be reduced by: • Providing a micropool at the outlet structure: o Use a reverse -sloped pipe that extends to a mid -depth of the permanent pool or micropool. o Install a downturned elbow or half -round CMP over a riser orifice (circular, rectangular, V -notch, etc.) to pull water from below the micropool surface. o The depth of the micropool shall be at least 4 ft deep, and the depth may not draw down by more than 2 ft during a 30 day summer. o Providing an over -sized forebay to trap sediment, trash and debris before it reaches the ED pond's low -flow orifice. o Installing a trash rack to screen the low -flow orifice. o Using a perforated pipe under a gravel blanket with an orifice control at the end in the riser structure to supplement the primary outlet. Emergency Spillway. ED ponds must be constructed with overflow capacity to pass the 100 -year design storm event through either the Primary Spillway or a vegetated or armored Emergency Spillway. Adequate Outfall Protection. The design must specify an outfall that will be stable for the 10 -year design storm event. The channel immediately below the pond outfall must be modified to prevent erosion and conform to natural dimensions in the shortest possible distance. This is typically done by placing appropriately sized riprap, over filter fabric, which can reduce flow velocities from the principal spillway to non-erosive levels (3.5 to 5.0 fps depending on the channel lining material). Flared pipe sections that discharge at or near the stream invert or into a step pool arrangement shall be used at the spillway outlet. Inlet Protection. Inlet areas shall be stabilized to ensure that non-erosive conditions exist during storm events up to the overbank flood event (i.e., the 10 -year storm event). Inlet pipe inverts shall generally be located at or slightly below the forebay pool elevation. Appendix 8 - GSP Specifications Drainage Criteria Manual i , On -Line ED Ponds must be designed to detain the required Tv and either manage or be capable of safely passing larger storm events conveyed to the pond (e.g., 1 -year channel protection detention, 10 -year flood protection, and/or the 100 -year design storm event). 6.6. Internal Design Features Side Slopes. Side slopes leading to the ED pond shall generally have a gradient of 4H:1V to SH:1V. The mild slopes promote better establishment and growth of vegetation and provide for easier maintenance and a more natural appearance. Long Flow Path. ED pond designs shall have an irregular shape and a long flow path from inlet to outlet to increase water residence time, treatment pathways, and pond performance. In terms of flow path geometry, there are two design considerations: (1) the overall flow path through the pond, and (2) the length of the shortest flow path (Hirschman et al., 2009): The overall flow path can be represented as the length -to -width ratio OR the flow path. These ratios must be at least 3L: 1W. Internal berms, baffles, or topography can be used to extend flow paths and/or create multiple pond cells. • The shortest flow path represents the distance from the closest inlet to the outlet. The ratio of the shortest flow to the overall length must be at least 0.7. In some cases - due to site geometry, storm sewer infrastructure, or other factors - some inlets may not be able to meet these ratios. However, the drainage area served by these "closer" inlets shall constitute no more than 20% of the total contributing drainage area. Treatment Volume Storage. The total Tv storage may be provided by a combination of the permanent pool (in the form of forebays, deep pools, and/or wetland area) and extended detention storage. Vertical Extended Detention Limits. The maximum Tv ED water surface elevation may not extend more than 4 ft above the basin floor or normal pool elevation. The maximum vertical elevation for ED and channel protection detention over shallow wetlands is 1 ft. The frequent bounce in water elevations caused by extended detention bounce effect is not as critical for larger flood control storms (e.g., the 10 -year design storm), and these events can exceed the 4 ft vertical limit if they are managed by a multi -stage outlet structure. Safety Features. • The principal spillway opening must be designed and constructed to prevent access by small children. • End walls above pipe outfalls greater than 48 inches in diameter must be fenced to prevent a hazard. • An emergency spillway and associated freeboard must be provided in accordance with applicable local or state dam safety requirements. The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges. • Both the safety bench and the aquatic bench shall be landscaped with vegetation that hinders or prevents access to the pool. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.7. Landscaping and Planting Plan A landscaping plan prepared by a licensed Landscape Architect must be provided that indicates the methods used to establish and maintain vegetative coverage within the ED pond and its buffer. Minimum elements of a plan include the following: • Delineation of pond-scaping zones within both the pond and buffer • Selection of corresponding plant species • The planting plan • The sequence for preparing the wetland bed, if one is incorporated with the ED pond (including soil amendments, if needed) • Sources of native plant material • The landscaping plan should provide elements that promote diverse wildlife and waterfowl use within the stormwater wetland and buffers. • The planting plan should allow the perimeter of the pond to mature into a native forest in the right places, but yet keep mowable turf along the embankment and all access areas. The wooded wetland concept proposed by Cappiella et al., (2005) may be a good option for many ED ponds. • Woody vegetation may not be planted or allowed to grow within 15 ft of the toe of the embankment nor within 25 ft from the principal spillway structure. • Avoid species that require full shade, or are prone to wind damage. Extra mulching around the base of trees and shrubs is strongly recommended as a means of conserving moisture and suppressing weeds. Detailed local plant selection and guidance can be found in Appendix D. For additional guidance on planting trees and shrubs in ED pond buffers, consult Cappiella et al (2006). 6.8. Maintenance Reduction Features Good maintenance access is needed so crews can remove sediments from the forebay, alleviate clogging and make riser repairs. The following ED pond maintenance issues can be addressed during design, in order to make on-going maintenance easier: • Adequate maintenance access must extend to the forebay, micropool, any safety benches, riser, and outlet structure and must have sufficient area to allow vehicles to turn around. • The riser shall be located within the embankment for maintenance access, safety and aesthetics. • Access roads must (1) be constructed of load-bearing materials or be built to withstand the expected frequency of use, (2) have a minimum width of 12 ft, and (3) have a profile grade that does not exceed 15%. Steeper grades are allowable if appropriate stabilization techniques are used, such as a gravel road. • A maintenance right -of- way or easement must extend to the ED pond from a public or private road. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.9. ED Pond Material Specifications ED ponds are generally constructed with materials obtained on-site, except for the plant materials, inflow and outflow devices (e.g., piping and riser materials), possibly stone for inlet and outlet stabilization, and filter fabric for lining banks or berms. The basic material specifications for earthen embankments, principal spillways, vegetated emergency spillways and sediment forebays shall be as specified in the Detention Design chapter of the Fayetteville Drainage Criteria Manual. An Arkansas Natural Resource Commission (ANRC) permit may apply to ponds with storage volumes and embankment heights large enough to fall under the regulation for dam safety, as applicable. Size emergency spillway for any overtopping of pond in case of rain event in excess of 100 -year storm and for instances of malfunction or clogging of primary outlet structure. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Steep Terrain The use of ED ponds is highly constrained at development sites with steep terrain. 7.2. Karst Terrain Karst regions are found in and around Fayetteville, which complicates both land development and stormwater design. For the most recent publicly available data, please refer to the Karst Area Sensitivity Map of Washington County (The Nature Conservancy, 2007) at: http://www.nwarpc.org/Pdf/GIS- Imagery/KASM WASHINGTON CO.pdf. In karst sensitive areas, a karst survey shall be performed by a qualified professional during the project planning stage. Because of the risk of sinkhole formation and groundwater contamination in karst regions, use of ED ponds is highly restricted there (see CSN Technical Bulletin No. 1, 2009). If geotechnical studies indicate that less than 3 ft of vertical separation exists between the bottom of the ED pond and the underlying soil -bedrock interface, ED ponds shall be constructed only with an acceptable liner. Even with a liner, ED ponds shall not be used adjacent to existing sinkholes. 7.3. Multi -Functional Uses Recreational and other uses may be provided between storm runoff events. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Figure 6.3. Multi -Use Dry Detention Doubling as Sports Fields (Source: Englewood, CO). SECTION 8: CONSTRUCTION 8.1. Construction Sequence The following is a typical construction sequence to properly install a dry ED pond. The steps may be modified to reflect different dry ED pond designs, site conditions, and the size, complexity and configuration of the proposed facility. Step 1: Use of ED pond for Erosion Protection and Sediment Control. An ED pond may serve as a sediment basin during project construction. If this is done, the volume shall be based on the more stringent sizing rule (erosion and sediment control requirement vs. water quality treatment requirement). Such use is not appropriate in sensitive karst areas. Installation of the permanent riser shall be initiated during the construction phase, and design elevations shall be set with final cleanout of the sediment basin and conversion to the post -construction ED pond in mind. The bottom elevation of the ED pond shall be lower than the bottom elevation of the temporary sediment basin. Appropriate procedures shall be implemented to prevent discharge of turbid waters when the basin is being converted into an ED pond. Step Z: Stabilize the Drainage Area. ED ponds shall only be constructed after the contributing drainage area to the pond is completely stabilized or if water is routed around them during construction. If the proposed pond site will be used as a sediment trap or basin during the construction phase, the construction notes shall clearly indicate that the facility will be dewatered, dredged and re -graded to design dimensions after the original site construction is complete. Step 3: Assemble Construction Materials on-site, make sure they meet design specifications, and prepare any staging areas. Step 4: Clear and Strip the project area to the desired sub -grade. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Step 5: Install Erosion and Sedimentation Controls prior to construction, including temporary de -watering devices and stormwater diversion practices. All areas surrounding the pond that are graded or denuded during construction must be planted with turf grass, native plantings, or other approved methods of soil stabilization. Step 6: Excavate the Core Trench and Install the Spillway Pipe. Step 7. Install the Riser or Outflow Structure and ensure the top invert of the overflow weir is constructed level at the design elevation. Step 8: Construct the Embankment and any Internal Berms in 8 to 12 -inch lifts and compact the lifts with appropriate equipment. Step 9: Excavate/Grade until the appropriate elevation and desired contours are achieved for the bottom and side slopes of the ED pond. Step 10: Construct the Emergency Spillway in cut or structurally stabilized soils. Step 11: Install Outlet Pipes, including downstream rip -rap apron protection. Step 12: Stabilize Exposed Soils with temporary seed mixtures appropriate for the pond buffer. All areas above the normal pool elevation shall be permanently stabilized by hydroseeding or seeding over straw. Step 13: Plant the Pond Buffer Area, following the pond-scaping plan (see Section 8.5 below). 8.2. Construction Inspection Multiple inspections are critical to ensure that stormwater ponds are properly constructed. Inspections are recommended during the following stages of construction: • Pre -construction meeting • Initial site preparation (including installation of US controls) • Excavation/Grading (interim and final elevations) • Installation of the embankment, the riser/primary spillway, and the outlet structure • Implementation of the pond -scraping plan and vegetative stabilization • Final inspection (develop a punch list for facility acceptance) If the ED pond has a permanent pool, then to facilitate maintenance the contractor shall measure the actual constructed pond depth at three areas within the permanent pool (forebay, mid -pond and at the riser), and they shall mark and geo-reference them on an as -built drawing. This simple data set will enable maintenance inspectors to determine pond sediment deposition rates in order to schedule sediment cleanouts. Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 9: MAINTENANCE 9.1. Maintenance The requirements for maintenance include the development of a Long Term Maintenance Plan (LTMP) by the design engineer. The LTMP contains a description of the stormwater system components and information on the inspection and maintenance activities. The owner is responsible for the cost of maintenance and annual inspections. The BMP owner must maintain and submit annual maintenance reports to the City. 9.2. Maintenance Inspections Maintenance of ED ponds is driven by annual inspections that evaluate the condition and performance of the pond, including the following: • Measure sediment accumulation levels in forebay. • Monitor growth of wetlands, trees and shrubs planted and note invasive plant species if present. • Inspect the condition of stormwater inlets to the pond for material damage, erosion or undercutting. • Inspect the banks of upstream and downstream channels for evidence of sloughing, animal burrows, boggy areas, woody growth, or gully erosion that may undermine embankment integrity. • Inspect pond outfall channel for erosion, undercutting, rip -rap displacement, woody growth, etc. • Inspect condition of principal spillway and riser for spalling, joint failure, leakage, corrosion, etc. • Inspect condition of all trash racks, reverse sloped pipes or flashboard risers for evidence of clogging, leakage, debris accumulation, etc. • Inspect maintenance access to ensure it is free of woody vegetation, and check to see whether valves, manholes and locks can be opened and operated. • Inspect internal and external side slopes of the pond for evidence of sparse vegetative cover, erosion, or slumping, and make needed repairs immediately. 9.3. Common Ongoing Maintenance Issues ED ponds are prone to a high clogging risk at the ED low -flow orifice. This component of the pond's plumbing shall be inspected at least twice a year after initial construction. The constantly changing water levels in ED ponds make it difficult to mow or manage vegetative growth. The bottom of ED ponds often become soggy, and water -loving trees such as willows may take over. The LTMP shall clearly outline how vegetation in the pond will be managed or harvested in the future. The LTMP shall schedule a cleanup at least once a year to remove trash and floatables that tend to accumulate in the forebay, micropool, and on the bottom of ED ponds. Frequent sediment removal from the forebay is essential to maintain the function and performance of an ED pond. Maintenance plans shall schedule cleanouts every 5 to 7 years, or when inspections indicate that 50% Appendix 8 - GSP Specifications Drainage Criteria Manual i , of the forebay capacity has been filled. Sediments excavated from ED ponds are not usually considered toxic or hazardous, and can be safely disposed by either land application or land filling. SECTION 10: AS -BUILT REQUIREMENTS After the pond is constructed, an as -built certification of the pond, performed by a registered Professional Engineer, must be submitted to the City Engineer. The as -built certification verifies that the BMP was installed as designed and approved. The following components must be addressed in the as -built certification: • Pretreatment for coarse sediments must be provided. • Surrounding drainage areas must be stabilized to prevent sediment from clogging the filter media. • Correct ponding depths and infiltration rates must be maintained to prevent killing vegetation. • A mechanism for overflow for large storm events must be provided. SECTION 11: COMMUNITY AND ENVIRONMENTAL CONCERNS Extended Detention Ponds can generate the following community and environmental concerns that need to be addressed during design. Aesthetics. ED ponds tend to accumulate sediment and trash, which residents are likely to perceive as unsightly and creating nuisance conditions. Fluctuating water levels in ED ponds also create a difficult landscaping environment. In general, designers should avoid designs that rely solely on dry ED ponds. Existing Wetlands. ED ponds should never be constructed within existing natural wetlands, nor should they inundate or otherwise change the drainage and inundation characteristics of existing wetlands. Existing Forests. Designers can expect a great deal of neighborhood opposition if they do not make a concerted effort to save mature trees in accordance with the Tree Preservation Ordinance during design and pond construction. Designers should also be aware that even modest changes in inundation frequency can kill upstream trees (Cappiella et al., 2007). Stream Warming Risk. ED ponds have less risk of stream warming than other pond options, but they can warm streams if they are unshaded or contain significant surface area in shallow pools. If an ED pond discharges to temperature -sensitive waters, it should be forested, contain the minimum pools to prevent clogging, and have a detention time of no longer than 12 hours. Safety Risk. ED ponds are generally considered to be safer than other pond options, since they have few deep pools. Steep side -slopes and unfenced headwalls, however, can still create some safety risks. Gentle side slopes shall be provided to avoid potentially dangerous drop-offs, especially where ED ponds are located near residential areas. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Mosquito Risk. The fluctuating water levels within ED ponds have potential to create conditions that lead to mosquito breeding. Mosquitoes tend to be more prevalent in irregularly flooded ponds than in ponds with a permanent pool (Santana et al., 1994). Designers can minimize the risk by combining ED with a wet pond or wetland and ensuring complete drainage within 72 hours after completion of rain events. SECTION 12: REFERENCES Cappiella, K., T. Schueler and T. Wright. 2006. Urban Watershed Forestry Manual: Part 2: Conserving and Planting Trees at Development Sites. USDA Forest Service. Center for Watershed Protection. Ellicott City, MD. Cappiella, K., T. Schueler, J. Tomlinson, T. Wright. 2007. Urban Watershed Forestry Manual: Part3: Urban Tree Planting Guide. USDA Forest Service. Center for Watershed Protection. Ellicott City, MD. Cappiella, K., L. Fraley -McNeal, M. Novotney and T. Schueler. 2008. "The Next Generation of Stormwater Wetlands." Wetlands and Watersheds Article No. S. Center for Watershed Protection. Ellicott City, MD. Center for Watershed Protection. 2004. Pond and Wetland Maintenance Guidebook. Ellicott City, MD. Chesapeake Stormwater Network (CSN). 2009. Technical Bulletin No. 1. Stormwater Guidance for Karst Terrain in the Chesapeake Bay Watershed, Version 2.0. Hirschman, D., L. Woodworth and S. Drescher. 2009. Technical Report. Stormwater BMPs in Virginia's James River Basin: An Assessment of Field Conditions & Programs. Center for Watershed Protection. Ellicott City, MD. Maryland Department of Environment (MDE). 2000. Maryland Stormwater Design Manual. Baltimore, MD. Santana, F.J., J.R. Wood, R.E. Parsons, & S.K. Chamberlain. 1994. Control of Mosquito Breeding in Permitted Stormwater Systems. Brooksville: Sarasota County Mosquito Control and Southwest Florida Water Management District. Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD. VADCR (Virginia Department of Conservation and Recreation) 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA. VADCR. 2011. Virginia DCR Stormwater Design Specification No. 15, Extended Detention (ED) Pond, Version 1.9, Division of Soil and Water Conservation. Richmond, VA. Appendix 8 - GSP Specifications Drainage Criteria Manual i , DOWNSPOUT DISCONNECTION Description: Refers to disconnecting roof downspouts away from storm drains and impervious areas such as driveways, parking lots, and roads that provide direct connections to a public stormwater system, and directing them instead to a storage facility or pervious areas for infiltration. Downspouts can be directed to rain barrels, rain gardens, on-site filters, vegetated filters and vegetated swales. Due to the difficulty of regulation and oversight, no credit for downspout disconnections is to be provided for residential grading permit projects unless the practice is included within a common area of a sub -division constructed with Green Infrastructure features and (Source: Low Impact Development Center) its protection and maintenance is included in the Home Owner's Association's Operation and Maintenance Agreement (or other agreement as specified within the Drainage Criteria Manual). • Cost effective • For appreciable volume and peak • Promotes discharge deduction, must be applied infiltration, reducing broadly runoff volume and • Requires owner buy -in and peak discharge maintenance to ensure performance • Vegetated areas for • Requires large on -lot pervious areas infiltration provide • Must avoid causing foundation aesthetics flooding or ice conditions • Increases public • Difficult to regulate and oversee for awareness and subdivision grading permit projects involvement • For Hydrologic Soil Groups (HSG) C or D, alternative runoff reduction practices (e.g., compost -amended filter path, bioretention, rainwater harvesting) are necessary to boost runoff reduction rate. • When designing simple disconnections, soil erodibility must be considered & clearly addressed in the site's Maintenance Plan or O&M Agreement. • Maintenance of disconnected downspouts usually involves regular lawn or landscaping maintenance in the filter path, unless directed to rain barrel or a natural, undisturbed setting. • Must be a minimum distance of 10 ft outside the water quality buffer or, where no buffer exists, 10 ft from the nearest stream or waterway. • Must be 500 ft or further from steep slopes or landslide -prone areas, and not increase potential off-site erosion. Appendix B — GSP Specifications Drainage Criteria Manual i , Selection Criteria: 25-50% Runoff Reduction Credit Land Use Considerations: © Residential (limited use) © Commercial © Industrial Maintenance: On-site systems need to be maintained to ensure proper drainage to avoid nuisance flooding. Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION This strategy involves managing runoff close to its source by intercepting, infiltrating, filtering, treating or reusing it as it moves from the impervious surface to the drainage system. Two kinds of disconnection are allowed: (1) simple disconnection, whereby rooftops and/or on -lot residential impervious surfaces are directed to pervious areas, and (2) disconnection leading to an alternative runoff reduction practice(s) adjacent to the roof (Figure 7.1). Alternative practices that take up less space can be used where space in not available for the disconnection practices described above. Applicable alternative runoff practices are shown in Table 7.1, below. SECTION 2: PERFORMANCE With proper design and maintenance, simple rooftop disconnection options can provide relatively high runoff reduction rates (Table 7.1). If an alternative runoff reduction practice is employed to achieve rooftop disconnection, the higher runoff reduction rate for that practice can be used for the contributing drainage area of the rooftop. Table 7.1. Annual Runoff FUNCTION PROVIDED BY SIMPLE ROOFTOP DISCONNECTION Volume Reduction d by Rooftop Level 1 HSG Soils C and D Level 2 HSG SOILS A and B Annual Runoff Volume Reduction (RR) 25% 50% NOTE: Stormwater functions of disconnection can be boosted if an acceptable alternative runoff reduction practice is employed. Acceptable practices and their associated runoff reduction rates are listed below. Designers shall consult the applicable specification number for design standards. Alternative Practice Specification No. Runoff Reduction Rate Soil compost -amended filter path See Section 4.2 50% Infiltration trench — Level 1 GSP-04 50% Infiltration trench — Level 2 GSP-04 90% Bioretention — Level 1 GSP-01 60% Bioretention — Level 2 GSP-01 90% Rainwater harvesting GSP-11 Defined by user Urban Bioretention GSP-02 60% 1. CWP and CSN (2008), CWP (2007). 2. When disconnecting a rooftop, it is possible to amend the soil or have the runoff flow into another GSP in order to improve volume reduction performance. If an alternative practice is employed, the runoff reduction rate assigned to the alternative practice shall be used, instead of the RR assigned to Level 1 or Level 2 soils. Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 3: TYPICAL DETAILS Figures 7.1 to 7.3 portray various rooftop disconnection and alternative runoff reduction options. COMMERCIAL ROOFTOP Figure 7.1. Roof Disconnection with Alternative Runoff Reduction Practices (Source: VADCR, 2011). SOIL COMPOST AMENDED FILTER PATH WIDTH AS PER SPEC SLIGHT DEPRESSION TO CONTAIN FLOW WITH FLAT BOTTOM TO HELP ENSURE SHEET FLOW 2« _ ¢,. COMPOST AMENDMENTS TILLED TO (Y-10" DEPTH Figure 7.2. Disconnection: Soil Compost Amended Filter Path (Source: VADCR, 2011). Appendix B — GSP Specifications Drainage Criteria Manual i , DISC:HARGF TO RAIN GARDEN OR OTHER TREATMENT PRACTICE (SEE SPECIFICATIONS FOR DETAILS BUILDING ROOF DRAIN MIN SETBACK NAIIVL GHASSBS AND SHRUBS COMPOST AMENDED FLOVV DOWNSPOUT PATH D SCCNNECTION MINIMUM LENGTH AS REQUIRED: -SIMPLE DISCONNECTION; -SOIL COMPOST AMENDED FILTER PATH PRETREATMENT WITH CONCENTRATED INFLOW TO RAIN GARDEN Figure 7.3. Rooftop Disconnection — Section View: a) Simple Disconnection with surface runoff to downstream Bioretention b) Disconnection — Alternative Practice: Over Compost Amended Flow Path to downstream Bioretention (Source: VADCR, 2011). Appendix B - GSP Specifications Drainage Criteria Manual i , CONVEYANCE TO GRASSED CHANNEL CURB CUT—""""" —GRAVEL DIAPHRAGM LL LL Z 2 2 D O CURB Q CE PERMEABLE BERM 0 TRFATMFNT TRAIN- PRFTRFATMFNT , D SOII COMPOST AMENDED FILTER PATH TO GRASS CHANNEL OR OTHER TREATMENT D SUHFACE 20FT MINLENGTH AS SPECIFIED IN SEL: I1ON 3 2 ■�/�fiiif� PRETREATMENT AND DESIGN COMPDNEN1S GRAVEL DIAPHRAGM, PLRMEAULE RFRM, COMPOST AMFN--OMFNT5 nIMFN91ONS, PER DESIGN SPECS AVSD suprACE Figure 7.4. Amended Filter Path to Downstream Grass Channel (or other treatment) (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , `�.p;Y" of {ECT F-LDW SC: GRASFD HANNFI ]e3� " I I I CFNTFRIINF III — Imo. GRAVEL DIAPHRAGM — —�— COM POST ATI ENDED– :fC I II— T� III 501L5' L :- _r �COM FOS T AMEN DED SUILU -II —,?` — _TF_ ''• I I— —� WITI IIN VTGI–TATE-[� F TFR - =STRIP R C: ANC] D) PERMEA$LE BERM = [HSLL IC L= ==� =1=J1= = L ' '=L= i_LJ _ I i=L=1ll=! Figure 7.4. Amended Filter Path to Downstream Grass Channel (or other treatment) (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , SECTION 4: DESIGN CRITERIA 4.1. Simple Rooftop Disconnection Table 7.2 provides the primary design criteria for simple rooftop disconnection. In general: • Simple disconnection is not credited for residential lots, except in the case of common areas within a development where the longevity and proper maintenance of the downspout disconnect has been ensure as described in page 1 of this GSP. • No more than half of a roof area (1,000 sq. ft. maximum) shall be discharged to any one point. • Care shall be taken to locate downspout disconnections in areas that slope away from structures. Downspouts may not be disconnected on slopes over 5%, or within 500 ft of steep slopes or landslide -prone areas. To reduce the possibility of erosion, flow shall also not be directed onto lawns or beds that are sloped more than 15%. • Simple disconnection can be used on any post -construction Hydrologic Soil Group (HSG). However, for HSG C or D, alternative runoff reduction practices (e.g., compost -amended filter path, bioretention, rainwater harvesting) can boost the runoff reduction rate. Also, erodibility of soils must be considered and, if erosive soils are present, addressed when designing simple disconnection. • Maintenance of disconnected downspouts usually involves regular lawn or landscaping maintenance in the filter path from the roof to the street. In some cases, runoff from a simple disconnection may be directed to a more natural, undisturbed setting (i.e., where lot grading and clearing is "fingerprinted" and the proposed filter path is protected). 1. For alternative runoff reduction practices, see the applicable specification for design criteria. See Table 7.1 herein for eligible practices and associated GSP specification numbers. 2. An alternative runoff reduction practice must be used when the disconnection length is less than 40 ft. 3. Turf reinforcement may include appropriate reinforcing materials that are confirmed by the designer to be non-erosive for the specific characteristics and flow rates anticipated at each individual application, and are acceptable to the City. 4. Note that the downspout extension of 5 ft is intended for simple shallow foundations. The use of a dry well or french drain adjacent to an in -ground basement or finished floor area should be coordinated with the design of the structure water -proofing system (foundation drains, etc.), or avoided altogether. Appendix 8 — GSP Specifications Drainage Criteria Manual i , 1 Table 7.2. Simple Rooftop DESIGN FACTOR Disconnection Design Criteria SIMPLE DISCONNECTION Maximum impervious (Rooftop) area treated 1,000 sq ft per disconnection Longest flow path (roof/gutter) 75 ft Disconnection length Equal to longest flow path, but no less than 40 ft Z Disconnection slope < 2%, or < 5% with turf reinforcement 3 Distance from buildings or foundations Extend downspouts 5 ft4 (15 ft in karst areas) away from building if grade away from building is less than 1%. Type of pretreatment External (leaf screens, etc) 1. For alternative runoff reduction practices, see the applicable specification for design criteria. See Table 7.1 herein for eligible practices and associated GSP specification numbers. 2. An alternative runoff reduction practice must be used when the disconnection length is less than 40 ft. 3. Turf reinforcement may include appropriate reinforcing materials that are confirmed by the designer to be non-erosive for the specific characteristics and flow rates anticipated at each individual application, and are acceptable to the City. 4. Note that the downspout extension of 5 ft is intended for simple shallow foundations. The use of a dry well or french drain adjacent to an in -ground basement or finished floor area should be coordinated with the design of the structure water -proofing system (foundation drains, etc.), or avoided altogether. Appendix 8 — GSP Specifications Drainage Criteria Manual i , 4.2. Soil Compost -Amended Filter Path The incorporation of compost amendments shall conform to the Soil Amendment portion of Appendix C, and shall include the following design elements: • Flow from the downspout shall be spread over a 10 -ft wide strip extending down -gradient along the flow path from the building to the street or conveyance system. • The filter path shall be at least 20 ft in length. • A pea gravel or river stone diaphragm, or other accepted flow spreading device shall be installed at the downspout outlet to distribute flows evenly across the filter path. • The strip shall have adequate "freeboard" so that flow remains within the strip and is not diverted away from the strip. In general, this means that the strip should be lower than the surrounding land area in order to keep flow in the filter path. Similarly, the flow area of the filter strip shall be level to discourage concentrating of flow in the middle of the filter path. • Use 2 to 4 inches of compost and till to a depth of 6 to 10 inches within the filter path. 4.3. Infiltration Infiltration trenches are excavations typically filled with stone to create an underground reservoir for stormwater runoff, allowing the runoff volume to gradually exfiltrate through the bottom and sides of the trench into the subsoil over a 2 -day period. By diverting runoff into the soil, infiltration trenches serve to both treat the water quality volume and to preserve the natural water balance on a site. Infiltration systems are limited to areas with highly porous soils where the water table and/or bedrock are located well below the bottom of the trench. The major design goal for Infiltration is to maximize runoff volume reduction. To this end, designers may choose to meet the requirements of a Level 1 baseline design or choose an enhanced design (Level 2) that maximizes runoff reduction, as described in GSP-04 (Infiltration) and summarized in Table 7.3. 1. A= Area, sq. ft. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 7.3. Level I and Level 2 Level 1 Design (RR:50) Infiltration Design Guidelines. Level 2 Design (RR:90) Sizing': Tv = [(Rv)(A)/121— the volume reduced by an upstream BMP Sizing: Tv = [1.1(Rv)(A)/121— the volume reduced by an upstream BMP At least two forms of pre-treatment (see GSP-04 Table 4.6 Pretreatment Elements) At least three forms of pre-treatment (see GSP-04 Table 4.6 Pretreatment Elements) Soil infiltration rate 0.5 to 1 in./hr., number of tests depends on the scale (Appendix C) Soil infiltration rates of 1.0 to 4.0 in/hr, number of tests depends on the scale (Appendix C) Minimum of 2 ft between the bottom of the infiltration practice and the seasonal high water table or bedrock Tv infiltrates within 24 to 48 hours Setbacks —see suggested minimum setbacks (Section 5.1 of GSP-01, see GSP-02 for urban applications) All designs are subject to potential area of high pollutant loading restrictions/prohibitions 1. A= Area, sq. ft. Appendix 8 — GSP Specifications Drainage Criteria Manual i , 4.4. Bioretention Providing a place for the water to soak in - such as with a compost -amended landscape area (see above), bioretention areas, or rock -filled trench - increases infiltration. Depending on soil properties, roof runoff may be filtered through a shallow bioretention area. The design for this option shall meet the requirements of Bioretention as described in GSP-01 and summarized in Table 7.4. TableBioretention DESIGN FACTOR Design Criteria'. BIORETENTION Impervious Area Treated 1,000 ft Type of Inflow Sheetflow or roof leader Minimum Soil Infiltration Rate 0.5 inches/hour (or use underdrain) Observation Well/ Cleanout Pipes No Type of Pretreatment External (leaf screens, etc) Underdrain and gravel layer Level 1: Yes; Level 2: Optional per soils' Minimum Filter Media Depth 24 inches (Level 1); 36 inches (Level 2) Media Source Mixed on-site (see Appendix C Soil Mixes) Hydraulic Head Required 1 to 3 ft Required Soil Borings One test pit/standard soil boring for every 1,000 sq ft only when an underdrain is not used Building Setbacks 10 ft down -gradient, 50 ft up -gradient 'Refer to GSP-01 for Level 1 and Level 2 Design Criteria, and sizing criteria for individual and multiple downspout applications. The bioretention media is 24 to 36 inches deep, and for Level 2 is located over a 12 inch deep (or greater) stone reservoir (as required by GSP-01). A perforated underdrain is located above the stone reservoir, to promote storage and recharge in poorly draining soils. In urban settings, the underdrain is directly connected into the storm drain pipe running underneath the street or in the street right-of-way. A trench needs to be excavated during construction to connect the underdrain to the street storm drain system. Appropriate approvals are required for making any connections to a common (or public) drainage system. Construction of the remainder of the front yard bioretention system is deferred until after the lot has been stabilized. The front yard design should reduce the risk of homeowner conversion because it allows the owners to choose whether they want turf or landscaping. Front yard bioretention requires regular mowing and/or landscape maintenance to perform effectively. It is recommended that the practice be located in an expanded right-of-way or stormwater easement so that it can be accessed in the event that it fails to drain properly. 4.5. Rain Tanks and Cisterns This form of disconnection must conform to the design requirements outlined in GSP-11 (Rain Tanks and Cisterns). The runoff reduction rates for rain tanks and cisterns depend on their storage capacity and ability to draw down water in between storms for reuse as grey -water or irrigation use. The actual runoff reduction rate for a particular design can be calculated using the information provided in GSP-11. All devices shall have a suitable overflow area to route extreme flows into the next treatment practice or the stormwater conveyance system. Appendix 8 — GSP Specifications Drainage Criteria Manual i , 4.6. Stormwater Planter (Urban Bioretention) This form of disconnection must conform to the design requirements for stormwater planters, as outlined in GSP-02, Urban Bioretention. Foundation planters are a useful option to disconnect and treat rooftop runoff, particularly in ultra -urban areas. They consist of confined planters that store and/or infiltrate runoff in a soil bed to reduce runoff volumes and pollutant loads. Stormwater planters combine an aesthetic landscaping feature with a functional form of stormwater treatment. Stormwater planters generally receive runoff from adjacent rooftop downspouts and are landscaped with plants that are tolerant to periods of both drought and inundation. The two basic design variations for stormwater planters are the infiltration planter and the filter planter. An infiltration planter filters rooftop runoff through soil in the planter followed by infiltration into soils below the planter. The recommended minimum depth is 30 inches, with the shape and length determined by architectural considerations. The planter shall be sized to temporarily store at least 0.5 inch of runoff from the contributing rooftop area in a reservoir above the planter bed. Infiltration planters shall be placed at least 10 ft away from a building to prevent possible flooding or basement seepage damage. A filter planter has an impervious liner on the bottom. The minimum planter depth is 30 inches, with the shape and length determined by architectural considerations. Runoff is temporarily stored in a reservoir located above the planter bed. Overflow pipes are installed to discharge runoff when maximum ponding depths are exceeded, to avoid water spilling over the side of the planter. In addition, an underdrain is used to carry runoff to the storm sewer system. Since a filter planter is self-contained and does not infiltrate into the ground, it can be installed with no setback, or right next to a building. All planters shall be placed at grade level or above ground. They shall be sized to allow captured runoff to drain out within four hours after a storm event. Plant materials shall be capable of withstanding seasonally moist and dry conditions. Planting media shall have an infiltration rate of at least 2 inches per hour. The sand and gravel on the bottom of the planter shall have a minimum infiltration rate of 5 inches per hour. The planter can be constructed of stone, concrete, brick, wood or other durable material. SECTION 5: MAINTENANCE The rooftop disconnection and supplementary treatment device must be covered by a drainage easement to allow inspection and maintenance and must also be included in the Long Term Maintenance Plan (LTMP) for the site. The LTMP shall include an inspection checklist for Downspout Disconnection. SECTION 6: AS-BUILTS During and after the downspout disconnection construction, the developer must have adequate inspection and an as -built certification prepared by a registered Professional Engineer and provided to the City. The as - built certification verifies that the BMP was installed as designed and approved. The following components must be addressed in the as -built certification: 1. Ensure disconnect is treating appropriate area size from either sheet flow or roof leader. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 2. Ensure filter media depth is properly sized. 3. Ensure building setbacks are 10 ft down -gradient, 50 ft up -gradient. 4. Ensure underdrain and gravel layer (if required) have been properly installed. 5. If alternative practices have been utilized, insure that as -built requirements for those GSPs are also certified using the as -built section of the utilized GSP. SECTION 7: REFERENCES City of Portland, Environmental Services, 2004. Portland Stormwater Management Manual. Portland, OR. http://www.portlandonline.com/bes/index.cfm?c=dfbbh CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD. Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia. Philadelphia Stormwater Management Guidance Manual, updated 2011. Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD. Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net VADCR (Virginia Department of Conservation and Recreation) 2011. Stormwater Design Specification No. 1: Rooftop (Impervious Surface) Disconnection, Version 1.9, Available at: http://vwrrc.vt.edu/swc/NonProprietaryBMPs.htmi. Appendix B — GSP Specifications Drainage Criteria Manual i , GRASS CHANNEL Description: Limited application structural control intended for small drainage areas. Open channels that are vegetated and are designed to filter stormwater runoff through settling and biological uptake mechanisms, as well as to slow water for treatment by another structural control. • Broad bottom channel on gentle slopes (4% or less) • Gentle side slopes (3H:1V or less) • Dense vegetation that assists in stormwater filtration • Check dams can be installed to maximize treatment • Provides pretreatment if used as part of runoff conveyance system • Provides partial infiltration of runoff in pervious soils • Cost effective — less expensive than curb and gutter • Good for small drainage areas • Wildlife habitat potential • Potential for thermal impacts downstream • Must be carefully designed to achieve low, non-erosive flow rates in the channel • May re -suspend sediment • May not be acceptable for some areas due to standing water in channels • Maximum drainage area of 5 acres • Requires slopes of 4% or flatter • Runoff velocities must be non—erosive • Appropriate for all except very low permeability soils • Requires vegetation that can withstand both relatively high velocity flows and wet and dry periods • Generally used in conjunction with downstream practices to increase runoff reduction • May not be used in a series of two Appendix B — GSP Specifications Drainage Criteria Manual i , Selection Criteria: Runoff Reduction Removal Credit LEVEL 1-10% for HSG soils C and D LEVEL 2 — 20% for HSG soils A and B Land Use Considerations: © Residential © Commercial © Industrial* *With City approval Maintenance: • Monitor sediment accumulation and periodically clean out • Inspect for and correct formation of rills and gullies • Remove debris from inlet and outlet structures • Maintain side slopes/remove invasive vegetation • Ensure that vegetation is well-established Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION Grass channels are conveyance channels that are designed to provide some treatment of runoff, as well as to slow runoff velocities for treatment in other structural controls. Grass channels are appropriate for a number of applications including treating runoff from paved roads and from other impervious areas. Grass channels can provide a modest amount of runoff filtering and volume attenuation within the stormwater conveyance system resulting in the delivery of less runoff and pollutants than a traditional system of curb and gutter, storm drain inlets and pipes. The performance of grass channels will vary depending on the underlying soil permeability as shown in Table 8.1. Grass channels, however, are not capable of providing the same stormwater functions as water quality swales as they lack the storage volume associated with the engineered soil media. Their runoff reduction performance can be increased when compost amendments are added to the bottom of the swale (Appendix C). Grass channels are a preferable alternative to both curb and gutter and storm drains as a stormwater conveyance system, where development density, topography and soils permit. SECTION 2: PERFORMANCE Table 8.1: Annual Runoff Volume Reduction by Grass Channels' Level 1 HSG Soils C and D Level 2 HSG Soils A and B Stormwater Function No CA With CA No CA With CA Annual Runoff Volume Reduction (RR) 10% 20% 20% 40% %.vvr aiiu wiv jcuuoj aiiu %-vvr kcuuij. z CA= Compost Amended Soils, see Appendix C— Soil Amendment section 3 Compost amendments are generally less applicable for A and B soils, although it may be advisable to incorporate them on mass -graded and/or excavated soils to maintain runoff reduction rates. In these cases, the 40% runoff reduction rate may be claimed. SECTION 3: DESIGN TABLE Grass channels must meet the minimum criteria outlined in Table 8.2 to qualify for the indicated level of runoff reduction. Design Criteria • The bottom width of the channel shall be between 4 to 8 ft wide. • The channel side slopes shall be 3H:1V or flatter. • The maximum total contributing drainage area to any individual grass channel is 5 acres. • The longitudinal slope of the channel shall be no greater than 4 %. • Check dams may be used to increase residence time. • The maximum flow velocity of the channel must be less than 1 ft per second during a 1 -inch storm event. • The channel shall be designed such that flow velocity is non-erosive during the 2 -year and 10 -year design storm events and the 10 -year design flow is contained within the channel (minimum of 6 inches of freeboard). • The vegetation used must withstand relatively high velocities as well as a range of moisture conditions from very wet to Appendix B — GSP Specifications Drainage Criteria Manual i , SECTION 4: TYPICAL DETAILS A eck Dam J• v l• W W 1• W •1• W -Y Y W W �Y -•Y �L W W V• V• ViW W VW •Y W J. ,L•V J- W •V W •Y W 1- W Channel Botfom •J• W v Y W V• •Y •Y Y• Y W •1• •J• W V V' v W W 4• 'Y W Y V J- 'Y V V •Y •1• W W 1• w W 4' w 'k +Y' v J- 4 Shoulder � LA f � Pretreatment Forebay PLAN Riprap Inflow r T— Appendix 8 — GSP Specifications Drainage Criteria Manual i , 10 Year 2 Year Limited Infiltration SECTION A -A Side Slope Figure 8.1. Grass Channel — Typical Plan and Section. CHANNEL LENGTH IS DIRECTLY PROPORTIONAL TO ROADWAY LENGTH —. PRETREATMENT FOREBAY (WHERE APPROPRIA'T'E) OPTIONAL CHECK DAM RlPRAP V INFLOW � MAXIMUM � � V,B' SLOPIIMUM —► C NNS —DTH CHANNEL BOTTOM ---� �G v � v v IV, � W V av u v V SHOULDER r ROADWAY—. 10 YEAR LEVEL blllllllml 1: 11 -- = 1 YEAR LEVEL - IIluu--- ' ,._ WO STORM PLAN VIEW SHOULDER - ROADWAY — VELOCITY LESS THAN 1.0 fps FOR 0.9" RAINFALL SECTION Figure 8.2. Grass Channel along Roadway — Typical Plan and Section (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual I , EZOIDAL NOTCH WEIR A r 3:1 MAX (5:1 OR j� LESS PREFFERED) CHANNEL WIDTH AS REQUIRED BY DESIGN CHECK DAM SPACING AS REQUIRED BY DESIGN 1 - 3 IN. COURSE AGGREGATE, ROUNDED COBBLE, OR OTHER LINING AS DESIGNED/SIZED FOR STABILITY MAX FLOW TOP OF DAM - 2 - 3 FT MIN.—{ r PROVIDE 1R' WEEP HOLES FLOW PLANVIEW PROFILE 3:1 MAX (5:1 OR LESS PREFFERED) WQV ELEV rNOTCH WEIR / rWQV ELEVATION r -FREE BOARD WIDTH AS REQUIRED BY DESIGN SECTION A -A' 181N. M A 1:I41:� 1:Id COATED #5 REBAR NOTE_ CHECK DAM CONSTRUCTED OF RAILROAD TIES, PRESSURE TREATED LOGS OR TIMBERS, OR CONCRETE. Figure 8.3. Grass Channel with Check Dams — Typical Plan, Profile, and Section. (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual i , NA • ..I Ilup!L.-U— ��jllrll it [l�I� li�--` IMI '"ll�ll ■ ■ • �, �' fl[.IILI�I mg f"1la] 1F: • , • X11=11=11=III[.=II=11=11 I I WELocrrr) T*�''•' Figure 8.4: Grass Channel with Compost Amendments — Section. (Source: VADCR, 2011). SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS Grass channels can be implemented on development sites where development density, topography and soils are suitable. The linear nature of grass channels makes them well-suited to treat highway runoff, low and medium density residential road runoff and small commercial parking areas or driveways. However, a Water Quality Swale (GSP-05) will provide much greater runoff reduction and pollutant removal performance. Key constraints for grass channels include: Land Use. Grass channels can be used in residential, commercial or institutional development settings. However, when grass channels are used for both conveyance and water quality treatment, they shall be applied only in linear configurations parallel to the contributing impervious cover, such as roads and small parking areas. • Large commercial site applications may require multiple channels in order to effectively break up the drainage areas and meet the design criteria. • The linear nature of grass channels makes them well suited to treat highway or low- and medium - density residential road runoff, if there is adequate right-of-way width and distance between driveways. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Contributing Drainage Area. The drainage area (contributing or effective) to a grass channel must be 5 acres or less. When grass channels treat and convey runoff from drainage areas greater than 5 acres, the velocity and flow depth through the channel becomes too great to treat runoff or prevent channel erosion. Available Space. Grass channels can be incorporated into linear development applications (e.g., roadways) by utilizing the footprint typically required for an open section drainage feature. The footprint required will likely be greater than that of a typical conveyance channel. However, the benefit of the runoff reduction may reduce the footprint requirements for stormwater management elsewhere on the development site. Longitudinal Slope. Grass channels shall be designed with slopes of less than 4%. Slopes steeper than 4% create rapid runoff velocities that can cause erosion and do not allow enough contact time for infiltration or filtering, unless check dams are used. Slopes of 1-2% are recommended, and slopes of less than 2% may eliminate the need for check dams. Channels designed with longitudinal slopes of less than 1% shall be monitored carefully during construction to ensure a continuous grade, in order to avoid flat areas with pockets of standing water. Soils. Grass channels can be used on most soils with some restrictions on the most impermeable soils. Grass channels situated on Hydrologic Soil Group C and D soils will require compost amendments in order to improve performance. Grass channels shall not be used on soils with infiltration rates less than 0.5 inches per hour if infiltration of small runoff flow is intended. Hydraulic Capacity. Grass channels are an on-line practice and must be designed with enough capacity to convey runoff from the 10 -year design storm event within the channel banks and be non-erosive during both the 2 -year and 10 -year design storm events. Larger flows shall be accommodated by the channel if dictated by the surrounding conditions. Refer to the applicable chapter(s) within the Drainage Criteria Manual for site hydraulic design requirements. Depth to Water Table. Designers shall ensure that the bottom of the grass channel is at least 2 ft above the known seasonally high water table to prevent a moist swale bottom and ensure that groundwater does not intersect the filter bed and possibly lead to groundwater contamination or practice failure. Utilities and Building Setbacks. Designers shall consult local utility design guidance for the horizontal and vertical clearance between utilities and the channels. Typically, utilities can cross grass channels if they are specially protected (e.g., double -casing) or are located below the channel invert. Arkansas One Call (811) shall be contacted before digging onsite begins. Grass channels shall be set back at least 10 ft down -gradient from building foundations, 50 ft from septic system fields and 100 ft from private wells. Forebays. A forebay is recommended in order to minimize the volume of sediment in the channel. Refer to Chapter 7 and Appendix F of the Drainage Criteria Manual for additional information. SECTION 6: DESIGN CRITERIA 6.1. Sizing of Grass Channels Unlike other stormwater practices, grass channels are designed based on a peak rate of flow. Designers must demonstrate channel conveyance and treatment capacity in accordance with the following guidelines: Appendix 8 - GSP Specifications Drainage Criteria Manual i , • The longitudinal slope of the channel should ideally be between 1% and 2% in order to avoid scour and short-circuiting within the channel. Longitudinal slopes up to 4% are acceptable; however, check dams will likely be required in order to meet the allowable maximum flow velocities. • A minimum residence time is of five minutes is required. • Hydraulic capacity shall be verified using Manning's Equation or an accepted equivalent method, such as erodibility factors and vegetal retardance (NOVA 2007). o The Flow Depth for the peak treatment volume shall be maintained at 3 inches or less. o Manning's "n" value shall be selected based on depth of flow, for overland flow or channel flow using the appropriate reference tables in Chapter 3. o Velocities and Peak Flow Rates for the 2 -year and 10 -year frequency storms must be non-erosive, and the 10 -year peak flow must be contained within the channel banks (with a minimum of 6 inches of freeboard). • Larger flows shall be accommodated by the channel if dictated by the surrounding conditions. Refer to the applicable chapter(s) within the Drainage Criteria Manual for site hydraulic design requirements. • Calculations for peak flow depth and velocity shall reflect any increase in flow along the length of the channel, as appropriate. If a single flow is used, the flow at the outlet shall be used. • The minimum length may be achieved with multiple swale segments connected by culverts with energy dissipaters. 1 Source: VADCR, 2011 Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 8.3. Maximum Permissible Velocities for Grass Channels' Cover Type Erosion Resistant Soils Highly Erodible (ft/sec) Soils (ft/sec) Bermuda grass 6 4.5 Buffalo grass Kentucky bluegrass 5 3.8 Reed canary grass Tall fescue Grass -legume mixture 4 3 Red Fescue 2.5 1.9 1 Source: VADCR, 2011 Appendix 8 — GSP Specifications Drainage Criteria Manual i , 6.2. Geometry and Site Layout • Grass channels shall generally be aligned adjacent to and the same length (minimum) as the contributing drainage area identified for treatment. • Grass channels shall be designed with a trapezoidal or parabolic cross section with relatively flat side slopes. A parabolic shape is preferred for aesthetic, maintenance and hydraulic reasons. • The bottom width of the channel shall be between 2 to 6 ft wide. If a channel will be wider than 8 ft, the designer shall incorporate benches, check dams, level spreaders or multi-level cross sections to prevent braiding and erosion along the channel bottom. The bottom width is a dependent variable in the calculation of velocity based on Manning's equation. If a larger channel is needed, the use of a compound cross section is recommended. • Grass channel side slopes shall be no steeper than 4 H:1 V for ease of mowing and routine maintenance. Flatter slopes are encouraged, where adequate space is available, to aid in pre- treatment of sheet flows entering the channel. Under no circumstances are side slopes to exceed 3 H:1 V. 6.3. Check dams Check dams may be used for pre-treatment, to break up slopes, and to increase the hydraulic residence time in the channel. Design requirements for check dams are as follows: • Check dams shall be spaced based on the channel slope, as needed to increase residence time, provide Tv storage volume, or any additional volume attenuation requirements. The ponded water at a downhill check dam should not touch the toe of the upstream check dam. • The maximum desired check dam height is 12 inches (for maintenance purposes). The average ponding depth throughout the channel shall be 12 inches. • Armoring may be needed at the downstream toe of the check dam to prevent erosion. • Check dams must be firmly anchored into the side -slopes to prevent outflanking; check dams must also be anchored into the channel bottom so as to prevent hydrostatic head from pushing out the underlying soils. • Check dams must be designed with a center weir sized to pass the channel design storm peak flow (10 -year storm event for man-made channels). • The check dam shall be designed so that it facilitates easy mowing. • Each check dam shall have a weep hole or similar drainage feature so it can dewater after storms. • Check dams shall be composed of wood, concrete, stone, or other non -erodible material, or shall be configured with elevated driveway culverts. • Individual channel segments formed by check dams or driveways shall generally be 25 to 40 ft in length. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.4. Compost Soil Amendments Soil compost amendments serve to increase the runoff reduction capability of a grass channel. The following design criteria apply when compost amendments are used: • The compost -amended strip shall extend over the length and width of the channel bottom, and the compost shall be incorporated to a depth as outlined in Appendix C. • The amended area shall be stabilized with grass. • Depending on the slope of the channel, it may be necessary to install a protective biodegradable geotextile fabric to protect the compost -amended soils. Care must be taken to consider the erosive characteristics of the amended soils when selecting an appropriate geotextile. • For redevelopment or retrofit applications, the final elevation of the grass channel (following compost amendment) must be verified as meeting the original design hydraulic capacity. 6.5. Planting Grass Channels Designers shall specify grass species that can withstand both wet and dry periods as well as relatively high - velocity flows within the channel. For applications along roads and parking lots, salt tolerant species should be chosen. Taller and denser grasses are preferable, though the species of grass is less important than good stabilization. Grass channels shall be seeded at such a density to achieve a 90 % turf cover after the second growing season. Grass channels shall be seeded and not sodded. Seeding establishes deeper roots and sod may have muck soil that is not conducive to infiltration (Wisconsin DNR, 2007). Grass channels shall be protected by a biodegradable erosion control fabric to provide immediate stabilization of the channel bed and banks. 6.6. Grass Channel Material Specifications The basic material specifications for grass channels are outlined in Table 8.4 below. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.7 Grass Channel as Pretreatment A number of structural controls such as bioretention areas and infiltration trenches may be supplemented by a grass channel that serves as pretreatment for runoff flowing to the device. The lengths of grass channels vary based on the drainage area imperviousness and slope. Channels must be no less than 20 ft long. Table 8.6 below gives the minimum lengths for grass channels based on slope and percent imperviousness. Table 8.4. Grass Channel Material Specifications Component Specification A dense cover of water -tolerant, erosion -resistant grass. The selection of an appropriate species or Channel Length Guidance" mixture of species is based on several factors including climate, soil type, topography and sun or Grass shade tolerance. Grass species should have the following characteristics: a deep root system to resist Between 33% and 67% Impervious scouring; a high stem density with well -branched top growth; water -tolerance; resistance to being Slope (max = 4%) flattened by runoff; an ability to recover growth following inundation; and, if receiving runoff from > 2% roadways, salt -tolerance. > 2% • All check dams shall be constructed of non-erosive material such as wood, gabions, riprap or > 2% concrete, and underlain with filter fabric conforming to local design standards. Check Dams • Wood used for check dams shall consist of pressure treated logs or timbers, or water-resistant 30 tree species such as cedar, hemlock, swamp oak or locust. 35 • Computation of check dam material is necessary, based on the surface area and depth used in the design computations. Diaphragm Pea gravel used to construct pre-treatment diaphragms shall consist of washed, open -graded, course aggregate between 3 and 10 mm in diameter and must conform to local design standards. Erosion Control Where flow velocities dictate, biodegradable erosion control netting or mats that are durable Fabric enough to last at least two growing seasons must be used. Needled, non -woven, polypropylene geotextile meeting the following specifications: Filter Fabric Grab Tensile Strength (ASTM D4632): > 120 lbs (check dams) Mullen Burst Strength (ASTM D3786): > 225 lbs/sq in Flow Rate (ASTM D4491): > 125 gpm/sq ft Apparent Opening Size (ASTM D4751): US #70 or #80 sieve 6.7 Grass Channel as Pretreatment A number of structural controls such as bioretention areas and infiltration trenches may be supplemented by a grass channel that serves as pretreatment for runoff flowing to the device. The lengths of grass channels vary based on the drainage area imperviousness and slope. Channels must be no less than 20 ft long. Table 8.6 below gives the minimum lengths for grass channels based on slope and percent imperviousness. 1 Source: Georgia Stormwater Management Manual. z Assumes 2 -ft bottom width. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 8.6. Grass Channel Length Guidance" Parameter <=33% Impervious Between 33% and 67% Impervious >=67% Impervious Slope (max = 4%) < 2% > 2% < 2% > 2% < 2% > 2% Grass channel minimum length (ft)Z 25 40 30 45 35 50 1 Source: Georgia Stormwater Management Manual. z Assumes 2 -ft bottom width. Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Steep Terrain Grass swales are not practical in areas of steep terrain, although terracing a series of grass swale cells may work on slopes from 5% to 10%. The drop in elevation between check dams shall be limited to 18 inches in these cases, and the check dams shall be armored on the down-slope side with suitably sized stone to prevent erosion. SECTION 8: CONSTRUCTION 8.1. Construction Sequence The following is a typical construction sequence to properly install a grass channel, although steps may be modified to reflect different site conditions. Grass channels shall be installed at a time of year that is best to establish turf cover without irrigation. Step 1: Protection during Site Construction. Ideally, grass channels shall remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. However, this is seldom practical, given that the channels are a key part of the drainage system at most sites. In these cases, temporary EPSC such as dikes, silt fences and other erosion control measures shall be integrated into the swale design throughout the construction sequence. Specifically, barriers shall be installed at key check dam locations, and erosion control fabric shall be used to protect the channel. Step Z. Grass channel installation may only begin after the entire contributing drainage area has been stabilized with vegetation. Any accumulation of sediments that does occur within the channel must be removed during the final stages of grading to achieve the design cross-section. Erosion and sediment control measures shall be installed as specified in the erosion and sediment control plan. Stormwater flows must not be permitted into the grass channel until the bottom and side slopes are fully stabilized. Step 3. Grade the grass channel to the final dimensions shown on the plan. Step 4. Install check dams, driveway culverts and internal pre-treatment features as shown on the plan. Fill material used to construct check dams shall be placed in 8- to 12 -inch lifts and compacted to prevent settlement. The top of each check dam shall be constructed level at the design elevation. Step 5 (Optional). Till the bottom of the channel to a depth of 1 ft and incorporate compost amendments in accordance Appendix C. Step 6. Add soil amendments as needed, hydro -seed the bottom and banks of the grass channel, and peg in erosion control fabric or blanket where needed. After initial planting, a biodegradable erosion control fabric shall be used. Step 7. Prepare planting holes for any trees and shrubs, then plant materials as shown in the landscaping plan and water them weekly in the first two months. The construction contract shall include a Care and Appendix 8 - GSP Specifications Drainage Criteria Manual i , Replacement Warranty to ensure vegetation is properly established and survives during the first growing season following construction. Step 8. Conduct the final construction inspection and develop a punch list for facility acceptance. 8.2. Construction Inspection Inspections during construction are needed to ensure that the grass channel is built in accordance with these specifications. Some common pitfalls can be avoided by careful post -storm inspection of the grass channel: • Make sure the desired coverage of turf or erosion control fabric has been achieved following construction, both on the channel beds and their contributing side -slopes. • Inspect check dams and pre-treatment structures to make sure they are at correct elevations, are properly installed, and are working effectively. • Make sure outfall protection/energy dissipation at concentrated inflows are stable. The real test of a grass channel occurs after its first big storm. Minor adjustments are normally needed as part of this post -storm inspection (e.g., spot reseeding, gully repair, added armoring at inlets or realignment of outfalls and check dams). SECTION 9: MAINTENANCE 9.1. Maintenance It is recommended that a Long Term Maintenance Plan (LTMP) be developed by the design engineer. The LTMP contains a description of the stormwater system components and information on the required inspection and maintenance activities. Basic maintenance requirements for grass channels include the following: • Maintain grass height of 3 to 4 inches. • Remove sediment build up in channel bottom when it accumulates to 25% of original total channel volume. • Ensure that rills and gullies have not formed on side slopes. Correct if necessary. • Remove trash and debris build up. • Replant areas where vegetation has not been successfully established. All grass channels must be covered by a drainage easement to allow inspection and maintenance. If a grass channel is located in a residential private lot, the existence and purpose of the grass channel shall be noted on the deed of record. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 9.2. Ongoing Maintenance Once established, grass channels have minimal maintenance needs outside of the spring cleanup, regular mowing, repair of check dams and other measures to maintain the hydraulic efficiency of the channel and a dense, healthy grass cover. Table 8.6. Suggested Spring Maintenance Inspections/Cleanups for Grass Channels' Activity Add reinforcement planting to maintain 90% turf cover. Reseed any dead vegetation. Remove any accumulated sand or sediment deposits behind check dams. Inspect upstream and downstream of check dams for evidence of undercutting or erosion, and remove and trash or blockages at weepholes. Examine channel bottom for evidence of erosion, braiding, excessive ponding or dead grass. Check inflow points for clogging and remove any sediment. Inspect side slopes and grass filter strips for evidence of any rill or gully erosion and repair. Look for any bare soil or sediment sources in the contributing drainage area and stabilize immediately. 1 Source: VADCR, 2011. SECTION 10. AS -BUILT REQUIREMENTS After the grass channel has been constructed, an as -built certification of the grass channel must be prepared by a registered Professional Engineer and submitted to the City. The as -built certification verifies that the BMP was installed as designed and approved. 1. The following components must be addressed in the as -built certification: 2. The channel must be adequately vegetated. 3. The channel flow velocities must not exceed allowable velocities for design storms. 4. A mechanism for overflow for large storm events must be provided. SECTION 11: ROADWAY APPLICATION Grass -lined channels have been widely used in roadway drainage systems for many years. They are easily constructed and maintained and work well in a variety of climates and soil conditions. Grass channels are applicable to: • Major Thoroughfares (Interstates and Other Freeways) • Major Urban Streets (Principal Arterials, Minor Arterials and Collectors) • Local Roads Appendix 8 — GSP Specifications Drainage Criteria Manual i , Figure 8.5. Typical Grass Channel Figure 8.6. Roadside Channel in Spokane, WA (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 12: REFERENCES Claytor, R. and T. Schueler, 1996. Design of stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD. CWP, 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD. Haan, C.T., Barfield, B.J., and Hayes, J.C., 1994. Design Hydrology and Sedimentology for Small Catchments. Academic Press, New York. Lantin, A., and M. Barrett, 2005. Design and Pollutant Reduction of Vegetated Strips and Swales. ASCE. Downloaded September, 2005. Maryland Department of Envirommnt (MDE), 2000. Maryland Stormwater Design Manual. Baltimore, MD. Northern Virginia Regional Commission, 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia Schueler, T., D. Hirschman, M. Novotney and J. Zielinski, 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD. Schueler, T., 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net VADCR (Virginia Department of Conservation and Recreation), 2011. Stormwater Design Specification No. 3: Grass Channel, Version 2.3, http://vwrrc.vt.edu/swc/NonProprietaryBMPs.html. VADCR, 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA. Wisconsin Department of Natural Resources, 2004. "Vegetated Infiltration Swale (1005)." Interim Technical Standard, Conservation Practice Standards. Standards Oversight Council, Madison, Wisconsin. Appendix B - GSP Specifications Drainage Criteria Manual i , SHEET FLOW Description: Impervious areas are disconnected and runoff is routed over a level spreader to sheet flow over adjacent vegetated areas. This slows runoff velocities, promotes infiltration, and allows sediment and attached pollutants to settle and/or be filtered by the vegetation. Variations: • Disconnection to vegetated filter strips. • Disconnection to conserved open space. • Engineered level spreader (ELS) — creates sheet flow • Vegetated filter strip or open space with minimal slope • Cost effective • Wildlife habitat potential • High community acceptance Appendix B — GSP Specifications Drainage Criteria Manual Selection Criteria: 50%-75% Runoff Reduction Credits See Table 9.1 Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Maintain dense, healthy vegetation to ensure sheet flow • Inspect regularly for signs of erosion Maintenance Burden © L = Low M = Moderate H = High • Must have slopes between 2% and 6% • Filter strips and conservation areas may be adjacent to and discharge to water quality buffers • Small drainage area • Sheet flow must be maintained to achieve design goals • Typically requires additional BMPs to achieve runoff reduction goals. SECTION 1: DESCRIPTION Filter strips are vegetated areas that treat sheet flow delivered from adjacent impervious areas by slowing runoff velocities and allowing sediment and attached pollutants to settle and/or be filtered by the vegetation. The two design variants of filter strips are (1) Conserved Open Space and (2) designed Vegetated Filter Strips. The design, installation, and management of these design variants are quite different, as outlined in this specification. In both instances, stormwater must enter the filter strip or conserved open space as sheet flow. If the inflow is from a pipe or channel, an engineered level spreader must be designed in accordance with the criteria contained herein to convert the concentrated flow to sheet flow. SECTION 2: PERFORMANCE With proper design and maintenance, these practices can provide relatively high runoff reduction as shown in Table 9.1. Table 9.1. Annual Runoff Volume Reduction by Filter Strips. Conservation Area Vegetated Filter Strip HSG Soils HSG Soils HSG Soils HSG Soils Stormwater Function A and B C and D A Bl, C and D Assume no CA in Conservation Area No CA' With CA Annual Runoff Volume Reduction (RR) 75% 1 50% 50% 50% 1CWP and CSN (2008); CWP (2007). z CA = Compost Amended Soils. 3 Compost amendments are generally not applicable for undisturbed A soils, although it may be advisable to incorporate them on mass -graded A or B soils and/or filter strips on B soils to maintain runoff reduction rates. SECTION 3: DESIGN TABLE Conserved Open Space and Vegetated Filter Strips do not have two levels of design. Instead, each must meet the appropriate minimum criteria outlined in Tables 9.1 and 9.2 and Section 6 to qualify for the indicated level of runoff reduction. In addition, designers must conduct a site reconnaissance prior to design to confirm topography and soil conditions. Appendix 8 — GSP Specifications Drainage Criteria Manual 1 A minimum of 1 % is recommended to ensure positive drainage. z For Conservation Areas with a varying slope, a pro -rated length may be computed only if the first 10 ft. is 2% or less. 3 Vegetative Cover is described in Section 6.2. 4 Where the Conserved Open Space is a mixture of native grasses, herbaceous cover and forest (or re -forested area), the length of the Engineered Level Spreader lip can be established by computing a weighted average of the lengths required for each vegetation type. Refer to Section 6.3 for design criteria. 5 The City may waive the requirement for compost amended soils for filter strips on B soils if adequate infiltration characteristics are met post -construction (see Section 6.1). 6 ELS = Engineered level spreader; GD = Gravel Diaphragm; PB = Permeable Berm. Appendix 8 — GSP Specifications Drainage Criteria Manual Table 9.2. Sheet Flow Design Guidance. Design Issue Conserved Open Space Vegetated Filter Strip Soil and Vegetative Cover Amended soils and dense turf cover or (Sections 6.1 and 6.2) Undisturbed soils and native vegetation landscaped with herbaceous cover, shrubs, and trees Overall Slope' and Width (perpendicular to the flow) Maximum flow length of 150 ft. from adjacent pervious areas; Maximum flow length of 75 ft. (Section 5) from adjacent impervious areas Length of ELSE Lip = 13 lin. ft. per each 1 cfs of Concentrated Flow inflow if area has 90% Vegatative Covera Length of ELSE Lip = 13 lin.ft. per each 1 cfs (Section 6.3) Length = 40 lin. ft. per 1 cfs where discharge to of inflow (13 lin.ft. min; 130 lin.ft. max.) conserved open space (forest/re-fo rested )4 Construction Stage Located outside the limits of disturbance and Prevent soil compaction by heavy (Section 7) protected by erosion and sediment control equipment Typical Applications Adjacent to stream or wetland buffer or forest Treat small areas of impervious cover (e.g., (Section 5) conservation area 5,000 sq. ft.) close to source Compost No Yes (B, C, and D Soils)5 Amendments (Section 6.1) Boundary Spreader (Section GD 6 at top of filter GD6 at top of filter 6.3) PB at toe of filter 1 A minimum of 1 % is recommended to ensure positive drainage. z For Conservation Areas with a varying slope, a pro -rated length may be computed only if the first 10 ft. is 2% or less. 3 Vegetative Cover is described in Section 6.2. 4 Where the Conserved Open Space is a mixture of native grasses, herbaceous cover and forest (or re -forested area), the length of the Engineered Level Spreader lip can be established by computing a weighted average of the lengths required for each vegetation type. Refer to Section 6.3 for design criteria. 5 The City may waive the requirement for compost amended soils for filter strips on B soils if adequate infiltration characteristics are met post -construction (see Section 6.1). 6 ELS = Engineered level spreader; GD = Gravel Diaphragm; PB = Permeable Berm. Appendix 8 — GSP Specifications Drainage Criteria Manual SECTION 4: TYPICAL DETAILS Figure 9.1 shows a typical approach for sheet flow to a Conserved Open Space (Cappiella et al., 2006). Figures 9.2 and 9.3 provide standard details for an engineered level spreader developed by North Carolina State University (Hathaway and Hunt, 2006). �•. } f-18 inches Gravel diapftiragm :� Ponding {12 by 24 Inch4m} for Forest prwtrde#ffiem tont Ponding Zone Runoff berm 1 -crest 4one Figure 9.1. Typical Sheetflow to Conserved Open Space (Source: Hathaway and Hunt 2006). 0 D versio-t I+�:Et,ert ro Leve Storrn,t�ate5 ¢rP, r Figure 9.2. Level Spreader Forebay (Source: Hathaway and Hunt 2006). Appendix 8 - GSP Specifications Drainage Criteria Manual Fri RDW Bypass Vagatated FAx Strip LeNlh =13• to 1W (parallel to oontour lines) __ tm Channel Reinforced Swale {necessary if IevA spreader is designed W the l inAn storm) Leonel Spreader Up F9GW p 057 • getated t Fillter Strip aannel 3 it Wbdc Underdram -:5=d strap of Por Ste Sad Fllxer Foibr~c Condit - Drain AppropnataCy `3�xad Into Ery)ms5 5wie Croncxae Fo: CV= Figure 9.3. Plan and Cross Section of Engineered Level Spreader (ELS) (Source: Hathaway 2006). Appendix 8 — GSP Specifications Drainage Criteria Manual �lVo JIL 11L JIL JIL JIL L11 W- LMC 44W AWL 'PUL. JL 4WI[- a1111F 11L Y Jlt JIL JIr- aCIJY. aW6 aWi1C 4Yla• aNYa- aCNYF ISCHARGE TO suct Owl& awl& -AW& swr. "W IUL& _ RECEIVING i i i i — i — _2-1 OR TREATMENT �] '` �` .��,. aU„FLATTER � MEASURE 1 sura e„� sour sow 4mia sur you m CLEAN WASHED GRAVEL OR OTHER LINING AS DESIGNEDYSIZED FOR STABILITY CLEAN WASHED GARVEL OR OTHER LINING AS DESIGNEDISIZED FOR STABILTfY FILTER F, DISCHARGE TO RECEIVING TREATMENT MEASURE 3 FT TREATED TIMBERS OR CONCRETE LIP PLAN VIEW VARIABLE (MIN. 7 FT) 'K6' TREATED T1MEERS OR CONCRETE LIP 475 REEAR TO SECURE TIMBERS r 6IN. MIN MIN 6 FT. 21 OR FLATTER Figure 9.4. Section - Level Spreader with Rigid Lip (Source: VADCR, 2011). Appendix 8 — GSP Specifications Drainage Criteria Manual VEGETATED UP OF JUTE, EXCELSIOR, OR PLAN VIEW EQUIVALENT STAPLED IN PLACE NOTE: CONSULT SPECIFICATIONS FOR APPROPRIATE USE OF JUTE, EXCELSIOR. OR VEGETATED LIP VERSUS RIGID LIP EOUIVAL.ENTSTAPLED IN PLACEI DISCHARGE TO URIED G'MIN. RECEIVING BURIED 6• MIN. TREATMENT r 6 IN. MIN MEASURE I e LEVEL LIP OF SPREADER MIR! 6 FT. PROFILE 2:i OR FLATTER Figure 9.5. Section - Alternative Level Spreader with Vegetated Lip (Source: VADCR, 2011). Appendix 8 — GSP Specifications Drainage Criteria Manual filmpf WN RIP i f**♦WE WRrR►iiiii;iii 11 *N*i?�*��i4�iE EE L «i�'iiii��ifii RE IV I iiW,r +l�iil��ii «iii+�► �M`*iii�{'itf'#i±�+ii+tai • OF IN WH i ii�+iii'*iiiiii*iii'�f Fy*ME.777`�`~��``llf II JUTE, EXCELSIOR, OR PLAN VIEW EQUIVALENT STAPLED IN PLACE NOTE: CONSULT SPECIFICATIONS FOR APPROPRIATE USE OF JUTE, EXCELSIOR. OR VEGETATED LIP VERSUS RIGID LIP EOUIVAL.ENTSTAPLED IN PLACEI DISCHARGE TO URIED G'MIN. RECEIVING BURIED 6• MIN. TREATMENT r 6 IN. MIN MEASURE I e LEVEL LIP OF SPREADER MIR! 6 FT. PROFILE 2:i OR FLATTER Figure 9.5. Section - Alternative Level Spreader with Vegetated Lip (Source: VADCR, 2011). Appendix 8 — GSP Specifications Drainage Criteria Manual SHEET FLOW FROM IMPERVIOUS OR TURF SHEET FLO W PAVEMENT SECTION PAVEMENT SUBGRADE 1' LINING AS SPECIFIED BY DESIGN ENGINEER TO PREVENT SUBGRADE SATURATION I� «I f P�l I 4' MIN — 2' 2' MINMIN 2"-4" ❑ RCf lidr lui, IF 5:1 MAX I I I I I �r�l I [—I I i— 3 FiLTER FABRIC _I I I_I I !—LI L CLEAN WASHED GRAVEL 1'M IN WITH 1:1 SIDE SLOPES Figure 9.6. Gravel Diaphragm - Sheet Flow Pre-treatment (Source: VADCR, 2011). RIPRAP PLUNGE POOL RIPRAP APRON �_ 7 ygk -3 IN. COURSE AGGREGATE OR OTHER LINING AS DESIGNED FOR S': ABILITY KEY LEVEL SPREADER IN i U LK S T IN 3 UHAUL _ 6'X 3' TREATED TIMBERS OR � f\QQN HL T L LII' LENGTH OF LEVEL SPREADER LIP TO BE BASED ON DESIGN FLOW AND ALI_OWARI-F VFIAXATY C3F' S]I- LLVLL ;iPHLAULH SHELL RE FLAT •I PLAN VIEW DIMENSION BASED IN UNDISTURBED EXISTING GRADE ULSIUN DISC HAHCaL 5% (20 1 ) OR LESS. IF VEGETATED FILTER STRIP ❑R C'ANSFRVFQ OPEN SPACE FWST If FEET LESS 2' MIN THAN :3' MIN 4 1 MIN i 3' MIN 12 IMIN. DEPTH BASED ON':.r — REQUIRED STONE SIZE ..,L! PROFILE Figure 9.7. Level Spreader: Pipe or Channel Flow to Filter Strip or Preserved Open Space (Source: VADCR, 2011). Appendix B - GSP Specifications Drainage Criteria Manual PROPOSE❑ PRESERVED OPEN SPACE VEGETE PLAN VIEW CONSERVED OPEN SPACE OR VEGETATED FILTER STIR P SHEET FLOW �� SECTION A -A' SIMPLE DISCONNECTION TO FILTER STRIP OR CONSERVED OPEN SPACE PROPOSED LEVEL SPREADER OR GRAVEL DIAPHRAGM (IF NECESSARY) Figure 9.8. Simple Disconnection to downstream Preserved Open Space or Vegetated Filter Strip (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS 5.1. Conserved Open Space Designers may apply a runoff reduction credit to any impervious area that is hydrologically connected and effectively treated by a protected Conserved Open Space that meets the following eligibility criteria: No major disturbance may occur within the conserved open space during or after construction. No clearing or grading shall occur except temporary disturbances associated with incidental utility construction of less than 10% total conserved open space area, restoration operations, or management of nuisance vegetation. The Conserved Open Space area shall not be stripped of topsoil. If required, some light grading may be performed at the boundary using tracked vehicles to prevent compaction. • The limits of disturbance shall be clearly shown on all construction drawings and protected by appropriate signage, fencing, and erosion control measures. • A long term vegetation management plan must be prepared to maintain the Conserved Open Space in a natural vegetative condition. Generally, Conserved Open Space management plans do not allow any active management. However, a specific plan shall be developed to manage the unintended consequences of passive recreation, control invasive species, provide for tree and understory maintenance, etc. • The Conserved Open Space must be protected by a perpetual easement or deed restriction that assigns the responsible party to ensure that no future development, disturbance, or clearing may occur within the area. • The practice does not apply to jurisdictional wetlands that are sensitive to increased stormwater inflows. 5.2. Vegetated Filter Strips Vegetated Filter Strips are best suited to treat runoff from small segments of impervious cover (usually less than 5,000 sq ft) adjacent to road shoulders, small parking lots and rooftops. Vegetated Filter Strips may also be used as pretreatment for another stormwater practice such as a dry Swale, bioretention, or infiltration areas (see Figure 9.9). If sufficient pervious area is available at the site, larger areas of impervious cover can be treated by vegetated filter strips, using an engineered level spreader to recreate sheet flow. Conserved Open Space and Vegetated Filter Strips can be used in a variety of situations; however there are several constraints to their use: • Filter Slopes and Widths. Maximum slope for both Conserved Open Space and Vegetated Filter Strips is 6%, in order to maintain sheet flow through the practice. In addition, the overall contributing drainage area must likewise be relatively flat to ensure sheet flow draining into the filter. Where this is not possible, alternative measures, such as an engineered level spreader, can be used. Minimum widths (flow path) for Conserved Open Space and Vegetated Filter Strips are dependent on slope, as specified in Table 9.2. Appendix 8 - GSP Specifications Drainage Criteria Manual • Soils. Vegetated Filter Strips are appropriate for all soil types, except fill soils without amendment. The applicable runoff reduction rate, however, is dependent on the underlying Hydrologic Soil Groups (see Table 9.1) and whether soils receive compost amendments. • Contributing Flow Path to Filter. Vegetated Filter Strips are used to treat very small drainage areas of a few acres or less. The limiting design factor is the length of flow directed to the filter. As a rule, flow tends to concentrate after traveling 75 ft for impervious surfaces, and 150 ft for pervious surfaces (Claytor, 1996). When flow concentrates, it moves too rapidly to be effectively treated by a Vegetated Filter Strip, unless an engineered level spreader is used. When the existing flow at a site is concentrated, a water quality Swale shall be used instead of a Vegetated Filter Strip (Lantin and Barrett, 2005). • Potential Areas of High Pollutant Loading. Vegetated Filter Strips shall not receive untreated runoff from these areas since the infiltrated runoff could cause groundwater contamination. • Proximity of Underground Utilities. Underground pipes and conduits that cross the Vegetated Filter Strip are acceptable. Care shall be taken in backfilling of utilities to assure that the surface drainage pattern is not disturbed. SECTION 6: DESIGN CRITERIA 6.1. Compost Soil Amendments Compost soil amendment details are provided in Appendix C to this chapter. Such amendments enhance the runoff reduction capability of a vegetated filter strip when they are located on hydrologic soil groups (HSG) B, C, and D, subject to the following design requirements: • The compost amendments shall extend over the full length and width of the filter strip. • The amount of approved compost material and the depth to which it must be incorporated shall be consistent with Appendix C. • The amended area will be raked to achieve the most level slope possible without using heavy construction equipment, and it will be stabilized rapidly with perennial grass and/or herbaceous species. • If slopes exceed 3%, a protective biodegradable fabric or matting shall be installed to stabilize the site prior to runoff discharge. • Compost amendments shall not be incorporated until the gravel diaphragm and/or engineered level spreader are installed (see Section 6.3). • The City may waive the requirement for compost amendments on HSG -B soils in order to receive credit as a filter strip if (1) the designer can verify adequate soil type, texture, and infiltration characteristics exista filter strip, and (2) the area designated for the filter strip is not disturbed during construction. Appendix B - GSP Specifications Drainage Criteria Manual 6.2. Planting and Vegetation Management Conserved Open Space. No grading or clearing of native vegetation is allowed within the Conserved Open Space other than for vegetation management in accordance with Section 5.1 of this GSP. Reforested Conserved Open Space. At some sites, the Conserved Open Space may be in turf or meadow cover, or overrun with invasive plants and vines. In these situations, a landscape architect shall prepare a reforestation plan for the Conserved Open Space utilizing the reforestation specifications as described under GSP-10, Reforestation, with any credits and associated plans receiving approval by the City. Vegetated Filter Strips. Vegetated Filter Strips shall be planted at such density as to achieve a 90% grass/herbaceous cover after the second growing season. Filter strips shall be seeded, not sodded, whenever possible. Seeding establishes deeper roots, and sod may have muck soil that is not conducive to infiltration (Wisconsin DNR, 2007). The filter strip vegetation may consist of turf grasses, meadow grasses, other herbaceous plants, shrubs, and trees, as long as the primary goal of at least 90% coverage with grasses and/or other herbaceous plants is achieved. Designers shall choose vegetation that stabilizes the soil and is salt tolerant. Vegetation at the toe of the filter, where temporary ponding may occur behind the permeable berm, shall be specified that is able to withstand both wet and dry periods. The planting areas can be divided into zones to account for differences in inundation and slope. Planting plans shall be prepared by a licensed Landscape Architect. Planting lists are provided in Appendix D. 6.3. Diaphragms, Berms and Level Spreaders Gravel Diaphragms: A diaphragm consisting of pea gravel at the top of the slope is required for both Conserved Open Space and Vegetated Filter Strips that receive sheet flow. The pea gravel diaphragm is created by excavating a 2 -ft wide and 1 -ft deep trench of constant elevation at the top of the filter strip. The diaphragm serves two purposes, acting as a pretreatment device to settle out sediment particles before they reach the practice, and as a level spreader, maintaining sheet flow as runoff flows over the Filter Strip. Refer to Figure 9.6. • The flow shall be directed over the impervious area and to the practice as sheet flow and then drop 2 to 4 inches onto the gravel diaphragm. The drop helps to prevent runoff from running laterally along the pavement edge, where grit and debris tend to build up (thus allowing by-pass of the Filter Strip). • A layer of filter fabric shall be placed between the gravel and the underlying soil trench. If the contributing drainage area is steep (6% slope or greater), then larger stone (B -Stone) shall be used in the diaphragm. Permeable Berm: Vegetated Filter Strips shall be designed with a permeable berm at the toe of the Filter Strip to create a shallow ponding area for runoff. Runoff then will gradually flow through outlet pipes in the berm or through a permeable lens or filter that contains a perforated pipe within the berm. During larger storms, runoff may overtop the berm (Cappiella et al., 2006) and shall be designed to accommodate erosive velocities up to and including a 50 -yr design storm. The permeable berm shall have the following properties: • A wide and shallow trench, 6 to 12 inches deep, shall be excavated at the upstream toe of the berm, parallel with the contours. Appendix B - GSP Specifications Drainage Criteria Manual • Media for the berm may consist of 40% excavated soil (if non -friable), 40% sand, and 20% pea gravel. • The 6 to 12 inch tall berm shall be located down gradient of the excavated depression and shall have gentle side slopes to promote easy mowing (Cappiella et al., 2006). • Stone may be needed to armor the top of berm to handle extreme storm events (up to and including the 50 -yr design storm). • A permeable berm is not needed when vegetated filter strips are used as pretreatment to another stormwater practice treating the same runoff. Engineered Level Spreaders. The design of engineered level spreaders shall conform to the following design criteria based on recommendations of Hathaway and Hunt (2006) in order to ensure non-erosive sheet flow into the vegetated area. Figure 9.3 represents a configuration that includes a bypass structure that diverts the design storm to the level spreader, and bypasses the larger storm events around the Conserved Open Space or Vegetated Filter Strip through an improved channel. An alternative approach involves pipe or channels discharging at the landward edge of a floodplain. The entire flow is directed through a stilling basin energy dissipater and then a level spreader such that the entire design storm for the conveyance system (typically a 10 -year frequency storm) is discharged as sheet flow through the floodplain. Key design elements of the engineered level spreader, as provided in Figures 9.2 and 9.3, include the following: • High Flow Bypass provides safe passage for larger design storms through the filter strip. The bypass channel design shall accommodate all peak flows greater than the water quality design flow. • The forebay shall have a maximum depth of 3 ft and gradually transition to a depth of 1 ft at the level spreader lip (Figure 9.2). The forebay is sized such that the surface area is 0.2% of the contributing impervious area. (A forebay is not necessary if the concentrated flow is from the outlet of an extended detention basin or similar practice). • The length of the level spreader shall be determined by the type of filter area and the design flow: 0 13 ft of level spreader length per every 1 cubic ft per second (cfs) of inflow for discharges to a Vegetated Filter Strip or Conserved Open Space consisting of native grasses or thick ground cover; 0 40 ft of level spreader length per every 1 cfs of inflow when the spreader discharges to a Conserved Open Space consisting of forested or reforested area (Hathaway and Hunt, 2006). o Where the Conserved Open Space is a mix of native grasses / thick ground cover and forest (or re -forested), establish the level spreader length based on a weighted average of the lengths required for each vegetation type. o The minimum level spreader length is 13 ft and the maximum is 130 ft. Appendix B - GSP Specifications Drainage Criteria Manual o For the purposes of determining the Level Spreader length, the peak discharge shall be determined for the entire contributing area, using the Rational Equation with an intensity of 1- inch/hour, or a more detailed method in conformance with relevant portions of the Drainage Criteria Manual. • The level spreader lip shall be concrete, wood or pre -fabricated metal, with a well -anchored footer, or other accepted rigid, non -erodible material. • The ends of the level spreader section shall be tied back into the slope to avoid scouring around the ends of the level spreader; otherwise, short-circuiting of the facility could occur. • The width of the level spreader channel on the up -stream side of the level lip shall be three times the diameter of the inflow pipe, and the depth shall be 9 inches or one-half the culvert diameter, whichever is greater. • The level spreader shall be placed 3 to 6 inches above the downstream natural grade elevation to avoid turf buildup. In order to prevent grade drops that re -concentrate the flows, a 3 -ft long section of coarse aggregate, underlain by filter fabric, shall be installed just below the spreader to transition from the level spreader to natural grade. Vegetated receiving areas down -gradient from the level spreader must be able to withstand the force of the flow coming over the lip of the device. It may be necessary to stabilize this area with temporary or permanent materials in accordance with the calculated velocity (on-line system peak, or diverted off-line peak) and material performance data, along with seeding and stabilization in conformance with the requirements of the Fayetteville Drainage Criteria manual. 6.4. Filter Design Material Specifications Table 9.3 describes materials specifications for the primary treatment within filter strips. Appendix 8 - GSP Specifications Drainage Criteria Manual SECTION 7: CONSTRUCTION 7.1. Construction Sequence for Conserved Open Space Areas The Conserved Open Space must be fully protected during the construction stage of development and kept outside the limits of disturbance on the Erosion and Sediment Control Plan. • No clearing, grading or heavy equipment access is allowed except temporary disturbances associated with incidental utility construction, restoration operations or management of nuisance vegetation. • The perimeter of the Conserved Open Space shall be protected by a silt fence, chain link fence, orange safety fence, signage,and other measures as needed to meet stormwater pollution prevention sediment discharge requirements. • The limits of disturbance and extent of Conserved Areas shall be clearly shown on site development plans, Grading Permit applications and/or concept plans and identified and clearly marked in the field. • Construction of the gravel diaphragm or engineered level spreader shall not commence until the contributing drainage area has been stabilized and perimeter erosion and sedimentation controls have been removed and cleaned. Appendix 8 — GSP Specifications Drainage Criteria Manual Table 9.3. Vegetated Filter Strip Material Specifications. Material Specification Quantity Gravel Pea Gravel (#8 or ASTM equivalent); where Diaphragm shall be 2 ft wide 1 ft deep,and Diaphragm steeper (6% +) use B -stone or ASTM 2 to 4 inches below the edge of pavement. equivalent (1 -inch maximum). Permeable 40% excavated soil (non -friable), 40% sand, and 20% pea gravel to serve as the media for the Berm berm. Needled, non -woven, polypropylene geotextile meeting the following specifications: Grab Tensile Strength (ASTM D4-632): > 120 lbs. Geotextile Mullen Burst Strength (ASTM D-3786): > 225 lbs./sq. in. Flow Rate (ASTM D-4491): > 125 gpm/sq. ft. Apparent Opening Size (ASTM D-4751): US #70 or #80 sieve Engineered Level spreader lip shall be concrete, metal, timber, or other rigid material; Reinforced channel Level spreader on upstream of lip. See Hathaway and Hunt (2006) Erosion Control Where flow velocities dictate, use woven biodegradable erosion control fabric or mats that are Fabric/ Matting durable enough to last at least 2 growing seasons. If existing topsoil is inadequate to support dense turf growth, imported top soil (loamy sand or sandy loam texture), with 5% or lower clay content, corrected pH at 6 to 7, a soluble salt Topsoil content not exceeding 500 ppm, and an organic matter content of at least 2% shall be used. Topsoil shall be uniformly distributed and lightly compacted to a minimum depth of 6 to 8 inches. Compost Compost shall be derived from plant material and consistent with other requirements as outlined in the Appendix - Soil Amendment. SECTION 7: CONSTRUCTION 7.1. Construction Sequence for Conserved Open Space Areas The Conserved Open Space must be fully protected during the construction stage of development and kept outside the limits of disturbance on the Erosion and Sediment Control Plan. • No clearing, grading or heavy equipment access is allowed except temporary disturbances associated with incidental utility construction, restoration operations or management of nuisance vegetation. • The perimeter of the Conserved Open Space shall be protected by a silt fence, chain link fence, orange safety fence, signage,and other measures as needed to meet stormwater pollution prevention sediment discharge requirements. • The limits of disturbance and extent of Conserved Areas shall be clearly shown on site development plans, Grading Permit applications and/or concept plans and identified and clearly marked in the field. • Construction of the gravel diaphragm or engineered level spreader shall not commence until the contributing drainage area has been stabilized and perimeter erosion and sedimentation controls have been removed and cleaned. Appendix 8 — GSP Specifications Drainage Criteria Manual • Some light grading may be needed at the area boundary; this shall be done with tracked vehicles to minimize compaction. • Stormwater shall not be diverted into the area until the gravel diaphragm and/or level spreader are installed and stabilized. 7.2. Construction Sequence for Vegetated Filter Strips Vegetated Filter Strips can be within the limits of disturbance during construction. Soil amendments may be required in accordance with previous sections. The following procedures shall be followed during construction: • Before site work begins, Vegetated Filter Strip boundaries shall be clearly marked. • Only vehicular traffic used for Filter Strip construction shall be allowed within 10 ft of the Filter Strip boundary (City of Portland, 2004). • Existing topsoil shall not be stripped during grading; if it is, it shall be stockpiled for later use. • Construction runoff shall be directed away from the proposed Filter Strip site, using perimeter silt fence, or, preferably, a diversion dike. • Construction of the gravel diaphragm or engineered level spreader shall not commence until the contributing drainage area has been stabilized and perimeter erosion and sedimentation controls have been removed and cleaned out. • Vegetated Filter Strips may require light grading to achieve desired elevations and slopes. This shall be done with tracked vehicles to prevent compaction. Topsoil and or compost amendments shall be incorporated evenly across the filter strip area, stabilized with seed, and protected by biodegradable erosion control matting or blankets. • Stormwater shall not be diverted into the Filter Strip until the turf cover is dense and well established. 7.3. Construction Inspection Construction inspection is critical to ensure construction is consistent with design, to ensure the gravel diaphragm or engineered level spreader is constructed level and constructed to the correct design elevation. As -built certification is required to ensure compliance with design standards. Inspectors shall evaluate the performance of the Filter Strip after the first big storm to look for evidence of gullies, shortcircuiting„ undercutting or sparse vegetative cover. Repairs shall be made as needed. SECTION 8: AS -BUILT REQUIREMENTS After the filter strip has been constructed, the developer must provide the City an as -built certification of the filter strip prepared by a registered Professional Engineer. The as -built certification verifies that the BMP was installed as designed and approved or documents exceptions and verifies functionality. The following components must be addressed in the as -built certification: Appendix 8 - GSP Specifications Drainage Criteria Manual 1. Ensure level spreader is properly installed to create sheet flow. 2. Ensure vegetated filter strip or open space that receives sheet flow has minimal slope. 3. Ensure appropriate drainage. 4. Ensure the proper vegetation has been established or protected - this may be established by a qualified Landscape Architect. 5. If using amended soils, verify the depth of mulch, amended soil and scarification meets requirements provided in Appendix C. SECTION 9: MAINTENANCE 9.1. Maintenance Requirements A Long Term Maintenance Plan (LTMP) for the Sheet Flow GSP shall be prepared by the design engineer to address inspection and maintenance. If the filter area is a natural Conserved Open Space, it must be protected by a perpetual easement or deed restriction that assigns a responsible party to ensure that no future development, disturbance or clearing may occur within the area, except as stipulated in the long-term vegetation management plan. 9.2. Maintenance Inspections Annual inspections are used to trigger maintenance operations such as sediment removal, re -vegetation and level spreader repair. Ideally, inspections shall be conducted in the non -growing season when it easier to see the flow path. Inspectors shall check that: • Flows through the Filter Strip do not short-circuit the overflow control section; • Debris and sediment does not build up at the top of the Filter Strip; • The gravel diaphragm performance is not compromised; • Erosion does not occur within the Filter Strip; • Trash and sediments are adequately removed from Level Spreader forebays and flow splitters; and • Vegetative density exceeds a 90% cover in the boundary zone or grass filter. 9.3. Ongoing Maintenance Once established, Vegetated Filter Strips have minimal maintenance needs outside of the spring clean up, regular mowing, repair of check dams and other measures to maintain the hydraulic efficiency of the strip and a dense, healthy grass cover. Vegetated Filter Strips that consist of grass/turf cover shall be mowed at least twice a year to prevent woody growth. Appendix 8 - GSP Specifications Drainage Criteria Manual Filter strip surrounding bioretention cell, Fort Bragg, NC. (Source: N. Weinstein, LIDC). SECTION 10: REFERENCES Cappiella, K., T. Schueler, and T. Wright, 2006. Urban Watershed Forestry Manual, Part 2. Conserving and Planting Trees at Development Sites. Center for Watershed Protection. Prepared for United States Department of Agriculture, Forest Service. City of Philadelphia, 2011 revision. Philadelphia Stormwater Management Guidance Manual. City of Portland, Environmental Services, 2004. Portland Stormwater Management Manual. Portland, OR. Available online at: http://www.portlandonline.com/bes/index.cfm?c=dfbbh Claytor, R. and T. Schueler, 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD. CWP, 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD. Hathaway, J. and B. Hunt, 2006. Level Spreaders: Overview, Design, and Maintenance. Department of Biological and Agricultural Engineering. NC State University. Raleigh, NC. Henrico County, Virginia, Henrico County Environmental Program Manual. North Carolina Department of Environment and Natural Resources, Division of Water Quality, 2007. Level Spreader Design Guidelines. Appendix B - GSP Specifications Drainage Criteria Manual North Carolina State University, Level Spreader Design Worksheet. Northern Virginia Regional Commission, 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook, Fairfax, Virginia. Schueler, T., D. Hirschman, M. Novotney and J. Zielinski, 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD. Schueler, T., 2008. Technical Support for the Baywide Runoff Reduction Method, Chesapeake Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net VADCR (Virginia Department of Conservation and Recreation), 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2, Richmond, VA. VADCR, 2011. Stormwater Design Specification No. 2: Sheet Flow To a Vegetated Filter Strip or Conserved Open Space, version 1.9, Richmond, VA. Appendix 8 - GSP Specifications Drainage Criteria Manual REFORESTATION Description: Reforestation refers to trees planted in groups in urban areas such as: parking lots, right of ways (ROW), parks, schools, public lands, vacant land, and neighborhood open spaces, to provide shade and stormwater retention and to add aesthetic value. Rv = Volumetric Runoff Coefficient. • Reduces effective impervious cover • Reduces stormwater runoff • Provides aesthetic value • Provides rainfall interception • Shade provides cooling and energy savings • Provides habitat • Provides pollutant removal • Provides flow attenuation Runoff Reduction Credit: • Rv for reforestation* is twice the forest Rv factor for the corresponding soil type. • If amended soils are used in conjunction with reforestation the Rv for reforestation* is equal to the forest Rv factor for the corresponding soil type. *This GSP is subject to Engineering Staff approval and shall be in accordance with the City of Fayetteville Tree and Landscape Manual and other relevant ordinances. Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Trees may require irrigation in dry periods. ©Maintenance Burden L = Low M = Moderate H = High • Poor quality urban soils may require soil amendments or remediation • Long-term maintenance is required • Must be implemented over large areas to provide significant reduction in stormwater runoff • Time required for trees to mature • Inadequate space may be a limitation in urban areas • Poor soils, improper planting methods, conflicts with paved areas and utilities, effects of road salt, lack of water, or disease can cause mortality Appendix B — GSP Specifications Drainage Criteria Manual i , • Trees may require irrigation in dry periods • All stormwater trees planted to obtain credit for runoff reduction shall be located and installed in accordance with the City of Fayetteville Tree and Landscape Technical Manual. • Stormwater trees are limited to areas where there is sufficient room to plant the tree, allow it to grow, and provide sufficient surface run-on at the base, as well as to provide space for pedestrians, street parking, utilities, and adequate distance from structures. • Planting groups of stormwater trees may be more feasible on public land, such as schools and parks, due to ownership and available space. This will be subject to City approval. • Stormwater trees provide the most benefit for locations with high annual rainfall and land uses with low impervious cover, and are most effective at reducing peak runoff rates in land uses with large amounts of open space. • Tree species with desirable stormwater control characteristics should be utilized. For trees receiving runoff, tree species must have a high tolerance for common urban pollutants. In cold climates, salt tolerance is particularly important. • Soils and mulch play a significant role in pollutant removal and tree health. These should be selected to allow water to infiltrate into the soil. • For individual trees, mulch shall be used as an added filtration mechanism. • Runoff Reduction Credit is subject to approval by City Engineer. SECTION 1: DESCRIPTION Site reforestation involves planting trees on existing turf or barren ground at a development site with the explicit goal of establishing a mature forest canopy that will intercept rainfall, increase evapotranspiration rates, and enhance soil infiltration rates. SECTION 2: DESIGN CRITERIA The overall runoff reduction credits for reforestation through lower runoff coefficients are summarized in Table 10.1. Table 10.1. Reforestation Runoff Coefficient Credit. Level 1 Design Level 2 Design Equal to the forest RV factor if amended soils are used in Twice the forest RV factor for the corresponding soil type. conjunction. A B C D A B C D 0.04 0.06 0.08 0.10 0.02 0.03 0.04 0.05 Reforestation areas at larger development sites are eligible under the following qualifying conditions: • The reforestation area must comply with the requirements of the City of Fayetteville Tree and Landscape Technical Manual and relevant ordinances. Appendix 8 — GSP Specifications Drainage Criteria Manual i , The minimum contiguous area of reforestation must be greater than 5,000 sq ft, with no more than 20% of any single tree species. The basic density of plantings is 200 large canopy trees per acre, approximately 15 ft on center. When shrubs are substituted for trees, there must be 10 shrubs per one large canopy tree. Two small canopy trees, such as Dogwoods or Red Buds, may be substituted for one large canopy tree. Adjustments can be made to these densities for areas of urban reforestation with the approval of the City of Fayetteville Urban Forester. Reforestation design shall consider the composition of area forests, and two thirds of selected trees must be large canopy. Reforestation methods should achieve 75% forest canopy within ten years. • The minimum size requirement for reforestation trees is 1"-2" caliper trees approximately 6-8 ft in height. The minimum size requirement for shrubs is 18-24 inches, or 3 gallon size. In addition, the entire reforestation area shall be covered with 2-4 inches of organic mulch or with a native seed mix in order to help retain moisture and provide a beneficial environment for the reforestation. • A long term vegetation management plan must be prepared and filed with the City of Fayetteville Urban Forester in order to maintain the reforestation area in a natural forest condition. The plan shall include a scale drawing showing the area to be planted, along with a plant list which includes species, size, number, and packaging. Plant lists are provided in Appendix D. In addition, the reforestation area shall be clearly identified on all construction drawings. • The reforestation area must be protected by a perpetual stormwater easement or deed restriction which stipulates that no future development or disturbance may occur within the area. • The planting plan must be approved by City of Fayetteville Urban Forester including any special site preparation needs. • The owner shall provide a care and replacement warranty extending at least three growing seasons, to ensure adequate growth and survival of the plant community. • The final size of the trees shall be considered when designing the planting plan. Arkansas One -Call (811) must be contacted prior to the submission of the planting plan to ensure that no utilities will be impacted by the tree planting. The planting plan must also avoid placing trees under overhead utilities. Perpetual easements required shall be separate from utility easements. SECTION 3: DESIGN CONSIDERATIONS Trees are often one of the most economical stormwater BMPs that can be introduced into urban ROWS. Tree canopies intercept rainfall before it becomes stormwater and the tree boxes into which trees are planted can be used to capture and treat runoff. Refer to the GSP-02 Urban Bioretention specification section for additional information about tree box planters. Trees also reduce the urban heat island effect, improve the urban aesthetic and improve air quality. Tree plantings within the ROW must receive approval from the City of Fayetteville Urban Forester. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 8' WIDE SIDEWALK r- r PLAN i 8 Ft. x 15 Ft. AREA OF STRUCTURAL SOIL (TYP.) NEENAH FOUNDRY COMPANY 50" METROPOLITAN SQUARE MODEL No. R-8707 MATERIAL: CAST GRAY IRON INSTALL PER MANUFACTURERS DETAILS AND SPECIFICATIONS DRAINAGE TO STORM SEWER VIA PERFORATED PIPE; WRAP IN FILTER FABRIC SECTION FOR THIS PROJECTr EACH TREE SHALL RECEIVE A MINIMUM AREA OF 8' x 15' OF CU STRUCTURAL SOIL INSTALLED PER THIS DETAIL AND DEPTH SHOWN: (3 FT. TYPICAL) CU STRUCTURAL SOIL PER AMEREQ, INC. (1-800-832-8788) SEE TREE PLANTING DETAIL DO NOT PILE MULCH rv,,. AGAINST TREE TRUNK 5'-0" x 5'-0" SQUARE OPENING LCH � 3' -6' SIDEWALK ?�Z. \- CU STRUCTURAL SU6GRADE SOIL 36" DEPTH RECOMMENDED Figure 10.1. Urban Tree Wells with Grate using Structural Soil in City of Fayetteville Tree and Landscape Manual. Appendix 8 - GSP Specifications Drainage Criteria Manual I , GSP-10: Reforestation GSPIO-S SECTION 4: REFERENCES Balusek, 2003. Quantifying decreases in stormwater runoff from deep -tilling, chisel -planting and compost amendments. Dane County Land Conservation Department. Madison, Wisconsin. Chollak, T. and P. Rosenfeld, 1998. Guidelines for Landscaping with Compost -Amended Soils. City of Redmond Public Works. Redmond, WA. City of Chesapeake, 2010. Chesapeake Landscape Specifications Manual: Tree and Shrub Planting Guidelines. Approved on October 16, 2008 and amended effective August 1, 2010.. City of Fayetteville, AR, 2010. Tree and Landscape Technical Manual, Fayetteville, AR. City of Portland, 2008. Soil Specification for Vegetated Stormwater Facilities, Portland Stormwater Management Manual. Portland, Oregon. Virginia Department of Conservation and Recreation, 2010. Design Specification No. 4: Soil Compost Amendment Version 1. 7, Appendix 4-A, Initial Minimum Design Criteria for Reforestation, Disconnection, Filter Strips, and Grass Channels, Richmond, VA. Appendix 8 - GSP Specifications Drainage Criteria Manual i , RAIN TANKS / CISTERNS Description: Rain tanks and cisterns are structures that store rooftop runoff and reuse it for future use. Rain Barrels do not qualify for Credit. Variations: • Aboveground Storage • Underground Storage • Roof surface • Collection and conveyance system • Pre-screening and first flush diverter • Storage tank • Distribution system • Overflow, filter path or secondary runoff reduction practice Selection Criteria: Up to 90% Runoff Reduction Credit *Credit is variable. Credit up to 90% is possible if all water from storms with rainfall of 1 inch or less is used through demand, and the tank is sized such that no overflow from this size event occurs. The total credit may not exceed 90%. Land Use Considerations: Residential © Commercial © Industrial Maintenance: • Gutters and downspouts should be kept clean and free of debris and rust. • Annual inspection Maintenance Burden © L = Low M = Moderate H = High • Water source for non -potable uses (toilet flushing, • Provides mosquito -breeding habitat unless properly irrigation) sealed Appendix B — GSP Specifications Drainage Criteria Manual i , • Underground storage tanks must be above groundwater level • Certain roof materials may leach metals or hydrocarbons, limiting potential uses for harvested rainwater • Underground tanks shall be set at least 10 ft from building foundations • Cistern overflows shall be designed to avoid soil saturation within 10 ft of building foundations • Systems must be designed for consistent drawdown year-round • Aboveground storage tanks shall be UV resistant and opaque to inhibit algae growth • Underground storage tanks must be designed to support anticipated loads • Hookups to municipal backup water supplies must be equipped with backflow prevention devices Appendix 8 - GSP Specifications Drainage Criteria Manual i , Roof Surface • The rooftop shall be made of smooth, non -porous material with efficient drainage either from a sloped roof or an efficient roof drain system. Collection and Conveyance System Gutters and downspouts shall be designed as they would be for a building without a rainwater harvesting system. Gutters shall be sized with slopes specified to contain the necessary amount of stormwater for treatment volume credit. • Pipes (connecting downspouts to the cistern tank) shall be at a minimum slope of 1.5% and sized/designed to convey the intended design storm. Pre -Screening and First Flush Diverter Inflow must be pre-screened to remove leaves, sediment, and other debris For large systems, the first flush (0.02 — 0.06 in.) of rooftop runoff shall be diverted to a secondary treatment practice to prevent sediment from entering the system Rooftop runoff shall be filtered to remove sediment before it is stored Storage Tank • Storage tanks to be sized based on consideration of indoor and outdoor water demand, long-term rainfall and rooftop capture area Distribution System The rainwater harvesting system shall be equipped with an appropriately -sized pump that produces sufficient pressure for all end -uses. Distribution lines shall be installed with shutoff valves and cleanouts, and shall be buried beneath the frost line or insulated to prevent freezing Overflow • The system must be designed with an overflow mechanism to divert runoff when the storage tanks are full • Overflows shall discharge to pervious areas set back from buildings and paved surfaces, or to secondary BMPs. SECTION 1: DESCRIPTION A cistern intercepts, diverts, stores and releases rainfall for future use. The term cistern is used in this specification, but it is also known as a rainwater harvesting system. Rainwater that falls on a rooftop is collected and conveyed into an above- or below -ground storage tank where it can be used for non -potable water uses and on-site stormwater disposal/infiltration. Non -potable uses may include flushing of toilets and urinals inside buildings, landscape irrigation, exterior washing (e.g., car washes, building facades, sidewalks, street sweepers, fire trucks, etc.), supply for chilled water cooling towers, replenishing and operation of laundry. All water reuse systems within the building shall be designed in accordance with Arkansas Plumbing Code and all applicable requirements of the Board of Health. In many instances, rainwater harvesting can be combined with a secondary (down -gradient) runoff reduction practice to enhance runoff volume reduction rates and/or provide treatment of overflow from the rainwater harvesting system. Some candidate secondary practices include: • Downspout Disconnection: GSP-07 (excluding rain tanks and cisterns). This may include release to a compost -amended filter path • Sheet Flow to a Vegetated Filter Strip or Conserved Open Space: GSP-09 • Grass Channel: GSP-08 • Infiltration Trench: GSP-04 • Bioretention: GSP-01 • Urban Bioretention: GSP-02 Storage and release in foundation planters. • Water Quality Swale: GSP-05 Section 5.3 (Physical Feasibility & Design Applications) provides more detail on system configurations, including the use of secondary practices. In addition, the actual runoff reduction rates for rainwater harvesting systems are "user defined," based on tank size, configuration, demand drawdown, and use of secondary practices. SECTION 2: PERFORMANCE The overall stormwater functions of the rainwater harvesting systems are described in Table 11.1. Table 11.1. Annual Runoff Volume Reduction Provided . Stormwater Function Performance Annual Runoff Volume Reduction (RR) Variable up to 90%1 \.reUIL IS Varla Ule. ldeUIL Uhl LU 7U70 IS PUSSIUle II all water HUM SLUHT]b WILH ralrllall UI 1 IrlCrl Ur ICSS 15 USeU LHWUgH Uerrlarl U, anU Ule lanK is sized such that no overflow from this size event occurs. The total credit may not exceed 90%. Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 3: DESIGN Rainwater harvesting system design does not have a design table. Runoff reduction credits are based on the total amount of annual internal water reuse, outdoor water reuse, and tank dewatering discharge calculated to be achieved by the tank system. SECTION 4: TYPICAL DETAILS Figures 11.1 through 11.3 of Section 5.3 provide typical schematics of cistern and piping system configurations, based on the design objectives (year-round internal use, external seasonal irrigation, etc.). Figures 11.4 through 11.6 of Section 5.4 provide typical schematics of Cistern tank configurations, based on the desired Treatment Volume and stormwater management objectives (Treatment Volume only, channel protection, etc.). SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS A number of site-specific features influence how rainwater harvesting systems are designed and/or utilized. These should not be considered comprehensive and conclusive considerations, but rather some recommendations that should be considered during the process of planning to incorporate rainwater harvesting systems into the site design. The following are key considerations. 5.1. Site Conditions Available Space. Adequate space is needed to house the tank and any overflow. Space limitations are rarely a concern with rainwater harvesting systems if they are considered during the initial building design and site layout of a residential or commercial development. Storage tanks can be placed underground, indoors, on rooftops or within buildings that are structurally designed to support the added weight, and adjacent to buildings. Designers can work with Architects and Landscape Architects to creatively site the tanks. Underground utilities or other obstructions shall always be identified prior to final determination of the tank location. Site Topography. Site topography and tank location shall be considered as they relate to all of the inlet and outlet invert elevations in the rainwater harvesting system. The total elevation drop will be realized beginning from the downspout leaders to the final mechanism receiving gravity -fed discharge and/or overflow from the cistern. These elevation drops will occur along the sloping lengths of the underground roof drains from roof drain leader downspouts at the building all the way to the cistern. A vertical drop occurs within the filter before the cistern. The cistern itself must be located sufficiently below grade and below the frost line, resulting in an additional elevation drop. When the cistern is used for additional volume detention for channel and/or flood protection, an orifice may be included with a low invert specified by the designer. An overflow outlet will always be present within the system, with an associated invert. Both the orifice (if specified) and the overflow will drain the tank during large storms, routing this water through an outlet pipe, the length and slope of which will vary from one site to another. Appendix 8 - GSP Specifications Drainage Criteria Manual i , All these components of the rainwater harvesting system have an elevation drop associated with them. The final invert of the outlet pipe must match the invert of the receiving system (natural channel, storm drain, etc.) that receives this overflow. These elevation drops and associated inverts shall be considered early in the design, in order to ensure that the rainwater harvesting system is feasible for the particular site. Site topography and tank location will also affect the amount of pumping needed. Locating storage tanks in low areas will make it easier to route roof drains from buildings to cisterns. However, it will increase the amount of pumping needed to distribute the harvested rainwater back into the building or to irrigated areas situated on higher ground. Conversely, placing storage tanks at higher elevations may require larger diameter roof drains with smaller slopes. However, this will also reduce the amount of pumping needed for distribution. In general, it is often best to locate the cistern close to the building, ensuring that minimum roof drain slopes and enclosure of roof drain pipes are sufficient. Available Hydraulic Head. The required hydraulic head depends on the intended use of the water. For residential landscaping uses, the cistern shall be sited up -gradient of the landscaping areas or on a raised stand. Pumps are commonly used to convey stored rainwater to the end use in order to provide the required head. When the water is being routed from the cistern to the inside of a building for non -potable use, often a pump is used to feed a much smaller pressure tank inside the building which then serves the internal demands through gravity -fed head. Cisterns can also use gravity- to accomplish indoor residential uses (e.g., laundry) that do not require high water pressure. In cases where cisterns are located on building roofs in order to operate under gravity -fed conditions, the structure must be designed to provide for the added weight of the rainwater harvesting system and stored water. Water Table. Underground storage tanks are most appropriate in areas where the tank can be buried above the water table. The tank shall be located in a manner that will not subject it to flooding. In areas where the tank is to be buried partially below the water table, buoyancy shall be computed on completely empty basis. The tank may need to be secured appropriately with fasteners or weighted to avoid uplift buoyancy (floating). The tank must also be installed according to the tank manufacturer's specifications. Soils. Storage tanks shall only be placed on native soils or on fill in accordance with the manufacturer's guidelines, and, as appropriate, in consultation with a geotechnical engineer.. Subgrade preparation consisting of SB -2 or a concrete seal slab may be required depending on the soils. Cistern supplies may also need a pH adjustment, since rainwater may corrode components. Proximity of Underground Utilities. All underground utilities must be taken into consideration during the design of underground rainwater harvesting systems. The rainwater harvesting system components and storm drains should be treated as typical stormwater facilities and pipes. The underground utilities must be marked and avoided during the installation of underground tanks and piping associated with the system. Appropriate minimum setbacks from septic drainfields shall be observed. Before digging, call Arkansas One Call (811) to get underground utility lines marked. Contributing Drainage Area. The contributing drainage area (CDA) to the cistern is the impervious area draining to the tank. In general, only rooftop surfaces should be included in the CDA. Areas of any size, including portions of roofs, can be used based on the sizing guidelines in this design specification. Runoff should be routed directly from rooftops to rainwater harvesting systems in closed roof drain systems or storm drain pipes, avoiding surface drainage, which could allow for increased contamination of the water. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Rooftop Material. The quality of the harvested rainwater will vary according to the roof material over which it flows. Water harvested from certain types of rooftops, such as asphalt sealcoats, tar and gravel, painted roofs, galvanized metal roofs, sheet metal or any material that may contain asbestos may leach trace metals and other toxic compounds. In general, harvesting rainwater from such roofs shall be avoided, unless new information determines that these materials are sufficient for the intended use and are allowed by the City Engineer. If a sealant or paint roof surface is desired, it is recommended to use one that has been certified for such purposes by the National Sanitation Foundation (ANSI/NSF standard). Water Quality of Rainwater. Designers should note that the pH of rainfall in the Arkansas tends to be acidic (as low as 5.0), which could result in leaching of metals from the roof surface, tank lining or water laterals to interior connections. Once rainfall runs off rooftop surfaces, pH levels tend to be slightly higher, ranging from 5.5 to 6.0. Limestone or other materials may be added in the tank to buffer acidity, if required. Land Uses with Potential for High Pollutant Loading. Harvesting rainwater can be an effective method to prevent contamination of rooftop runoff that would result from mixing it with ground -level runoff from an area with potential for high pollutant loading. In some cases, however, industrial roof surfaces may also be contaminated and be designated as an area with high pollutant loading. This would exclude harvesting rainwater as a stormwater management option from that particular rooftop. Setbacks from Buildings. Cistern overflow devices shall be designed to avoid causing ponding or soil saturation within 10 ft of building foundations. Storage tanks shall be designed to be watertight to prevent water damage when placed near building foundations. In general, it is recommended that underground tanks be set at least 10 ft from any building foundation. Vehicle Loading. Whenever possible, underground rainwater harvesting systems should be placed in areas without vehicle traffic or be designed to support live loads from heavy trucks, a requirement that may significantly increase construction costs. 5.2. Stormwater Uses The capture and reuse of rainwater can significantly reduce stormwater runoff volumes and pollutant loads. By providing a reliable and renewable source of water to end users, rainwater harvesting systems can also have environmental and economic benefits beyond stormwater management (e.g., increased water conservation, water supply during drought and mandatory municipal water supply restrictions, decreased demand on municipal or groundwater supply, decreased water costs for the end-user, potential for increased groundwater recharge, etc). To enhance their runoff reduction and nutrient removal capability, rainwater harvesting systems can be combined with other rooftop disconnection practices, such as infiltration trenches (GSP-04) and bioretention or foundation planters (GSP-01 and GSP-02). In this specification, these allied practices are referred to as "secondary runoff reduction practices." While the most common uses of captured rainwater are for non -potable purposes, such as those noted above, in some limited cases rainwater can be treated to potable standards. This is not permitted in Fayetteville at this time. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 5.3. Design Objectives and System Configurations Many rainwater harvesting system variations can be designed to meet user demand and stormwater objectives. This specification focuses on providing a design framework for addressing the Treatment Volume (Tv) objectives. From a rainwater harvesting standpoint, there are numerous potential configurations that could be implemented. However, in terms of the goal of addressing the design treatment volume, this specification adheres to the following concepts in order to properly meet the stormwater volume reduction goals: • Credit is only available for dedicated year-round drawdown/demand for the water. While seasonal practices (such as irrigation) may be incorporated into the site design, they are not considered to contribute to the treatment volume credit (for stormwater purposes) unless a drawdown at an equal or greater rate is also realized during non -seasonal periods (e.g., treatment in a secondary runoff reduction practice during non -irrigation months). • System design is encouraged to use rainwater as a resource to meet on-site demand or in conjunction with other runoff reduction practices (especially those that promote groundwater recharge). • Pollutant load reduction is realized through reduction of the volume of runoff leaving the site. • Peak flow reduction is realized through reduced volume and temporary storage of runoff. Therefore, the rainwater harvesting system design configurations presented in this specification are targeted for continuous (year-round) use of rainwater through (1) internal use, and (2) irrigation and/or treatment in a secondary practice. Three basic system configurations are described below. Configuration 1: Year-round indoor use with optional seasonal outdoor use (Figure 11.1). The first configuration is for year round indoor use along with optional seasonal outdoor use, such as irrigation. Because there is no on-site secondary runoff reduction practice incorporated into the design for non- seasonal (or non -irrigation) months, the system must be designed and treatment volume awarded for the interior use only. (However, it should be noted that the seasonal irrigation will provide an economic benefit in terms of water usage). Stormwater credit can be enhanced by adding a secondary runoff reduction practice (see Configuration 3 below). Configuration 2: Seasonal outdoor use and approved year-round secondary runoff reduction practice (Figure 11.2). The second configuration uses stored rainwater to meet a seasonal or intermittent water use, such as irrigation. However, because these uses are only intermittent or seasonal, this configuration also relies on an approved secondary practice for stormwater credit. Compared to a stand-alone BMP (without the upgradient tank), the size and/or storage volume of the secondary practice can be reduced based on the storage in the tank. The tank's drawdown and release rate shall be designed based on the infiltration properties, surface area, and capacity of the receiving secondary runoff reduction practice. The release rate therefore is typically much less than the flow rate that would result from routing a detention facility. The secondary practice shall serve as a "backup" facility, especially during non -irrigation months. In this regard, the tank provides some meaningful level of storage and reuse, accompanied by a small flow to the secondary practice. This is especially important if the size and/or storage volume of the secondary practice is reduced compared to using that practice in a "stand-alone" design (i.e., without an upgradient cistern). See Section 5.4 Tank Design 3 for more information. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Configuration 3: Year-round indoor use, seasonal outdoor irrigation, and non -seasonal treatment in a secondary runoff reduction practice (Figure 11.3). The third configuration provides for a year-round internal non -potable water demand, and a seasonal outdoor, automated irrigation system demand. In addition, this configuration incorporates a secondary practice during non -irrigation (or non -seasonal) months in order to yield a greater stormwater credit. In this case, the drawdown due to seasonal irrigation must be compared to the drawdown due to water released to the secondary practice. The minimum of these two values is used for system modeling and stormwater credit purposes. I Figure 11.1. Configuration 1: Year-round indoor use with optional seasonal outdoor use (Source: VADCR, 2011). Appendix 8 — GSP Specifications Drainage Criteria Manual i , Figure 11.2. Configuration 2: Seasonal outdoor use and approved year-round secondary practice (Source: VADCR, 2011). Figure 11.3. Configuration 3: Year-round indoor use, seasonal outdoor irrigation, and non -seasonal on-site treatment in secondary practice (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual i , 5.4. Design Objectives and Tank Design Set -Ups Pre -fabricated rainwater harvesting cisterns typically range in size from 250 to over 30,000 gallons. There are three basic tank design configurations used to meet the various rainwater harvesting system configurations that are described in Section 5.3. Tank Design 1. The first tank set-up (Figure 11.4) maximizes the available storage volume associated with the Treatment Volume (Tv) to meet the desired level of treatment credit. This layout also maximizes the storage that can be used to meet a demand. An emergency overflow exists near the top of the tank as the only gravity release outlet device (not including the pump, manway or inlets). It should be noted that it is possible to address channel and flood protection volumes with this tank configuration, but the primary purpose is to address Tv. Figure 11.4. Tank Design 1: Storage Associated with Treatment Volume (Tv) only (Source: VADCR, 2011). Tank Design 2. The second tank set-up (Figure 11.5) uses tank storage to meet the Treatment Volume (Tv) objectives as well as using an additional detention volume above the treatment volume space to also meet some or all of the channel and/or flood protection volume requirements. An orifice outlet is provided at the top of the design storage for the Tv storage level, and an emergency overflow is located at the top of the detention volume level. This specification only addresses the storage for the Tv. However, it may possible to model and size the Channel Protection and Flood Protection (detention) volumes. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Figure 11.5. Tank Design 2: Storage Associated with Treatment, Channel Protection and Flood Volume (Source: VADCR, 2011). Tank Design 3. The third tank set-up (Figure 11.6) creates a constant drawdown within the system. The small orifice at the bottom of the tank needs to be routed to an appropriately designed secondary practice (e.g., rain garden, micro -scale infiltration, urban bioretention, etc.) that will allow the rainwater to be treated and allow for groundwater recharge over time. The release shall not be discharged to a receiving channel or storm drain without treatment, and maximum specified drawdown rates from this constant drawdown shall be adhered to, since the primary function of the system is not intended to be detention. For the purposes of this tank design, the secondary practice must be considered a component of the rainwater harvesting system with regard to the runoff reduction percentage. In other words, the runoff reduction associated with the secondary practice must not be added (or double -counted) to the rainwater harvesting percentage. The reason for this is that the secondary practice is an integral part of a rainwater harvesting system with a constant drawdown. The exception to this would be if the secondary practice were also sized to capture and treat impervious area beyond the area treated by rainwater harvesting (for instance, the adjacent yard or a driveway). In this case, only these additional areas should be added to receive credit for the secondary practice. While a small orifice is shown at the bottom of the tank in Figure 11. 6, the orifice could be replaced with a pump that would serve the same purpose, conveying a limited amount of water to a secondary practice on a routine basis. Appendix 8 — GSP Specifications Drainage Criteria Manual i , "how VOPAG[ A96p{WW WRHOW99L nancTwfr ANP"ODYT.IE,UME — OIPFICE (WIFT STOWEASSOCIAM WRHTREATMWTVOLVW Figure 11.5. Tank Design 2: Storage Associated with Treatment, Channel Protection and Flood Volume (Source: VADCR, 2011). Tank Design 3. The third tank set-up (Figure 11.6) creates a constant drawdown within the system. The small orifice at the bottom of the tank needs to be routed to an appropriately designed secondary practice (e.g., rain garden, micro -scale infiltration, urban bioretention, etc.) that will allow the rainwater to be treated and allow for groundwater recharge over time. The release shall not be discharged to a receiving channel or storm drain without treatment, and maximum specified drawdown rates from this constant drawdown shall be adhered to, since the primary function of the system is not intended to be detention. For the purposes of this tank design, the secondary practice must be considered a component of the rainwater harvesting system with regard to the runoff reduction percentage. In other words, the runoff reduction associated with the secondary practice must not be added (or double -counted) to the rainwater harvesting percentage. The reason for this is that the secondary practice is an integral part of a rainwater harvesting system with a constant drawdown. The exception to this would be if the secondary practice were also sized to capture and treat impervious area beyond the area treated by rainwater harvesting (for instance, the adjacent yard or a driveway). In this case, only these additional areas should be added to receive credit for the secondary practice. While a small orifice is shown at the bottom of the tank in Figure 11. 6, the orifice could be replaced with a pump that would serve the same purpose, conveying a limited amount of water to a secondary practice on a routine basis. Appendix 8 — GSP Specifications Drainage Criteria Manual i , OVERFLOW STORAGE ASSOMM W11H CHANNEL PROWrION AND FLOOPYOLWNE OMME OUTLET 4TORAGEASSWMIED Wml7REAnYlEli7Y0tUAlE Figure 11.6. Tank Design 3: Constant drawdown, Storage Associated with Treatment, Channel Protection and Flood Volume (Source: VADCR, 2011). 5.5. On -Site Treatment in a Secondary Practice Recent rainwater harvesting system design materials do not include guidance for on-site stormwater infiltration or "disposal". The basic approach is to provide a dedicated secondary runoff reduction practice on-site that will ensure water within the tank will gradually drawdown at a specified design rate between storm events. Secondary runoff reduction practices may include the following: • Downspout Disconnection (GSP-07), excluding rain tanks and cisterns. This may include release to a compost -amended filter path. • Sheet Flow (GSP-09) • Grass Channel (GSP-08) • Infiltration (GSP-04) • Bioretention (GSP-01) • Stormwater planter (GSP-02) • Water Quality Swale (GSP-05) The secondary practice approach is useful to help achieve the desired treatment volume when demand is not enough to sufficiently draw water levels in the tank down between storm events. Of course, if demand for the harvested rainwater is relatively high, then a secondary practice may not be needed or desired. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Use of a secondary practice may be particularly beneficial to employ in sites that use captured rainwater for irrigation during part of the year, but have no other use for the water during non -irrigation months. During non -irrigation months, credit cannot be realized unless on-site infiltration/treatment or another drawdown mechanism creates a year-round drawdown, since no stormwater benefit would be realized during non- seasonal periods. The design of the secondary practice shall account for soil types, ground surface areas, release rates, methods of conveyance (gravity fed or pumped), time periods of operation, and invert elevations to determine the disposal rate and sizing of the practice (both storage volume and surface area). 5.6. System Components There are six primary components of a rainwater harvesting system (Figure 11.7): • Roof surface • Collection and conveyance system (e.g., gutter and downspouts) • Pre-screening and first flush diverter • Storage tank • Distribution system • Overflow, filter path or secondary runoff reduction practice Appendix 8 - GSP Specifications Drainage Criteria Manual i , Grade .. Each of these system components is discussed below. Rooftop Surface. The rooftop shall be made of smooth, non -porous material with efficient drainage either from a sloped roof or an efficient roof drain system. Slow drainage of the roof leads to poor rinsing and a prolonged first flush, which can decrease water quality. If the harvested rainwater is selected for uses with significant human exposure (e.g., pool filling, watering vegetable gardens), care should be taken in the choice of roof materials. Some materials may leach toxic chemicals making the water unsafe for humans. Collection and Conveyance System. The collection and conveyance system consists of the gutters, downspouts and pipes that channel stormwater runoff into storage tanks. Gutters and downspouts should be designed as they would for a building without a rainwater harvesting system. Aluminum, round -bottom gutters and round downspouts are generally recommended for rainwater harvesting. Minimum slopes of gutters shall be specified. At a minimum, gutters shall be sized with slopes specified to contain the 1 -inch storm at a rate of 1-inch/hour for treatment volume credit. If volume credit will also be sought for channel and flood protection, the gutters shall be designed to convey the 2 and 10 -year storm, using the appropriate 2 and 10 year storm intensities, specifying size and minimum slope. In all cases, gutters shall be hung at a minimum of 0.5% for 2/3 of the length and at 1% for the remaining 1/3 of the length. Appendix 8 — GSP Specifications Drainage Criteria Manual I , U .yO. n Rainwater collec€ionpoint (rocfdrains, gutters,etcj FloatingAainlessstoo Isuction filter Rainwater enters the vortex filter and is processed G 3ulbmersiblefeed pump �Pussibla 90% diverted to storage tank. { Remaining wa€er from voilex fitter to.averflow Low water cut offfloat switch s—' Smoothing inlet—stain€esssteel'tlow-calming'deMce to N8rflori i s alirninare Wrbulence of lha inonnng water es it enters the tank pressu€e tank RAINWATER HARVESTING SYSTEM DETAIL NQT TQ SCALE Figure 11.7. Sample Rainwater Harvesting System Detail (Source: VADCR, 2011). Each of these system components is discussed below. Rooftop Surface. The rooftop shall be made of smooth, non -porous material with efficient drainage either from a sloped roof or an efficient roof drain system. Slow drainage of the roof leads to poor rinsing and a prolonged first flush, which can decrease water quality. If the harvested rainwater is selected for uses with significant human exposure (e.g., pool filling, watering vegetable gardens), care should be taken in the choice of roof materials. Some materials may leach toxic chemicals making the water unsafe for humans. Collection and Conveyance System. The collection and conveyance system consists of the gutters, downspouts and pipes that channel stormwater runoff into storage tanks. Gutters and downspouts should be designed as they would for a building without a rainwater harvesting system. Aluminum, round -bottom gutters and round downspouts are generally recommended for rainwater harvesting. Minimum slopes of gutters shall be specified. At a minimum, gutters shall be sized with slopes specified to contain the 1 -inch storm at a rate of 1-inch/hour for treatment volume credit. If volume credit will also be sought for channel and flood protection, the gutters shall be designed to convey the 2 and 10 -year storm, using the appropriate 2 and 10 year storm intensities, specifying size and minimum slope. In all cases, gutters shall be hung at a minimum of 0.5% for 2/3 of the length and at 1% for the remaining 1/3 of the length. Appendix 8 — GSP Specifications Drainage Criteria Manual I , Pipes (connecting downspouts to the cistern tank) shall be at a minimum slope of 1.5% and sized/designed to convey the intended design storm, as specified above. In some cases, a steeper slope and larger sizes may be recommended and/or necessary to convey the required runoff, depending on the design objective and design storm intensity. Gutters and downspouts shall be kept clean and free of debris and rust. Pre -Treatment. Screening, First Flush Diverters and Filter Efficiencies. Pre -filtration is required to keep sediment, leaves, contaminants and other debris from the system. Leaf screens and gutter guards meet the minimal requirement for pre -filtration of small systems, although direct water filtration is preferred. All pre - filtration devices shall be low -maintenance or maintenance -free. The purpose of pre -filtration is to significantly cut down on maintenance by preventing organic buildup in the tank, thereby decreasing microbial food sources. For larger tank systems, the initial first flush must be diverted from the system before rainwater enters the storage tank. Designers should note that the term "first flush" in rainwater harvesting design does not have the same meaning as has been applied historically in the design of stormwater treatment practices. In this specification, the term "first flush diversion" is used to distinguish it from the traditional stormwater management term "first flush". The amount can range between the first 0.02 to 0.06 inches of rooftop runoff. The diverted flows (first flush diversion and overflow from the filter) must be directed to an acceptable pervious flow path that will not cause erosion during a 2 -year storm or to an appropriate BMP on the property, for infiltration. Preferably the diversion will be conveyed to the same secondary runoff reduction practice that is used to receive tank overflows. Various first flush diverters are described below. In addition to the initial first flush diversion, filters have an associated efficiency curve that estimates the percentage of rooftop runoff that will be conveyed through the filter to the storage tank. If filters are not sized properly, a large portion of the rooftop runoff may be diverted and not conveyed to the tank at all. A design intensity of 1-inch/hour shall be used for the purposes of sizing pre -tank conveyance and filter components. This design intensity captures a significant portion of the total rainfall during a large majority of rainfall events (NOAA 2004). If the system will be used for channel and flood protection, the 2- and 10 -year storm intensities shall be used for the design of the conveyance and pre-treatment portion of the system. For the 1 -inch storm treatment volume, a minimum of 95% filter efficiency is required. This efficiency includes the first flush diversion. For the 2- and 10 -year storms, a minimum filter efficiency of 90% should be met. First Flush Diverters. First flush diverters direct the initial pulse of stormwater runoff away from the storage tank. While leaf screens effectively remove larger debris such as leaves, twigs and blooms from harvested rainwater, first flush diverters can be used to remove smaller contaminants such as dust, pollen and bird and rodent feces (Figure 11.8). Simple first flush diverters require active management, by draining the first flush water volume to a pervious area following each rainstorm. First flush diverters may be the preferred pretreatment method if the water is to be used for indoor purposes. A vortex filter may serve as an effective pre -tank filtration device and first flush diverter. • Leaf Screens. Leaf screens are mesh screens installed over either the gutter or downspout to separate leaves and other large debris from rooftop runoff. Leaf screens must be regularly cleaned to be effective; if not maintained, they can become clogged and prevent rainwater from flowing into the storage tanks. Built-up debris can also harbor bacterial growth within gutters or downspouts (TWD B, 2005). Appendix 8 - GSP Specifications Drainage Criteria Manual i , Roof Washers. Roof washers are placed just ahead of storage tanks and are used to filter small debris from harvested rainwater (Figure 11.9). Roof washers consist of a tank, usually between 25 and 50 gallons in size, with leaf strainers and a filter with openings as small as 30 -microns (TWDB, 2005). The filter functions to remove very small particulate matter from harvested rainwater. All roof washers must be cleaned on a regular basis. Inlet First slush charnte r Duti et i H uGe Bibb 8 VY Clean-�LlC drip 8 � RIu9 i Qmffia Figure 11.8. First Flush Diverter. Figure 11.9. Roof Washer. (Source: VADCR, 2011). fitter Vortex Filters. For large scale applications, vortex filters can provide filtering of rooftop rainwater from larger rooftop areas. Two images of the vortex filter are displayed below. The first image (Figure 11.10) provides a plan view photograph showing the interior of the filter with the top off. The second image (Figure 11.11) displays the filter just installed in the field prior to the backfill. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Figure 11.10. Interior of Vortex Filter (Source: VADCR, 2011). Figure 11.11. Installation of Vortex Filter prior to backfill (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual i , Storage Tanks. The storage tank is the most important and typically the most expensive component of a rainwater harvesting system. Cistern capacities range from 250 to over 30,000 gallons. Multiple tanks can be placed adjacent to each other and connected with pipes to balance water levels and increase overall storage on-site as needed. Typical rainwater harvesting system capacities for residential use range from 1,500 to 5,000 gallons. Storage tank volumes are calculated to meet the water demand and stormwater treatment volume objectives, as described in Section 6 of this specification. While many of the graphics and photos in this specification depict cisterns with a cylindrical shape, the tanks can be made of many materials and configured in various shapes, depending on the type used and the site conditions where the tanks will be installed. For example, configurations can be rectangular, L-shaped or step vertically to match the topography of a site. The following factors that shall be considered when designing a rainwater harvesting system and selecting a storage tank: • Aboveground storage tanks shall be UV and impact resistant. • Underground storage tanks must be designed to support the overlying sediment and any other anticipated loads (e.g., vehicles, pedestrian traffic, etc.). • Underground rainwater harvesting systems shall have a standard size manhole or equivalent opening to allow access for cleaning, inspection, and maintenance purposes. This access point shall be secured/locked to prevent unwanted access. • All rainwater harvesting systems shall be sealed using a water -safe, non-toxic substance. • Rainwater harvesting systems may be ordered from a manufacturer or can be constructed on site from a variety of materials. Table 11.2 compares the advantages and disadvantages of different storage tank materials. • Storage tanks shall be opaque or otherwise protected from direct sunlight to inhibit algae growth and shall be screened to discourage mosquito breeding and reproduction. • Dead storage below the outlet to the distribution system and an air gap at the top of the tank shall be included in the total volume. For gravity -fed systems, a minimum of 6 inches of dead storage shall be provided. For systems using a pump, the dead storage depth will be based on the pump specifications. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Table 11.2. Advantages and Disadvantages Tank Material Advantages Disadvantages Commercially available, alterable and Must be installed on smooth, solid, Fiberglass moveable; durable with little level footing; pressure proof for below - maintenance; light weight; integral ground installation; expensive in fittings (no leaks); broad application smaller sizes Commercially available, alterable, Can be UV -degradable; must be moveable, affordable; available in wide painted or tinted for above -ground Polyethylene range of sizes; can install above or installations; pressure -proof for below - below ground; little maintenance; broad ground installation application Can modify to topography; can alter Longevity may be less than other Modular Storage footprint and create various shapes to materials; higher risk of puncturing of fit site; relatively inexpensive water tight membrane during construction Plastic Barrels Commercially available; inexpensive Low storage capacity (20 to 50 gallons); limited application Commercially available, alterable and Possible external corrosion and rust; Galvanized Steel moveable; available in a range of sizes; must be lined for potable use; can only film develops inside to prevent install above ground; soil pH may limit corrosion underground applications Small storage capacity; prone to Commercially available, alterable and corrosion, and rust can lead to leaching Steel Drums moveable of metals; verify prior to reuse for toxics; water pH and soil pH may also limit applications Durable and immoveable; suitable for FerroConcrete above or below ground installations; Potential to crack and leak; expensive neutralizes acid rain Durable, immoveable, versatile; suitable Potential to crack and leak; permanent; Cast in Place Concrete for above or below ground installations; will need to provide adequate platform neutralizes acid rain and design for placement in clay soils Stone or concrete Block Durable and immoveable; keeps water Difficult to maintain; expensive to build cool in summer months Source: Cabell Brand, 2007, 2009. Appendix 8 — GSP Specifications Drainage Criteria Manual i , The images below in Figures 11.12 to 11.14 display three examples of various materials and shapes of cisterns discussed in Table 11.2 above. Figure 11.12. Example of Multiple Fiberglass Cisterns in Series (Source: VADCR, 2011). Figure 11.13. Example of two Polyethylene Cisterns (Source: VADCR, 2011). Appendix 8 - GSP Specifications Drainage Criteria Manual i , Figure 11.14. Example of Modular Units (Source: VADCR, 2011). Distribution Systems. Most distribution systems require a pump to convey harvested rainwater from the storage tank to its final destination, whether inside the building, an automated irrigation system, or gradually discharged to a secondary runoff reduction practice. The rainwater harvesting system shall be equipped with an appropriately -sized pump that produces sufficient pressure for all end -uses. Separate plumbing labeled as non -potable may be required. The typical pump and pressure tank arrangement consists of a multi -stage centrifugal pump, which draws water out of the storage tank and sends it into the pressure tank, where it is stored for distribution. When water is drawn out of the pressure tank, the pump activates to supply additional water to the distribution system. The backflow preventer is required to separate harvested rainwater from the main potable water distribution lines. Distribution lines from the rainwater harvesting system shall be buried beneath the frost line. Lines from the rainwater harvesting system to the building shall have shut-off valves that are accessible when snow cover is present. A drain plug or cleanout sump, also draining to a pervious area, shall be installed to allow the system to be completely emptied, if needed. Above -ground outdoor pipes shall be insulated or heat -wrapped to prevent freezing and ensure uninterrupted operation during winter. Overflow, Filter Path and Secondary Runoff Reduction Practice. An overflow mechanism shall be included in the rainwater harvesting system design in order to handle an individual storm event or multiple storms in succession that exceed the capacity of the tank. Overflow pipes shall have a capacity equal to or greater than the inflow pipe(s) and have a diameter and slope sufficient to drain the cistern while maintaining an adequate freeboard height. The overflow pipe shall be screened to prevent access to the tank by rodents and birds. Appendix 8 - GSP Specifications Drainage Criteria Manual i , The filter path is a pervious or grass corridor that extends from the overflow to the next runoff reduction practice, the street, an adequate existing or proposed channel, or the storm drain system. The filter path must be graded with a slope that results in sheet flow conditions. If compacted or impermeable soils are present along the filter path, compost amendments may be needed (see GSP-01 Appendix - Soil Amendments,). It is also recommended that the filter path be used for first flush diversions. In many cases, rainwater harvesting system overflows are directed to a secondary runoff reduction practice to boost overall runoff reduction rates. These options are addressed in Section 5.5. SECTION 6: DESIGN CRITERIA 6.1. Sizing of Rainwater Harvesting Systems The rainwater harvesting cistern sizing criteria presented in this section was developed using best estimates of indoor and outdoor water demand, long-term rainfall data, and rooftop capture area data. 6.2. Incremental Design Volumes within Cistern Rainwater tank sizing is determined by accounting for varying precipitation levels, captured rooftop runoff, first flush diversion (through filters) and filter efficiency, low water cut-off volume, dynamic water levels at the beginning of various storms, storage needed for treatment volume (permanent storage), storage needed for channel protection and flood volume (temporary detention storage), seasonal and year-round demand use and objectives, overflow volume, and freeboard volumes above high water levels during very large storms. See Figure 11.15 for a graphical representation of these various incremental design volumes. Appendix 8 - GSP Specifications Drainage Criteria Manual i , EMENGENU STORAGE ASSOCIATED WITH CHANNEL AND FLOOD PROTECTION VOLUME (DETENTION STORAGE WITH ORIFICE CONTROLLED RELEASE) STORAGE ASSOCIATED WITH (TV,) TREATMENT VOLUME TANK STORAGE CAPACITY (TOTAL STORAGE AVAILABLE FOR RE -USE) � R'l�.iTEL �BEGIh]A]INri --- Figure 11.15. Incremental Design Volumes associated with tank sizing (Source: VADCR, 2011). The "Storage Associated with the Treatment Volume" is the storage within the tank that is modeled and available for reuse. While the Treatment Volume will remain the same for a specific rooftop capture area, the "Storage Associated with the Treatment Volume" may vary depending on demand and runoff reduction credit objectives. It includes the variable water level at the beginning of a storm and the low water cut-off volume that is necessary to satisfy pumping requirements. 6.3. Cistern Design Guidance There are various proprietary software programs available to assist with the design of the rainwater harvesting system. The software must be capable of providing a printed report showing the variables and calculations and is subject to the approval of the City Engineer. 6.4. Rainwater Harvesting Material Specifications The basic material specifications for rainwater harvesting systems are presented in Table 11.3. Designers should consult with experienced rainwater harvesting system and irrigation installers on the choice of recommended manufacturers of prefabricated tanks and other system components. Appendix 8 — GSP Specifications Drainage Criteria Manual I , Table 11.3. Design Specifications for Rainwater Harvesting Systems. Tank Material Specification Materials commonly used for gutters and downspouts include PVC pipe, vinyl, aluminum and galvanized steel. Lead shall not be used as gutter and downspout Gutters solder, since rainwater can dissolve the lead and contaminate the water supply. ppI y. • The length of gutters and downspouts is determined by the size and layout Downspout of the catchment and the location of the storage tanks. • Be sure to include needed bends and tees. At least one of the following (all rainwater to pass through pre-treatment): • First flush diverter Pre- • Vortex filter Treatment . Roof washer • Leaf and mosquito screen (1 mm mesh size) • Materials used to construct storage tanks shall be structurally sound. • Tanks shall be constructed in areas of the site where native soils can support the load associated with stored water. • Storage tanks shall be water tight and sealed using a water -safe, non-toxic substance. Storage Tanks . Tanks shall be opaque to prevent the growth of algae. . Re -used tanks shall be fit for potable water or food -grade products. • Underground rainwater harvesting systems shall have a minimum of 18 to 24 inches of soil cover and be located below the frost line. • The size of the rainwater harvesting system(s) is determined during the design calculations. Note: This table does not address indoor systems or oumos. SECTION 7: SPECIAL CASE DESIGN ADAPTATIONS 7.1. Steep Terrain Rainwater harvesting systems can be useful in areas of steep terrain where other stormwater treatments are inappropriate, provided the systems are designed in a way that protects slope stability. Cisterns shall be located in level areas atop suitable bearing materials. Harvested rainwater shall not be discharged over steep slopes; rather, the rainwater shall be used for indoor non -potable applications or outdoor irrigation. SECTION 8: CONSTRUCTION 8.1. Construction Sequence It is advisable to have a single contractor install the rainwater harvesting system, outdoor irrigation system and secondary runoff reduction practices. The contractor should be familiar with rainwater harvesting Appendix 8 — GSP Specifications Drainage Criteria Manual i , system sizing, installation, and placement. A licensed plumber is required to install the rainwater harvesting system components to the plumbing system. A standard construction sequence for proper rainwater harvesting system installation is provided below. This can be modified to reflect different rainwater harvesting system applications or expected site conditions. • Choose the tank location on the site • Route all downspouts or roof drains to pre-screening devices and first flush diverters • Properly Install the tank • Install the pump (if needed) and piping to end -uses (indoor, outdoor irrigation, or tank dewatering release) • Route all pipes to the tank • Stormwater should not be diverted to the rainwater harvesting system until the overflow filter path has been stabilized with vegetation. 8.2. Construction Inspection The following items shall be inspected prior to final sign -off and acceptance of a rainwater harvesting system: • Rooftop area matches plans • Diversion system is properly sized and installed • Pretreatment system is installed • Mosquito screens are installed on all openings • Overflow device is directed as shown on plans • Rainwater harvesting system foundation is constructed as shown on plans • Catchment area and overflow area are stabilized • Secondary runoff reduction practice(s) is installed as shown on plans SECTION 9: AS -BUILT REQUIREMENTS After the cistern has been installed, the developer must have an as -built certification of the cistern conducted by a registered professional engineer. The as -built certification verifies that the BMP was installed as designed and approved. The components listed below are vital to ensure a properly working cistern and must be addressed in the as -built certification. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Incorporation of Rainwater Harvesting System into Site Plan Grading and Storm Sewer Plan Construction Documents • Include a roof plan of the building that will be used to capture rainwater, showing slope direction and roof material. • Include a roof plan of the building that will be used to capture rainwater, showing slope direction and roof material. • Display downspout leaders from the rooftops being used to capture rainwater. • Display the storm drain pipe layout (pipes between building downspouts and the tank) in plan view, specifying materials, diameters, slopes and lengths, to be included on typical grading and utilities or storm sewer plan sheets. • Include a detail or note specifying the minimum size, shape configuration and slope of the gutter(s) that convey rainwater Rainwater Harvesting System Construction Document sheet • The Cistern or Storage Unit material and dimensions in a scalable detail (use a cut sheet detail from Manufacturer, if appropriate). Include the specific Filter Performance specification and filter efficiency curves. Runoff estimates from the rooftop area captured for 1 -inch storm shall be estimated and compared to filter efficiencies for the 1 -inch storm. It is assumed that the first flush diversion is included in filter efficiency curves. A minimum of 95% filter efficiency is required to receive the Treatment Volume credit. If this value is altered (increased), the value shall be reported. Filter curve cut sheets are normally available from the manufacturer. Show the specified materials and diameters of inflow and outflow pipes. • Show the inverts of the orifice outlet, the emergency overflows, and, if applicable, the receiving secondary runoff reduction practice or on-site infiltration facility. • Include a cross section of the storage unit displaying the inverts associated with the various incremental volumes (if requested by the reviewer). SECTION 10: MAINTENANCE 10.1. Maintenance Plans It is recommended that a Long Term Maintenance Plan (LTMP) be developed by the design engineer. The LTMP contains a description of the stormwater system components and information on the required inspection and maintenance activities. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 10.2. Maintenance Inspections All rainwater harvesting systems components shall be inspected by the property owner twice per year (preferably Spring and the Fall). A comprehensive inspection by a professional engineer or landscape architect shall occur every five years. 10.3. Rainwater harvesting system Maintenance Schedule Maintenance requirements for rainwater harvesting systems vary according to use. Systems that are used to provide supplemental irrigation water have relatively low maintenance requirements, while systems designed for indoor uses have much higher maintenance requirements. Table 11.4 describes routine maintenance tasks to keep rainwater harvesting systems in working condition. Table 11.4. Suggested Maintenance Tasks for Rainwater Harvesting Activity Systems. Frequency Keep gutters and downspouts free of leaves and other debris O: Twice a year Inspect and clean pre-screening devices and first flush diverters O: Four times a year Inspect and clean storage tank lids, paying special attention to vents and screens on inflow and outflow spigots. Check mosquito screens and patch holes or gaps immediately O: Once a year Inspect condition of overflow pipes, overflow filter path and/or secondary runoff reduction practices O: Once a year Inspect tank for sediment buildup I: Every third year Clear overhanging vegetation and trees over roof surface I: Every third year Check integrity of backflow preventer I: Every third year Inspect structural integrity of tank, pump, pipe and electrical system I: Every third year Replace damaged or defective system components I: Every third year Key: O = Owner I = qualified third party inspector. SECTION 11: COMMUNITY & ENVIRONMENTAL CONCERNS Although rainwater harvesting is an ancient practice, it is becoming more popular due to benefits especially in urban environments. Some common concerns associated with rainwater harvesting that must be addressed during design include: Winter Operation. Rainwater harvesting systems can be used throughout the year if they are located underground or indoors to prevent problems associated with freezing, ice formation and subsequent system damage. Alternately, an outdoor system can be used seasonally, or year round if special measures and design considerations are incorporated. Plumbing Codes. Designer and plan reviewers shall consult building codes to determine if they explicitly allow the use of harvested rainwater for toilet and urinal flushing. In the cases where a Metro backup supply is used, rainwater harvesting systems are required to have backflow preventers or air gaps to keep harvested water separate from the main water supply. Pipes and spigots using rainwater must be clearly labeled as non -potable. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Mosquitoes. In some situations, poorly designed rainwater harvesting systems can create habitat suitable for mosquito breeding and reproduction. Designers shall provide screens on above- and below -ground tanks to prevent mosquitoes and other insects from entering the tanks. If screening is not sufficient in deterring mosquitoes, dunks or pellets containing larvicide can be added to cisterns when water is intended for landscaping use. Child Safety. Above -grade residential rainwater harvesting systems cannot have unsecured openings large enough for children to enter the tank. For underground cisterns, manhole access shall be secured to prevent unwanted access. SECTION 12: REFERENCES Cabell Brand Center, 2009. Virginia Rainwater Harvesting Manual, Version 2.0. Salem, VA. (Draft Form) http:://www.cabellbrandcenter.org Cabell Brand Center, 2007. Virginia Rainwater Harvesting Manual. Salem, VA. http://www.cabellbrandcenter.org Center for Watershed Protection (CWP), 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Ellicott City, MD. City of Portland, Environmental Services, 2004. Portland Stormwater Management Manual. Portland, OR. http://www.portlandonline.com/bes/index.cfm?c=dfbcc City of Tucson, AZ, 2005. Water Harvesting Guidance Manual. City of Tucson, AZ. Tucson, AZ. Coombes, P., 2004. Water Sensitive Design in the Sydney Region. Practice Note 4: Rainwater Tanks. Published by the Water Sensitive Design in the Sydney Region Project. Credit Valley Conservation, 2008. Credit River Stormwater Management Manual. Mississauga, Ontario. Foraste, J. Alex and Hirschman David, 2010. A Methodology for using Rainwater Harvesting as a Stormwater Management BMP. ASCE International Low Impact Development Conference, Redefining Water in the City. San Francisco, CA. Gowland, D. and T. Younos, 2008. Feasibility of Rainwater Harvesting BMP for Stormwater Management. Virginia Water Resources Research Center. Special Report SR38-2008. Blacksburg, VA. National Oceanic and Atmospheric Administration (NOAH), 2004. NOAA Atlas 14 Precipitation -Frequency Atlas of the United States, Volume 2, Version 3.0. Revised 2006. Silver Spring, MD. North Carolina Division of Water Quality, 2008. Technical Guidance: Stormwater Treatment Credit for Rainwater Harvesting Systems. Revised September 22, 2008. Raleigh, NC. Northern Virginia Regional Commission, 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia. Nova Scotia Environment, 2009. The Drop on Water. Cisterns. Nova Scotia. Schueler, T., D. Hirschman, M. Novotney and J. Zielinski, 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Schueler, T., 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net VADCR (Virginia Department of Conservation and Recreation), 2011. Stormwater Design Specification No. 6: Rainwater Harvesting, Version 1.9.S, Available at: http://vwrrc.vt.edu/swc/NonProprietaryBMPs.html. Appendix B - GSP Specifications Drainage Criteria Manual i , GREEN ROOF Description: A green roof is a layer of vegetation installed on top of a specially designed flat or slightly sloped roof that consists of waterproofing material, root permeable filter fabric, growing media, and specially selected plants. Variations: • Extensive green roofs have a thin layer of growing medium and are usually composed of sedums. • Intensive green roofs have a thicker layer of growing medium and contain shrubs, trees and other vegetation. • Runoff volume reduction • Provides flow attenuation • Extends the life of a conventional roof by up to 20 yrs • Provides increased insulation and energy savings • Reduces air pollution • Provides habitat for wildlife • Increases aesthetic value • Provides sound insulation • Provides water quality treatment • Reduces urban heat island effect Appendix B — GSP Specifications Drainage Criteria Manual i , Selection Criteria: LEVEL 1— 80% Runoff Reduction Credit LEVEL 2 — 90% Runoff Reduction Credit Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • May include watering, fertilizing, and weeding, typically greatest in the first two years when plants are becoming established. • Maintenance largely depends on the type of green roof system installed and the type of vegetation planted. ©Maintenance Burden L = Low M = Moderate H = High • Cost may be greater than a conventional roof, and feasibility is limited by load-bearing capacity of roof • Must obtain necessary permits and comply with local building codes • Requires more maintenance than a conventional roof • Plant survival and waterproofing are potential issues • May require irrigation SECTION 1: DESCRIPTION Vegetated roofs (also known as green roofs, living roofs or ecoroofs) are alternative roof surfaces that typically consist of waterproofing and drainage materials and an engineered growing media that is designed to support plant growth. Vegetated roofs capture and temporarily store stormwater runoff in the growing media before it is conveyed into the storm drain system. A portion of the captured stormwater evaporates or is taken up by plants, which helps reduce runoff volumes, peak runoff rates and pollutant loads on development sites. There are two different types of vegetated roof systems: intensive vegetated roofs and extensive vegetated roofs. Intensive systems have a deeper growing media layer that ranges from 6 inches to 4 feet thick, which is planted with a wider variety of plants, including trees. By contrast, extensive systems typically have much shallower growing media (under 6 inches), which is planted with carefully selected drought tolerant vegetation. Extensive vegetated roofs are much lighter and less expensive than intensive vegetated roofs and are recommended for use on most development and redevelopment sites. NOTE; This specification is intended for situations where the primary design objective of the vegetated roof is stormwater management and, unless specified otherwise, addresses extensive roof systems. Hence tree planting recommendations are not provided in this document Designers may wish to pursue other design objectives for vegetated roofs, such as energy efficiency, green building or LEED points, architectural considerations, visual amenities and landscaping features, which are often maximized with intensive vegetated roof systems. However, these design objectives are beyond the scope of this specification. Vegetated roofs typically contain a layered system of roofing, which is designed to support plant growth and retain water for plant uptake while preventing ponding on the roof surface. The roofs are designed so that water drains vertically through the media and then horizontally along a waterproofing layer towards the outlet. Extensive vegetated roofs are designed to have minimal maintenance requirements. Plant species are selected so that the roof does not need supplemental irrigation or fertilization after vegetation is initially established. Tray systems are also available with removable dividers allowing the media to meld together creating a seamless appearance but with less difficulty in construction. SECTION 2: PERFORMANCE The overall stormwater functions of vegetated roofs are summarized in Table 12.1. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Stormwater Function Level 1 Design Level 2 Design Annual Runoff Volume Reduction (RR) 1 80% 90% Appendix 8 — GSP Specifications Drainage Criteria Manual i , SECTION 3: DESIGN TABLE The major design goal for vegetated roofs is to maximize nutrient removal and runoff volume reduction. The rooftops have little TSS loading or loading removal. Designers may choose the baseline design (Level 1) or choose an enhanced (Level 2) design that maximizes nutrient and runoff reduction. In general, most intensive vegetated roof designs will automatically qualify as being Level 2. Table 12.2 lists the design criteria for Level 1 and 2 designs. Table 12.2. Green Level 1 Design (RR:80) Roof Design Guidance. Level 2 Design (RR: 90) Tv = 1.0 (Rv)l (A)/12 Tv = 1.1 (Rv)1 (A)/12 Depth of media up to 6 inches Media depth > 6 inches No more than 15% organic matter in media No more than 15% organic matter in media All Designs: Must be in conformance to ASTM (2005) International Green (Vegetated) Roof Standards. 1Rv represents the runoff coefficient for a conventional roof, which will usually be 0.95. The runoff reduction rate applied to the vegetated roof is for "capturing" the Treatment Volume (Tv) compared to what a conventional roof would produce as runoff. SECTION 4: TYPICAL DETAILS Figure 12.1. Photos of Vegetated Roof Cross -Sections (Source: B. Hunt, NCSU). Appendix B — GSP Specifications Drainage Criteria Manual i , i nw Plants sedumslherhs (typ.) Erosion control (wind blanket or jute meshy '* 3" to 6" growth medium (typ.) - Filter Fabric (typ.) Drainage Layer. 2" 1ightweighl granular mix (optional: mat or plate system) Filter Fabric (optional) — Aluminum Curb (typ.) Gravel (optional) Vegetation -free strip -1 1 �gravel, pavers (typ.) Thermal insulation (optional) Leak Detection System (optional) Proter_lion Layer (typ-) Root Barrier (typ.) Waterproof Membrane (typ.) Roof Deck with Vapor Barrier and Roof Structure Perforated aluminum curb (typ.) wl drainage fabric Roof drain with parapet well Emergency overflow CROSS SECTION VIEW (NTS) Figure 12.2. Typical Section — Extensive Vegetated Roof (Source: Northern VA Regional Commission). Appendix 8 - GSP Specifications Drainage Criteria Manual i , Plants: perennials and shrubs Erosion control (wind blanket 0r jute mesh) 6" to 12" growth medium (typ.) Filter Fabric Drainage: 4" to 6" granular — (optional: mal or plate system) Filter Fabric (optional) Aluminum Curbing — Gravel (optional) rVegetation-free strip gravel, pavers (typ.) Thermal Insulalion (optional) Leak detection System (optional) Protection Layer (typ.) Root Barrier (typ.) Waterproof Membrane (typ.) Roof Deck with Vapor Barrier and Roof Structure Perforated aluminum curb (typ.) wl drainage fabric Roof drain with parapet well Emergency ovorflow GROSS SECTION (NTS) Figure 12.3. Typical Section - Intensive Vegetated Roof (Source: Northern VA Regional Commission). Appendix 8 - GSP Specifications Drainage Criteria Manual i , SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS 5.1. Typical applications Vegetated roofs are ideal for use on commercial, institutional, municipal and multi -family residential buildings. They are particularly well suited for use on ultra -urban development and redevelopment sites. Vegetated roofs can be used on a variety of rooftops, including the following: • Non-residential buildings (e.g., commercial, industrial, institutional and transportation uses) • Multi -family residential buildings (e.g., condominiums or apartments) • Mixed-use buildings 5.2. Common Site Constraints Structural Capacity of the Roof. When designing a vegetated roof, designers must not only consider the stormwater storage capacity of the vegetated roof, but also its structural capacity to support the weight of the additional water. A conventional rooftop typically must be designed to support an additional 15 to 30 pounds per square foot (psf) for an extensive vegetated roof. As a result, a licensed structural engineer shall be involved with all vegetated roof designs to ensure that the building has enough structural capacity to support a vegetated roof. Roof Pitch. Treatment volume (Tv) is maximized on relatively flat roofs (a pitch of 1 to 2%). Some pitch is needed to promote positive drainage and prevent ponding and/or saturation of the growing media. Vegetated roofs can be installed on rooftops with slopes up to 25% if baffles, grids, or strips are used to prevent slippage of the media. The effective treatment volume (Tv), however, diminishes on rooftops with steep pitches (Van Woert et al., 2005). Roof Access. Adequate access to the roof must be available to deliver construction materials and perform routine maintenance. Roof access can be achieved either by an interior stairway through a penthouse or by an alternating tread device with a roof hatch or trap door not less than 16 square feet in area and with a minimum dimension of 24 inches (NVRC, 2007). Designers should also consider how they will get construction materials up to the roof (e.g., by elevator or crane), and how and where construction materials will be stockpiled in the confined space. Non -Vegetated Areas. Roof access paths, mechanical equipment, photovoltaic panels, and skylights are counted as part of the green roof for calculation purposes. These areas shall not exceed 20% of the roof area counted as green roof. Roof Type. Vegetated roofs can be applied to most roof surfaces, although concrete roof decks are preferred. Certain roof materials, such as exposed treated wood and uncoated galvanized metal, may not be appropriate for vegetated rooftops due to pollutant leaching through the media (Clark et al.., 2008). Retrofitting Green Roofs. Key feasibility factors to consider when evaluating a retrofit include the area, age and accessibility of the existing roof, and the capability of the building's owners to maintain it. Options for Appendix 8 - GSP Specifications Drainage Criteria Manual i , green roof retrofits are described in Profile Sheet RR -3 of Schueler et al. (2007). The structural capacity of the existing rooftop can be a major constraint to a green roof retrofits. Building Codes. The vegetated roof design shall comply with all applicable building codes for Fayetteville with respect to roof drains and emergency overflow devices. If the green roof is designed to be accessible, the access must not only be convenient for installation and maintenance purposes but also must adhere to applicable building codes and other regulations for access and safety. Construction Cost. When viewed strictly as stormwater treatment systems, vegetated roofs can cost between $12 and $25 per square foot (Moran et al., 2005, Schueler et al. 2007). These cost analyses, however, do not include life cycle cost savings relating to increased energy efficiency, higher rents due to green building scores and increased roof longevity. These benefits over the life cycle of a vegetated roof may make it a more attractive investment. Risks of Leaky Roofs. Although well designed and installed green roofs purportedly have less problems with roof leaks than traditional roofs, there is a perception among property managers, insurers and product fabricators that this emerging technology could have a greater risk of problems. For an excellent discussion on how to properly manage risk in vegetated roof installations, see Chapter 9 in Weiler and Scholz -Barth (2009). SECTION 6: DESIGN CRITERIA 6.1. Overall Sizing Vegetated roof areas shall be sized to capture a portion of the Treatment Volume (Tv). The required size of a vegetated roof will depend on several factors, including the porosity and hydraulic conductivity of the growing media and the underlying drainage materials. Site designers and planners shall consult with vegetated roof manufacturers and material suppliers for specific sizing guidelines. As a general sizing rule, the following equation can be used to determine the water quality treatment storage volume retained by a vegetated roof: Equation 12.1. Treatment Volume for Green Roof Tv = (RA * D * P)/12 Where, T, = storage volume (cu. ft.) RA = vegetated roof area (sq. ft.) D = media depth (in.) P = media porosity (usually 0.3, but consult manufacturer specifications) The resulting Tv can then be compared to the required Tv for the entire rooftop area (including all non- vegetated areas) to determine if it meets or exceeds the required Tv for Level 1 or Level 2 design, as shown in Table 12.2 above. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.2. Structural Capacity of the Roof Vegetated roofs must be designed and constructed fo the additional weight of the fully saturated growing medium and mature plants, in terms of the physical capacity of the roof to bear structural loads. The designer shall consult with a licensed structural engineer or architect to ensure that the building will be able to support the additional live and dead structural load and determine the maximum depth of the vegetated roof system and any needed structural reinforcement. In most cases, fully -saturated extensive vegetated roofs have a maximum load of about 30 lbs./sq. ft., which is fairly similar to traditional new rooftops (12 to 15 lbs./sq. ft.) that have a waterproofing layer anchored with stone ballast. For an excellent discussion of vegetated roof structural design issues, consult Chapter 9 in Weiler and Scholz -Barth (2009) and ASTM E-2397, Standard Practice for Determination of Dead Loads and Live Loads Associated with Green (Vegetated) Roof Systems. 6.3. Functional Elements of a Vegetated Roof System A vegetated roof is composed of up to eight different systems or layers, from bottom to top, that are combined together to protect the roof and maintain a vigorous cover. Designers can employ a wide range of materials for each layer, which can differ in cost, performance, and structural load. The entire system as a whole must be assessed to meet design requirements. Some manufacturers offer proprietary vegetated roofing systems, whereas in other cases, the designer or architect must assemble their own system, in which case they are advised to consult Weiler and Scholz -Barth (2009), Snodgrass and Snodgrass (2006) and Dunnett and Kingsbury (2004). 1. Deck Layer. The roof deck layer is the foundation of a vegetated roof. It and may be composed of concrete, wood, metal, plastic, gypsum or a composite material. The type of deck material determines the strength, load bearing capacity, longevity and potential need for insulation in the vegetated roof system. In general, concrete decks are preferred for vegetated roofs, although other materials can be used as long as the appropriate system components are matched to them. Z. Waterproofing Layer. All vegetated roof systems must include an effective and reliable waterproofing layer to prevent water damage through the deck layer. A wide range of waterproofing materials can be used, including built up roofs, modified bitumen, single -ply, and liquid -applied methods (see Weiler and Scholz -Barth, 2009 and Snodgrass and Snodgrass, 2006). The waterproofing layer must be 100% waterproof and have an expected life span as long as any other element of the vegetated roof system. 3. Insulation Layer. Many vegetated rooftops contain an insulation layer, usually located above, but sometimes below, the waterproofing layer. The insulation increases the energy efficiency of the building and/or protects the roof deck (particularly for metal roofs). According to Snodgrass and Snodgrass (2006), the trend is to install insulation on the outside of the building, in part to avoid mildew problems. 4. Root Barrier (Optional). The next layer of a vegetated roof system is an optional root barrier that protects the waterproofing membrane from root penetration. A wide range of root barrier options are described in Weiler and Scholz -Barth (2009). Chemical root barriers or physical root barriers that have been impregnated with pesticides, metals or other chemicals that could leach into stormwater runoff shall be avoided. Appendix B - GSP Specifications Drainage Criteria Manual i , 5. Drainage Layer and Drainage System. A drainage layer is then placed between the optional root barrier and the growing media to quickly remove excess water from the vegetation root zone. The drainage layer shall consist of synthetic or inorganic materials (e.g., gravel, recycled polyethylene, etc.) that are capable of retaining water and providing efficient drainage. A wide range of prefabricated water cups or plastic modules can be used, as well as a traditional system of protected roof drains, conductors and roof leader. The required depth of the drainage layer is governed by both the required stormwater storage capacity and the structural capacity of the rooftop. ASTM E2396 and E2398 can be used to evaluate alternative material specifications. 6. Root -Permeable Filter Fabric. A semi -permeable polypropylene filter fabric is normally placed between the drainage layer and the growing media to prevent the media from migrating into the drainage layer and clogging it. 7. Growing Media. The next layer in an extensive vegetated roof is the growing media, which is typically 4 to 6 inches deep for extensive roofs and 6 inches or more for intensive roofs. The depth and composition of the media is described in Section 6.5. 8. Plant Cover. The top layer of a vegetated roof consists of slow-growing, shallow -rooted, perennial, succulent plants that can withstand harsh conditions at the roof surface.. Guidance on selecting the appropriate vegetated roof plants for hardiness zones in Fayetteville can be found in Snodgrass and Snodgrass (2006). A mix of base ground covers (usually Sedum species) and accent plants can be used to enhance the visual amenity value of a green roof. 6.4. Pretreatment Pretreatment is not needed for green roofs. 6.5. Filter Media Composition The recommended growing media for extensive vegetated roofs is composed of approximately 80% to 90% lightweight inorganic materials, such as expanded slates, shales or clays, pumice, scoria or other similar materials. The remaining media shall contain no more than 15% organic matter, normally well -aged compost. The percentage of organic matter shall be limited, since it can leach nutrients into the runoff from the roof and clog the permeable filter fabric. The growing media shall have a maximum water retention capacity of around 30%. It is advisable to mix the media in a batch facility prior to delivery to the roof. More information on growing media can be found in Weiler and Scholz -Barth (2009) and Snodgrass and Snodgrass (2006). The composition of growing media for intensive vegetated roofs may be different, and it is often much greater in depth (e.g., 6 inches to 4 feet). If trees are included in the vegetated roof planting plan, the growing media must provide enough volume for the root structure of mature trees. 6.6. Conveyance and Overflow The drainage layer below the growth media shall be designed to convey the 10 -year storm without backing water up to into the growing media. The drainage layer shall convey flow to an outlet or overflow system such as a traditional rooftop drainage system with inlets set slightly above the elevation of the vegetated roof Appendix 8 - GSP Specifications Drainage Criteria Manual i , surface. Roof drains immediately adjacent to the growing media shall be boxed and protected by flashing extending at least 3 inches above the growing media to prevent clogging. 6.7. Vegetation and Surface Cover A planting plan must be prepared for a vegetated roof by a licensed Landscape Architect, botanist or other professional experienced with vegetated roofs, and it must be reviewed and approved by the City of Fayetteville. Plant selection for vegetated rooftops is an integral design consideration, which is governed by local climate and design objectives. The primary ground cover for most vegetated roof installations is a hardy, low - growing succulent, such as Sedum, Delosperma, Talinum, Semperivum or Hieracium that is matched to the local climate conditions and can tolerate the difficult growing conditions found on building rooftops (Snodgrass and Snodgrass, 2006). Fayetteville lies in the USDA Plant Hardiness Zone 6b (AHS, 2003). A detailed list of options for local conditions is provided in the Green Roof Plant List, Table 12.3. • Plant choices can be much more diverse for deeper intensive vegetated roof systems. Herbs, forbs, grasses, shrubs and even trees can be used, but designers should understand they have higher watering, weeding and landscape maintenance requirements. • The species and layout of the planting plan shall be based on considerations including height, exposure to wind, snow loading, heat stress, orientation to the sun, and shading by surrounding buildings. In addition, plants should be selected that are fire resistant and able to withstand heat, cold and high winds. • Designers shall also designate species suited to the expected rooting depth of the growing media, which can also provide enough lateral growth to stabilize the growing media surface. The planting plan should usually include several accent plants to provide diversity and seasonal color. For a comprehensive resource on vegetated roof plant selection, consult Snodgrass and Snodgrass (2006). • Given the limited number of vegetated roof plant nurseries in the region, designers should order plants 6 to 12 months prior to the expected planting date. It is also advisable to have plant materials contract -grown. • When appropriate species are selected, most vegetated roofs will not require supplemental irrigation, except during the first year that the vegetated roof is being established or during periods of drought. Irrigation shall thus be provided as needed for full establishment and during drought periods. The planting window extends from the spring to early fall, although it is important to allow plants to root thoroughly before the first killing frost. • Plants can be established using cuttings, plugs, mats, and, more rarely, seeding or containers. Several vendors also sell mats, rolls, or proprietary vegetated roof planting modules. For the pros and cons of each method, see Snodgrass and Snodgrass (2006). • The goal for vegetated roof systems designed for stormwater management is to establish a full and vigorous cover of low -maintenance vegetation that is self-sustaining and requires minimal mowing, trimming and weeding. • The vegetated roof design shall include non -vegetated walkways (e.g., permeable paver blocks) to allow for easy access to the roof for weeding and making spot repairs. Appendix 8 - GSP Specifications Drainage Criteria Manual i , 6.8. Material Specifications Standards specifications for North American vegetated roofs continue to evolve, and no universal material specifications exist that cover the wide range of roof types and system components currently available. The American Society for Testing and Materials (ASTM) has recently issued several overarching vegetated roof standards, which are described and referenced in Table 12.4 below. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 12.3. Green Roof Plant List. Latin Name Common Name Size Spacing Moisture Color Height Flowering Perennials Plugs— 1 plant/ Asclepias tuberasa Butterfly milkweed 1 gal. 18 in. o.c. Dry -moist Orange 2' Pale Plugs— 1 plant/18 Echinacea pallida purple coneflower 1 gal. in. o.c. dry Purple 2-3' Echinacea purpurea Purple coneflower Plugs— 1 plant/18 Moist -dry Purple 34 1 gal. in. o.c. Plugs— 1 plant/24 Liatrisspicata Dense blazingstar Moist -dry Purple 1.5' 1 gal. in. o.c. Plugs— 1 plant/24 Monarda didyma Bee balm Wet -moist Red 3' 1 gal. in. o.c. Plugs— 1 plant/181 Monardafistulosa Wild bergamot Moist -dry Purple 1-3' g al in.o.c. Plugs— 1 plant/181 Oenethera speciosa Evening primrose Moist -dry Pink 1-2' g al in. o.c. Penstemon digitalis Smooth white Plugs— 1 plant/24 Wet White 2-3' beardtongue 1 gal in. o.c. Plugs— 1 plant/181 Rudbeckia hirta Black-eyed Susan Moist -dry Yellow 3' gal in. o.c. Grasses/Sedges Plugs— 1 plant/18Wet Equisetum hyemale Horsetail Green 3' 1 gal. in. o.c. Carex sp. Sedge 1 gal. 1 plant/24 Moist Green 2-3' in. o.c. Panicum virgatum Switchgrass 1-3 gal. 1 plant/48 Moist - dry Yellow 5-7' in. o.c. Schizachyrium Little Blue Stem 1 gal. 1 plant/24 p / Moist -dry Yellow 3' scoparium in. o.c. Sorghastrum nutans Indian grass 1 gal. 1 plant/24 Moist -dry Green 2-4' in. o.c. Native Shrubs Callicarpa americana American Beautyberry NA NA Dry -moist Lilac 4-6' Hypericum prolificum Shrubby St. John's Wort NA NA Dry -moist Yellow 3' Rhododendron sp. Rhododendron/azalea NA NA Moist -dry Various 4-6' 6.8. Material Specifications Standards specifications for North American vegetated roofs continue to evolve, and no universal material specifications exist that cover the wide range of roof types and system components currently available. The American Society for Testing and Materials (ASTM) has recently issued several overarching vegetated roof standards, which are described and referenced in Table 12.4 below. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Designers and reviewers shall also fully understand manufacturer specifications for each system component listed in Section 6.3, particularly if they choose to install proprietary "complete" vegetated roof systems or modules. SECTION 7: CONSTRUCTION 7.1. Construction Sequence Given the diversity of extensive vegetated roof designs, there is no typical step-by-step construction sequence for proper installation. The following general construction considerations are noted: • Construct the roof deck with the appropriate slope and material. • Install the waterproofing method, according to manufacturer's specifications. • Conduct a flood test to ensure the system is water tight by placing at least 2 inches of water over the membrane for 48 hours to confirm the integrity of the waterproofing system. • Add additional system components (e.g., insulation, optional root barrier, drainage layer and interior drainage system, and filter fabric), taking care not to damage the waterproofing. Drain collars and protective flashing shall be installed to ensure free flow of excess stormwater. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Table 12.4. Green Roof Design Guidance. Material Specification Structural Capacity shall conform to ASTM E-2397-05, Practice for Determination of Live Loads and Dead Loads Associated with Green (Vegetated) Roof Systems. In addition, use standard test Roof methods ASTM E2398-05 for Water Capture and Media Retention of Geocomposite Drain Layers for Green (Vegetated) Roof Systems, and ASTME 2399-05 for Maximum Media Density for Dead Load Analysis. See Chapter 6 of Weiler and Scholz -Barth (2009) for waterproofing options that are designed to Waterproof Membrane convey water horizontally across the roof surface to drains or gutter. This layer may sometimes act as a root barrier. Root Barrier(Optional) Impermeable liner that impedes root penetration of the membrane. 1 to 2 in. layer of clean, washed granular material, such as ASTM D 448 size No. 8 stone. Roof Drainage Layer drains and emergency overflow shall be designed in accordance with Fayetteville Building Safety Codes. Needled, non -woven, polypropylene geotextile. Filter Fabric Density (ASTM D3776) > 16 oz./sq. yd., or approved equivalent. Puncture resistance (ASTM D4833) > 220 lbs., or approved equivalent. 85% lightweight inorganic materials and 15% organic matter (e.g., well -aged compost). Media Growth Media shall have a maximum water retention capacity of around 30%. Media shall provide sufficient nutrients and water holding capacity to support the proposed plant materials. Determine acceptable saturated water permeability using ASTM E2396-05. Low plants such as sedum, herbaceous plants, and perennial grasses that are shallow -rooted, Plant Materials self-sustaining, and tolerant of direct sunlight, drought, wind, and frost. See ASTM E2400-06, Guide for Selection, Installation and Maintenance of Plants for Green (Vegetated) Roof Systems. SECTION 7: CONSTRUCTION 7.1. Construction Sequence Given the diversity of extensive vegetated roof designs, there is no typical step-by-step construction sequence for proper installation. The following general construction considerations are noted: • Construct the roof deck with the appropriate slope and material. • Install the waterproofing method, according to manufacturer's specifications. • Conduct a flood test to ensure the system is water tight by placing at least 2 inches of water over the membrane for 48 hours to confirm the integrity of the waterproofing system. • Add additional system components (e.g., insulation, optional root barrier, drainage layer and interior drainage system, and filter fabric), taking care not to damage the waterproofing. Drain collars and protective flashing shall be installed to ensure free flow of excess stormwater. Appendix 8 — GSP Specifications Drainage Criteria Manual i , • The growing media shall be mixed prior to delivery to the site. Media should be spread evenly over the filter fabric surface. The growing media shall be covered until planting to prevent weeds from growing. Sheets of exterior grade plywood can also be laid over the growing media to accommodate foot or wheelbarrow traffic. Foot traffic and equipment traffic should be limited over the growing media to reduce compaction. • The growing media shall be moistened prior to planting, and then planted with the ground cover and other plant materials, per the planting plan, or in accordance with ASTM E2400. Plants shall be watered immediately after installation and routinely during establishment. • It generally takes 12 to 18 months to fully establish the vegetated roof. An initial fertilization using slow release fertilizer (e.g., 14-14-14) with adequate minerals is often needed to support growth. Watering is needed during the first summer. Hand weeding is also critical in the first two years (see Table 10.1 of Weiler and Scholz -Barth, 2009, for a photo guide of common rooftop weeds). • Most construction contracts should contain a Care and Replacement Warranty that specifies a 75% minimum survival after the first growing season of species planted and a minimum effective vegetative ground cover of 75% for flat roofs and 90% for pitched roofs. 7.2. Construction Inspection Inspections during construction are needed to ensure that the vegetated roof is built in accordance with these specifications. Detailed inspection checklists shall be used that include sign -offs by qualified individuals at critical stages of construction and confirm that the contractor's interpretation of the plan is consistent with the intent of the designer and/or manufacturer. An experienced installer shall be retained to construct the vegetated roof system. The vegetated roof shall be constructed in sections for easier inspection and maintenance access to the membrane and roof drains. Careful construction supervision is needed during several steps of vegetated roof installation, as follows: • During placement of the waterproofing layer, to ensure that it is properly installed and watertight; • During placement of the drainage layer and drainage system; • During placement of the growing media, to confirm that it meets the specifications and is applied to the correct depth; • Upon installation of plants, to ensure they conform to the planting plan; • Before issuing use and occupancy approvals; and • At the end of the first or second growing season to ensure desired surface cover specified in the Care and Replacement Warranty has been achieved. SECTION 8: AS -BUILT REQUIREMENTS During and after the green roof construction, the developer must have inspections and an as -built certification of the green roof conducted by a registered professional engineer and provided to the City before receipt of the Certificate of Occupancy. The as -built certification verifies that the BMP was installed as Appendix 8 - GSP Specifications Drainage Criteria Manual i , designed and approved. The following components are vital components of a properly working green roof and must be addressed in the as -built certification: • Protection of vulnerable areas (abutting vertical walls, roof vent pipes, outlets, air conditioning units and perimeter areas) from leakage; • Profile view of facility including typical cross-sections with dimensions; • Growing medium specification including dry and saturated weight; • Filter fabric specification; • Drainage layer specification; • Waterproof membrane specification, including root barriers; • Stormwater piping associated with the site, including pipe materials, sizes, slopes, invert elevations at bends and connections; and • Planting and irrigation plan. SECTION 9: MAINTENANCE 9.1. Maintenance Plans It is recommended that a Long Term Maintenance Plan (LTMP) be developed by the design engineer. The LTMP contains a description of the stormwater system components and information on the required inspection and maintenance activities. 9.2. Maintenance Inspections and Ongoing Operations A vegetated roof shall be inspected twice a year during the growing season to assess vegetative cover, and to look for leaks, drainage problems and any rooftop structural concerns (see Table 12.5 below). In addition, the vegetated roof shall be hand -weeded to remove invasive or volunteer plants, and plants/media shall be added to repair bare areas (refer to ASTM E2400). Many practitioners also recommend an annual application of slow release fertilizer in the first five years after the vegetated roof is installed. If a roof leak is suspected, it is advisable to perform an electric leak survey (i.e., Electrical Field Vector Mapping) to pinpoint the exact location, make localized repairs, and then reestablish system components and ground cover. The use of herbicides, insecticides, and fungicides shall be avoided, since their presence could hasten degradation of the waterproof membrane. Also, power -washing and other exterior maintenance operations shall be avoided so that cleaning agents and other chemicals do not harm the vegetated roof plant communities. Appendix 8 - GSP Specifications Drainage Criteria Manual i , Table 12.5. Typical Maintenance Activities Associated with Activity Green R.. Schedule Water to promote plant growth and survival. As needed Inspect the vegetated roof and replace any dead or dying vegetation. (Following Construction) Inspect the waterproof membrane for leaking or cracks. Semi-annually Annual fertilization (first five years). Annually for first five years Weeding to remove invasive plants. As needed Inspect roof drains, scuppers and gutters to ensure they are not overgrown or have organic matter deposits. Remove any accumulated organic matter or debris. Semi-annually Inspect the green roof for dead, dying or invasive vegetation. Plant replacement vegetation as needed. Annually SECTION 10: REFERENCES American Horticultural Society (AHS). 2003. United States Department of Agriculture Plant Hardiness Zone Map. Alexandria, VA. ASTM International. 2005. Standard Test Method for Maximum Media Density for Dead Load Analysis of Green (Vegetated) Roof Systems. Standard E2399-05. ASTM, International. West Conshohocken, PA. ASTM International. 2005. Standard Test Method for Saturated Water Permeability of Granular Drainage Media [Falling -Head Method] for Green (Vegetated) Roof Systems. Standard E2396- 05. ASTM, International. West Conshohocken, PA. ASTM International. 2005. Standard Test Method for Water Capture and Media Retention of Geocomposite Drain Layers for Green (Vegetated) Roof Systems. Standard E2398-05. ASTM, International. West Conshohocken, PA. ASTM International. 2005. Standard Practice for Determination of Dead Loads and Live Loads Associated with Green (Vegetated) Roof Systems. Standard E2397-05. ASTM, International. West Conshohocken, PA. ASTM International. 2006. Standard Guide for Selection, Installation and Maintenance of Plants for Green (Vegetated) Roof Systems. Standard E2400-06. ASTM, International. West Conshohocken, PA. Berhage, R., A. Jarrett, D. Beattie and others. 2007. Quantifying evaporation and transpiration water losses from green roofs and green roof media capacity for neutralizing acid rain. Final Report. National Decentralized Water Resource Capacity Development Project Research Project. Pennsylvania State University. Clark, S., B. Long, C. Siu, J. Spicher and K. Steele. 2008. "Early -life runoff quality: green versus traditional roofs." Low Impact Development 2008. Seattle, WA. American Society of Civil Engineers. Dunnett, N. and N. Kingsbury. 2004. Planting Green Roofs and Living Walls. Timber Press. Portland, Oregon. Maryland Department of Environment. (MDE). 2008. Chapter 5. Environmental Site Design. "Green Roofs." Baltimore, MD. Appendix 8 — GSP Specifications Drainage Criteria Manual i , Miller, C. 2008. Green roofs as stormwater best management practices: Preliminary computation of runoff coefficients: sample analysis in the Mid -Atlantic states. Roofscapes, Inc. Philadelphia, PA. Moran, A., W. Hunt and G. Jennings. 2004. Green roof research of stormwater runoff quantity and quality in North Carolina. NWQEP Notes. No. 114. North Carolina State University. Raleigh, NC. North Carolina State University (NCSU). 2008. Green Roof Research Web Page. Department of Biological and Agricultural Engineering. http://www.bae.ncsu.edu/greenroofs. Northern Virginia Regional Commission (NVRC). 2007. Low Impact Development Manual. "Vegetated Roofs." Fairfax, VA. Schueler et al. 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD. Snodgrass, E. and L. Snodgrass. 2006. Green Roof Plants: a resource and planting guide. Timber Press. Portland, OR. Van Woert, N., D. Rowe, A. Andersen, C. Rugh, T. Fernandez and L. Xiao. 2005. "Green roof stormwater retention: effects of roof surface, slope, and media depth." Journal of Environmental Quality. 34: 1036-1044. VADCR (Virginia Department of Conservation and Recreation). 2011. Stormwater Design Specification No. 5: Vegetated Roof, Version 2.3, Richmond, VA. http://vwrrc.vt.edu/swc/NonProprietaryBMPs.html. Weiler, S. and K. Scholz -Barth 2009. Green Roof Systems: A Guide to the Planning, Design, and Construction of Landscapes over Structure. Wiley Press. New York, NY. Appendix B - GSP Specifications Drainage Criteria Manual i , APPENDIX C SOIL INFILTRATION AND SOIL AMENDMENTS Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual INFILTRATION SOIL TESTING PROCEDURE Test Pit/Boring Procedures 1. One (1) test pit or standard soil boring should be provided for every 1,000 sq ft of the proposed infiltration area. 2. The location of each test pit or standard soil boring should correspond to the location of the proposed infiltration area. 3. Excavate each test pit or penetrate each standard soil boring to a depth at least 2 ft below the bottom of the proposed infiltration area. 4. If the groundwater table is located within 2 ft of the bottom of the proposed facility, determine the depth to the groundwater table immediately upon excavation and again 24 hours after excavation is completed. S. Determine the Unified Soil Classification System (USCS) texture at the base of the proposed infiltration area to a depth 2 ft below the base. Soil components should be classified and described in general accordance with USCS or modified USCS from ground surface to 2 ft below anticipated design depth. 6. If bedrock is located within 2 ft of the bottom of the proposed infiltration area, determine the depth to the bedrock layer. 7. Test pit/soil boring stakes should be left in the field to identify where soil investigations were performed. 8. A map of the test pit / boring locations and logs shall be provided with stratigraphic classifications (in accordance with USCS) and test results. Infiltration Testing Procedures 1. One infiltration test should be conducted for every 1,000 sq ft of surface area for the infiltration area. 2. The location of each infiltration test should correspond to the location of the proposed infiltration area. 3. Install a test casing (e.g., a rigid, 4 to 6 inch diameter pipe) to a depth 2 ft below the bottom of the proposed infiltration area. 4. Remove all loose material from the sides of the test casing and any smeared soil material from the bottom of the test casing to provide a natural soil interface into which water may percolate. If desired, a 2 -inch layer of coarse sand or fine gravel may be placed at the bottom of the test casing to prevent clogging and scouring of the underlying soils. Fill the test casing with clean water to 2 ft above the top of the soil interface, and allow the underlying soils to presoak for 24 hours. S. After 24 hours, refill the test casing with another 2 ft of clean water and measure the drop in water level within the test casing after one hour. Repeat the procedure three (3) additional times by filling the test casing with clean water and measuring the drop in water level after one hour. A total of four (4) observations must be completed. The infiltration rate of the underlying soils may be reported either as the average of all four observations or the value of the last observation. The infiltration rate should be reported in terms of inches per hour. F - Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual I , 6. Infiltration testing may be performed within an open test pit or a standard soil boring. After infiltration testing is completed, the test casing should be removed and the test pit or soil boring should be backfilled and restored. 7. Together with the map of test pit / boring and infiltration test locations, calculation pages reporting infiltration test measurements and results shall be provided. This information shall include verification that the test holes have been appropriately backfilled. Geotechnical logs, calculations and infiltration test results shall be certified by a qualified registered professional engineer or geologist. DESIGN CRITERIA FOR AMENDING SOILS WITH COMPOST AND SOIL MIXES SECTION 1. DESCRIPTION Soil amendments described herein consist of post -construction restoration. This is a practice applied after construction that consists of deeply tilling compacted soils to restore their porosity by amending them with compost. These soil amendments can reduce the generation of runoff from compacted urban lawns or other existing lawnscapes, and may also be used to enhance the infiltration characteristics of downspout disconnections, grass channels, and filter strips. SECTION 2. PHYSICAL FEASIBILITY & DESIGN APPLICATIONS Compost amended soils are suitable as a pervious layer where soils have been or will be compacted by the grading and construction process. This technique can also be used in areas where construction and development have occurred in the past, if infiltration in such areas is low. Soil amendment should be considered when existing soils have low infiltration rates (typically Hydrologic Soil Groups C and D - applicable throughout much of the City of Fayetteville) and when the pervious area will be used to filter runoff (downspout disconnections and grass channels). The area or strip of amended soils should be hydraulically connected to the stormwater conveyance system. In new construction, soil amendment is recommended if mass grading of more than a foot of cut and fill will occur across the site. Compost amendments are not recommended where: • Existing soils have high infiltration rates (e.g., HSG A and B), although compost amendments may be needed to restore mass -graded B soils in order to maintain runoff reduction rates. • The water table or bedrock is located within 1.5 ft of the soil surface. • Slopes exceed 10% for longer than 25 ft in downslope direction. Compost amendments may be considered for such slopes if permanent erosion control measures are designed and approved by the City Engineer. • Existing surface soils are saturated or wet for the extent of the wet season. • They would harm roots of existing trees (keep amendments outside the tree drip line). • The surface drainage is toward an existing or proposed building foundation. F_ Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual I , • The contributing impervious surface area exceeds the surface area of the amended soils. • Areas of additional jurisdiction apply (i.e., Streamside Protection Area, FEMA Special Flood Hazard Area, USACE jurisdictional Wetlands Areas) Compost amendments can be applied to the entire pervious area of a development or be applied only to select areas of the site to enhance the performance of runoff reduction practices. Some common design applications include: • Reduce runoff from compacted lawns. • Enhance rooftop disconnections on poor soils by amending the soils and redirecting roof runoff from impervious surfaces to amended soil areas. • Increase runoff reduction within a grass channel. • Increase runoff reduction within a vegetated filter strip. • Increase the runoff reduction function of a tree cluster or reforested area of the site. SECTION 3. DESIGN CRITERIA Soil Testing Soil tests are required during two stages of the compost amendment process. Initial testing is done to ascertain soil properties in proposed amendment areas prior to restoration activities. The initial testing is to obtain samples and determine soil properties to a depth 1 ft below the proposed amendment area. Soil tests should include bulk density, organic content, moisture content, pH, salts, and soil nutrients (NPK). An infiltration test may also be performed. These tests should be conducted every 5000 sq ft, and are used to determine what, if any, further soil amendments are needed and to characterize potential drainage problems. The second soil test is taken at least one week after the compost has been incorporated into the soils. This soil analysis should be conducted to determine whether any further nutritional requirements, pH adjustment, and organic matter adjustments are necessary for plant growth. This soil analysis should be done in conjunction with the final construction inspection to ensure tilling or subsoiling has achieved design depths and should include the parameters required for initial testing as described herein. If improved infiltration characteristics are included in anticipated runoff reduction, an infiltration test must be performed to confirm improvement. In such cases, antecedent moisture conditions must be documented. Determining Depth of Compost Incorporation The depth of compost amendment is based on the relationship of the surface area of the soil amendment to the contributing area of impervious cover that drains to the amended surface area. Table CA presents some general guidance derived from soil modeling by Holman -Dodds (2004) that evaluates the required depth to which compost must be incorporated. Some adjustments to the recommended incorporation depth were made to reflect alternative recommendations of Roa Espinosa (2006), Balousek (2003), Chollak and Rosenfeld (1998) and others. F_ Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual I , 1. IC = contrib. impervious cover (sq. ft.) and SA = surface area of compost amendment (sq ft). 2. For amendment of compacted lawns that do not receive off-site runoff. 3. In general, IC/SA ratios greater than 1 should be avoided. 4. Average depth of compost added. 5. Lower end for B soils, higher end for C/D soils. Once the area and depth of the compost amendments are known, the designer can estimate the total amount of compost needed, based on the equation below: Where: Equation 8.1. Compost Quantity Estimation C = compost needed (cu. yds.) A = area of soil amended (sq. ft.) D = depth of compost added (in.) C=A*D*0.0031 3.3 Compost Specifications The basic material specifications for compost amendments are outlined below: Compost shall be derived from plant material and provided to meet the minimum following requirements: The compost shall be the result of the biological degradation and transformation of plant -derived materials under conditions that promote anaerobic decomposition. The material shall be well composted, free of viable weed seeds, and stable with regard to oxygen consumption and carbon dioxide generation. The compost shall have a moisture content that has no visible free water or dust produced when handling the material. It shall meet the following criteria, as reported by the U.S. Composting Council STA Compost Technical Data Sheet provided by the vendor: 0 100% of the material must pass through a half inch screen 0 The pH of the material shall be between 6 and 8 0 Manufactured inert material (plastic, concrete, ceramics, metal, etc.) shall be less than 1.0% by weight 0 The organic matter content shall be between 35% and 65% 0 Soluble salt content shall be less than 6.0 mmhos/cm 0 Maturity should be greater than 80% 0 Stability shall be 7 or less F_ Appendix C— Soil Infiltration and Soil Amendments Drainage Criteria Manual I I o Carbon/nitrogen ratio shall be less than 25:1 Heavy metals test results within limits of the USEPA 40CFR Part 503, for applicable sources The compost must have a dry bulk density ranging from 40 to 50 lbs./cu.ft. SECTION 4. CONSTRUCTION Construction Sequence The construction sequence for compost amendments differs depending whether the practice will be applied to a narrow filter strip, such as in a rooftop disconnection or grass channel or a large area. For larger areas, a typical construction sequence is as follows: Step 1. Prior to building, the proposed area should be deep tilled to a depth of 2 to 3 ft using a tractor and sub-soiler with two deep shanks (curved metal bars) to create rips perpendicular to the direction of flow. (This step is usually omitted when compost is used for narrower filter strips.) Step 2. A second deep tilling to a depth of 12 to 18 inches is needed after final grading of individual building lots is complete. Step 3, if required. Dewater to ensure dry conditions at the site prior to incorporating compost. Step 4. Using a roto -tiller or similar equipment, incorporate a compost mix meeting the requirements per Section 3 into the soil, at the volumetric rate of 1 part compost to 2 parts soil. Step 5. The site should be leveled and seeds or sod used to assist in establishing a vigorous grass cover, where individual seedling or container planting is not required. Lime or irrigation may initially be needed to help the grass grow quickly. A general fertilizer such as 13-13-13 would work as a starter and is readily available if needed. Do not add a high nitrogen fertilizer as it can burn seedlings. Step 6. Employ simple erosion control measures, such as silt fence, to reduce the potential for erosion of and trap sediment from areas receiving compost amendments and exceeding 2500 sq ft in size. SECTION 5. ENGINEERED SOIL MIXES FOR BIORETENTION Engineered soils are an option for Bioretention GSP soil media when infiltration rates of the post construction soils do not meet acceptable infiltration rates. As stated in the Bioretention GSP-01 specification, post construction soils should exhibit a minimum infiltration rate of 1 inch/hr in accordance with the infiltration testing procedure in this manual. Engineered Soil Requirements Two soil mix designs are provided herein. These soil mixes may be used for soil filter media in bioretention practices. The soil mix design should be selected based upon the desired infiltration characteristics and regional availability of materials. Since the post -construction bioretention feature must provide adequate infiltration capacity, pre- and post -construction infiltration testing of the mix design should be performed F_ Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual I , using materials known to be readily available. For economical reasons, on-site soil material should be used provided infiltration characteristics can be met. In this case, additional pre -construction tests should include particle size, or gradation analyses, of these soils in accordance with ASTM D-422, and Atterberg Limits tests on the fines in accordance with ASTM D-4318. Soil Mix A For the best infiltration characteristics and longevity (minimal clogging of pore space), a relatively well - graded sand -compost mixture with minimum clay content (to restrict infiltration rates and achieve the desired 24-hour retention time) is recommended, following MPCA (2005): Concrete Sand (Sand meeting ASTM C-33 gradation): 60%-80% Compost': 15%-30% Natural soil materialz: 5%-10% 1. With a required organics component of the compost ranging from 30%-60%, the proportion of compost should be limited so the total organic content of the resulting soil mix does not exceed 8%, if species from the native vegetation list are used. Where non -natives are specified and approved, higher organic content may be permissible. 2. The total clay content of the resulting soil mix may not exceed 5%. If used, clay must below plasticity (CL) per Unified Soil Classification System (USCS). If topsoil containing clay is used, post -construction infiltration tests must be performed to ensure adequate infiltration rate. Soil Mix B This soil mix allows for incorporation of additional natural soil material with the expectation that existing on-site soil material containing silt and clay will be used where possible. On-site silts and clays of low plasticity (ML and CL) may only be used if infiltration and hydraulic conductivity analyses indicate the design infiltration rate can be achieved. During mix preparation, the on-site soil material must be judiciously combined with the concrete sand to achieve the desired sand and fine ratio, to ensure permeability of the resulting soil mix is not too low. The compost material may be added last. The exact composition of organic matter and soil material will vary depending on the native soils available for use, thus an exact design is difficult to develop without a prior evaluation of available materials. (MWS, 2011). After construction, infiltration tests should be performed (see the recommended ratio per GSP-01) to show that the minimum initial infiltration rate is achieved: Concrete Sand (Sand meeting ASTM C-33 gradation): 40%-70% Silt: 0%-40% Compost': 15%-30% Clayz: 0%-20% 1. With a required organics component of the compost ranging from 30%-60%, the proportion of compost should be limited so the total organic content of the resulting soil mix does not exceed 8%, if species from the native vegetation list are used. Where non -natives are specified and approved, higher organic content may be permissible. F_ Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual I , 2. The total clay content of the resulting soil mix may not exceed 5%. If used, clay must below plasticity (CL) per Unified Soil Classification System (USCS). If soil containing clay is used, post -construction infiltration tests must be performed to ensure adequate infiltration rate. REFERENCES Minnesota Pollution Control Agency, 2008, Minnesota Stormwater Manual, Version 2, Chapter 12-6 Bioretention, St. Paul, MN. Virginia Department of Conservation and Recreation, 2011, Virginia DCR Stormwater Design Specification No. 4 Soil Compost Amendment, Version 1.8, Richmond, VA. F_ Appendix C- Soil Infiltration and Soil Amendments Drainage Criteria Manual I , APPENDIX D NATIVE PLANTS FOR USE IN BIORETENTION r Appendix D - Native Plants for Use in eioretention Drainage Criteria Manual NATIVE PLANTS FOR USE IN BIORETENTION The following is a list of plant options for installation in bioretention basins. Additional plants can be recommended by the licensed Landscape Architect. Planting plans are subject to City approval. Latin Name Table D.1. Native Perennials Common Name for Bioretention Size — Full Sun Spacing Moisture Color Height Amsonia tabernaemontana Bluestar Plugs —1 gal. 1plant/18" o.c. Moist Blue 2-3' Asclepias incarnata Marsh milkweed Plugs —1 gal. 1 plant/24" o.c. Wet Pink 34 Asclepias synaca Common milkweed Plugs —1 gal. 1 plant/18" o.c. Moist -dry Orange 2-5' Asclepias tuberosa Butterfly milkweed Plugs —1 gal. 1 plant/18" o.c. Dry -moist Orange 2' Asclepias verdis Green milkweed Plugs —1 gal. 1 plant/18" o.c. moist Green 2' Asclepias verdicillata Whorled milkweed Plugs —1 gal. 1 plant/18" o.c. moist White 2.5' Asterlaevis Smooth aster Plugs —1 gal. 1 plant/18" o.c. moist Blue 24 Astersericeus Silky aster Plugs —1 gal. 1 plant/18" o.c. dry Purple 1-2' Baptisia australis Wild indigo Plugs —1gal. 1 plant/24" o.c. Dry Blue 1.5-2.5' Chamaecrista fasciculata Partridge pea Plugs —1 gal. 1 plant/18" o.c. dry Yellow 1-2' Conoclinium coelestinum Mist flower Plugs —1 gal. 1 plant/18" o.c. Moist -dry Blue 1-2' Coreopsis tinctoria Coreopsis Plugs —1 gal. 1 plant/18" o.c. Moist -dry Yellow 2-3' Echinacea pallida Pale purple coneflower Plugs —1 gal. 1 plant/18" o.c. dry Purple 2-3' Echinacea purpurea Purple coneflower Plugs —1 gal. 1 plant/18" o.c. Moist -dry Purple 34 Eupatorium perfoliatum Boneset Plugs —1 gal. 1 plant/24" o.c. wet White 3-5' Eupatorium purpureum Sweet Joe-Pye Weed Plugs —1 gal. 1 plant/24" o.c. Wet -moist Purple 3-6' Hibiscus moscheutos Swamp mallow Plugs —1 gal. 1 plant/36" o.c. Wet White -Pink 3-5' Iris virginica Flag Iris Plugs —1 gal. 1 plant/18" o.c. Moist -Wet Blue 2' Liatrisspicata Dense blazingstar Plugs —1 gal. 1 plant/24" o.c. Moist -dry Purple 1.5' Lobelia cardinalis Cardinal flower Plugs —1 gal. 1 plant/18" o.c. Wet -moist Red 24 Monarda didyma Bee balm Plugs —1 gal. 1 plant/24" o.c. Wet -moist Red 3' Monarda fistulosa Wild bergamot Plugs -1 gal 1 plant/18" o.c. Moist -dry Purple 1-3' Oenethera speciosa Evening primrose Plugs -1 gal 1 plant/18" o.c. Moist -dry Pink 1-2' Penstemon digitalis Smooth white beardtongue Plugs -1 gal 1 plant/24" o.c. Wet White 2-3' Pycanthemum albescens Mountain mint Plugs -1 gal 1 plant/18" o.c. Moist White 1.5-2.5' Physostegia virginiana False dragonhead Plugs —1gal 1 plant/18" o.c. Wet -moist White 4-6' Rudbeckia hirta Black-eyed Susan Plugs -1 gal 1 plant/18" o.c. Moist -dry Yellow 3' Solidago sp. Goldenrod Plugs —1 gal. 1 plant/18" o.c. Moist -dry Yellow 1-6' Tradescantia sp. Spiderwort Plugs -1 gal. 1 plant/18" o.c. Moist -dry Blue 1.5-3' Verbena canadensis Verbena Plugs -1 gal. 1 plant/18" o.c. Moist -dry Purple 0.5-1.5' Veronia gigantea Giant ironweed Plugs -1 gal. 1 plant/24" o.c. Wet -moist Purple 34 Yucca sp. Yucca 1 gal. 1plant/24" o.c. Dry White 2-5' Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I Latin Name Table D.2. Native Common Name Perennials for Size Bioretention — Shade. Spacing Moisture Color Height Adiantum pedatum Maidenhair fern 1 gal. 1 plant/18" o.c. Moist Green 1-1.5' Aquilegia canadensis Wild columbine Plugs -1 gal. 1 plant/18" o.c. Moist -dry Pink 1-2.5' Athyrium filix femina Lady Fern 1 gal. 1 plant/18" o.c. Moist Green 3' Arisaema triphyllum Jack-in-the-pulpit Plugs -1 gal. 1 plant/18" o.c. Moist Green 1.5-2.5' Arisaema dricontium Green dragon Plugs -1 gal. 1 plant/18" o.c. Wet -moist Green 3' Asarum canadense Wild ginger Plugs -1 gal. 1 plant/18" o.c. Wet -moist Red -brown 0.5-1' Aster cardifolius Blue wood aster Plugs -1 gal. 1 plant/18" o.c. Moist -dry Blue 1-3' Coreopsis lanceolata Tickseed coreopsis Plugs -1 gal. 1 plant/18" o.c. Moist -dry Yellow 3' Delphinium sp. Larkspur Plugs -1 gal. 1 plant/18" o.c. Moist -dry Blue -purple 14 Geranium maculatum Wild geranium Plugs -1 gal. 1 plant/18" o.c. Moist Pink 2' Heuchera americana Alumroot Plugs -1 gal. 1 plant/18" o.c. Moist -dry Purple 1' Iris cristata Dwarf crested iris Plugs -1 gal. 1 plant/18" o.c. Moist -dry Purple 4" Lobeliasiphilicata Great blue lobelia Plugs -1 gal. 1 plant/18" o.c. Wet -moist Blue 1.54 Lobelia cardinalis Cardinal flower Plugs -1 gal. 1 plant/18" o.c. Wet -moist Red 24 Onoclea sensibilis Sensitive fern 1 gal. 1 plant/24" ox Wet -moist Green 2-3.5' Osmunda cinnamomea Cinnamon Fern 1 gal. 1 plant/24" o.c. Wet -moist Green 34 Phlox divaricata Blue phlox Plugs -1 gal. 1 plant/18" o.c. moist Blue 0.5-2' Polygonatum biflorum Solomon's seal Plugs -1 gal. 1 plant/18" o.c. Moist Green 2-4' Polystichum acrostichoides Christmas fern Plugs -1 gal. 1 plant/24" o.c. Moist -dry Evergreen 2' Sedges,Table D.3. Native Grasses, Latin Name Common Name Size Spacing Moisture Color Height Acorus calamus Sweet flag 1 gal. 1 plant/24" o.c. Moist Green 3-5' Andropogon gerardii Big bluestem 1 gal. 1 plant/24" o.c. Moist Green 4-7' Andropogon virginicus Broom -sedge 1 gal. 1 plant/24" o.c. Moist -dry Green 24 Carex sp. Sedge 1 gal. 1 plant/24" o.c. Moist Green 24 Carex grayi Gray's Sedge 1 gal. 1 plant/24" o.c. Moist Green 3' Carex muskingumensis Palm Sedge 1 gal. 1 plant/24" o.c. Moist Green 3' Carex stricta Tussock Sedge 1 gal. 1 plant/24" o.c. Moist Green 34 Chasmanthium latifolium Upland Sea Oats Plugs —1 gal. 1 plant/18" o.c. Moist -dry Green 4' Juncus effesus Soft Rush Plugs —1 gal. 1 plant/24" o.c. Wet -dry Green 3-5' Muhlenbergia capallaris Muhly Grass 1 gal. 1 plant/24" o.c. Moist Pink 3' Panicum virgatum Switchgrass 1-3 gal. 1 plant/48" o.c. Moist -dry Yellow 5-7' Schizachyrium scoparium Little Blue Stem 1 gal. 1 plant/24" o.c. Moist -dry Yellow 3' Sorghastrum nutans Indian grass 1 gal. 1 plant/24" o.c. Moist -dry Green 24 Stipa sp. Needle grass 1 gal. 1 plant/24" o.c. Dry Green 1-3' Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I DT = Drought Tolerant. FT = Flood Tolerant. Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I Table D.4. Native Trees for .. FCo Latin Name Common Name Light Moisture Notes Height FDT orr Acerrubrum Red Maple DT -FT Sun -shade Dry -wet Fall color 50-70' Sun -pt Acersaccharum Sugar Maple Moist Fall color 50-75' shade Ameleanchier Sun -pt Serviceberry Moist -wet Eatable berries White 15-25' Canadensis shade Sun -pt Asimina triloba Paw Paw Moist Eatable fruits Maroon 15-30' shade Sun-pt Betula nigra River Birch FT Exfoliating bark 40-70' shadeMoist-wet Sun-pt Carpinuscaroliniana Ironwood Interesting trunk White 40-60' shadeMoist Carya aquatica Water Hickory FT -DT Sun Moist Fall color 35-50' Mockernut Carya tomentosa Sun Moist Fall color, fruit 60-80' Hickory Pea -like flowers, CercisCanadensis Redbud DT Sun -shade Moist purple 20-30' seed pods Cladratis lutea Yellowwood DT Sun Dry -moist Fall color White 30-45' Flowering Cornus flonda Part shade Moist Red fruit, wildlife White 15-30' Dogwood American Sun -Pt Cotinus obovatus DT Dry Fall color Yellow 15-30' smoketree Shade Diospyros Sun -Pt Moist- Fall color, edible Persimmon FT 30-50' virginiana shade Wet fruit Liquidambar Sun -pt 60- Sweetgum DT -FT Dry -moist Spiny fruit styraciflua shade 100' Nyssasylvatica Black Gum Sun -Shade Moist Fall color 35-50' 70- Platanus occidentalis Sycamore FT Moist White mottled bark shade 00' Prunus serotina Black Cherry DT Sun Dry -moist Fruit, wildlife White 60-80' Quercus alba White Oak Sun Dry -Moist Bark, Acorns 65-85' Swamp White Sun -pt Quercus bicolor DT Moist -wet Acorns 50-60' Oak shade Quercus lyrata Overcup Oak FT Sun Moist Acorns 40-60' Quercus rubra Northern Red Oak Sun Dry -Moist Acorns, Fall Color 70-90' Carolina Rhamnuscaroliniana Sun Moist Black fruit 15-30' Buckthorn Sun -pt Sassafras albidum Sassafras Moist -dry Fall Color 20-50' shade Ulmus americana Sun -pt American Elm DT -FT Moist Fall color 50-80' var. "Valley Forge" shade DT = Drought Tolerant. FT = Flood Tolerant. Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I DT = Drought Tolerant. FT = Flood Tolerant. Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I Table D.5. Native Shrubs for Bioretention DT Flower Latin Name Common Name Light Moisture Notes Height FT Color Sun- Buddleia davidii Butterfly Bush DT Dry -moist Non-native Blue 5' pt shade Sun- Callicarpa americana American Beautyberry DT Dry -moist Showy purple fruit Lilac 4-6' pt shade Cephalanthus Moist- Button Bush FT Sun -shade Attracts wildlife White 6-12' occidentalis wet Chionanthus Sun- Panicled, fragrant Fringetree Moist White 12-20 , virginicus pt shade flowers Moist- Blue berries, Cornus amomum Silky Dogwood FT Sun -shade White 6-12' wet wildlife Sun- Eatable nuts, Corylus americana American Hazelnut Dry -moist Yellow 8-15' pt shade wildlife Hamemelis Sun- u n- Witch -hazel Dry -moist Winter bloom Yellow 10' virginiana vi pt shade Shrubby St. John's Sun- Hypericum prolificum DT Dry -moist Semi -evergreen Yellow 3' Wort pt shade flex decidua Deciduous holly FT Sun- Moist Red berries Green 15-20' pt shade Pt shade— Moist- Lindera benzoin Spicebush DT Butterflies, wildlife Yellow 6-12' shade wet Sun - Ptelea trifoliata Wafer -ash Moist Interesting fruit White 6-15' pt shade Pt shade- Showy blooms, Rhododendron sp. Rhododendron/azalea DT Moist -dry Various 4-6' shade butterflies Salix humilis Prairie willow DT Sun Moist -dry Fruit, fall color Green 2-8' Sambucus Sun- American elderberry Moist Purple berries White 6-10' cqnadensis pt shade Pt shade- Showy fruit, Staphlea trifolia Bladdernut Moist White 4-12, shade butterflies Viburnum dentatum Arrowwood Viburnum Sun -shade Dry -wet Wildlife White 6-8' Sun- Viburnum rufidulum Blackhaw Viburnum DT Dry -moist Wildlife White 10-15' pt shade DT = Drought Tolerant. FT = Flood Tolerant. Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I POPULAR PLANTS SUITABLE FOR TREE PLANTERS IN FAYETTEVILLE Hydrophytic species -wet gardens Upland species -well -drained gardens Trees Chionanthus virginiana* fringe tree Ilex decidua deciduous holly Trees Fraxinus pennsylvanica green ash Sassafras albidum sassafras Diospyros virginiana persimmon Cercis canadensis* redbud Acer rubrum red maple Amelanchier arborea serviceberry Liquidambarstyraciflua sweetgum Carya tomentosa mockernut hickory Betula nigra river birch Cornus florida* flowering dogwood Asimina triloba* paw -paw Cotinus obovatus American smoketree Celtis laevigata sugarberry Prunus serotina black cherry Penstomen digitalis foxglove beardtongue Quercus alba white oak Yucca Lobelia silphilitica big lobelia Quercus rubra red oak Lobelia cardinalis cardinal flower Verbena canadensis Fraxinus americana white ash Shrubs Shrubs Cephalanthus occidentalis* buttonbush Chionanthus virginiana* fringe tree Ilex decidua deciduous holly Virburnum molle* viburnum Lindera benzoin* spicebush Ptelea trifoliata wafer ash Staphlea trifolia bladdernut Buddlea davidii** butterfly bush Hamamelis virginiana* witch hazel Rhodendron sp. (Azalea)* azalea/rhododendron Sambucus canadensis elderberry Rhus coppalinum smooth sumac Salix humilis prairie willow Herbaceous Herbaceous Asclepias incarnata swamp milkweed Rudbeckia hirta black-eyed susan Physostegia virginiana false dragonhead Liatris spicata spiked gayfeather Pycnanthemum albescens mountain -mint Asclepias tuberosa butterfly milkweed Solidago sp. goldenrod Solidago sp. goldenrod Aster sp. Aster Coreopsis tinctoria golden tickseed Amsonia tabernaemontana bluestar Penstomen digitalis foxglove beardtongue Eupatorium sp. joe-pye weed,boneset Yucca sp. Yucca Lobelia silphilitica big lobelia Baptisia australis wild indigo Lobelia cardinalis cardinal flower Verbena canadensis verbena Hibiscus moscheutos swamp mallow Monarda sp. bee balm Acorus calamus sweet flag Echinacea purpurea purple coneflower Arisaema triphyllum jack-in-the-pulpit Aster sp. Aster Onoclea sensibilis* sensitive fern Tradescantia sp. * spiderwort Juncus effusus soft rush Delphinium sp. larkspur Andropogon gerardii big bluestem Phlox sp. * Phlox Andropogon virginicus broom -sedge Oenothera speciosa evening primrose Carex sp. (various caric sedges) Caric sedge Panicum virgatum switchgrass Cyperus sp. flatsedge Sorghastrum nutans Indian grass Iris sp. Iris Stipa sp. needle grass Equisetum hymale horsetail Andropogon virginicus broom -sedge Polygonatum biflorum Solomon's seal Adian tum pedatum* northern maiden -hair fern F_ Appendix D — Native Plants for Use in Bioretention Drainage Criteria Manual I APPENDIX E DETENTION STRUCTURAL CONTROLS DRY DETENTION/DRY ED BASINS Description: A surface storage basin or facility designed to provide water quantity control through detention and/or extended detention of stormwater runoff. Maintenance Burden © L = Low M = Moderate H = High Stormwater Management Capability: • Reduction in peak rate of runoff • Provides peak flow • Does not provide water discharge attenuation quality treatment Land Use Considerations: Design Considerations: Residential • Applicable for drainage areas up to 75 acres • Typically less costly than stormwater (wet) ponds for equivalent Commercial flood storage, as less excavation is required • Used in conjunction with water quality structural controls Industrial • Recreational and other open space opportunities between storm runoff events Maintenance: • Remove debris from basin surface to minimize outlet clogging and improve aesthetics • Removed sediment buildup • Repair and revegetate eroded areas • Perform any needed structural repairs to inlet and outlets • Mow to limit unwanted vegetation Maintenance Burden © L = Low M = Moderate H = High SECTION 1: DESCRIPTION Dry detention and dry extended detention (ED) basins are surface facilities intended to provide for the temporary storage of stormwater runoff to reduce downstream water quantity impacts. These facilities temporarily detain stormwater runoff, releasing the flow over a period of time. They are designed to completely drain following a storm event and are normally dry between rain events. Dry detention basins are intended to provide overbank flood protection (peak flow reduction of the 25 -year storm, Qpzs) and can be designed to control the extreme flood (100 -year, Qf) storm event. Dry ED basins provide downstream channel protection through extended detention of the channel protection volume (CP,), and can also provide Qpzs and Qf control. Both dry detention and dry ED basins provide limited pollutant removal benefits and are not intended for water quality treatment. Detention -only facilities should be used in a treatment train approach with other structural controls that provide treatment of the WQ„ (see Chapter 4, Water Quality). Compatible multi -objective use of dry detention facilities is strongly encouraged. SECTION 2: PLANNING AND DESIGN CRITERIA Location Dry detention and dry ED basins are to be located downstream of other structural stormwater controls providing treatment of the water quality volume (WQ,). See Chapter 4 for more information on the use of multiple structural controls in a treatment train. • The maximum contributing drainage area to be served by a single dry detention or dry ED basin is 75 acres. General Design • Dry detention basins are sized to temporarily store the volume of runoff required to provide overbank flood (Qpzs) protection (i.e., reduce the post -development peak flow of the 25 -year storm event to the pre -development rate), and control the 100 -year storm (Qf) if required. • Dry ED basins are sized to provide extended detention of the channel protection volume (CPv) over 24 hours and can also provide additional storage volume for normal detention (peak flow reduction) Of Qpzs and Qf. • Routing calculations must be used to demonstrate that the storage volume is adequate. See Chapter 7 for procedures on the design of detention storage. • Storage volumes greater than 50 acre-feet are subject to the requirements of the Arkansas Natural Resources Commission (ANRC) Title VII, Rules Governing Design and Operation of Dams. • Vegetated embankments shall be less than 20 ft in height and shall have side slopes no steeper than 2:1 (horizontal to vertical) although 3:1 is preferred. Riprap-protected embankments shall be no steeper than 2:1. Geotechnical slope stability analysis is recommended for embankments greater than 10 feet in height and is mandatory for embankment slopes steeper than those given above. • The maximum depth of the basin should not exceed 10 feet. Areas above the normal high water elevations of the detention facility should be sloped toward the basin to allow drainage and to prevent standing water. Careful finish grading is required to avoid creation of upland surface depressions that may retain runoff. The bottom area of storage facilities should be graded toward the outlet to prevent standing water conditions. A low flow or pilot channel across the facility bottom from the inlet to the outlet (often constructed with riprap) is recommended to convey low flows and prevent standing water conditions. • Adequate maintenance access must be provided for all dry detention and dry ED basins. Inlet and Outlet Structures • Inflow channels are to be stabilized with flared riprap aprons, or the equivalent. A sediment forebay sized to 0.1 inches per impervious acre of contributing drainage should be provided for dry detention and dry ED basins that are in a treatment train with off-line water quality treatment structural controls. • For a dry detention basin, the outlet structure is sized for Qpzs control (based upon hydrologic routing calculations) and can consist of a weir, orifice, outlet pipe, combination outlet, or other acceptable control structure. Small outlets that will be subject to clogging or are difficult to maintain are not acceptable. For a dry ED basin, a low flow orifice capable of releasing the channel protection volume over 24 hours must be provided. The channel protection orifice should have a minimum diameter of 3 inches and should be adequately protected from clogging by an acceptable external trash rack. The orifice diameter may be reduced to 1 inch if internal orifice protection is used (e.g., an overperforated vertical stand pipe with 0.5 -inch orifices or slots that are appropriately protected. Recommended details for clogging prevention are provided in Appendix G. Adjustable gate valves can also be used to achieve this equivalent diameter. See Appendix G (Detention Outlet Structure Design) for more information on the design of outlet works. • Seepage control or anti -seep collars should be provided for all outlet pipes. • Riprap, plunge pools or pads, or other energy dissipaters are to be placed at the end of the outlet to prevent scouring and erosion. If the basin discharges to a channel with dry weather flow, care should be taken to minimize tree clearing along the downstream channel, and to reestablish a forested riparian zone in the shortest possible distance. See Chapter 6 for more guidance on energy dissipation design. • An emergency spillway is to be included in the stormwater pond design to safely pass the extreme flood flow. The spillway prevents pond water levels from overtopping the embankment and causing structural damage. The emergency spillway must be designed to convey the 100 -year storm event and must be located so that downstream structures will not be impacted by spillway discharges. • A minimum of 1 ft of freeboard must be provided, measured from the top of the water surface elevation for the extreme flood, to the lowest point of the dam embankment not counting the emergency spillway. SECTION 3. TYPICAL SCHEMATIC DETAILS INFI RIPRAP NKMENT EMERGENCY SPILLWAY PLAN VIEW EMBANKMENT EMERGENCY INFLOW 100 YEAR LEVEL SPILLWAY 25 YEAR LEVEL - - - - - - STABLE OUTFALL BARREL / PROFILE Figure DSC -E.1. Schematic of dry detention basin. INFI NKMENT MERGENCY SPILLWAY PLAN VIEW RIPRAP EMBANKMENT 1 EMERGENCY INFLOW i 100 YEAR LEVEL SPILLWAY 25 YEAR LEVEL ------ CP, LEVEL -----CP„LEVEL STABLE OUTFALL RISER --- BARREL / LOW FLOW/_ ORIFICE PROFILE Figure DSC -E.2. Schematic of dry extended detention basin. SECTION 4: INSPECTION AND MAINTENANCE REQUIREMENTS Table DSC -E.1. Typical Maintenance Activities for Dry Detention / Dry ED Basins Urban(Source: Denver Activity Schedule • Remove debris from basin surface to minimize outlet clogging and improve Annually and following aesthetics. significant storm events • Remove sediment buildup. As needed based on • Repair and revegetate eroded areas. inspection • Perform structural repairs to inlet and outlets. • Mow to limit unwanted vegetation. Routine SECTION 5: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume Z: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf. Urban Drainage and Flood Control District. 1999. Criteria Manual, Denver, CO. MULTI-PURPOSE DETENTION AREAS Description: A facility designed primarily for another purpose, such as parking lots and rooftops that can provide water quantity control through detention of stormwater runoff. • Provides peak flow • Does not provide water quality attenuation treatment • Allows for multiple uses of site areas and reduces the need for downstream detention facilities • Should be used in conjunction with water quality structural controls • Adequate grading and drainage must be provided to allow full use of facility's primary purposes following a storm event Stormwater Management Capability: • Reduction in peak rate of runoff discharge Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Remove debris from ponding area to minimize outlet clogging and improve aesthetics • Remove sediment buildup • Repair and revegetate eroded areas • Perform structural repairs to inlet and outlets • Perform additional maintenance activities specific to the type of facility © Maintenance Burden L = Low M = Moderate H = High SECTION 1. DESCRIPTION Multi-purpose detention areas are site areas primarily used for one or more specific activities that are also designed to provide for the temporary storage of stormwater runoff to reduce downstream water quantity impacts. Example of multi-purpose detention areas include: • Parking Lots • Rooftops • Sports Fields • Recessed Plazas Multi-purpose detention areas are normally dry between rain events, and by their very nature must be useable for their primary function the majority of the time. As such, multi-purpose detention areas should not be used for extended detention (CPQ control). Multi-purpose detention areas are not intended for water quality treatment and must be used in a treatment train approach with other structural controls that provide treatment of the WQ, (see Chapter 4, Section 4.4, Using Structural Stormwater Controls in Series). SECTION 2: PLANNING AND DESIGN CRITERIA Location Multi-purpose detention areas can be located upstream or downstream of other structural stormwater controls providing treatment of the water quality volume (WQ„). See Section 4.4 for more information on the use of multiple structural controls in a treatment train. General Design • Multi-purpose detention areas are sized to temporarily store the peak flow from the 2-, 10-, and 25 -year storm events and be capable of safely conveying the 100 -year storm (Qf). The 2- and 10 -year storm events must be detained since extended detention for the 1 -year event is not provided with this structural practice. • Routing calculations must be used to demonstrate that the storage volume is adequate. See Chapter 7 for procedures on the design of detention storage. • All multi-purpose detention facilities must be designed to minimize potential safety risks, potential property damage, and inconvenience to the facility's primary purposes. Emergency overflows are to be provided for the 50- and 100 -year events. The overflow must not create a significant adverse impact to downstream properties or the conveyance system. If adverse impact to downstream properties is possible, then the 100 -year event must also be detained. • All types of multi -use detention is subject to the approval of the City Engineer. Parking Lot Storage Parking lot detention can be implemented in areas where portions of large, paved lots can be temporarily used for runoff storage without significantly interfering with normal vehicle and pedestrian traffic. Parking lot detention can be created in two ways: by using ponding areas along sections of raised curbing, or through depressed areas of pavement at drop inlet locations. • The maximum depth of detention ponding in a parking lot, except at a flow control structure, should be 6 inches for a 10 -year storm, and 9 inches for a 100 -year storm. The maximum depth of ponding at a flow control structure is 12 inches for a 100 -year storm. • The storage area (portion of the parking lot subject to ponding) must have a minimum slope of 0.5% towards the outlet to ensure complete drainage following a storm. A slope of 1% or greater is recommended. • Fire lanes used for emergency equipment must be free of ponding water for runoff events up to the extreme storm (100 -year) event. • Flows are typically backed up in the parking lot using a raised inlet. Rooftop Storage • Rooftops can be used for detention storage as long as the roof support structure is designed to address the weight of ponded water and is sufficiently waterproofed to achieve a minimum service life of 30 years. All rooftop detention designs must meet Arkansas Fire Prevention Code and the City of Fayetteville's building code requirements. • The minimum pitch of the roof area subject to ponding is 0.25 inches per foot. • The rooftop storage system must include another mechanism for draining the ponding area in the event that the primary outlet is clogged. Snorts Fields • Athletic facilities such as football and soccer fields and tracks can be used to provide stormwater detention. This is accomplished by constructing berms around the facilities, which in essence creates very large detention basins. Outflow can be controlled through the use of an overflow weir or other appropriate control structure. Proper grading must be performed to ensure complete drainage of the facility. Public Plazas • In high-density areas, recessed public common areas such as plazas and pavilions can be utilized for stormwater detention. These areas shall be designed to flood no more than once or twice annually (i.e., 1 -year or 2 -year storm events), and provide important open recreation space during the rest of the year. SECTION 3. INSPECTION AND MAINTENANCE REQUIREMENTS Multi-PurposeTable DSC E.1 Typical Maintenance Activities for Urban(Source: Denver Activity Schedule • Remove debris from ponding area to minimize outlet clogging and improve Annually and following aesthetics. significant storm events • Remove sediment buildup. As needed based on • Repair and revegetate eroded areas. inspection • Perform structural repairs to inlet and outlets. • Perform additional maintenance activities specific to the type of facility. As required SECTION 4. REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Urban Drainage and Flood Control District. 1999. Criteria Manual, Denver, CO. UNDERGROUND DETENTION Description: Detention storage located in underground tanks, vaults, or pipe systems designed to provide water quantity control through detention and/or extended detention of stormwater runoff. Source: City of Clemson, South Carolina • Provides peak flow • Does not provide water quality attenuation treatment • Can be used in space limited applications • Does not take up surface space • Should be used in conjunction with water quality structural control • Concrete vaults or pipe/tank systems can be used • Maintenance access must be considered during design of the system Stormwater Management Capability: • Reduction in peak rate of runoff discharge Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Remove any trash/debris and sediment buildup in the underground vaults or tanks • Perform structural repairs to inlets and outlets ©Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION • Detention vaults are box -shaped underground stormwater storage facilities typically constructed with reinforced concrete. Detention tanks are underground storage facilities typically constructed with large diameter metal or plastic pipe. Both serve as an alternative to surface dry detention for stormwater quantity control, particularly for space -limited areas where there is not adequate land for a dry detention basin or multi-purpose detention area. • Both underground vaults and tanks can provide channel protection through extended detention of the channel protection volume (CP,), and overbank flood Qpzs (and in some cases extreme flood Qf) control through normal detention. Basic storage design and routing methods are the same as for detention basins except that the bypass for high flows is typically included. • Underground detention vaults and tanks are not intended for water quality treatment and should be used in a treatment train approach with other structural controls that provide treatment of the WQv (see Chapter 4). This will prevent the underground vault or tank from becoming clogged with trash or sediment and significantly reduces the maintenance requirements for an underground detention system. • Prefabricated concrete vaults are available from commercial vendors. In addition, several pipe manufacturers have developed packaged detention systems. SECTION 2. PLANNING AND DESIGN CRITERIA Location • Underground detention systems are to be located downstream of other structural stormwater controls that provide treatment of the water quality volume (WQ,). The maximum contributing drainage area to be served by a single underground detention vault or tank is 25 acres. General Design Underground detention systems are sized to provide extended detention of the channel protection volume over 24 hours and temporarily store the volume of runoff required to provide overbank flood (Qp25) protection (i.e., reduce the post -development peak flow of the 25 -year storm event to the pre - development rate). Due to the storage volume required, underground detention vaults and tanks are typically not used to control the 100 -year storm (Qf) except for very small drainage areas (<1 acre). If Qf is not detained, a properly designed bypass must be provided. • Routing calculations must be used to demonstrate that the storage volume is adequate. See Chapter 7 for procedures on the design of detention storage. Detention Vaults: Minimum 3,000 psi structural reinforced concrete may be used for underground detention vaults. All construction joints must be provided with water stops. Cast -in-place wall sections must be designed as retaining walls. The maximum depth from finished grade to the vault invert should be 20 ft. • Detention Tanks: The minimum pipe diameter for underground detention tanks is 36 inches. • Underground detention vaults and tanks must meet structural requirements for overburden support and traffic loading if appropriate. • Adequate maintenance access must be provided for all underground detention systems. Access must be provided over the inlet pipe and outflow structure. Access openings can consist of a standard frame, grate and solid cover, or a removable panel. Vaults with widths of 10 ft or less should have removable lids. Inlet and Outlet Structures • A separate sediment sump or vault chamber sized to 0.1 inches per impervious acre of contributing drainage should be provided at the inlet for underground detention systems that are in a treatment train with off-line water quality treatment structural controls. For CP„ control, a low flow orifice capable of releasing the channel protection volume over 24 hours must be provided. The channel protection orifice should have a minimum diameter of 3 inches and should be adequately protected from clogging by an acceptable external trash rack. The orifice diameter may be reduced to 1 inch if internal orifice protection is used (i.e., an overperforated vertical stand pipe with 0.5 -inch orifices or slots that are protected by wirecloth and a stone filtering jacket). Adjustable gate valves can also be used to achieve this equivalent diameter. • For overbank flood protection and extreme flood flow, additional outlets are sized for Qpzs and Qe control (based upon hydrologic routing calculations) and can consist of a weir, orifice, outlet pipe, combination outlet, or other acceptable control structure. • See Appendix G for more information on the design of outlet works. • Riprap, plunge pools or pads, or other energy dissipaters are to be placed at the end of the outlet to prevent scouring and erosion. See Chapter 6 for more guidance on energy dissipation. SECTION 3: TYPICAL SCHEMATIC DETAILS MBnlfold ConCrm Control Structure Welr Mall oulkt Plpe Lowllow orttldl Figure DSC -3.1 Example underground detention tank system (Source: Atlanta Regional Commission). NOTIEE- At vauR areas must be WMM W of an ao pviR 10 ---__— ..__---- oum CyVMOSM M I N I�� 10 001ioao! AI PULN VIEW V x 7 tY sucaaa uaLk t $ iba�ed.in j I! _� 1 j now_ A and round &Ad oowrs nnadoed IMIW wt& loc" boRs. wd'bnge DESHM itrt f - W.S. : :. WAWL �[ow low restrfu�or steps or taddar C sod wa i SiOrap ftoW 12• � - depadty of OUSM Pk* �, [tot #tee than devwkn*d SECTION -A IOQ-yr design flog HTS minlmuM rtoor grate wi#h 2' x 7 Waged access door 11'x Me gahmntaad Iq@bars) Figure DSC -3.2 Schematic of typical underground detention vault (Source: WDE, 2000) SECTION 4: INSPECTION AND MAINTENANCE REQUIREMENTS Table DSC -3.1 Typical Maintenance Activities for Detention Systems. Activity Schedule • Remove any trash/debris and sediment buildup in the underground vaults or Annually tanks. As needed, based on • Perform structural repairs to inlet and outlets. inspection SECTION 5. REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume Z: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Washington State Department of Ecology, 2000. Stormwater Management Manual for Western Washington. APPENDIX F WATER QUALITY STRUCTURAL CONTROLS STORMWATER PONDS Description: Constructed stormwater retention basin that has a permanent pool (or micropool). Runoff from each rain event is detained and treated in the pool primarily through settling and biological uptake mechanisms. • Moderate to high removal • Potential for thermal rate of urban pollutants impacts/downstream warming • High community • Dam height restrictions for acceptance areas with high relief terrain • Opportunity for wildlife • Pond drainage can be a habitat problematic for low relief terrain Design Criteria: • A sediment forebay or equivalent upstream pretreatment must be provided • Minimum length to width ratio for the pond is 1.5:1 • Maximum depth of the permanent pool should not exceed 8 ft • Side slopes to the pond should not exceed 3:1 (h:v) Stormwater Management Capability: • Reduction in peak rate of runoff discharge • Water quality benefits can provide 80% TSS removal. Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Remove debris from inlet and outlet structures • Maintain side slopes / remove invasive vegetation • Monitor sediment accumulation and remove periodically ©Maintenance Burden L = Low M = Moderate H = High SECTION 1. DESCRIPTION Stormwater ponds (also referred to as retention ponds, wet ponds, or wet extended detention ponds) are constructed stormwater retention basins that have a permanent (dead storage) pool of water throughout the year. They can be created by excavating an already existing natural depression or through the construction of embankments. In a stormwater pond, runoff from each rain event is detained and treated in the pool through gravitational settling and biological uptake until it is displaced by runoff from the next storm. The permanent pool also serves to protect deposited sediments from resuspension. Above the permanent pool level, additional temporary storage (live storage) is provided for runoff quantity control. The upper stages of a stormwater pond are designed to provide extended detention of the 1 -year storm for downstream channel protection, as well as normal detention of larger storm events. Stormwater ponds are among the most cost-effective and widely used stormwater practices. A well-designed and landscaped pond can be an aesthetic feature on a development site when planned and located properly. There are several different variants of stormwater pond design, the most common of which include the wet pond, the wet extended detention pond, and the micropool extended detention pond. In addition, multiple stormwater ponds can be placed in series or parallel to increase performance or meet site design constraints. Below are descriptions of each design variant: • Wet Pond - Wet ponds are stormwater basins constructed with a permanent (dead storage) pool of water equal to the water quality volume. Stormwater runoff displaces the water already present in the pool. Temporary storage (live storage) can be provided above the permanent pool elevation for larger flows. • Wet Extended Detention (ED) Pond - A wet extended detention pond is a wet pond where the water quality volume is split evenly between the permanent pool and extended detention (ED) storage provided above the permanent pool. During storm events, water is detained above the permanent pool and released over 24 hours. This design has similar pollutant removal to a traditional wet pond, but consumes less space. • Micropool Extended Detention (ED) Pond - The micropool extended detention pond is a variation of the wet ED pond where only a small "micropool" is maintained at the outlet to the pond. The outlet structure is sized to detain the water quality volume for 24 hours. The micropool prevents resuspension of previously settled sediments and also prevents clogging of the low flow orifice. • Multiple Pond Systems - Multiple pond systems consist of constructed facilities that provide water quality and quantity volume storage in two or more cells. The additional cells can create longer pollutant removal pathways and improved downstream protection. Figure 1.1 shows a number of examples of stormwater pond variants. Section 3 provides plan view and profile schematics for the design of a wet pond, wet extended detention pond, micropool extended detention pond, and multiple pond system. Wet Pond Micropool ED Pond Wet ED Pond Wet ED Pond Figure 1.1. Stormwater Pond Examples. SECTION 2. STORMWATER MANAGEMENT SUITABILITY Stormwater ponds are designed to control both stormwater quantity and quality. Thus, a stormwater pond can be used to address Minimum Standards 1, 2, 3 and 4. Minimum Standard #1 Stormwater ponds treat incoming stormwater runoff by physical, biological, and chemical processes. The primary removal mechanism is gravitational settling of particulates, organic matter, metals, bacteria and organics as stormwater runoff resides in the pond. Another mechanism for pollutant removal is uptake by algae and wetland plants in the permanent pool—particularly of nutrients. Volatilization and chemical activity also work to break down and eliminate a number of other stormwater contaminants such as hydrocarbons. Section 3 of this specification provides median pollutant removal efficiencies that can be used for planning and design purposes. Minimum Standard #2 A portion of the storage volume above the permanent pool in a stormwater pond can be used to provide control of the channel protection volume. This is accomplished by releasing the 1 -year, 24-hour storm runoff volume over 24 hours (extended detention). Minimum Standards #3 and #4 A stormwater pond can also provide storage above the permanent pool to reduce the post -development peak flow of the 25-, and 100 -year storms to pre -development levels (detention). SECTION 3. POLLUTANT REMOVAL CAPABILITIES All of the stormwater pond design variants are presumed to be able to remove 80% of the total suspended solids (TSS) load in typical urban post -development runoff when sized, designed, constructed and maintained in accordance with the recommended specifications. Undersized or poorly designed ponds can reduce TSS removal performance. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. In a situation where a removal rate is not deemed sufficient, additional controls may be put in place at the given site in a series or "treatment train" approach. • Total Suspended Solids - 80% • Total Phosphorus - 50% • Total Nitrogen - 30% • Fecal Coliform - 70% (if no resident waterfowl population present) • Heavy Metals - 50% For additional information and data on pollutant removal capabilities for stormwater ponds, see the National Pollutant Removal Performance Database (2nd Edition) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org SECTION 4: TYPICAL SCHEMATIC DETAILS POND BUFFER (25 FEET MINIMUM) 2e' OVERFLOW— SPILLWAY VERFLOW SPILLWAY -- _ ��---- r i _ HARDENED PAD FOREBAY IRREGULAR POOL SHAPE 6 to 8 FEET DEEP MAINTENANCE---- AINTENANCE--- ----- ```\ --- - - --- - - / ( •-.- - \ -- ACCESS ROAD T -� - ------ - - AQUATIC BENCH NATIVE LANDSCAPING SAFETY BENCH AROUND POOL PLAN EMBANKMENT RISER— EXTREME FLOOD PROTECTION NOD Year) LEVEL V OVERBANK FLOOD PROTECTION (25 -Year) LEVEL CHANNEL PROTECTION LEVEL *SAFETY BENCH — WATER QUALITY VOLUME LEVEL AQUATIC BENCH INFLOW PERMANENT POOL FOREBAY POND DRAIN OVERFLOW SPILLWAY REVERSE PIP BARREL– *4'- 6' WIDE BENCH, 2' MAX. DEPTH, NO 3:1 SLOPES ANTI -SEEP COLLAR OR ALLOWED COMING OUT OF FILTER DIAPHRAGM WATER DDnCII C Figure 1.2. Schematic of Wet Pond (Source: Center for Watershed Protection). RISER/ BARREL RISER IN EMBANKMENT STABLE OUTFALL POND BUFFER (25 FEET MINIMUM) i PRESERVE RIPARIAN / CANOPY FOREBAY PERMANENT POOL 6 to 8 FEET DEEP r i OUTFALL RISER/BARREL --" RISER IN MAINTENANCE ' ______- EMBANKMENT ACCESS ROAD MAXIMUM ED LIMIT AQUATIC BENCH SAFETY BENCH MAXIMUM SAFETY PLAN STORM LIMIT EMBANKMENT *SAFETY RISER BENCH V EXTREME FLOOD PROTECgz?il LEVEL V OVERBANK FLOOD PROar) LEVEL CHANNEL PROTECTI0 WATER QUALITYBENCH STABLE -N –_ ERMANENT POOLNFLOW i i / I OUTFALL L_ OVERFLOW POND DRAIN— SPILLWAY REVERSE PIPE—/BARREL- 4'- IPE BARREL- 4'- 6' WIDE BENCH, 2' MAX. ANTI -SEEP COLLAR OR DEPTH, NO 3:1 SLOPES FILTER DIAPHRAGM ALLOWED COMING OUT OF WATER Figure 1.3. Schematic of Wet Extended Detention Pond (Source: Center for Watershed Protection). MAXIMUM ELEVATION OF SAFETY STORM MAXIMUM ELEVATION OF ED POOL EXISTING VEGETATION RETAINED RIP RAP PILOT CHANNEL INFLOW FOREBAY OUTFALL SAFETY BENCH -- � ROp004 of AAI EMBANKMENT RISER EXTREME FLOOD PROTECTION (100 Year) LEVEL V OVERBANK FLOOD PROTECTION (25 -Year) LEVEL 0 CHANNEL PROTECTION LEVEL1,11 III — Q WATER QUALITY VOLUM LEVEL _ STABLE 0 PERMANENT _ _ `OUTFALL INFLOW i _ I I - POOL I—I / FOREBAY—/ MICROPOOL—/ BARREL ANTI -SEEP COLLAR OR FILTER DIAPHRAM Figure 1.4. Schematic of Micropool Extended Detention Pond (Source: Center for Watershed Protection). SAFETY RISER/ BENCH BARREL MgiNTF�q/VC `---- ,ACCESSSQq CELL 1 — (FOREBAY) OVERFLOW SPILLWAY (TYPICAL) AQUATIC BENCH) SAFETY BENCH CELL 2 PLAN V 25 YEAR LEVEL 0 CP„ LEVEL AQUATIC BENCH-) n EMBANKMENT RISER - SAFETY BENCH n T - k CELL 1 J i ' ' i ' ' iii i ' (FOREBAY) CELL 2 CELL 3 BARREL (WET POOL) (WET POOL) POND DRAIN REVERSE PIPE ANTI -SEEP COLLAR OR FILTER DIAPHRAM PROFILE Figure 1.5. Schematic of Multiple Pond System (Source: Center for Watershed Protection). EMERGENCY SPILLWAY EMERGENCY SPILLWAY STABLE OUTFALL 9 - SEDIMENT FOREBAY EMBANKMENT i STABILIZED 5 INFLOW CHANNEL / + STABILIZED / OVERFLOW SEDIMENTSPILLWAY FOREBAY v as /NORMAL WATER RIP—RAP + SURFACE ELEVATION INLET CHANNEL — PROTECTION —-------- CONSTRUCTION/ \ MAINTENANCE ACCESS _ PLAN STABILIZED OVERFLOW EXISTING SPILLWAY (CONC.) GRADE NORMAL WATER SURFACE ELEVATION NORMAL WATER — _ FOREBAY SURFACE ELEVATION ' RETENTION BASIN PROPOSED GRADE — RIPRAP INLETS � - CHANNEL PROTECTION NOTE: PROFILE STABILIZED OVERFLOW SPILLWAY SHOWN AS CONCRETE. RIPRAP, GABION BASKETS, OR OTHER TYPES OF ARMORING MAY BE USED, BASED ON DESIGN VELOCITIES. Figure 1.6. Typical Sediment Forebay Plan and Section. EXISTING NORMAL WATER SURFACE GRADE ELEVATION-FOREBAY NORMAL WATER SURFACE --- — ELEVATION -CONSTRUCTED WEIR PROPOSED GRADE ---- _ °go --- --- — _ -- RIPRAP INLET CHANNEL PROTECTION ALTERNATE SECTION No scale STABILIZED OVERFLOW SPILLWAY (GABION BASKETS) EXISTING VARIABLE WATER GRADESURFACE ELEVATION - FOREBAY VARIABLE WATER PROPOSED SURFACE ELEVATION GRADE --- — -- RIPRAP INLET CHANNEL PROTECTION GABION BASKET WEIR ALTERNATE SECTION No scale Figure 1.7. Typical Sediment Forebay Alternate Sections. SECTION 5. SITE FEASIBILITY & DESIGN APPLICATIONS Stormwater ponds are generally applicable to most types of new development and redevelopment, and can be used in both residential and nonresidential areas. Ponds can also be used in retrofit situations. The following criteria should be evaluated to ensure the suitability of a stormwater pond for meeting stormwater management objectives on a site or development. General Feasibility Suitable for Residential Subdivision Usage - YES • Suitable for High Density/Ultra-Urban Areas - Land requirements may preclude use • Regional Stormwater Control - YES Physical Feasibility - Physical Constraints at Project Site • Drainage Area - A minimum of 25 acres is needed for wet pond and wet ED pond to maintain a permanent pool, 10 acres minimum for micropool ED pond. A smaller drainage area may be acceptable with an adequate water balance and anti -clogging device. • Space Required - Approximately 2 to 3% of the tributary drainage area • Site Slone - There should not be more than 15% slope across the pond site. • Minimum Head - Elevation difference needed at a site from the inflow to the outflow: 6 to 8 ft • Minimum Depth to Water Table - If used on a site with an underlying water supply aquifer or when treating an area with potential for high pollutant loading, a separation distance of 2 ft is required between the bottom of the pond and the elevation of the seasonally high water table. • Soils - Underlying soils of hydrologic group "C" or "D" should be adequate to maintain a permanent pool. Most group "A" soils and some group "B" soils will require a pond liner. Evaluation of soils should be based upon an actual subsurface analysis and permeability tests. SECTION 6. PLANNING AND DESIGN CRITERIA The following criteria are to be considered minimum standards for the design of a stormwater pond facility. Location and Siting Stormwater ponds should have a minimum contributing drainage area of 25 acres or more for wet pond or wet ED pond to maintain a permanent pool. For a micropool ED pond, the minimum drainage area is 10 acres. A smaller drainage area can be considered when water availability can be confirmed (such as from a groundwater source or areas with a high water table). In these cases a water balance may be performed. Ensure that an appropriate anti -clogging device is provided for the pond outlet. • A stormwater pond should be sited such that the topography allows for maximum runoff storage at minimum excavation or construction costs. Pond siting should also take into account the location and use of other site features such as buffers and undisturbed natural areas and should attempt to aesthetically "fit" the facility into the landscape. Bedrock close to the surface may prevent excavation. • Stormwater ponds should not be located on steep (>15%) or unstable slopes. • Stormwater ponds cannot be located within a stream or any other navigable waters of the U.S., including wetlands, without obtaining a Section 404 permit under the Clean Water Act, and any other applicable State permit. • Minimum setback requirements for stormwater pond facilities: From a property line - 10 ft o From a private well - 100 ft; if well is downgradient from a land use area with potential for high pollutant loading, then the minimum setback is 250 ft o From a septic system tank/leach field - 50 ft • All utilities should be located outside of the pond/basin site. General Design • A well-designed stormwater pond consists of: o Permanent pool of water, o Overlying zone in which runoff control volumes are stored, and Shallow littoral zone (aquatic bench) along the edge of the permanent pool that acts as a biological filter. • In addition, all stormwater pond designs need to include a sediment forebay at the inflow to the basin to allow heavier sediments to drop out of suspension before the runoff enters the permanent pool. A sediment forebay schematic can be found in Section 4 above. • Additional pond design features include an emergency spillway, maintenance access, safety bench, pond buffer, and appropriate native landscaping. • Figures 1.2 thru 1.5 in this specification provide plan view and profile schematics for the design of a wet pond, wet ED pond, micropool ED pond and multiple pond system. Physical Specifications / Geometry In general, pond designs are unique for each site and application. However, there are number of geometric ratios and limiting depths for pond design that must be observed for adequate pollutant removal, ease of maintenance, and improved safety. • Permanent pool volume is typically sized as follows: o Standard wet ponds: 100% of the water quality treatment volume (1.0 WQv) o Wet ED ponds: 50% of the water quality treatment volume (0.5 WQ„) o Micropool ED ponds: Approximately 0.1 inch per impervious acre • Proper geometric design is essential to prevent hydraulic short-circuiting (unequal distribution of inflow), which results in the failure of the pond to achieve adequate levels of pollutant removal. The minimum length -to -width ratio for the permanent pool shape is 1.5:1, and should ideally be greater than 3:1 to avoid short-circuiting. In addition, ponds should be wedge-shaped when possible so that flow enters the pond and gradually spreads out, improving the sedimentation process. Baffles, pond shaping or islands can be added within the permanent pool to increase the flow path. • Maximum depth of the permanent pool should generally not exceed 8 ft to avoid stratification and anoxic conditions. Minimum depth for the pond bottom should be 3 to 4 ft. Deeper depths near the outlet will yield cooler bottom water discharges that may mitigate downstream thermal effects. • Side slopes to the pond should not usually exceed 3:1 (h:v) without safety precautions or if mowing is anticipated and should terminate on a safety bench (see Figure 1.8). The safety bench requirement may be waived if slopes are 4:1 or gentler. SAFETY BENCH AQUATIC BENCH average 15' 3 average 15' 1 15 NORMAL POOL 1� ELEVATON 12"-18" EMERGENT 1 3 WETLAN D VEGETAT O N BAS I N FLOOR Figure 1.8. Typical Stormwater Pond Geometry Criteria. The perimeter of all deep pool areas (4 ft or greater in depth) should be surrounded by two benches: safety and aquatic. For larger ponds, a safety bench extends approximately 15 ft outward from the normal water edge to the toe of the pond side slope. The maximum slope of the safety bench should be 6%. An aquatic bench extends inward from the normal pool edge (15 ft on average) and has a maximum depth of 18 inches below the normal pool water surface elevation (see Figure 1.8). • The contours and shape of the permanent pool should be irregular to provide a more natural landscaping effect. Pretreatment / Inlets Each pond should have a sediment forebay or equivalent upstream pretreatment. A sediment forebay is designed to remove incoming sediment from the stormwater flow prior to dispersal in a larger permanent pool. The forebay should consist of a separate cell, formed by an acceptable barrier. A forebay is to be provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the pond. In some design configurations, the pretreatment volume may be located within the permanent pool. The forebay is sized to contain 0.1 inches per impervious acre of contributing drainage and should be 4 to 6 ft deep. The pretreatment storage volume is part of the total WQ„ requirement and may be subtracted from WQ, for permanent pool sizing. • A fixed vertical sediment depth marker shall be installed in the forebay to measure sediment deposition over time. The bottom of the forebay may be hardened (e.g., using concrete, paver blocks, etc.) to make sediment removal easier. • Inflow channels are to be stabilized with flared riprap aprons, or the equivalent. Inlet pipes to the pond can be partially submerged. Exit velocities from the forebay must be nonerosive. Outlet Structures • Flow control from a stormwater pond is typically accomplished with the use of a concrete or corrugated metal riser and barrel. The riser is a vertical pipe or inlet structure that is attached to the base of the pond with a watertight connection. The outlet barrel is a horizontal pipe attached to the riser that conveys flow under the embankment (see Figure 1.9). The riser should be located within the embankment for maintenance access, safety and aesthetics. Figure 1.9. Typical Pond Outlet Structure. • A number of outlets at varying depths in the riser provide internal flow control for routing of the water quality, channel protection, and overbank flood protection runoff volumes. The number of orifices can vary and is usually a function of the pond design. For example, a wet pond riser configuration is typically comprised of a channel protection outlet (usually an orifice) and overbank flood protection outlet (often a slot or weir). The channel protection orifice is sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). Since the water quality volume is fully contained in the permanent pool, no orifice sizing is necessary for this volume. As runoff from a water quality event enters the wet pond, it simply displaces that same volume through EMBANKMENT 100 YEAR LEVEL _ _ 1�\ ..X 25 YEAR LEVEL ` EMERGENCY SPILLWAY Cp, LEVEL = HOOD/TRASH RACK/ SKIMMER NORMAL POOL _ - - - - - - - 'MULTI -STAGE RISER �� ELEVATION REVERSE -SLOPE PIPE w/ VALVE BARREL POND DRAIN w/ VALVE ANTI -SEEP COLLAR Figure 1.9. Typical Pond Outlet Structure. • A number of outlets at varying depths in the riser provide internal flow control for routing of the water quality, channel protection, and overbank flood protection runoff volumes. The number of orifices can vary and is usually a function of the pond design. For example, a wet pond riser configuration is typically comprised of a channel protection outlet (usually an orifice) and overbank flood protection outlet (often a slot or weir). The channel protection orifice is sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). Since the water quality volume is fully contained in the permanent pool, no orifice sizing is necessary for this volume. As runoff from a water quality event enters the wet pond, it simply displaces that same volume through the channel protection orifice. Thus an off-line wet pond providing only water quality treatment can use a simple overflow weir as the outlet structure. In the case of a wet ED pond or micropool ED pond, there is generally a need for an additional outlet (usually an orifice) that is sized to pass the extended detention water quality volume that is surcharged on top of the permanent pool. Flow will first pass through this orifice, which is sized to release the water quality ED volume in 24 hours. The preferred design is a reverse slope pipe attached to the riser, with its inlet submerged 1 ft below the elevation of the permanent pool to prevent floatables from clogging the pipe and to avoid discharging warmer water at the surface of the pond. The next outlet is sized for the release of the channel protection storage volume. The outlet (often an orifice) invert is located at the maximum elevation associated with the extended detention water quality volume and is sized to release the channel protection storage volume over a 24-hour period. Alternative hydraulic control methods to an orifice can be used and include the use of a broad - crested rectangular, V -notch, proportional weir, or an outlet pipe protected by a hood that extends at least 12 inches below the normal pool. • The water quality outlet (if design is for a wet ED or micropool ED pond) and channel protection outlet should be fitted with adjustable gate valves or other mechanism that can be used to adjust detention time. • Higher flows (overbank and extreme flood protection) flows pass through openings or slots protected by trash racks further up on the riser. • After entering the riser, flow is conveyed through the barrel and is discharged downstream. Anti - seep collars should be installed on the outlet barrel to reduce the potential for pipe failure. • Riprap, plunge pools or pads, or other energy dissipaters are to be placed at the outlet of the barrel to prevent scouring and erosion. If a pond daylights to a channel with dry weather flow, care should be taken to minimize tree clearing along the downstream channel, and to reestablish a forested riparian zone in the shortest possible distance. • Each pond must have a bottom drain pipe with an adjustable valve that can completely or partially drain the pond within 24 hours. • The pond drain should be sized one pipe size greater than the calculated design diameter. The drain valve is typically a handwheel activated knife or gate valve. Valve controls shall be located inside of the riser at a point where they (a) will not normally be inundated and (b) can be operated in a safe manner. See the design procedures in Chapter 7 and Appendix G for additional information and specifications on pond routing and outlet works. Emergency Spillway • An emergency spillway is to be included in the stormwater pond design to safely pass the extreme flood flow. The spillway prevents pond water levels from overtopping the embankment and causing structural damage. The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges. 0 A minimum of 1 ft of freeboard must be provided, measured from the top of the water surface elevation for the extreme flood to the lowest point of the dam embankment, not counting the emergency spillway. Maintenance Access A maintenance right of way or easement must be provided to a pond from a public or private road. Maintenance access should be at least 12 ft wide, have a maximum slope of no more than 15%, and be appropriately stabilized to withstand maintenance equipment and vehicles. • The maintenance access must extend to the forebay, safety bench, riser, and outlet and, to the extent feasible, be designed to allow vehicles to turn around. Access to the riser is to be provided by lockable manhole covers, and manhole steps within easy reach of valves and other controls. Safety Features All embankments and spillways for ponds with a volume over 50 acre -ft must be obtain an Arkansas Natural Resources Commission permit, in accordance with ANRC Title VII, Rules Governing Design and Operation of Dams. • The safety bench should be landscaped to deter access to the pool. • The principal spillway opening should not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter should be fenced to prevent access. Warning signs should be posted near the pond to prohibit swimming and fishing in the facility. Landscaping Aquatic vegetation can play an important role in pollutant removal in a stormwater pond. In addition, vegetation can enhance the appearance of the pond, stabilize side slopes, serve as wildlife habitat, and can temporarily conceal unsightly trash and debris. Therefore, wetland plants should be encouraged in a pond design, along the aquatic bench (fringe wetlands), the safety bench and side slopes (ED ponds), and within shallow areas of the pool itself. The best elevations for establishing wetland plants, either through transplantation or volunteer colonization, are within 6 inches (plus or minus) of the normal pool elevation. • Woody vegetation may not be planted on the embankment or allowed to grow within 15 ft of the toe of the embankment and 25 ft from the principal spillway structure. • A pond buffer should be provided that extends 25 ft outward from the maximum water surface elevation of the pond. The pond buffer should be contiguous with other buffer areas that are required by existing regulations (e.g., stream buffers) or that are part of the overall stormwater management concept plan. No structures should be located within the buffer, and an additional setback to permanent structures may be provided. • Existing trees should be preserved in the buffer area during construction. It is desirable to locate forest conservation areas adjacent to ponds. To discourage resident geese populations, the buffer can be planted with trees, shrubs and native ground covers. • The soils of a pond buffer are often severely compacted during the construction process to ensure stability. The density of these compacted soils is so great that it effectively prevents root penetration and therefore may lead to premature mortality or loss of vigor. Consequently, it is advisable to excavate large and deep holes around the proposed planting sites and backfill these with uncompacted topsoil. A fountain or solar -powered aerator may be used for oxygenation of water in the permanent pool. • Compatible multi -objective use of stormwater pond locations is strongly encouraged. Additional Site -Specific Design Criteria and Issues Physiographic Factors - Local terrain design constraints o Low Relief - Maximum normal pool depth is limited; providing pond drain can be problematic o High Relief - Embankment heights restricted o Karst - Requires poly or clay liner to sustain a permanent pool of water and protect aquifers; limits on ponding depth; geotechnical tests may be required • Soils Hydrologic group "A" soils generally require pond liner; group "B" soils may require infiltration testing SECTION 7. DESIGN PROCEDURES Step 1. Compute runoff control volumes from the Unified Stormwater Sizing Criteria • Calculate the Water Quality Volume (WQv), Channel Protection Volume (Cps), Overbank Flood Protection Volume (Qp), and the Extreme Flood Volume (Qf). Details on the Stormwater Sizing Criteria are found in Chapter 2. Step 2. Determine if the development site and conditions are appropriate for the use of a stormwater pond • Consider the Application and Site Feasibility Criteria in Section 6 of this specification. Step 3. Confirm local design criteria and applicability • Consider any special site-specific design conditions/criteria from Section 6 of this specification. • Check with City Engineer to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply. Step 4. Determine pretreatment volume A sediment forebay is provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the pond. The forebay should be sized to contain 0.1 inches per impervious acre of contributing drainage and should be 4 to 6 ft deep. The forebay storage volume counts toward the total WQv requirement and may be subtracted from the WQv for subsequent calculations. Step S. Determine permanent pool volume (and water quality ED volume) Wet Pond: Size permanent pool volume to 1.0 WQv Wet ED Pond: Size permanent pool volume to 0.5 WQ,. Size extended detention volume to 0.5 WQ,. Micropool ED Pond: Size permanent pool volume to 25 to 30% of WQv. Size extended detention volume to remainder of WQv. Step 6. Determine pond location and preliminary geometry. Conduct pond grading and determine storage available for permanent pool (and water quality extended detention if wet ED pond or micropool ED pond) This step involves initially grading the pond (establishing contours) and determining the elevation - storage relationship for the pond. • Include safety and aquatic benches. • Set WQv permanent pool elevation (and WQv- ED elevation for wet ED and micropool ED pond) based on volumes calculated earlier. See Section 6 of this specification for more details. Step 7. Compute extended detention orifice release rate(s) and size(s), and establish Cpv elevation Wet Pond: The Cp„ elevation is determined from the stage -storage relationship and the orifice is then sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). The channel protection orifice should have a minimum diameter of 3 inches and should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the riser, with its inlet submerged 1 ft below the elevation of the permanent pool, is a recommended design. The orifice diameter may be reduced to 1 inch if internal orifice protection is used (i.e., an over -perforated vertical stand pipe with Y2 -inch orifices or slots that are protected by wirecloth and a stone filtering jacket). Adjustable gate valves can also be used to achieve this equivalent diameter. Wet ED Pond and Micropool ED Pond: Based on the elevations established in Step 6 for the extended detention portion of the water quality volume, the water quality orifice is sized to release this extended detention volume in 24 hours. The water quality orifice should have a minimum diameter of 3 inches and should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the riser, with its inlet submerged 1 ft below the elevation of the permanent pool, is a recommended design. Adjustable gate valves can also be used to achieve this equivalent diameter. The Cp, elevation is then determined from the stage -storage relationship. The invert of the channel protection orifice is located at the water quality extended detention elevation, and the orifice is sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). Step 8. Calculate Qpzs (25 -year storm) release rate and water surface elevation Set up a stage -storage -discharge relationship for the control structure for the extended detention orifice(s) and the 25 -year storm. Step 9. Design embankment(s) and spillway(s). Size emergency spillway, calculate 100 -year water surface elevation, set top of embankment elevation, and analyze safe passage of the Extreme Flood Volume (Qf). At final design, provide safe passage for the 100 -year event. Step 10. Investigate potential pond hazard classification The design and construction of stormwater management ponds are required to follow the latest version of the State of Georgia dam safety rules (see Appendix H). Step 11. Design inlets, sediment forebay(s), outlet structures, maintenance access, and safety features. See Section 6 of this specification for more details. Step 12. Prepare Vegetation and Landscaping Plan A landscaping plan for a stormwater pond and its buffer should be prepared to indicate how aquatic and terrestrial areas will be stabilized and established with vegetation. See Section 6 of this specification for more details. SECTION 8. INSPECTION AND MAINTENANCE REQUIRMENTS PondsTable 3.1. Typical Maintenance Activities for Activity Schedule • Clean and remove debris from inlet and outlet structures. Monthly • Mow side slopes. • If wetland components are included, inspect for invasive vegetation. Semiannual Inspection • Inspect for damage, paying particular attention to the control structure. • Check for signs of eutrophic conditions. • Note signs of hydrocarbon build-up, and remove appropriately. Annual • Monitor for sediment accumulation in the facility and forebay. Inspection • Examine to ensure that inlet and outlet devices are free of debris and operational. • Check all control gates, valves or other mechanical devices. Repair undercut or eroded areas. As Needed Perform wetland plant management and harvesting. Annually (if needed) 5 to 7 years Remove sediment from the forebay. or after 50% of the total forebay capacity has been lost Monitor sediment accumulations, and remove sediment when the pool volume 10 to 20 years or after 25% has become reduced significantly, or the pond becomes eutrophic. of the permanent pool volume has been lost SECTION 9. REFERENCES Atlanta Regional Commission, 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. Watershed Management Institute (WMI), 1997. Operation. Maintenance. and Management of Stormwater Management Systems. Prepared for US EPA, Office of Water. STORMWATER WETLANDS Description: Constructed wetland systems used for stormwater management. Runoff volume is both stored and treated in the wetland facility. • Good nutrient removal • Requires large land area • Provides natural wildlife • Needs continuous baseflow for habitat viable wetland • Relatively low maintenance • Sediment regulation is critical costs to sustain wetlands Design Criteria: • Minimum dry weather flow path of 2:1 (length:width) should be provided from inflow to outflow • Minimum of 35% of total surface area should have a depth of 6 inches or less; 10% to 20% of surface area should be deep pool (1.5- to 6 -ft depth) Stormwater Management Capability: • Reduction in peak rate of runoff discharge • Water quality benefits can provide 80% TSS removal. Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Replace wetland vegetation to maintain at least 50% surface area coverage • Remove invasive vegetation • Monitor sediment accumulation and remove periodically ©Maintenance Burden L = Low M = Moderate H = High SECTION 1. DESCRIPTION Stormwater wetlands (also referred to as constructed wetlands) are constructed shallow marsh systems that are designed to both treat urban stormwater and control runoff volumes. As stormwater runoff flows through the wetland facility, pollutant removal is achieved through settling and uptake by marsh vegetation. Wetlands are among the most effective stormwater practices in terms of pollutant removal and also offer aesthetic value and wildlife habitat. Constructed stormwater wetlands differ from natural wetland systems in that they are engineered facilities designed specifically for the purpose of treating stormwater runoff and typically have less biodiversity than natural wetlands both in terms of plant and animal life. However, as with natural wetlands, stormwater wetlands require a continuous base flow or a high water table to support aquatic vegetation. There are several design variations of the stormwater wetland, each design differing in the relative amounts of shallow and deep water, and dry storage above the wetland. These include the shallow wetland, the extended detention shallow wetland, pond/wetland system and pocket wetland. Below are descriptions of each design variant: • Shallow Wetland - In the shallow wetland design, most of the water quality treatment volume is in the relatively shallow high marsh or low marsh depths. The only deep portions of the shallow wetland design are the forebay at the inlet to the wetland, and the micropool at the outlet. One disadvantage of this design is that, since the pool is very shallow, a relatively large amount of land is typically needed to store the water quality volume. Extended Detention (ED) Shallow Wetland - The extended detention (ED) shallow wetland design is the same as the shallow wetland; however, part of the water quality treatment volume is provided as extended detention above the surface of the marsh and released over a period of 24 hours. This design can treat a greater volume of stormwater in a smaller space than the shallow wetland design. In the extended detention wetland option, plants that can tolerate both wet and dry periods need to be specified in the ED zone. Pond/Wetland Systems - The pond/wetland system has two separate cells: a wet pond and a shallow marsh. The wet pond traps sediments and reduces runoff velocities prior to entry into the wetland, where stormwater flows receive additional treatment. Less land is required for a pond/wetland system than for the shallow wetland or the ED shallow wetland systems. • Pocket Wetland - A pocket wetland is intended for smaller drainage areas of 5 to 10 acres and typically requires excavation down to the water table for a reliable water source to support the wetland system. Shallow Wetland Newly Constructed Shallow Wetland Shallow ED Wetland Pocket Wetland Figure 2.1. Stormwater Wetland Examples. SECTION 2. STORMWATER MANAGEMENT SUITABILITY Similar to stormwater ponds, stormwater wetlands are designed to control both stormwater quantity and quality. Thus, a stormwater wetland can be used to address Minimum Standards 1, 2, and 3. Minimum Standard #1 Pollutants are removed from stormwater runoff in a wetland through uptake by wetland vegetation and algae, vegetative filtering, and through gravitational settling in the slow moving marsh flow. Other pollutant removal mechanisms are also at work in a stormwater wetland, including chemical and biological decomposition, and volatilization. Section 3 of this specification provides median pollutant removal efficiencies that can be used for planning and design purposes. Minimum Standard #2 The storage volume above the permanent pool/water surface level in a stormwater wetland is used to provide control of the channel protection volume (Cps). This is accomplished by releasing the 1 -year, 24 hour storm runoff volume over 24 hours (extended detention). It is best to do this with minimum vertical water level fluctuation, as extreme fluctuation may stress vegetation. Minimum Standard #3 A stormwater wetland can also provide storage above the permanent pool to reduce the post -development peak flow of the 25- and 100 -year storms to pre -development levels (detention). SECTION 3. POLLUTANT REMOVAL CAPABILITIES All of the stormwater wetland design variants are presumed to be able to remove 80% of the total suspended solids load in typical urban post -development runoff when sized, designed, constructed and maintained in accordance with the recommended specifications. Undersized or poorly designed wetland facilities can reduce TSS removal performance. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. In a situation where a removal rate is not deemed sufficient, additional controls may be put in place at the given site in a series or "treatment -train" approach. • Total Suspended Solids - 80% • Total Phosphorus - 40% • Total Nitrogen - 30% • Fecal Coliform - 70% (if no resident waterfowl population present) • Heavy Metals - 50% For additional information and data on pollutant removal capabilities for stormwater wetlands, see the National Pollutant Removal Performance Database (2nd Edition) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org SECTION 4. TYPICAL SCHEMATIC DETAILS WETLAND BUFFER (25 FEET MINIMUM) LIMIT 25% OF POND - ss PERIMETER OPEN GRASS ------ j ------------ EMERGENCY SAFETY BENCH T SPILLWAY tiXn- ,; WEIR nX ^ ri ,ix WALL FOREBAY ��'�i;;�'� �'i7 MICROPOOL OUTFALL 3F -7C 7 WA ERF v �OS� ti I ISLAND�: RISER BARREL ,1 ____,=� i—♦ a.,i. a.a RISER IN ♦ - —%� �� i� EMBANKMENT MAINTENANCE '--�— ACCESS ROAD ♦ ' ------------- ------------------------------ HIGH MARSH 25' WETLAND BUFFER LANDSCAPED WITH z5' (LESS THAN 6" WATER DEPTH) NATIVE TREES I SHRUBS FOR HABITAT 1 LOW MARSH (WATER DEPTH BETWEEN 6" and 18") PLAN VIEW WETLANDS EMBANKMENT HIGH MARSH RISER 100 YEAR LEVEL EMERGENCY UI_ 25 YEAR LEVEL SPILLWAY _IIID 1111 —_ llillll— Cp„ LEVEL PERMANENT WQv LEVEL —_ POOL �♦ 171111= — - i — INFLOW VIII— X11111 —lFr M —L': III EIII-I II 1111=III—IIII IIII— 11111 2111 fiET�lillIlillfGfl --1Til STABLE FOREBAY _ IIII=III IIII ll11i1 OUTFALL =IIiL1=girl Illi IH jjp�lllll� l�Lflil —III II IIII— �� GABION WALL POND DRAIN _-1 111-11111 LOW MARSH REVERSE PIPE BARREL � -1111 7111—rQ ANTI -SEEP COLLAR or F1TER DIAPHRAGM PROFILE Figure 2.2. Schematic of Shallow Wetland (Source: Center for Watershed Protection). POND BUFFER (25 FEET MINIMUM) t 25' MAXIMUM ED LIMIT— ----L FOREBAY 1 e j„ Jv �� WETLANDS HIGH MARSH 100 YEAR LEVEL Q 25 YEAR LEVEL ILI— C LEVEL —;�— p __Ih 1 _. - WQ,r-ED ELEVATION INFLOW —111111111- �I =1111 IIII 1111V iF 1I 111_11= 1 FIT rT —fil �l =(n FOREBAY —ru-111111711 rip SAFETY BENCH (LESS THAN 6" WATER DEPTH) LOW MARSH (WATER DEPTH BETWEEN 6" and 18") EMBANKMENT RISER--\ PERMANENT POOL / 911111.11 Illi.= i VIII =11111 .hill=j�l.- �l POND DRAW LOW MARSH REVERSE PIPEBARREL ANTI -SEEP COLLAR or FILTER DIAPHRAGM Figure 2.3. Schematic of Extended Detention Shallow Wetland (Source: Center for Watershed Protection). EMERGENCY SPILLWAY OUTFALL RISER BARREL RISER IN EMBANKMENT PLAN VIEW EMERGENCY SPILLWAY STABLE OUTFALL PROFILE Figure 2.4. Schematic of Pond/Wetland System (Source: Center for Watershed Protection). I POND BUFFER (25 FEET MINIMUM) xs Aa CONCRETE EMERGENCY SPILLWAY SPILLWAY HIGH MARSH WEDGES .F i ♦I ♦ �� k� M INFLOW- lE WET POND `: • PLUNGEr POOL 1 r- MICRa POOL OUTFALL AQUATIC ENCH (` --------------- t- .�`-_- r r♦I.r r 1 BISER I i ARREL - ' LOW MARSH ZONE �fc jiE �, 'AFS'A* J✓'r RISER IN EMBANIONENT MAINTENANCE ACCESS ROAD j SAFETY BENCH MAXIMUM SAFETY STORM LIMIT--/ PLAN VIEW EMBANIOIIENT� RISER 100 YEAR LEVEL EMERGENCY 25 YEAR LEVEL SPILLWAY llll= - SAFETY = CP, LEVEL BENCH fill—l — HIGH MARSH MARSH rl WOLEVEL pOP pMANENT 1=_ IIw il— I I - - I — — �i 11- f@=1 Ip�l _ — 1 lli STABLE ur _OUTFALL WET POOL 11= n =1 yE 11— — rII =1 =fl131� 1I�I111= MICROPOOL POND IN!r — M- REVERSE PIPE BARREL m�1 R JIit>w ANTI -SEEP COLLAR or FILTER DIAPHRAGM PROFILE Figure 2.4. Schematic of Pond/Wetland System (Source: Center for Watershed Protection). MAINTENANCE ACCESS SWALE�,,��,,y uw�+Y�ipmw '�y _ FOREBAY iA—ham t LOW MARSH ZONE AD SEDIMENT, DISPOSAL AREA . MICROPOOL HIGH MARSH WEDGES — ' % \ C j X _ _ _' , i� , Iij I xT,gg� 'I SAFETY Bli $UFFenHALF ROUND /BROAD TRASH RACK CRESTED MAXIMUM SAFETY STORM LIMIT ---- — — - WEIR --- 0 100 YEAR LEVEL D 25 YEAR LEVEL Q Cp„ LEVEL SWALE �l WQv LEVEL 0 1=11 IIII- -1U1 I— I— Jill IIII--1111=1111=— �T(f=11f11-iTITI III_ II Tf¶=11111— FOREBAY MICROPOOL GROUND WATER LOW MARSH TABLE EMBANKMENT HIGH MARSH TRASH RACK III ILI 11FM 1TH �Iiu�1Tl- _I1➢1 '1111-UII1 POND DRAIN BARREL ANTI -SEEP COLLAR or FILTER DIAPHRAGM Figure 2.5. Schematic of Pocket Wetland (Source: Center for Watershed Protection). l II PLAN VIEW BROAD CRESTED WEIR STABLE OUTFALL PROFILE SECTION 5. SITE FEASIBILITY & DESIGN APPLICATIONS Stormwater wetlands are generally applicable to most types of new development and redevelopment, and can be utilized in both residential and nonresidential areas. However, due to the large land requirements, wetlands may not be practical in higher density areas. The following criteria should be evaluated to ensure the suitability of a stormwater wetland for meeting stormwater management objectives on a site or development. General Feasibility • Suitable for Residential Subdivision Usage - YES • Suitable for High Density/Ultra Urban Areas - Land requirements may preclude use Regional Stormwater Control - YES Physical Feasibility - Physical Constraints at Project Site • Drainage Area - A minimum of 25 acres and a positive water balance is needed to maintain wetland conditions; 5 acres for pocket wetland • Space Required - Approximately 3 to 5% of the tributary drainage area • Site Slone - There should be no more than 8% slope across the wetland site • Minimum Head - Elevation difference needed at a site from the inflow to the outflow: 3 to 5 ft; 2 to 3 ft for pocket wetland • Minimum Depth to Water Table - If used on a site with an underlying water supply aquifer or when treating an area with potential for high pollutant loading, a separation distance of 2 ft is recommended between the bottom of the wetland and the elevation of the seasonally high water table; pocket wetland is typically below water table. • Soils - Permeable soils are not well suited for a constructed stormwater wetland without a high water table. Underlying soils of hydrologic group "C" or "D" should be adequate to maintain wetland conditions. Most group "A" soils and some group "B" soils will require a liner. Evaluation of soils should be based upon an actual subsurface analysis and permeability tests. SECTION 6. PLANNING AND DESIGN CRITERIA The following criteria are to be considered minimum standards for the design of a stormwater wetland facility. Location and Siting • Stormwater wetlands should normally have a minimum contributing drainage area of 25 acres or more. For a pocket wetland, the minimum drainage area is 5 acres. A continuous base flow or high water table is required to support wetland vegetation. A water balance must be performed to demonstrate that a stormwater wetland can withstand a 30 -day drought at summer evaporation rates without completely drawing down. • Wetland siting should also take into account the location and use of other site features such as natural depressions, buffers, and undisturbed natural areas, and should attempt to aesthetically "fit" the facility into the landscape. Bedrock close to the surface may prevent excavation. • Stormwater wetlands cannot be located within navigable waters of the U.S., including wetlands, without obtaining a Section 404 permit under the Clean Water Act, and any other applicable State permit. In some isolated cases, a wetlands permit may be granted to convert an existing degraded wetland in the context of local watershed restoration efforts. • If a wetland facility is not used for overbank flood protection, it should be designed as an off-line system to bypass higher flows rather than passing them through the wetland system. • Minimum setback requirements for stormwater wetland facilities (when not specified by local ordinance or criteria): From a property line - 10 ft o From a private well - 100 ft; if well is downgradient from an area with potential for high pollutant loading land use then the minimum setback is 250 ft o From a septic system tank/leach field - 50 ft • All utilities should be located outside of the wetland site. General Design • A well-designed stormwater wetland consists of: o Shallow marsh areas of varying depths with wetland vegetation, o Permanent micropool, and o Overlying zone in which runoff control volumes are stored. Pond/wetland systems also include a stormwater pond facility (see WSC-01, Stormwater Ponds, in Appendix F for pond design information). • In addition, all wetland designs must include a sediment forebay at the inflow to the facility to allow heavier sediments to drop out of suspension before the runoff enters the wetland marsh. • Additional pond design features include an emergency spillway, maintenance access, safety bench, wetland buffer, and appropriate wetland vegetation and native landscaping. Figures 2.2 through 2.5 in Section 4 of this specification provide plan view and profile schematics for the design of a shallow wetland, ED shallow wetland, pond/wetland system, and pocket wetland. Physical Specifications / Geometry In general, wetland designs are unique for each site and application. However, there are number of geometric ratios and limiting depths for the design of a stormwater wetland that must be observed for adequate pollutant removal, ease of maintenance, and improved safety. Table 2.1 provides the recommended physical specifications and geometry for the various stormwater wetland design variants. 2.1 Recommended Design Criteria for Stormwater Wetlands ModifiedTable Design Criteria Shallow Wetland ED Shallow Wetland Pond/ Pocket Wetland Wetland Length to Width Ratio 2:1 2:1 2:1 2:1 (minimum) Extended Detention (ED) No Yes Optional Optional Allocation of WQv Volume 70/30/0 (pool/marsh/ED) in % 25/75/0 25/25/50 (includes pond 25/75/0 volume) Allocation of Surface Area 45/25/25/5 (deepwater/low marsh/high 20/35/40/5 10/35/45/10 (includes pond 10/45/40/5 marsh/semi-wet) in % surface area) Forebay Required Required Required Optional Micropool Required Required Required Required Reverse -slope pipe or Reverse -slope pipe or Reverse -slope pipe or Hooded broad - Outlet Configuration hooded broad- hooded broad -crested hooded broad - crested weir crested weir weir crested weir Depth: Deepwater: 1.5 to 6 ft below normal pool elevation Low marsh: 6 to 18 inches below normal pool elevation High marsh: 6 inches or less below normal pool elevation Semi -wet zone: Above normal pool elevation • The stormwater wetland should be designed with the recommended proportion of "depth zones." Each of the four wetland design variants has depth zone allocations which are given as a percentage of the stormwater wetland surface area. Target allocations are found in Table 2.1. The four basic depth zones are: o Deepwater zone From 1.5 to 6 ft deep. Includes the outlet micropool and deepwater channels through the wetland facility. This zone supports little emergent wetland vegetation, but may support submerged or floating vegetation. Low marsh zone From 6 to 18 inches below the normal permanent pool or water surface elevation. This zone is suitable for the growth of several emergent wetland plant species. High marsh zone From 6 inches below the pool to the normal pool elevation. This zone will support a greater density and diversity of wetland species than the low marsh zone. The high marsh zone should have a higher surface area to volume ratio than the low marsh zone. Semi -wet zone Those areas above the permanent pool that are inundated during larger storm events. This zone supports a number of species that can survive flooding. A minimum dry weather flow path of 2:1 (length to width) is required from inflow to outlet across the stormwater wetland and should ideally be greater than 3:1. This path may be achieved by constructing internal dikes or berms, using marsh plantings, and by using multiple cells. Finger dikes are commonly used in surface flow systems to create serpentine configurations and prevent short- circuiting. Microtopography (contours along the bottom of a wetland or marsh that provide a variety of conditions for different species needs and increases the surface area to volume ratio) is encouraged to enhance wetland diversity. • A 4- to 6 -ft deep micropool must be included in the design at the outlet to prevent the outlet from clogging and resuspension of sediments, and to mitigate thermal effects. • Maximum depth of any permanent pool areas should generally not exceed 6 ft. • The volume of the extended detention must not comprise more than 50% of the total WQ, and its maximum water surface elevation must not extend more than 3 ft above the normal pool. Qp and/or Cp, storage can be provided above the maximum WQ„ elevation within the wetland. • The perimeter of all deep pool areas (4 ft or greater in depth) should be surrounded by safety and aquatic benches similar to those for stormwater ponds. • The contours of the wetland should be irregular to provide a more natural landscaping effect. Pretreatment / Inlets • Sediment regulation is critical to sustain stormwater wetlands. A wetland facility should have a sediment forebay or equivalent upstream pretreatment. A sediment forebay is designed to remove incoming sediment from the stormwater flow prior to dispersal into the wetland. The forebay should consist of a separate cell, formed by an acceptable barrier. A forebay is to be provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the wetland facility. • The forebay is sized to contain 0.1 inches per impervious acre of contributing drainage and should be 4 to 6 ft deep. The pretreatment storage volume is part of the total WQv requirement and may be subtracted from WQv for wetland storage sizing. • A fixed vertical sediment depth marker shall be installed in the forebay to measure sediment deposition over time. The bottom of the forebay may be hardened (e.g., using concrete, paver blocks, etc.) to make sediment removal easier. • Inflow channels are to be stabilized with flared riprap aprons, or the equivalent. Inlet pipes to the pond can be partially submerged. Exit velocities from the forebay must be nonerosive. Outlet Structures • Flow control from a stormwater wetland is typically accomplished with the use of a concrete or corrugated metal riser and barrel. The riser is a vertical pipe or inlet structure that is attached to the base of the micropool with a watertight connection. The outlet barrel is a horizontal pipe attached to the riser that conveys flow under the embankment (see Figure 3.2.2-2) The riser should be located within the embankment for maintenance access, safety and aesthetics. • A number of outlets at varying depths in the riser provide internal flow control for routing of the water quality, channel protection, and overbank flood protection runoff volumes. The number of orifices can vary and is usually a function of the pond design. For shallow and pocket wetlands, the riser configuration is typically comprised of a channel protection outlet (usually an orifice) and overbank flood protection outlet (often a slot or weir). The channel protection orifice is sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). Since the water quality volume is fully contained in the permanent pool, no orifice sizing is necessary for this volume. As runoff from a water quality event enters the wet pond, it simply displaces that same volume through the channel protection orifice. Thus an off-line shallow or pocket wetland providing only water quality treatment can use a simple overflow weir as the outlet structure. In the case of a extended detention (ED) shallow wetland, there is generally a need for an additional outlet (usually an orifice) that is sized to pass the extended detention water quality volume that is surcharged on top of the permanent pool. Flow will first pass through this orifice, which is sized to release the water quality ED volume in 24 hours. The preferred design is a reverse slope pipe attached to the riser, with its inlet submerged 1 ft below the elevation of the permanent pool to prevent floatables from clogging the pipe and to avoid discharging warmer water at the surface of the pond. The next outlet is sized for the release of the channel protection storage volume. The outlet (often an orifice) invert is located at the maximum elevation associated with the extended detention water quality volume and is sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). o Alternative hydraulic control methods to an orifice can be used and include the use of a broad - crested rectangular, V -notch, proportional weir, or an outlet pipe protected by a hood that extends at least 12 inches below the normal pool. EMBANKMENT 100 YEAR LEVEL 25 YEAR LEVEL `EMERGENCY SPILLWAY Cps, LEVEL = HOOD/TRASH RACK/ SKIMMER NORMAL POOL — — — — — — ' ' ELEVATION � .MULTI—STAGE RISER REVERSE—SLOPE PIPE w/ VALVE BARREL WETLAND D w/ VALVE ANTI—SEEP COLLAR Figure 2.6. Typical Wetland Facility Outlet Structure. The water quality outlet (if design is for an ED shallow wetland) and channel protection outlet should be fitted with adjustable gate valves or other mechanism that can be used to adjust detention time. • Higher flows (overbank and extreme flood protection) flows pass through openings or slots protected by trash racks further up on the riser. • After entering the riser, flow is conveyed through the barrel and is discharged downstream. Anti - seep collars should be installed on the outlet barrel to reduce the potential for pipe failure. o Riprap, plunge pools or pads, or other energy dissipators are to be placed at the outlet of the barrel to prevent scouring and erosion. If a wetland facility daylights to a channel with dry weather flow, care should be taken to minimize tree clearing along the downstream channel, and to reestablish a forested riparian zone in the shortest possible distance. See Section 4.5 (Energy Dissipation Design) for more guidance. o The wetland facility must have a bottom drain pipe located in the micropool with an adjustable valve that can completely or partially dewater the wetland within 24 hours. (This requirement may be waived for coastal areas, where positive drainage is difficult to achieve due to very low relief) o The wetland drain should be sized one pipe size greater than the calculated design diameter. The drain valve is typically a handwheel activated knife or gate valve. Valve controls shall be located inside of the riser at a point where they (a) will not normally be inundated and (b) can be operated in a safe manner. See the design procedures in Chapter 7 and Appendix G for additional information and specifications on pond routing and outlet works. Emergency Spillway • An emergency spillway is to be included in the stormwater wetland design to safely pass flows that exceed the design storm flows. The spillway prevents the wetland's water levels from overtopping the embankment and causing structural damage. The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges. A minimum of 1 ft of freeboard must be provided, measured from the top of the water surface elevation for the extreme flood to the lowest point of the dam embankment, not counting the emergency spillway. Maintenance Access • A maintenance right of way or easement must be provided to the wetland facility from a public or private road. Maintenance access should be at least 12 ft wide, have a maximum slope of no more than 15%, and be appropriately stabilized to withstand maintenance equipment and vehicles. • The maintenance access must extend to the forebay, safety bench, riser, and outlet and, to the extent feasible, be designed to allow vehicles to turn around. Access to the riser is to be provided by lockable manhole covers, and manhole steps within easy reach of valves and other controls. Safety Features • All embankments and spillways must be designed to State of Georgia guidelines for dam safety (see Appendix H). • Fencing of wetlands is not generally desirable, but may be required by the local review authority. A preferred method is to manage the contours of deep pool areas through the inclusion of a safety bench (see above) to eliminate dropoffs and reduce the potential for accidental drowning. • The principal spillway opening should not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter should be fenced to prevent a hazard. Landscaping • A landscaping plan should be provided that indicates the methods used to establish and maintain wetland coverage. Minimum elements of a plan include: delineation of landscaping zones, selection of corresponding plant species, planting plan, sequence for preparing wetland bed (including soil amendments, if needed) and sources of plant material. Landscaping zones include low marsh, high marsh, and semi -wet zones. The low marsh zone ranges from 6 to 18 inches below the normal pool. This zone is suitable for the growth of several emergent plant species. The high marsh zone ranges from 6 inches below the pool up to the normal pool. This zone will support greater density and diversity of emergent wetland plant species. The high marsh zone should have a higher surface area to volume ratio than the low marsh zone. The semi -wet zone refers to those areas above the permanent pool that are inundated on an irregular basis and can be expected to support wetland plants. • The landscaping plan should provide elements that promote greater wildlife and waterfowl use within the wetland and buffers. • Woody vegetation may not be planted on the embankment or allowed to grow within 15 ft of the toe of the embankment and 25 ft from the principal spillway structure. • A wetland buffer shall extend 25 ft outward from the maximum water surface elevation, with an additional 15 -ft setback to structures. The wetland buffer should be contiguous with other buffer areas that are required by existing regulations (e.g., stream buffers) or that are part of the overall stormwater management concept plan. No structures should be located within the buffer, and an additional setback to permanent structures may be provided. • Existing trees should be preserved in the buffer area during construction. It is desirable to locate forest conservation areas adjacent to ponds. To discourage resident geese populations, the buffer can be planted with trees, shrubs and native ground covers. • The soils of a wetland buffer are often severely compacted during the construction process to ensure stability. The density of these compacted soils is so great that it effectively prevents root penetration and therefore may lead to premature mortality or loss of vigor. Consequently, it is advisable to excavate large and deep holes around the proposed planting sites and backfill these with uncompacted topsoil. Additional Site -Specific Design Criteria and Issues Physiographic Factors - Local terrain design constraints o Low Relief - Providing wetland drain can be problematic o High Relief - Embankment heights restricted o Karst - Requires poly or clay liner to sustain a permanent pool of water and protect aquifers; limits on ponding depth; geotechnical tests may be required • Soils o Hydrologic group "A" soils and some group "B" soils may require liner (not relevant for pocket wetland) SECTION 7. DESIGN PROCEDURES Step 1. Compute runoff control volumes from the Unified Stormwater Sizing Criteria Calculate the Water Quality Volume (WQv), Channel Protection Volume (Cpv), Overbank Flood Protection Volume (Qp), and the Extreme Flood Volume (Qf). Details on the Stormwater Sizing Criteria are found in Chapter 2. Step 2. Determine if the development site and conditions are appropriate for the use of a stormwater wetland Consider the Application and Site Feasibility Criteria in Section 6 of this specification (Location and Siting). Step 3. Confirm local design criteria and alplicability Consider any special site-specific design conditions/criteria from Section 6 of this specification (Additional Site -Specific Design Criteria and Issues). Check with the City Engineer to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply. Step 4. Determine pretreatment volume A sediment forebay is provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the pond. The forebay should be sized to contain 0.1 inches per impervious acre of contributing drainage and should be 4 to 6 ft deep. The forebay storage volume counts toward the total WQv requirement and may be subtracted from the WQv for subsequent calculations. Step 5. Allocate the W0, volume among marsh, micropool. and ED volumes Use recommended criteria from Table 2.1 of this specification Step 6. Determine wetland location and preliminary geometry, including distribution of wetland depth zones This step involves initially laying out the wetland design and determining the distribution of wetland surface area among the various depth zones (high marsh, low marsh, and deepwater). Set WQ, permanent pool elevation (and WQ,- ED elevation for ED shallow wetland) based on volumes calculated earlier. See Section 6 of this specification (Physical Specification / Geometry) for more details. Step 7. Compute extended detention orifice release rate(s) and size(s), and establish Cp, elevation Shallow Wetland and Pocket Wetland: The Cp, elevation is determined from the stage -storage relationship and the orifice is then sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). The channel protection orifice should have a minimum diameter of 3 inches and should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the riser, with its inlet submerged 1 ft below the elevation of the permanent pool is a recommended design. The orifice diameter may be reduced to 1 inch if internal orifice protection is used (i.e., an over -perforated vertical stand pipe with 1/z -inch orifices or slots that are protected by wirecloth and a stone filtering jacket). Adjustable gate valves can also be used to achieve this equivalent diameter. ED Shallow Wetland: Based on the elevations established in Step 6 for the extended detention portion of the water quality volume, the water quality orifice is sized to release this extended detention volume in 24 hours. The water quality orifice should have a minimum diameter of 3 inches, and should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the riser, with its inlet submerged 1 ft below the elevation of the permanent pool, is a recommended design. Adjustable gate valves can also be used to achieve this equivalent diameter. The Cpv elevation is then determined from the stage -storage relationship. The invert of the channel protection orifice is located at the water quality extended detention elevation, and the orifice is sized to release the channel protection storage volume over a 24-hour period (12 -hour extended detention may be warranted in some cold water streams). Step 8. Calculate Qpzs (25 -year storm) release rate and water surface elevation Set up a stage -storage -discharge relationship for the control structure for the extended detention orifice(s) and the 25 -year storm. Step 9. Design embankment(s) and spillway(s) Size emergency spillway, calculate 100 -year water surface elevation, set top of embankment elevation, and analyze safe passage of the Extreme Flood Volume (Qf). At final design, provide safe passage for the 100 -year event. Attenuation may not be required. Step 10. Design inlets, sediment forebay(s), outlet structures, maintenance access, and safety features. See Section 6 of this specification for more details. Step 11. Prepare Vegetation and Landscaping Plan A landscaping plan for the wetland facility and its buffer should be prepared to indicate how aquatic and terrestrial areas will be stabilized and established with vegetation. See Section 6 of this specification (Landscaping) for more details. SECTION 8. INSPECTION AND MAINTENANCE REQUIREMENTS Additional Maintenance Considerations and Requirements Maintenance requirements for constructed wetlands are particularly high while vegetation is being established. Monitoring during these first years is crucial to the future success of the wetland as a stormwater structural control. Wetland facilities should be inspected after major storms (greater than 2 inches of rainfall) during the first year of establishment to assess bank stability, erosion damage, flow channelization, and sediment accumulation within the wetland. For the first 3 years, inspections should be conducted at least twice a year. A sediment marker should be located in the forebay to determine when sediment removal is required. Accumulated sediments will gradually decrease wetland storage and performance. The effects of sediment deposition can be mitigated by the removal of the sediments. • Sediments excavated from stormwater wetlands that do not receive runoff from designated hotspots are not considered toxic or hazardous material and can be safely disposed of by either land application or landfilling. Sediment testing may be required prior to sediment disposal when a hotspot land use is present. Sediment removed from stormwater wetlands should be disposed of according to an approved erosion and sediment control plan. 2.2. Typical Maintenance Activities for Wetlands (AdaptedTable Activity Schedule • Replace wetland vegetation to maintain at least 50% surface area coverage in One -Time Activity wetland plants after the second growing season. • Clean and remove debris from inlet and outlet structures. Frequently • Mow side slopes. (3 to 4 times/year) • Monitor wetland vegetation and perform replacement planting as necessary. Semi-annual Inspection(first 3 years) • Examine stability of the original depth zones and microtopographical features. • Inspect for invasive vegetation, and remove where possible. • Inspect for damage to the embankment and inlet/outlet structures. Repair as necessary. Annual • Note signs of hydrocarbon build-up, and remove appropriately. Inspection • Monitor for sediment accumulation in the facility and forebay. • Examine to ensure that inlet and outlet devices are free of debris and operational. • Repair undercut or eroded areas. As Needed • Harvest wetland plants that have been "choked out" by sediment build-up. Annually 5 to 7 years • Removal of sediment from the forebay. or after 50% of the total forebay capacity has been lost • Monitor sediment accumulations, and remove sediment when the pool 10 to 20 years or after volume has become reduced significantly, plants are "choked" with sediment, 25% of the wetland or the wetland becomes eutrophic. volume has been lost Additional Maintenance Considerations and Requirements Maintenance requirements for constructed wetlands are particularly high while vegetation is being established. Monitoring during these first years is crucial to the future success of the wetland as a stormwater structural control. Wetland facilities should be inspected after major storms (greater than 2 inches of rainfall) during the first year of establishment to assess bank stability, erosion damage, flow channelization, and sediment accumulation within the wetland. For the first 3 years, inspections should be conducted at least twice a year. A sediment marker should be located in the forebay to determine when sediment removal is required. Accumulated sediments will gradually decrease wetland storage and performance. The effects of sediment deposition can be mitigated by the removal of the sediments. • Sediments excavated from stormwater wetlands that do not receive runoff from designated hotspots are not considered toxic or hazardous material and can be safely disposed of by either land application or landfilling. Sediment testing may be required prior to sediment disposal when a hotspot land use is present. Sediment removed from stormwater wetlands should be disposed of according to an approved erosion and sediment control plan. • Periodic mowing of the wetland buffer is only required along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow (mowing every other year) or forest. • Regular inspection and maintenance is critical to the effective operation of stormwater wetlands as designed. SECTION 9. REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. Massachusetts Department of Environmental Protection / Massachusetts Office of Coastal Zone Management, 1997. Stormwater Management -- Volume One: Stormwater Policy Handbook. and Volume Two: Stormwater Technical Handbook. Watershed Management Institute (WMI), 1997. Operation, Maintenance. and Management of Stormwater Management Systems. Prepared for US EPA, Office of Water. SAND FILTERS Description: Multi-chamber structure designed to treat stormwater runoff through filtration, using a sediment forebay, a sand bed as its primary filter media and, typically, an underdrain collection system. • Applicable to small • High maintenance burden drainage areas • Not recommended for areas • Good for highly impervious with high sediment content in areas stormwater or clay/silt runoff • Good retrofit capability areas • Relatively costly • Possible odor problems Design Criteria: • Typically requires 2 to 6 ft of head • Maximum contributing drainage area of 10 acres for surface sand filter; 2 acres for perimeter sand filter • Sand filter media with underdrain system Stormwater Management Capability: • Water quality benefits can provide 80% TSS removal. Land Use Considerations: ■ Residential © Commercial © Industrial Maintenance: • Inspect for clogging — rake first inch of sand • Remove sediment from forebay/chamber • Replace sand filter media as needed ©Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION Sand filters (also referred to as filtration basins) are structural stormwater controls that capture and temporarily store stormwater runoff and pass it through a filter bed of sand. Most sand filter systems consist of two -chamber structures. The first chamber is a sediment forebay or sedimentation chamber, which removes floatables and heavy sediments. The second is the filtration chamber, which removes additional pollutants by filtering the runoff through a sand bed. The filtered runoff is typically collected and returned to the conveyance system, though it can also be partially or fully exfiltrated into the surrounding soil in areas with porous soils. Because they have few site constraints beside head requirements, sand filters can be used on development sites where the use of other structural controls may be precluded. However, sand filter systems can be relatively expensive to construct and install. There are two primary sand filter system designs, the surface sand filter and the perimeter sand filter. Below are descriptions of these filter systems: • Surface Sand Filter - The surface sand filter is a ground -level open air structure that consists of a pretreatment sediment forebay and a filter bed chamber. This system can treat drainage areas up to 10 acres in size and is typically located off-line. Surface sand filters can be designed as an excavation with earthen embankments or as a concrete or block structure. • Perimeter Sand Filter - The perimeter sand filter is an enclosed filter system typically constructed just below grade in a vault along the edge of an impervious area such as a parking lot. The system consists of a sedimentation chamber and a sand bed filter. Runoff flows into the structure through a series of inlet grates located along the top of the control. A third design variant, the underground sand filter, is intended primarily for extremely space limited and high density areas and is thus considered a limited application structural control. See subsection 4.6.10 for more details. Surface Sand Filter Perimeter Sand Filter Figure 3.1. Sand Filter Examples. SECTION 2: STORMWATER MANAGEMENT SUITABILITY Sand filter systems are designed primarily as off-line systems for stormwater quality (i.e., the removal of stormwater pollutants) and will typically need to be used in conjunction with another structural control to provide downstream channel protection, overbank flood protection, and extreme flood protection, if required. However, under certain circumstances, filters can provide limited runoff quantity control, particularly for smaller storm events. Minimum Standard #1 In sand filter systems, stormwater pollutants are removed through a combination of gravitational settling, filtration and adsorption. The filtration process effectively removes suspended solids and particulates, biochemical oxygen demand (BOD), fecal coliform bacteria, and other pollutants. Surface sand filters with a grass cover have additional opportunities for bacterial decomposition as well as vegetation uptake of pollutants, particularly nutrients. Section 4.6.4.3 provides median pollutant removal efficiencies that can be used for planning and design purposes. Minimum Standard #2 For smaller sites, a sand filter may be designed to capture the entire channel protection volume Cpv in either an off- or on-line configuration. Given that a sand filter system is typically designed to completely drain over 40 hours, the requirement of extended detention of the 1 -year, 24-hour storm runoff volume will be met. For larger sites -or- where only the WQv is diverted to the sand filter facility, another structural control must be used to provide Cps extended detention. Minimum Standard #3 Another structural control must be used in conjunction with a sand filter system to reduce the post - development peak flow of the 25 -year storm (Qp) to pre -development levels (detention). Sand filter facilities must provide flow diversion and/or be designed to safely pass extreme storm flows (100 -year storm event) and protect the filter bed and facility. SECTION 3: POLLUTANT REMOVAL CAPABILITIES Both the surface and perimeter sand filters are presumed to be able to remove 80% of the total suspended solids load in typical urban post -development runoff when sized, designed, constructed and maintained in accordance with the recommended specifications. Undersized or poorly designed sand filters can reduce TSS removal performance. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. In a situation where a removal rate is not deemed sufficient, additional controls may be put in place at the given site in a series or "treatment train" approach. • Total Suspended Solids - 80% • Total Phosphorus - 50% Total Nitrogen - 25% Fecal Coliform - 40% • Heavy Metals - 50% For additional information and data on pollutant removal capabilities for sand filters, see the National Pollutant Removal Performance Database (3rd Edition) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org SECTION 4: TYPICAL SCHEMATIC DETAILS UNDERDRAIN COLLECTION SYSTEM IRFLOW LWAY PLAN VIEW FLOW DIVERSION PERFORATED STANDPIPE STRUCTURE DETENTION STRUCTURE INFLOW 1111111111111,1111-wi FILTER BED Nil I I OVERFLOW SPILLWAY PRETREATMENT —rn W'7� OUTFLOW 10 UNDERDRAIN COLLECTION SYSTEM 3" TOPSOIL FILTER FABRIC 18" CLEAN WASHED "CONCRETE" SAND FILTER FABRIC 6" PERFORATED PIPE/ GRAVEL UNDERDRAIN SYSTEM TYPICAL SECTION PROFILE Figure 3.2. Schematic of Surface Sand Filter (Source: Center for Watershed Protection). OUTLET CURB PARKING LOT SHEET FLOW 1 1 1 1 INLET GRATES `OVERFLOW WEIRS CLEAR WELL .. .. ..... .. .. .. .. SAND CHAMBER OUTLET PIPE COLLECTION SYSTEM - ACCESS GRATES CURB STOPS UNDERDRAIN TEMPORARY PONDING 6" - 12" F 7. 7. 7. w. 7. 7. 7. 7. 18" CLEAN WASHED SAND FILTER FABRIC 4" PERFORATED PIPE IN 6" GRAVEL JACKET TYPICAL SECTION PLAN VIEW PIPE OUTLET PIPES PROFILE Figure 3.3. Schematic of Perimeter Sand Filter. (Source: Center for Watershed Protection). SECTION 5: SITE FEASIBILITY & DESIGN APPLICATIONS Sand filter systems are well suited for highly impervious areas where land available for structural controls is limited. Sand filters should primarily be considered for new construction or retrofit opportunities for commercial, industrial, and institutional areas where the sediment load is relatively low, such as: parking lots, driveways, loading docks, gas stations, garages, airport runways/taxiways, and storage yards. Sand filters may also be feasible and appropriate in some multi -family or higher density residential developments. To avoid rapid clogging and failure of the filter media, the use of sand filters should be avoided in areas with less than 50% impervious cover, or high sediment yield sites with clay/silt soils. The following basic criteria should be evaluated to ensure the suitability of a sand filter facility for meeting stormwater management objectives on a site or development. General Feasibility • Suitable for Residential Subdivision Usage - NO • Suitable for High Density/Ultra Urban Areas - YES • Regional Stormwater Control - NO Physical Feasibility - Physical Constraints at Project Site • Drainage Area - 10 acres maximum for surface sand filter; 2 acres maximum for perimeter sand filter • Space Required - Function of available head at site • Site Slope - No more than 6% slope across filter location • Minimum Head - Elevation difference needed at a site from the inflow to the outflow: 5 ft for surface sand filters; 2 to 3 ft for perimeter sand filters • Minimum Depth to Water Table - For a surface sand filter with exfiltration (earthen structure), 2 ft are required between the bottom of the sand filter and the elevation of the seasonally high water table • Soils - No restrictions; Group "A" soils generally required to allow exfiltration (for surface sand filter earthen structure) Other Constraints / Considerations • Aquifer Protection - Do not allow exfiltration of filtered runoff into groundwater from areas with potential for high pollutant loading. SECTION 6: PLANNING AND DESIGN CRITERIA The following criteria are to be considered minimum standards for the design of a sand filter facility. Location and Siting • Surface sand filters should have a contributing drainage area of 10 acres or less. The maximum drainage area for a perimeter sand filter is 2 acres. • Sand filter systems are generally applied to land uses with a high percentage of impervious surfaces. Sites with less than 50% imperviousness or high clay/silt sediment loads must not use a sand filter without adequate pretreatment due to potential clogging and failure of the filter bed. Any disturbed areas within the sand filter facility drainage area should be identified and stabilized. Filtration controls should only be constructed after the construction site is stabilized. • Surface sand filters are generally used in an off-line configuration where the water quality volume (WQv) is diverted to the filter facility through the use of a flow diversion structure and flow splitter. Stormwater flows greater than the WQ, are diverted to other controls or downstream using a diversion structure or flow splitter. • Perimeter sand filters are typically sited along the edge, or perimeter, of an impervious area such as a parking lot. • Sand filter systems are designed for intermittent flow and must be allowed to drain and reaerate between rainfall events. They should not be used on sites with a continuous flow from groundwater, sump pumps, or other sources. General Design • Surface Sand Filter o A surface sand filter facility consists of a two -chamber open-air structure, which is located at ground -level. The first chamber is the sediment forebay (a.k.a sedimentation chamber) while the second chamber houses the sand filter bed. Flow enters the sedimentation chamber where settling of larger sediment particles occurs. Runoff is then discharged from the sedimentation chamber through a perforated standpipe into the filtration chamber. After passing though the filter bed, runoff is collected by a perforated pipe and gravel underdrain system. Figure 3.2 provides plan view and profile schematics of a surface sand filter. • Perimeter Sand Filter A perimeter sand filter facility is a vault structure located just below grade level. Runoff enters the device through inlet grates along the top of the structure into the sedimentation chamber. Runoff is discharged from the sedimentation chamber through a weir into the filtration chamber. After passing though the filter bed, runoff is collected by a perforated pipe and gravel underdrain system. Figure 3.3 provides plan view and profile schematics of a perimeter sand filter. Physical Specificiations / Geometry • Surface Sand Filter o The entire treatment system (including the sedimentation chamber) must temporarily hold at least 75% of the WQv prior to filtration. Figure 3.4 illustrates the distribution of the treatment volume (0.75 WQ,) among the various components of the surface sand filter, including: 0 VS - volume within the sedimentation basin ■ Vf - volume within the voids in the filter bed • Vf_temp - temporary volume stored above the filter bed ■ A, - the surface area of the sedimentation basin ■ Af - surface area of the filter media ■ hs - height of water in the sedimentation basin ■ hf - average height of water above the filter media ■ df - depth of filter media o The sedimentation chamber must be sized to at least 25% of the computed WQ„ and have a length -to -width ratio of at least 2:1. Inlet and outlet structures should be located at opposite ends of the chamber. o The filter area is sized based on the principles of Darcy's Law. A coefficient of permeability (k) of 3.5 ft/day for sand should be used. The filter bed is typically designed to completely drain in 40 hours or less. SEDIMENTATION SAND FILTER BASIN AREA: BED AREA As Af PLAN INFLOW PIPE 17 hs VS — Vf - temp 2 • hf Vf df PROFILE SAND BED Figure 3.4. Surface Sand Filter Volumes (Source: Clayton and Schueler, 1996). o The filter media consists of an 18 -inch layer of clean washed medium sand (meeting ASTM C-33 concrete sand) on top of the underdrain system. Three inches of topsoil are placed over the sand bed. Permeable filter fabric is placed both above and below the sand bed to prevent clogging of the sand filter and the underdrain system. Figure 3.6 illustrates a typical media cross section. ■ The filter bed is equipped with a 6 -inch perforated PVC pipe (AASHTO M 252) underdrain in a gravel layer. The underdrain must have a minimum grade of 1/8 -inch per foot (1% slope). Holes should be 3/8 -inch diameter and spaced approximately 6 inches on center. Gravel should be clean washed aggregate with a maximum diameter of 3.5 inches and a minimum diameter of 1.5 inches with a void space of about 40%. Aggregate contaminated with soil shall not be used. ■ The structure of the surface sand filter may be constructed of impermeable media such as concrete, or through the use of excavations and earthen embankments. When constructed with earthen walls/embankments, filter fabric should be used to line the bottom and side slopes of the structures before installation of the underdrain system and filter media. • Perimeter Sand Filter o The entire treatment system (including the sedimentation chamber) must temporarily hold at least 75% of the WQv prior to filtration. Figure 3.5 illustrates the distribution of the treatment volume (0.75 WQv) among the various components of the perimeter sand filter, including: ■ VW - wet pool volume within the sedimentation basin ■ Vf - volume within the voids in the filter bed • Vtemp - temporary volume stored above the filter bed ■ AS - the surface area of the sedimentation basin ■ At - surface area of the filter media ■ ht - average height of water above the filter media (1/2 htemp) ■ df - depth of filter media o The sedimentation chamber must be sized to at least 50% of the computed WQv. o The filter area is sized based on the principles of Darcy's Law. A coefficient of permeability (k) of 3.5 ft/day for sand should be used. The filter bed is typically designed to completely drain in 40 hours or less. ■ The filter media should consist of a 12- to 18 -inch layer of clean washed medium sand (meeting ASTM C-33 concrete sand) on top of the underdrain system. Figure 3.6 illustrates a typical media cross section. The perimeter sand filter is equipped with a 4 inch perforated PVC pipe (AASHTO M 252) underdrain in a gravel layer. The underdrain must have a minimum grade of 1/8 inch per foot (1% slope). Holes should be 3/8 -inch diameter and spaced approximately 6 inches on center. A permeable filter fabric should be placed between the gravel layer and the filter media. Gravel should be clean washed aggregate with a maximum diameter of 3.5 inches and a minimum diameter of 1.5 inches with a void space of about 40%. Aggregate contaminated with soil shall not be used. OUTLET CHAMBER SEDIMENTATION BASIN AREA: Al As SAND FILTER BED AREA Af PLAN h temp 2Xhf Ut—P (2Xhf) df V V 2- w PROFILE Figure 3.5. Perimeter Sand Filter Volumes (Source: Clayton and Schueler,1996). Pretreatment / Inlets Pretreatment of runoff in a sand filter system is provided by the sedimentation chamber. Inlets to surface sand filters are to be provided with energy dissipators. Exit velocities from the sedimentation chamber must be nonerosive. Figure 3.7 shows a typical inlet pipe from the sedimentation basin to the filter media basin for the surface sand filter. Outlet Structures Outlet pipe is to be provided from the underdrain system to the facility discharge. Due to the slow rate of filtration, outlet protection is generally unnecessary (except for emergency overflows and spillways). Emergency Spillway • An emergency or bypass spillway must be included in the surface sand filter to safely pass flows that exceed the design storm flows. The spillway prevents filter water levels from overtopping the embankment and causing structural damage. The emergency spillway should be located so that downstream buildings and structures will not be impacted by spillway discharges. HORIZONTAL SURFACE SAND BED 18"-24" ASTM C-33 MEDIUM AGGREGATE CONCRETE SAND) FILTER FABRIC OR 4" PEA / GRAVEL LAYER IN LIEU OF IMPERMEABLE LINER FILTER FABRIC WHERE NECESSARY HORIZONTAL SURFACE SAND BED 3" TOPSOIL LAYER (PEA GRAVEL WINDOW FOR POCKET SAND FILTER ONLY) ✓2�� Y GRAVEL LAYER PERFORATED 6" PVC PIPE (LATERAL SPACES AT 10' O.C.) 3" TOPSOIL LAYER (PEA GRAVEL WINDOW FOR POCKET SAND FILTER ONLY) Figure 3.6.Typical Sand Filter Media Cross Sections (Source: Clayton and Schueler, 1996). Maintenance Access • Adequate access must be provided for all sand filter systems for inspection and maintenance, including the appropriate equipment and vehicles. Access grates to the filter bed need to be included in a perimeter sand filter design. Facility designs must enable maintenance personnel to easily replace upper layers of the filter media. Safety Features Surface sand filter facilities can be fenced to prevent access. Inlet and access grates to perimeter sand filters may be locked. Landscaping • Surface filters can be designed with a grass cover to aid in pollutant removal and prevent clogging. The grass should be capable of withstanding frequent periods of inundation and drought. SOLID CAP TRASH RACK, MIN. OPENING SIZE = 3 x ; PERFORATION DIAMETER, F1 IJ fll O BOTTOM OF SEDIMENTATION CHAMBER 1" DIAMETER PERFORATIONS SPACED VERTICALLY AT 2 1/2" CENTERS PIPE HANGERS /CONCRETE WALL CAP WITH LOW FLOW ORIFICE SIZED FOR 2"± _ i 24 HOUR DETENTION FLOW DISTRIBUTION VAULT BROAD CRESTED WEIR, "V" NOTCH WEIR, OR MULTIPLE ORIFICES @ CONSTANT ELEV. 18" ± SAND BED _ EROSION --- PROTECTION (RIP RAP OR EQUIV.) Figure 3.7. Surface Sand Filter Perforated Stand -Pipe (Source: Clayton and Schueler, 1996). Additional Site -Specific Design Criteria and Issues • Physiographic Factors - Local terrain design constraints o Low Relief - Use of surface sand filter may be limited by low head o High Relief - Filter bed surface must be level o Karst - Use polyliner or impermeable membrane to seal bottom of earthen surface sand filter or use watertight structure PARTIALLY UT 12" SUBMERGED DEEP OUTLET MIN. PERFORATION SCHEDULE PIPE # OF SIZE (IN.) PERF./ROW 6 7 8 10 10 12 TYPICAL DETAIL NTS BROAD CRESTED WEIR, "V" NOTCH WEIR, OR MULTIPLE ORIFICES @ CONSTANT ELEV. 18" ± SAND BED _ EROSION --- PROTECTION (RIP RAP OR EQUIV.) Figure 3.7. Surface Sand Filter Perforated Stand -Pipe (Source: Clayton and Schueler, 1996). Additional Site -Specific Design Criteria and Issues • Physiographic Factors - Local terrain design constraints o Low Relief - Use of surface sand filter may be limited by low head o High Relief - Filter bed surface must be level o Karst - Use polyliner or impermeable membrane to seal bottom of earthen surface sand filter or use watertight structure • Soils o No restrictions • Special Downstream Watershed Considerations o Aquifer Protection - Use polyliner or impermeable membrane to seal bottom of earthen surface sand filter or use watertight structure; no exfiltration of filter runoff into groundwater SECTION 7: DESIGN PROCEDURES Step 1. Compute runoff control volumes from the Unified Stormwater Sizing Criteria Calculate the Water Quality Volume (WQ„), Channel Protection Volume (Cp„), Overbank Flood Protection Volume (Qp), and the Extreme Flood Volume (Qf). Details on the Stormwater Sizing Criteria are found in Chapter 2. Step 2. Determine if the development site and conditions are appropriate for the use of a surface or perimeter sand filter. Consider the Application and Site Feasibility Criteria in Section 6 of this specification (Location and Siting). Step 3. Confirm local design criteria and applicability Consider any special site-specific design conditions/criteria from Section 6 of this specification (Additional Site -Specific Design Criteria and Issues). Check with City Engineer to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply. Step 4. Compute WQ„ peak discharge fOwj The peak rate of discharge for water quality design storm is needed for sizing of off-line diversion structures (see Chapter 3). 1. Using WQ,,, compute CN 2. Compute time of concentration using TR -55 method 3. Determine appropriate unit peak discharge from time of concentration 4. Compute Qwq from unit peak discharge, drainage area, and WQ,,. Step 5. Size flow diversion structure, if needed A flow regulator (or flow splitter diversion structure) should be supplied to divert the WQ„ to the sand filter facility. Size low flow orifice, weir, or other device to pass Qwq. Step 6. Size filtration basin chamber The filter area is sized using the following equation (based on Darcy's Law): Af = (WQ-,) (df) / [(k) (hf+ dr) (tf)] where: Af = surface area of filter bed (ftz) df = filter bed depth (typically 18 inches, no more than 24 inches) k = coefficient of permeability of filter media (ft/day) (use 3.5 ft/day for sand) hf = average height of water above filter bed (ft) (1/2 hmax, which varies based on site but hmax is typically <_ 6 ft) tf = design filter bed drain time (days) (1.67 days or 40 hours is recommended maximum) Set preliminary dimensions of filtration basin chamber. See subsection 4.6.4.5-C (Physical Specifications/Geometry) for filter media specifications. Step 7. Size sedimentation chamber Surface sand filter: The sedimentation chamber should be sized to at least 25% of the computed WQ„ and have a length -to -width ratio of 2:1. The Camp -Hazen equation is used to compute the required surface area: AS = - (Qo/w) * Ln (1-E) Where: AS = sedimentation basin surface area (ftz) Q,, = rate of outflow = the WQ„ over a 24-hour period w = particle settling velocity (ft/sec) E = trap efficiency Assuming: • 90% sediment trap efficiency (0.9) • particle settling velocity (ft/sec) = 0.0033 ft/sec for imperviousness < 75% • particle settling velocity (ft/sec) = 0.0004 ft/sec for imperviousness >_ 75% • average of 24 hour holding period Then: AS = (0.066) (WQ„) ftz for I < 75% AS = (0.0081) (WQ,) ftz for I >_ 75% Set preliminary dimensions of sedimentation chamber. Perimeter sand filter: The sedimentation chamber should be sized to at least 50% of the computed WQ,. Use same approach as for surface sand filter. Step 8. Compute Vmin Vmin =0.75 * WQ, Step 9. Compute storage volumes within entire facility and sedimentation chamber orifice size Surface sand filter: Vmin = 0.75 WQv = Vs + Vf + Vf-temp 1. Compute Vf = water volume within filter bed/gravel/pipe = Af * df * n Where: n = porosity = 0.4 for most applications 2. Compute Vf-temp = temporary storage volume above the filter bed = 2 * hf * Af 3. Compute Vs = volume within sediment chamber = Vmin - Vf - Vf-temp 4. Compute hs = height in sedimentation chamber = Vs/AS 5. Ensure hs and hf fit available head and other dimensions still fit - change as necessary in design iterations until all site dimensions fit. 6. Size orifice from sediment chamber to filter chamber to release Vs within 24 -hours at average release rate with 0.5 hs as average head. 7. Design outlet structure with perforations allowing for a safety factor of 10 (see example) 8. Size distribution chamber to spread flow over filtration media - level spreader weir or orifices. Perimeter sand filter: 1. Compute Vf = water volume within filter bed/gravel/pipe = Af * df * n 2. Where: n = porosity = 0.4 for most applications 3. Compute VW = wet pool storage volume As * 2 ft minimum 4. Compute Vtemp = temporary storage volume = Vmin - (Vf + Vw) 5. Compute htemp = temporary storage height = Vtemp / (Af + AS) 6. Ensure htemp >_ 2 * hf, otherwise decrease hf and re -compute. Ensure dimensions fit available head and area - change as necessary in design iterations until all site dimensions fit. 7. Size distribution slots from sediment chamber to filter chamber. Step 10. Design inlets, pretreatment facilities, underdrain system, and outlet structures See Section 6 of this specification for more details. Step 11. Compute overflow weir sizes Surface sand filter: 1. Size overflow weir at elevation hs in sedimentation chamber (above perforated stand pipe) to handle surcharge of flow through filter system from 25 -year storm (see example). Plan inlet protection for overflow from sedimentation chamber and size overflow weir at elevation hf in filtration chamber (above perforated stand pipe) to handle surcharge of flow through filter system from 25 -year storm (see example). Perimeter sand filter: 1. Size overflow weir at end of sedimentation chamber to handle excess inflow, set at WQ, elevation. SECTION 8: INSPECTION AND MAINTENANCE REQUIRMENTS Table 3.1. Typical Maintenance Activities for Sand Filters (Source: WMI, 1997; Pitt, 1997.) Activity Schedule • Ensure that contributing area, facility, inlets and outlets are clear of debris. • Ensure that the contributing area is stabilized and mowed, with clippings removed. • Remove trash and debris. • Check to ensure that the filter surface is not clogging (also check after Monthly moderate and major storms). • Ensure that activities in the drainage area minimize oil/grease and sediment entry to the system. • If permanent water level is present (perimeter sand filter), ensure that the chamber does not leak, and normal pool level is retained. • Check to see that the filter bed is clean of sediment, and the sediment chamber is not more than 50% full or 6 inches, whichever is less, of sediment. Remove sediment as necessary. • Make sure that there is no evidence of deterioration, spalling or cracking of concrete. • Inspect grates (perimeter sand filter). • Inspect inlets, outlets and overflow spillway to ensure good condition and no Annually evidence of erosion. • Repair or replace any damaged structural parts. • Stabilize any eroded areas. • Ensure that flow is not bypassing the facility. • Ensure that no noticeable odors are detected outside the facility. • If filter bed is clogged or partially clogged, manual manipulation of the surface layer of sand may be required. Remove the top few inches of sand, roto -till or otherwise cultivate the surface, and replace media with sand meeting the As needed design specifications. • Replace any filter fabric that has become clogged. Additional Maintenance Considerations and Requirements • A record should be kept of the dewatering time for a sand filter to determine if maintenance is necessary. • When the filtering capacity of the sand filter facility diminishes substantially (i.e., when water ponds on the surface of the filter bed for more than 48 hours), then the top layers of the filter media (topsoil and 2 to 3 inches of sand) will need to be removed and replaced. This will typically need to be done every 3 to 5 years for low sediment applications, more often for areas of high sediment yield or high oil and grease. Removed sediment and media may usually be disposed of in a landfill. SECTION 9: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. Clayton, R., & Schueler, T. (1996). Design of stormwater filtering systems. Ellicott City, MD. Center for Watershed Protection. Pitt, R., M. Lilbum, S. Nix, S. Durrans, and S. Burian,1997. Guidance Manual for Integrated Wet Weather Flow Collection and Treatment Systems for Newly Urbanized Areas. US EPA. Office of Research and Development. Watershed Management Institute (WMI), 1997. Operation. Maintenance. and Management of Stormwater Management Systems. Prepared for US EPA, Office of Water. ORGANIC FILTER Description: Design variant of the surface sand filter using organic materials in the filter media. • Can be used in high • High maintenance density/ultra urban areas requirements • High pollutant removal • Severe clogging potential capability if exposed soil surfaces exist upstream Design Considerations: • Intended for areas with potential for high pollutant loading or space limited applications requiring enhanced pollutant removal capability • Removal of dissolved pollutants is greater than sand filter due to cation exchange capacity • Minimum head requirement of 5 to 8 ft Stormwater Management Capability: • Water quality benefits can provide 80% TSS removal. Land Use Considerations: ■ Residential © Commercial © Industrial Maintenance: • Sediment and debris must be removed periodically • Check media for clogging • Replace media as needed ©Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION The organic filter is a design variant of the surface sand filter, which uses organic materials such as leaf compost or a peat/sand mixture as the filter media. The organic material enhances pollutant removal by providing adsorption of contaminants such as soluble metals, hydrocarbons, and other organic chemicals. As with the surface sand filter, an organic filter consists of a pretreatment chamber, and one or more filter cells. Each filter bed contains a layer of leaf compost or the peat/sand mixture, followed by filter fabric and a gravel/perforated pipe underdrain system. The filter bed and subsoils can be separated by an impermeable polyliner or concrete structure to prevent movement into groundwater. Organic filters are typically used in high-density applications, or for areas requiring an enhanced pollutant removal ability. Maintenance is typically higher than the surface sand filter facility due to the potential for clogging. In addition, organic filter systems have a higher head requirement than sand filters. SECTION 2: POLLUTANT REMOVAL CAPABILITIES Peat/sand filter systems provide good removal of bacteria and organic waste metals. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. • Total Suspended Solids - 80% • Total Phosphorus - 60% • Total Nitrogen - 40% • Fecal Coliform - 50% • Heavy Metals - 75% SECTION 3: DESIGN CRITERIA AND SPECIFICATIONS • Organic filters are typically used on relatively small sites (up to 10 acres), to minimize potential clogging. • The minimum head requirement (elevation difference needed at a site from the inflow to the outflow) for an organic filter is 5 to 8 ft. • Organic filters can utilize a variety of organic materials as the filtering media. Two typical media bed configurations are the peat/sand filter and compost filter (see Figure 4.1). The peat filter includes an 18 -inch 50/50 peat/sand mix over a 6 -inch sand layer and can be optionally covered by 3 inches of topsoil and vegetation. The compost filter has an 18 -inch compost layer. Both variants utilize a gravel underdrain system. • The type of peat used in a peat/sand filter is critically important. Fibric peat in which undecomposed fibrous organic material is readily identifiable is the preferred type. Hemic peat containing more decomposed material may also be used. Sapric peat made up of largely decomposed matter should not be used in an organic filter. • Typically, organic filters are designed as "off-line" systems, meaning that the water volume (WQ„) is diverted to the filter facility through the use of a flow diversion structure and flow splitter. Stormwater flows greater than the WQv are diverted to other controls or downstream using a diversion structure or flow splitter. • Consult the design criteria for the surface sand filter (see WSC-03, Sand Filters, in Appendix F) for the organic filter sizing and design steps. SECTION 4: TYPICAL SCHEMATIC DETAILS UNDERDRAIN COLLECTION SYSTEM BYPASS FLOW DIVERSION STRUCTURE FILTER BED /PRETREATMENT OUTFLOW '�SEDIMENTATION r------- 71 CHAMBER —OVERFLOW SPILLWAY PLAN VIEW FLOW DIVERSION PERFORATED STANDPIPE STRUCTURE DETENTION STRUCTURE INFLOW OVERFLOW 1bIfl FILTER BED SPILLWAY —IIH Ul 1111, =I ffl7=1%1IIffIII1I PRETREATMENT IT 164", Wt --IIIIIIIF OUTFLOW UI 171 =IIIILj—H 1-11-INDERDRAIN COLLECTION SYSTEM TOPSOIL 50/50 PEAT/SAND III III III IIII�IIIII�IIIIIIIIIIIII III III III III III IIII�IIIIIIIIIIIII MIXTURE LEAF COMPOST SAND III III ISI III ISI III ISI III ISI III III IF PERFORATED ... FILTER FABRIC PERFORATED PIPE IN GRAVEL _T PIPE IN GRAVEL V X X JACKET • JACKET TYPICAL SECTIONS PROFILE Figure 4.1. Schematic of Organic Filter (Source: Center for Watershed Protection). SECTION 5: INSPECTION AND MAINTENANCE REQUIREMENTS The inspection and maintenance requirements for organic filters are similar to those for surface sand filter facilities (see WSC-03, Sand Filter, in Appendix F) SECTION 6: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. UNDERGROUND SAND FILTER Description: Design variant of the surface sand filter using organic materials in the filter media. • Can be used in high • High maintenance density/ultra urban areas requirements • High pollutant removal capability Design Considerations: • Intended for areas with potential for high pollutant loading or space limited applications requiring enhanced pollutant removal capability • High removal rates for sediment, BOD, and fecal coliform bacteria • Precast concrete shells are available, which decrease construction costs Stormwater Management Capability: • Water quality benefits can provide 80% TSS removal. Land Use Considerations: ■ Residential © Commercial © Industrial Maintenance: • Monitor water level in sand filter chamber • Sedimentation chamber should be cleaned out when the sediment depth reaches 12 inches • Remove accumulated oil and floatables in �e sedimentation chamber Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION The underground sand filter is a design variant of the sand filter located in an underground vault designed for high-density land use or ultra -urban applications where there is not enough space for a surface sand filter or other structural stormwater controls. The underground sand filter is a three -chamber system. The initial chamber is a sedimentation (pretreatment) chamber that temporarily stores runoff and utilizes a wet pool to capture sediment. The sedimentation chamber is connected to the sand filter chamber by a submerged wall that protects the filter bed from oil and trash. The filter bed is 18 to 24 inches deep and may have a protective screen of gravel or permeable geotextile to limit clogging. The sand filter chamber also includes an underdrain system with inspection and clean out wells. Perforated drain pipes under the sand filter bed extend into a third chamber that collects filtered runoff. Flows beyond the filter capacity are diverted through an overflow weir. Due to its location below the surface, underground sand filters have a high maintenance burden and should only be used where adequate inspection and maintenance can be ensured. SECTION 2: GUIDELINES FOR USING PROPRIETARY SYSTEMS Underground sand filter pollutant removal rates are similar to those for surface and perimeter sand filters (see WSC-03, Sand Filters, in Appendix F). SECTION 3: DESIGN CRITERIA AND SPECIFICATIONS • Underground sand filters are typically used on highly impervious sites of 1 acre or less. The maximum drainage area that should be treated by an underground sand filter is 5 acres. • Underground sand filters are typically constructed on-line, but can be constructed off-line. For off- line construction, the overflow between the second and third chambers is not included. • The underground vault should be tested for water tightness prior to placement of filter layers. • Adequate maintenance access must be provided to the sedimentation and filter bed chambers. • Compute the minimum wet pool volume required in the sedimentation chamber as: VH, = AS * 3 ft minimum • Consult the design criteria for the perimeter sand filter (see Section 4.6.4) for the rest of the underground filter sizing and design steps. SECTION 4: TYPCIAL SCHEMATIC DETAILS INLET PIPE ]FIV W UNDERDRAIN ACCESS GRATES PIPE SYSTEM OUTLETRI PIPE . . . . . . . . ... .. ....... ..... OVERFLOW CHAMBER WET POOL CHA B R .............................................. ACCESS GRATES TEMPORAR) PONDING (VARIABLE) DEBRIS SCREEN (1 ") 24" CLEAN WASHEDSAND 6" PERFORATED PIPE IN 11" GRAVEL JACKET TYPICAL SECTION PIPE PLAN VIEW PROFILE Figure 4.1. Schematic of Underground Sand Filter (Source: Center for Watershed Protection). STEPS OVERFLOW TEMPORARY (TYP) WEIR INLET PIPE PONDING IIII-f[n�; PERMANENT DEBRIS SCRE E CLEANOUTS 1111— POOL =1F1 -GRAVEve 111— SUBMERGE. WALL TEMPORAR) PONDING (VARIABLE) DEBRIS SCREEN (1 ") 24" CLEAN WASHEDSAND 6" PERFORATED PIPE IN 11" GRAVEL JACKET TYPICAL SECTION PIPE PLAN VIEW PROFILE Figure 4.1. Schematic of Underground Sand Filter (Source: Center for Watershed Protection). SECTION 5: INSPECTION AND MAINTENANCE REQUIREMENTS Table 5.1 Typical Maintenance Activities for Underground Sand Filters (Source: CWP, 1996). Activity Schedule • Monitor water level in sand filter chamber. Quarterly and following large storm events • Sedimentation chamber should be cleaned out when the sediment depth reaches 12 inches. As needed • Remove accumulated oil and floatables in sedimentation chamber. As needed, (typically every 6 months) Additional inspection and maintenance requirements for organic filters are similar to those for surface sand filter facilities (see WSC-03, Sand Filters, in Appendix F) SECTION 6: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. SUBMERGED GRAVEL WETLANDS Description: One or more cells filled with crushed rock designed to support wetland plants. Stormwater flows subsurface through the root zone of the constructed wetland where pollutant removal takes place. .`"""— " Stormwater Management Capability: • Can be used in high • High maintenance density/ultra urban areas requirements • Water quality benefits can provide • Limited performance data 80% TSS removal. exists Land Use Considerations: ■ Residential © Commercial Design Considerations: • Generally requires low land consumption, and can fit within an © Industrial area that is typically devoted to landscaping • Can be located in low -permeability soils with a high water table • Periodic sediment removal required to prevent clogging of gravel Maintenance: base • Ensure that inlets and outles to each submerged gravel wetland cell are free from debris and not clogged • Check for sediment buildup in gravel bed • Replace with clean gravel and replant vegetation as needed. ©Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION The submerged gravel wetland system consists of one or more treatment cells that are filled with crushed rock or gravel and is designed to allow stormwater to flow subsurface through the root zone of the constructed wetland. The outlet from each cell is set at an elevation to keep the rock or gravel submerged. Wetland plants are rooted in the media, where they can directly take up pollutants. In addition, algae and microbes thrive on the surface area of the rocks. In particular, the anaerobic conditions on the bottom of the filter can foster the denitrification process. Although widely used for wastewater treatment in recent years, only a handful of submerged gravel wetland systems have been designed to treat stormwater. Mimicking the pollutant removal ability of nature, this structural control relies on the pollutant -stripping ability of plants and soils to remove pollutants from runoff. SECTION 2: POLLUTANT REMOVAL CAPABILITIES The pollution removal efficiency of the submerged gravel wetland is similar to a typical wetland. Recent data show a TSS removal rate in excess of the 80% goal. This reflects the settling environment of the gravel media. These systems also exhibit removals of about 60% TP, 20% TN, and 50% Zn. The growth of algae and microbes among the gravel media has been determined to be the primary removal mechanism of the submerged gravel wetland. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. • Total Suspended Solids - 80% • Total Phosphorus - 50% • Total Nitrogen - 20% • Fecal Coliform - 70% • Heavy Metals - 50% SECTION 3: DESIGN CRITERIA AND SPECIFICATIONS • Submerged gravel wetlands should be designed as off-line systems designed to handle only water quality volume. • Submerged gravel wetland systems need sufficient drainage area to maintain vegetation. • The local slope should be relatively flat (<2%). While there is no minimum slope requirement, there does need to be enough elevation drop from the inlet to the outlet to ensure that hydraulic conveyance by gravity is feasible (generally about 3 to 5 ft). • All submerged gravel wetland designs should include a sediment forebay or other equivalent pretreatment measures to prevent sediment or debris from entering and clogging the gravel bed. • Unless they receive runoff from areas with potential for high pollutant loading, submerged gravel wetland systems can be allowed to intersect the groundwater table. • See WSC-02, Stormwater Wetlands in Appendix F for additional planning and design guidance. SECTION 4: TYPICAL SCHEMATIC DETAILS Awe5ecmc ZOVE WAT6e SW5ACE FLEVA77 v AEkl e zave IN&KdW f e 6T.4VDPr) . :�r p 0C1TLE% r e � v e INC e o v K;79W AIL CK LAYER 1=614 IMAIERME+4BLE PIPE INLET IAINQCLILA7701V LINER Figure 6.1. Schematic of Submerged Gravel Wetland System (Source: Center for Watershed Protection; Roux Associates, Inc.). SECTION 5: INSPECTION AND MAINTENANCE REQUIREMENTS The inspection and maintenance requirements for organic filters are similar to those for surface sand filter facilities (see WSC-03, Sand Filter, in Appendix F) SECTION 6: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume Z: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. GRAVITY (OIL -GRIT) SEPARATOR Description: Hydrodynamic separation device designed to remove settleable solids, oil and grease, debris and floatables from stormwater runoff through gravitational settling and trapping of pollutants. • Can be used in high • Cannot alone achieve the density/ultra urban areas 80% TSS removal target • Limited performance data Design Considerations: I • Intended for the removal of settleable solids (grit and sediment) and floatable matter, including oil and grease • Dissolved pollutants are not effectively removed • Frequent maintenance required • Performance dependant on design and frequency of inspection and cleanout of unit Stormwater Management Capability: • Water quality benefits can provide 80% TSS removal. Land Use Considerations: ■ Residential © Commercial © Industrial Maintenance: • Inspect the gravity separator unit regularly • Clean out sediment, oil and grease, atable as needed Maintenance Burden L = Low M = Moderate H = High SECTION 1: DESCRIPTION Gravity separators (also known as oil -grit separators) are hydrodynamic separation devices that are designed to remove grit and heavy sediments, oil and grease, debris and floatable matter from stormwater runoff through gravitational settling and trapping. Gravity separator units contain a permanent pool of water and typically consist of an inlet chamber, separation/storage chamber, a bypass chamber, and an access port for maintenance purposes. Runoff enters the inlet chamber where heavy sediments and solids drop out. The flow moves into the main gravity separation chamber, where further settling of suspended solids takes place. Oil and grease are skimmed and stored in a waste oil storage compartment for future removal. After moving into the outlet chamber, the clarified runoff is then discharged. The performance of these systems is based primarily on the relatively low solubility of petroleum products in water and the difference between the specific gravity of water and the specific gravities of petroleum compounds. Gravity separators are not designed to separate other products such as solvents, detergents, or dissolved pollutants. The typical gravity separator unit may be enhanced with a pretreatment swirl concentrator chamber, oil draw -off devices that continuously remove the accumulated light liquids, and flow control valves regulating the flow rate into the unit. Gravity separators are best used in commercial, industrial and transportation land uses and are intended primarily as a pretreatment measure for high-density or ultra urban sites, or for use in hydrocarbon hotspots, such as gas stations and areas with high vehicular traffic. However, gravity separators cannot be used for the removal of dissolved or emulsified oils and pollutants such as coolants, soluble lubricants, glycols and alcohols. Since resuspension of accumulated sediments is possible during heavy storm events, gravity separator units are typically installed off-line. Gravity separators are available as prefabricated proprietary systems from a number of different commercial vendors. SECTION 2: POLLUTANT REMOVAL CAPABILITIES Testing of gravity separators has shown that they can remove between 40 and 50% of the TSS loading when used in an off-line configuration (Curran, 1996 and Henry, 1999). Gravity separators also provide removal of debris, hydrocarbons, trash and other floatables. They provide only minimal removal of nutrients and organic matter. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. • Total Suspended Solids - 40% • Total Phosphorus - 5% • Total Nitrogen - 5% • Fecal Coliform - insufficient data • Heavy Metals - insufficient data Actual field testing data and pollutant removal rates from an independent source should be obtained before using a proprietary gravity separator system. SECTION 3: DESIGN CRITERIA AND SPECIFICATIONS • The use of gravity (oil -grit) separators should be limited to the following applications: • Pretreatment for other structural stormwater controls • High-density, ultra urban or other space -limited development sites • Hotspot areas where the control of grit, floatables, and/or oil and grease are required • Gravity separators are typically used for areas less than 5 acres. It is recommended that the contributing area to any individual gravity separator be limited to 1 acre or less of impervious cover. • Gravity separator systems can be installed in almost any soil or terrain. Since these devices are underground, appearance is not an issue and public safety risks are low. • Gravity separators are rate -based devices. This contrasts with most other stormwater structural controls, which are sized based on capturing and treating a specific volume. • Gravity separator units are typically designed to bypass runoff flows in excess of the design flow rate. Some designs have built-in high flow bypass mechanisms. Other designs require a diversion structure or flow splitter ahead of the device in the drainage system. An adequate outfall must be provided. • The separation chamber should provide for three separate storage volumes: • A volume for separated oil storage at the top of the chamber • A volume for settleable solids accumulation at the bottom of the chamber • A volume required to give adequate flow-through detention time for separation of oil and sediment from the stormwater flow • The total wet storage of the gravity separator unit should be at least 400 cu ft per contributing impervious acre. • The minimum depth of the permanent pools should be 4 ft. • Horizontal velocity through the separation chamber should be 1 to 3 ft/min or less. No velocities in the device should exceed the entrance velocity. • A trash rack should be included in the design to capture floating debris, preferably near the inlet chamber to prevent debris from becoming oil impregnated. • Ideally, a gravity separator design will provide an oil draw -off mechanism to a separate chamber or storage area. • Adequate maintenance access to each chamber must be provided for inspection and cleanout of a gravity separator unit. • Gravity separator units should be watertight to prevent possible groundwater contamination. • The design criteria and specifications of a proprietary gravity separator unit should be obtained from the manufacturer. SECTION 4: TYPCIAL SCHEMATIC DETAILS Figure 7.1. Schematic of an Example Gravity (Oil -Grit) Separator (Source: NVRC, 1992[1]). SECTION 5: INSPECTION AND MAINTENANCE REQUIREMENTS Table 7.1 Typical Maintenance Activities for Gravity Separators Activity Schedule • Inspect the gravity separator unit. Regularly (quarterly) • Clean out sediment, oil and grease, and floatables, using catch basin cleaning As Needed equipment (vacuum pumps). Manual removal of pollutants may be necessary. Additional Maintenance Considerations and Requirements • Additional maintenance requirements for a proprietary system should be obtained from the manufacturer. • Failure to provide adequate inspection and maintenance can result in the resuspension of accumulated solids. Frequency of inspection and maintenance is dependent on land use, climatological conditions, and the design of gravity separator. • Proper disposal of oil, solids and floatables removed from the gravity separator must be ensured. SECTION 6: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume Z: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf Center for Watershed Protection (CWP), 1996. Design of Stormwater Filtering Systems. Prepared for Chesapeake Research Consortium. Northern Virginia Regional Commission (NVRC), 1992. The Northern Virginia BMP Handbook. Annandale, VA. PROPRIETARY STRUCTURAL CONTROLS Description: Manufactured structural control systems available from commercial vendors designed to treat stormwater runoff and/or provide water quantity control. • Can be used in high Depending on the system there density/ultra urban areas may be: • Can provides pretreatment • Limited performance data prior to discharging to • Application constraints other water quality • High maintenance controls requirements • Higher costs than other structural control alternatives Design Considerations: • Independent performance data must be available to prove a demonstrated capability of meeting stormwater management goals • System or device must be appropriate for use in Fayetteville • Installation and operations/maintenance requirements must be understood by all parties approving and using the system or device in question SECTION 1: DESCRIPTION Stormwater Management Capability: • Water quality benefits can provide 80% TSS removal. Land Use Considerations: © Residential © Commercial © Industrial Maintenance: • Depends on the proprietary system • Sediment and debris must be removed periodically • Check inlets and outlets for clogging F -m I Maintenance Burden L = Low M = Moderate H = High There are many types of commercially -available proprietary stormwater structural controls available for both water quality treatment and quantity control. These systems include: • Hydrodynamic systems such as gravity and vortex separators • Filtration systems • Catch basin media inserts • Chemical treatment systems • Package treatment plants • Prefabricated detention structures Many proprietary systems are useful on small sites and space -limited areas where there is not enough land or room for other structural control alternatives. Proprietary systems can often be used in pretreatment applications in a treatment train. However, proprietary systems are often more costly than other alternatives and may have high maintenance requirements. Perhaps the largest difficulty in using a proprietary system is the lack of adequate independent performance data. Below are general guidelines that should be followed before considering the use of a proprietary commercial system. SECTION 2: GUIDELINES FOR USING PROPRIETARY SYSTEMS In order for use as a limited application control, a proprietary system must have a demonstrated capability of meeting the stormwater management goals for which it is being intended. This means that the system must provide: 1. Independent third -party scientific verification of the ability of the proprietary system to meet water quality treatment objectives and/or to provide water quantity control (channel or flood protection) 2. Proven record of longevity in the field 3. Proven ability to function in Fayetteville conditions (e.g., climate, rainfall patterns, soil types, etc.) For a propriety system to meet (1) above for water quality goals, the following monitoring criteria should be met for supporting studies: • At least 15 storm events must be sampled • The study must be independent or independently verified (i.e., may not be conducted by the vendor or designer without third -party verification) • The study must be conducted in the field, as opposed to laboratory testing • Field monitoring must be conducted using standard protocols which require proportional sampling both upstream and downstream of the device • Concentrations reported in the study must be flow -weighted • The propriety system or device must have been in place for at least one year at the time of monitoring Although local data is preferred, data from other regions can be accepted as long as the design accounts for the local conditions. A poor performance record or high failure rate is valid justification for not allowing the use of a proprietary system or device. SECTION 3: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook. Atlanta, GA. http://www.georgiastormwater.com/GSMMVol2.pdf APPENDIX G OUTLET STRUCTURES OUTLET STRUCTURES SECTION 1: PRIMARY OUTLETS Primary outlets provide the critical function of the regulation of flow for structural stormwater controls. There are several different types of outlets that may consist of a single stage outlet structure, or several outlet structures combined to provide multi -stage outlet control. For a single stage system, the stormwater facility can be designed as a simple pipe or culvert. For multistage control structures, the inlet is designed considering a range of design flows. A stage -discharge curve is developed for the full range of flows that the structure would experience. The outlets are housed in a riser structure connected to a single outlet conduit. An alternative approach would be to provide several pipe or culvert outlets at different levels in the basin that are either discharged separately or are combined to discharge at a single location. This section provides an overview of outlet structure hydraulics and design for stormwater storage facilities. Energy dissipation structures are discussed in Chapter 6. The design engineer is referred to an appropriate hydraulics text for additional information on outlet structures not contained in this manual. (a) PIPE OR BOX CULVERT (b) RISER STRUCTURE (single and multi-level outlets) (c) DROP INLET (d) WEIR OVERFLOW SPILLWAY Side Elevation Front Elevation (e) SLOTTED OUTLET Figure 1.1. Typical Primary Outlets. SECTION 2: OUTLET STRUCTURE TYPES There are a wide variety of outlet structure types, the most common of which are covered in this section. Descriptions and equations are provided for the following outlet types for use in stormwater facility design: • Orifices • Perforated risers • Pipes / Culverts • Sharp -crested weirs • Broad -crested weirs • V -notch weirs • Proportional weirs • Combination outlets Each of these outlet types has a different design purpose and application: • Water quality and channel protection flows are normally handled with smaller, more protected outlet structures such as reverse slope pipes, hooded orifices, orifices located within screened pipes or risers, perforated plates or risers, and V -notch weirs. • Larger flows, such as overbank protection and extreme flood flows, are typically handled through a riser with different sized openings, through an overflow at the top of a riser (drop inlet structure), or a flow over a broad crested weir or spillway through the embankment. Overflow weirs can also be of different heights and configurations to handle control of multiple design flows. Orifices An orifice is a circular or rectangular opening of a prescribed shape and size. The flow rate depends on the height of the water above the opening and the size and edge treatment of the orifice. For a single orifice, as illustrated in Figure 1.2 (a), the orifice discharge can be determined using the standard orifice equation below. Q = CA (2gH)o.s (Eq. 1.1) Where: Q = the orifice flow discharge (cfs) C = discharge coefficient A = cross-sectional area of orifice or pipe (ftz) g = acceleration due to gravity (32.2 ft/S2) D = diameter of orifice or pipe (ft) H = effective head on the orifice, from the center of orifice to the water surface If the orifice discharges as a free outfall, then the effective head is measured from the center of the orifice to the upstream (headwater) surface elevation. If the orifice discharge is submerged, then the effective head is the difference in elevation of the headwater and tailwater surfaces as shown in Figure 1.2(b). Headwater _ H Tailwater n #D I1 (b) 4{ H, Hz D"A� 0 p I Hs 0 0 0 0 0 0 0--- (C) Figure 1.2 Orifice Definitions Figure 1.3 Perforated Riser O S12 0 0 H, 0 T _0 "` H S12 0 f batty m L orifice prate Figure 1.3 Perforated Riser When the orifice material thickness is than the orifice diameter, with sharp edges, a coefficient of 0.6 should be used. For square -edged entrance conditions the generic orifice equation can be simplified: Q = 0.6A (2gH)o.s = 3.78D2Ho.s (Eq. 1.2) When the material is thicker than the orifice diameter a coefficient of 0.80 should be used. If the edges are rounded, a coefficient of 0.92 can be used. Flow through multiple orifices, such as the perforated plate shown in Figure 1.2(c), can be computed by summing the flow through individual orifices. For multiple orifices of the same size and under the influence of the same effective head, the total flow can be determined by multiplying the discharge for a single orifice by the number of openings. Perforated orifice plates for the control of discharge can be of any size and configuration. However, the Denver Urban Drainage and Flood Control District has developed standardized dimensions that have worked well. Table 1.1 gives appropriate dimensions. The vertical spacing between hole centerlines is always 4 inches. Source: Urban Drainage and Flood Control District, Denver, CO For rectangular slots the height is normally 2 inches with variable width. Only one column of rectangular slots is allowed. Hole Diameter (in) Table 1.1. Circular Minimum Column Hole Centerline Spacing (in) Perforation Sizing. Flow Area per Row (in") 1 column 2 columns 3 columns 1/4 1 0.05 0.1 0.15 5/16 2 0.08 0.15 0.23 3/8 2 0.11 0.22 0.33 7/16 2 0.15 0.3 0.45 1/2 2 0.2 0.4 0.6 9/16 3 0.25 0.5 0.75 5/8 3 0.31 0.62 0.93 11/16 3 0.37 0.74 1.11 3/4 3 0.44 0.88 1.32 13/16 3 0.52 1.04 1.56 7/8 3 0.6 1.2 1.8 15/16 3 0.69 1.38 2.07 1 4 0.79 1.58 2.37 11/16 4 0.89 1.78 2.67 11/8 4 0.99 1.98 2.97 13/16 4 1.11 2.22 3.33 11/4 4 1.23 2.46 3.69 15/16 4 1.35 2.7 4.05 13/8 4 1.48 2.96 4.44 17/16 4 1.62 3.24 4.86 11/2 4 1.77 3.54 5.31 19/16 4 1.92 3.84 5.76 15/8 4 2.07 4.14 6.21 111/16 4 2.24 4.48 6.72 13/4 4 2.41 4.82 7.23 113/16 4 2.58 5.16 7.74 17/8 4 2.76 5.52 8.28 115/16 4 2.95 5.9 8.85 2 4 3.14 6.28 9.42 Number of columns refers to parallel columns of holes Minimum steel plate thickness 1/4" 5/16" 3/8" Source: Urban Drainage and Flood Control District, Denver, CO For rectangular slots the height is normally 2 inches with variable width. Only one column of rectangular slots is allowed. Figure 1.4 provides a schematic of an orifice plate outlet structure for a wet ED pond showing the design pool elevations and the flow control mechanisms. Figure 1.4. Schematic of Orifice Plate Outlet Structure. Perforated Risers A special kind of orifice flow is a perforated riser as illustrated in Figure 1.3. In the perforated riser, an orifice plate at the bottom of the riser, or in the outlet pipe just downstream from the elbow at the bottom of the riser, controls the flow. It is important that the perforations in the riser convey more flow than the orifice plate so as not to become the control. Referring to Figure 1.3, a shortcut formula has been developed to estimate the total flow capacity of the perforated section (McEnroe, 1988): 2A Q CP 3HP 2gH Z s (Eq. 1.3) Where: Q = discharge (cfs) Cp = discharge coefficient for perforations (normally 0.61) Ap = cross-sectional area of all the holes (ft') HS = distance from S/2 below the lowest row of holes to S/2 above the top row (ft) H = head on riser pipe measured from S/2 below the centerline of the lowest row of holes (ft) Pipes and Culverts Discharge pipes are often used as outlet structures for stormwater control facilities. The design of these pipes can be for either single or multi -stage discharges. A reverse -slope underwater pipe is often used for water quality or channel protection outlets. Pipes smaller than 12 inches in diameter may be analyzed as a submerged orifice as long as HID is greater than 1.5. Note: For low flow conditions when the flow reaches and begins to overflow the pipe, weir flow controls (see subsection on Sharp Crested Weirs below). As the stage increases the flow will transition to orifice flow. Pipes greater than 12 inches in diameter should be analyzed as a discharge pipe with headwater and tailwater effects taken into account. The outlet hydraulics for pipe flow can be determined from the procedures given in Chapter 6 regarding culvert design, or by using equation 1.4 (NRCS, 1984). The following equation is a general pipe flow equation that is derived through the use of the Bernoulli and continuity principles. Q = a[(2gH) / (1 + km + kpL)]0.5 Where: Q = discharge (cfs) = pipe cross sectional area (ft') g = acceleration of gravity (ft/sz) H = elevation head differential (ft) km = coefficient of minor losses (use 1.0) kp = pipe friction coefficient = 5087n2 /D4/3 L = pipe length (ft) Sharp -Crested Weirs (Eq. 1.4) If the overflow portion of a weir has a sharp, thin leading edge such that the water springs clear as it overflows, the overflow is termed a sharp -crested weir. If the sides of the weir also cause the through flow to contract, it is termed an end -contracted sharp -crested weir. Sharp -crested weirs have stable stage -discharge relations and are often used as a measurement device. A sharp -crested weir with no end contractions is illustrated in Figure 1.5(a). The discharge equation for this configuration is (Chow, 1959): Q = [(3.27 + 0.4(H/Hc)] LH'.' (Eq. 1.5) Where: Q = discharge (cfs) H = head above weir crest excluding velocity head (ft) He = height of weir crest above channel bottom (ft) L = horizontal weir length (ft) (a) No end contractions (b) With end contractions H $ Hil f #HZ HC HC (c) Section view Figure 1.5. Sharp -Crested Weir. A sharp -crested weir with two end contractions is illustrated in Figure 1.5(b). The discharge equation for this configuration is (Chow, 1959): Q = [(3.27 + 0.04(H/Hc)] (L - 0.2H) HI -S (Eq. 1.6) Where: Q = discharge (cfs) H = head above weir crest excluding velocity head (ft) He = height of weir crest above channel bottom (ft) L = horizontal weir length (ft) A sharp -crested weir will be affected by submergence when the tailwater rises above the weir crest elevation. The result will be that the discharge over the weir will be reduced. The discharge equation for a sharp -crested submerged weir is (Brater and King, 1976): Qs = Qf(1 - (Hz/Hl)l.$)o.aas (Eq. 1.7) Where: Qs = submergence flow (cfs) Qf = free flow (cfs) H1 = upstream head above crest (ft) Hz = downstream head above crest (ft) Broad -Crested Weirs A weir in the form of a relatively long raised channel control crest section is a broad -crested weir. The flow control section can have different shapes, such as triangular or circular. True broad -crested weir flow occurs when upstream head above the crest is between the limits of about 1/20 and 1/2 the crest length in the direction of flow. For example, a thick wall or a flat stop log can act like a sharp -crested weir when the approach head is large enough that the flow springs from the upstream corner. If upstream head is small enough relative to the top profile length, the stop log can act like a broad -crested weir (USBR, 1997). The equation for the broad -crested weir is (Brater and King, 1976): Where: Q = CLH1.s Q C L H (Eq. 1.8) = discharge (cfs) = broad -crested weir coefficient = broad -crested weir length perpendicular to flow (ft) = head above weir crest (ft) If the upstream edge of a broad -crested weir is so rounded as to prevent contraction and if the slope of the crest is as great as the loss of head due to friction, flow will pass through critical depth at the weir crest; this gives the maximum C value of 3.087. For sharp corners on the broad -crested weir, a minimum C value of 2.6 should be used. Information on C values as a function of weir crest breadth and head is given in Table 1.2. Figure 1.6. Broad -Crested Weir. * Measured at least 2.5H upstream of the weir. Source: Brater and King (1976) V -Notch Weirs The discharge through a V -notch weir (Figure 1.7) can be calculated from the following equation (Brater and King, 1976). Q = 2.5 tan (0/2) Hz.s Where: Q = discharge (cfs) 0 = angle of V -notch (degrees) H = head on apex of notch (ft) (1.9) Broad-CrestedTable 1.2 Measured Head (H)* Weir Crest Breadth (b) in feet In feet 0.50 0.75 1.00 1.50 2.00 2.50 3.00 4.00 5.00 10.00 15.00 0.2 2.80 2.75 2.69 2.62 2.54 2.48 2.44 2.38 2.34 2.49 2.68 0.4 2.92 2.80 2.72 2.64 2.61 2.60 2.58 2.54 2.50 2.56 2.70 0.6 3.08 2.89 2.75 2.64 2.61 2.60 2.68 2.69 2.70 2.70 2.70 0.8 3.30 3.04 2.85 2.68 2.60 2.60 2.67 2.68 2.68 2.69 2.64 1.0 3.32 3.14 2.98 2.75 2.66 2.64 2.65 2.67 2.68 2.68 2.63 1.2 3.32 3.20 3.08 2.86 2.70 2.65 2.64 2.67 2.66 2.69 2.64 1.4 3.32 3.26 3.20 2.92 2.77 2.68 2.64 2.65 2.65 2.67 2.64 1.6 3.32 3.29 3.28 3.07 2.89 2.75 2.68 2.66 2.65 2.64 2.63 1.8 3.32 3.32 3.31 3.07 2.88 2.74 2.68 2.66 2.65 2.64 2.63 2.0 3.32 3.31 3.30 3.03 2.85 2.76 2.27 2.68 2.65 2.64 2.63 2.5 3.32 3.32 3.31 3.28 3.07 2.89 2.81 2.72 2.67 2.64 2.63 3.0 3.32 3.32 3.32 3.32 3.20 3.05 2.92 2.73 2.66 2.64 2.63 3.5 3.32 3.32 3.32 3.32 3.32 3.19 2.97 2.76 2.68 2.64 2.63 4.0 3.32 3.32 3.32 3.32 3.32 3.32 3.07 2.79 2.70 2.64 2.63 4.5 3.32 3.32 3.32 3.32 3.32 3.32 3.32 2.88 2.74 2.64 2.63 5.0 3.32 3.32 3.32 3.32 3.32 3.32 3.32 3.07 2.79 2.64 2.63 5.5 3.32 3.32 3.32 3.32 3.32 3.32 3.32 3.32 2.88 2.64 2.63 * Measured at least 2.5H upstream of the weir. Source: Brater and King (1976) V -Notch Weirs The discharge through a V -notch weir (Figure 1.7) can be calculated from the following equation (Brater and King, 1976). Q = 2.5 tan (0/2) Hz.s Where: Q = discharge (cfs) 0 = angle of V -notch (degrees) H = head on apex of notch (ft) (1.9) 4 n H I Section A -A Figure 1.7. V -Notch Weir. Proportional Weirs Although more complex to design and construct, a proportional weir may significantly reduce the required storage volume for a given site. The proportional weir is distinguished from other control devices by having a linear head -discharge relationship achieved by allowing the discharge area to vary nonlinearly with head. A typical proportional weir is shown in Figure 1.8. Design equations for proportional weirs are (Sandvik, 1985): Q = 4.97 aO.5 b (H - a/3) (Eq. 1.10) x/b = 1- (1/3.17) (arctan (y/a)0.5) (Eq. 1.11) Where: Q = discharge (cfs) Dimensions a, b, H, x, and y are shown in Figure 1.8 b Figure 1.8. Proportional Weir Dimensions. Combination Outlets Combinations of orifices, weirs and pipes can be used to provide multi -stage outlet control for different control volumes within a storage facility (i.e., water quality volume, channel protection volume, overbank flood protection volume, and/or extreme flood protection volume). There are generally two types of combination outlets: shared outlet control structures and separate outlet controls. Shared outlet control is typically a number of individual outlet openings (orifices), weirs or drops at different elevations on a riser pipe or box which all flow to a common larger conduit or pipe. Figure 1.9 shows an example of a riser designed for a wet ED pond. The orifice plate outlet structure in Figure 1.4 is another example of a combination outlet. Separate outlet controls are less common and may consist of several pipe or culvert outlets at different levels in the storage facility that are either discharged separately or are combined to discharge at a single location. The use of a combination outlet requires the construction of a composite stage -discharge curve (see example in Figure 1.10) suitable for control of multiple storm flows. The design of multi -stage combination outlets is discussed later in this section. 10' MIN. TOP OF EMBANKMENT TOP WIDTH \7 100 YR 3 0 25 YR 1 SLOT vCpv Figure 1.9. Schematic of Combination Outlet Structure. GATE y WQv ED VALVE 3 1 MAX GATE PROPOSED V PERM. POOL VALVE GRADE D.I.P. CONCRETE E.D. PIPE HEADWALL CONC. SAND FILTER EXIST. BOX RISER DIAPHRAGM GROUND GATE POND VALVE BOTTOM I RCP BgRf?EL POND DRAIN PIPE s RIP -RAP CHANNEL IMPERMEABLE CUTOFF TRENCH Figure 1.9. Schematic of Combination Outlet Structure. 103.5 103.0 102.5 102.0 c 0 j 101.5 d W 101.0 100.5 100.0 t Secondary Outlet (Spillway) 0 Riser Capacity t Primary Outlet Total Outflow 10 15 20 25 30 Discharge (cfs) Figure 1.10. Composite Stage -Discharge Curve. SECTION 3. EXTENDED DETENTION (WATER QUALITY AND CHANNEL PROTECTION) OUTLET DESIGN Introduction Extended detention orifice sizing is required in design applications that provide extended detention for downstream channel protection or the ED portion of the water quality volume. In both cases an extended detention orifice or reverse slope pipe can be used for the outlet. For a structural control facility providing both WQv extended detention and CPQ control (wet ED pond, micropool ED pond, and shallow ED wetland), there will be a need to design two outlet orifices - one for the water quality control outlet and one for the channel protection drawdown. (This following procedures are based on the water quality outlet design procedures included in the Virginia Stormwater Management Handbook, 1999) The outlet hydraulics for peak control design (overbank flood protection and extreme flood protection) is usually straightforward in that an outlet is selected that will limit the peak flow to some predetermined maximum. Since volume and the time required for water to exit the storage facility are not usually considered, the outlet design can easily be calculated and routing procedures used to determine if quantity design criteria are met. In an extended detention facility for water quality treatment or downstream channel protection, however, the storage volume is detained and released over a specified amount of time (e.g., 24 hours). The release period is a brim drawdown time, beginning at the time of peak storage of the water quality volume until the entire calculated volume drains out of the basin. This assumes that the brim volume is present in the basin prior to any discharge. In reality, however, water is flowing out of the basin prior to the full or brim volume being reached. Therefore, the extended detention outlet can be sized using either of the following methods: 1. Use the maximum hydraulic head associated with the storage volume and maximum flow, calculate the orifice size needed to achieve the required drawdown time, and route the volume through the basin to verify the actual storage volume used and the drawdown time. 2. Approximate the orifice size using the average hydraulic head associated with the storage volume and the required drawdown time. These two procedures are outlined in the examples below and can be used to size an extended detention orifice for water quality and/or channel protection. Method 1: Maximum Hydraulic Head with Routing A wet ED pond sized for the required water quality volume will be used here to illustrate the sizing procedure for an extended -detention orifice. Given the following information, calculate the required orifice size for water quality design. Given: Water Quality Volume (WQ„) = 0.76 ac ft = 33,106 ft3 Maximum Hydraulic Head (Hmax) = 5.0 ft (from stage vs. storage data) (Step 1) Determine the maximum discharge resulting from the 24-hour drawdown requirement. It may be approximated by dividing the Water Quality Volume (or Channel Protection Volume) by the required time to find the average discharge, and then multiplying by two to obtain the maximum discharge. Qavg = 33,106 ft3 / (24 hr)(3,600 s/hr) = 0.38 cfs Qmax=2*Qavg=2*0.38=0.76 cfs (Step 2) Determine the required orifice diameter by using the orifice equation (1.1) and Qmax and Hmax: Q = CA(2gH)0.5, or A = Q / C(2gH)o.s A = 0.76 / 0.6[(2)(32.2)(5.0)]0.5 = 0.071 ft3 Determine pipe diameter from A = 3.14d2/4, then d = (4A/3.14)0.5 D = [4(0.071)/3.14]0.5 = 0.30 ft = 3.61 in Use a 3.6 -inch diameter water quality orifice. Routing the water quality volume of 0.76 ac ft through the 3.6 -inch water quality orifice will allow the designer to verify the drawdown time, as well as the maximum hydraulic head elevation. The routing effect will result in the actual drawdown time being less than the calculated 24 hours. Judgment should be used to determine whether the orifice size should be reduced to achieve the required 24 hours or if the actual time achieved will provide adequate pollutant removal. Method 2: Average Hydraulic Head and Average Discharge Using the data from the previous example use Method 2 to calculate the size of the outlet orifice. Given: Water Quality Volume (WQ„) = 0.76 ac ft = 33,106 ft3 Average Hydraulic Head (ha,g) = 2.5 ft (from stage vs storage data) (Step 1) Determine the average release rate to release the water quality volume over a 24-hour time period. Q = 33,106 ft3 / (24 hr)(3,600 s/hr) = 0.38 cfs (Step 2) Determine the required orifice diameter by using the orifice equation (1.1) and the average head on the orifice: Q = CA(2gH)0-5, or A = Q / C(2gH)0.5 A = 0.38 / 0.6[(2)(32.2)(2.5)]0.5 = 0.05 ft3 Determine pipe diameter from A = 3.14r2 = 3.14d2/4, then d = (4A/3.14)0.5 D = [4(0.05)/3.14]0.5 = 0.252 ft = 3.03 in Use a 3 -inch diameter water quality orifice. Use of Method 1, utilizing the maximum hydraulic head and discharge and routing, results in a 3.6 -inch diameter orifice (though actual routing may result in a changed orifice size) and Method 2, utilizing average hydraulic head and average discharge, results in a 3.0 -inch diameter orifice. SECTION 4: MULTI -STAGE OUTLET DESIGN Introduction A combination outlet such as a multiple orifice plate system or multi -stage riser is often used to provide adequate hydraulic outlet controls for the different design requirements (e.g., water quality, channel protection, overbank flood protection, and/or extreme flood protection) for stormwater ponds, stormwater wetlands and detention -only facilities. Separate openings or devices at different elevations are used to control the rate of discharge from a facility during multiple design storms. Figures 1.4 and 1.9 are examples of multi -stage combination outlet systems. A design engineer may be creative to provide the most economical and hydraulically efficient outlet design possible in designing a multi -stage outlet. Many iterative routings are usually required to arrive at a minimum structure size and storage volume that provides proper control. The stage -discharge table or rating curve is a composite of the different outlets that are used for different elevations within the multi- stage riser (see Figure 1.10) Multi -Stage Outlet Design Procedure Below are the steps for designing a multi -stage outlet. Note that if a structural control facility will not control one or more of the required storage volumes (WQv, CPQ, Qpzs, and Qf), then that step in the procedure is skipped. (Step 1) Determine Stormwater Control Volumes. Using the procedures from Chapter 3, estimate the required storage volumes for water quality treatment (WQv), channel protection (CPQ), and overbank flood control (Qpzs)and extreme flood control (Qf). (Step 2) Develop Stage -Storage Curve. Using the site geometry and topography, develop the stage -storage curve for the facility in order to provide sufficient storage for the control volumes involved in the design. (Step 3) Design Water Quality Outlet. Design the water quality extended detention (WQv- ED) orifice using either Method 1 or Method 2 outlined in subsections above. If a permanent pool is incorporated into the design of the facility, a portion of the storage volume for water quality will be above the elevation of the permanent pool. The outlet can be protected using a reverse slope pipe, a hooded protection device, or another acceptable method (see Section 5 of this appendix). (Step 4) Design Channel Protection Outlet. Design the stream channel protection extended detention outlet (CPQ ED) using either method from subsections above. For this design, the storage needed for channel protection will be "stacked" on top of the water quality volume storage elevation determined in Step 3. The total stage -discharge rating curve at this point will include water quality control orifice and the outlet used for stream channel protection. The outlet should be protected in a manner similar to that for the water quality orifice. (Step 5) Design Overbank Flood Protection Outlet. The overbank protection volume is added above the water quality and channel protection storage. Establish the Qpzs maximum water surface elevation using the stage -storage curve and subtract the CPv elevation to find the 25 -year maximum head. Select an outlet type and calculate the initial size and geometry based upon maintaining the predevelopment 25 -year peak discharge rate. Develop a stage -discharge curve for the combined set of outlets (WQv, CPQ and Qpzs). This procedure is repeated for control (peak flow attenuation) of the 100 -year storm (Qf), if required. (Step 6) Check Performance of the Outlet Structure. Perform a hydraulic analysis of the multi -stage outlet structure using reservoir routing to ensure that all outlets will function as designed. Several iterations may be required to calibrate and optimize the hydraulics and outlets that are used. Also, the structure should operate without excessive surging, vibration, or vortex action at any stage. This usually requires that the structure have a larger cross-sectional area than the outlet conduit. The hydraulic analysis of the design must take into account the hydraulic changes that will occur as depth of storage changes for the different design storms. As shown in Figure 1. 11, as the water passes over the rim of a riser, the riser acts as a weir. However, when the water surface reaches a certain height over the rim of a riser, the riser will begin to act as a submerged orifice. The designer must compute the elevation at which this transition from riser weir flow control to riser orifice flow control takes place for an outlet where this change in hydraulic conditions will change. Also note in Figure 1.11 that as the elevation of the water increases further, the control can change from barrel inlet flow control to barrel pipe flow control. Figure 1.12 shows another condition where weir flow can change to orifice flow, which must be taken into account in the hydraulics of the rating curve as different design conditions results in changing water surface elevations. (Step 7) Size the Emergency Spillway. It is recommended that all stormwater impoundment structures have a vegetated emergency spillway (see Section 6 of this appendix). An emergency spillway provides a degree of safety to prevent overtopping of an embankment if the primary outlet or principal spillway should become clogged, or otherwise inoperative. The 100 -year storm should be routed through the outlet devices and emergency spillway to ensure the hydraulics of the system will operate as designed. (Step 8) Design Outlet Protection. Design necessary outlet protection and energy dissipation facilities to avoid erosion problems downstream from outlet devices and emergency spillway(s). See Chapter 6 of the Drainage Criteria manual for more information regarding energy dissipation. (Step 9) Perform Buoyancy Calculations. Perform buoyancy calculations for the outlet structure and footing. Flotation will occur when the weight of the structure is less than or equal to the buoyant force exerted by the water. (Step 10) Provide Seepage Control. Seepage control should be provided for the outflow pipe or culvert through an embankment. The two most common devices for controlling seepage are (1) filter and drainage diaphragms and (2) anti -seep collars. Water surface elevation (WSE) Water surface elevation (WSE) A. Riser Weir Flow -Control B. Riser Orifice Flow Control water surface /elevation (WSE) C. Barrel Inlet Flow Control Water surface /elevation (WSE) D. Barrel Pipe Flow Control Figure 1.11. Riser Flow Diagrams (Source: VDCR, 1999). Water surface HT Weir crest Weir Flow Water surface _ Rectangular opening Orifice Flow Figure 1.12. Weir and Orifice Flow (Source: VDCR, 1999). SECTION 5: EXTENDED DETENTION OUTLET PROTECTION Small low flow orifices such as those used for extended detention applications can easily clog, preventing the structural control from meeting its design purpose(s) and potentially causing adverse impacts. Therefore, extended detention orifices need to be adequately protected from clogging. There are a number of different anti -clogging designs, including: • The use of a reverse slope pipe attached to a riser for a stormwater pond or wetland with a permanent pool (see Figure 1.13). The inlet is submerged 1 foot below the elevation of the permanent pool to prevent floatables from clogging the pipe and to avoid discharging warmer water at the surface of the pond. • The use of a hooded outlet for a stormwater pond or wetland with a permanent pool (see Figures 1.14 and 1.15). • Internal orifice protection through the use of an over -perforated vertical stand pipe with 1/z -inch orifices or slots that are protected by wirecloth and a stone filtering jacket (see Figure 1.16). • Internal orifice protection through the use of an adjustable gate valves can achieve an equivalent orifice diameter. • Internal orifice protection through the use of a gravel filtration box (Figure 1.17). 1 3 FLANGED DIP CL 50 @ 23.65 1 PERMANENT POOL TOP ELEV. FLANGED DIP BEND Figure 1.13. Reverse Slope Pipe Outlet. CONCRETE EXTENDED DETENTION ORIFICE BOX RISER \ I �A \ 25 YR. ORIFICE c: I� PERMANENTS POOL LEVEL I' ! g -1 I 1 "I I� A PROFILE Figure 1.14. Hooded Outlet. �p N HOOD WIRE MESH SECTION 'A -A' 12"-18" ORIFICE Figure 1.15. Half -Round CMP Orifice Hood. IFICE L) AD CMP HOOF 4 To 01 aa.r,.,—kia A V110 ML1F_l7 0J aRJI IM Figure 1.16. Internal Control for Orifice Protection. OUTFALL STRUCTURE EXTREME FLOOD PROTECTION (100 -Year) LEVEL O OVERBANK FLOOD PROTECTION _ (25-Year)LEVEL 25 YR ORIFICE MIN. ELEVATION CALCULATED A CPvolume _ — - - - - - - - - - - ORIFICE GRAVEL FILTRATION BOX FILTRATION BOXES Ci� (INNER AND OUTER) #4 REBAR @ 3" O.C. O EACH WAY) CO $„ 12" $„ FOR ORIFICES UP TO 6" DIAMETER CLEAN AGGREGATE WITH MAX. DIAMETER OF 3.5' AND A MIN. DIAMETER OF 1.5' REMOVABLE AND LOCKABLE GRATE FILTER FABRIC C O O O O N O O = 0 o co ATTACH BOXES FIRMLY �- \ TO CONCRETE PAD CONCRETE PAD WITH REMOVABLE BOLTS SECTION B -B Figure 1.17. Filtration Box for Internal Orifice Protection. SECTION 6: TRASH RACKS AND SAFETY GRATES Introduction CPV ORIFICE The susceptibility of larger inlets to clogging by debris and trash needs to be considered when estimating their hydraulic capacities. In most instances trash racks will be needed. Trash racks and safety grates are a critical element of outlet structure design and serve several important functions: • Keeping debris away from the entrance to the outlet works where they will not clog the critical portions of the structure • Capturing debris in such a way that relatively easy removal is possible • Ensuring that people and large animals are kept out of confined conveyance and outlet areas • Providing a safety system that prevents anyone from being drawn into the outlet and allows them to climb to safety When designed properly, trash racks serve these purposes without interfering significantly with the hydraulic capacity of the outlet (or inlet in the case of conveyance structures) (ASCE, 1985; Allred-Coonrod, 1991). The location and size of the trash rack depends on a number of factors, including head losses through the rack, structural convenience, safety and size of outlet. Well-designed trash racks can also have an aesthetically pleasing appearance. An example of trash racks used on a riser outlet structure is shown in Figure 1.18. The inclined vertical bar rack is most effective for lower stage outlets. Debris will ride up the trash rack as water levels rise. This design also allows for removal of accumulated debris with a rake while standing on top of the structure. Figure 1.18. Example of Various Trash Racks Used on a Riser Outlet Structure (Source: VADCR, 1999.) Trash Rack Design Trash racks must be large enough such that partial plugging will not adversely restrict flows reaching the control outlet. There are no universal guidelines for the design of trash racks to protect detention basin outlets, although a commonly used "rule -of -thumb" is to have the trash rack area at least ten times larger than the control outlet orifice. The surface area of all trash racks should be maximized and the trash racks should be located a suitable distance from the protected outlet to avoid interference with the hydraulic capacity of the outlet. The spacing of trash rack bars must be proportioned to the size of the smallest outlet protected. However, where a small orifice is provided, a separate trash rack for that outlet should be used, so that a simpler, sturdier trash rack with more widely spaced members can be used for the other outlets. Spacing of the rack bars should be wide enough to avoid interference, but close enough to provide the level of clogging protection required. To facilitate removal of accumulated debris and sediment from around the outlet structure, the racks should have hinged connections. If the rack is bolted or set in concrete it will preclude removal of accumulated material and will eventually adversely affect the outlet hydraulics. Since sediment will tend to accumulate around the lowest stage outlet, the inside of the outlet structure for a dry basin should be depressed below the ground level to minimize clogging due to sedimentation. Depressing the outlet bottom to a depth below the ground surface at least equal to the diameter of the outlet is recommended. Trash racks at entrances to pipes and conduits should be sloped at about 3H:1V to 5H:1V to allow trash to slide up the rack with flow pressure and rising water level—the slower the approach flow, the flatter the angle. Rack opening rules -of -thumb are found in literature. Figure 1.19 gives opening estimates based on outlet diameter (UDFCD, 1992). Judgment should be used in that an area with higher debris (e.g., a wooded area) may require more opening space. The bar opening space for small pipes should be less than the pipe diameter. For larger diameter pipes, openings should be 6 inches or less. Collapsible racks have been used in some places if clogging becomes excessive or a person becomes pinned to the rack. Alternately, debris for culvert openings can be caught upstream from the opening by using pipes placed in the ground or a chain safety net (USBR, 1978; UDFCD, 1992). Racks can be hinged on top to allow for easy opening and cleaning. The control for the outlet should not shift to the grate, nor should the grate cause the headwater to rise above planned levels. Therefore head losses through the grate should be calculated. A number of empirical loss equations exist though many have difficult to estimate variables. Two will be given to allow for comparison. Metcalf & Eddy (1972) give the following equation (based on German experiments) for losses. Grate openings should be calculated assuming a certain percentage blockage as a worst case to determine losses and upstream head. Often 40 to 50% is chosen as a working assumption. Hg = Kgl (w/x)4/3 (V.2 /2g) sin Og (Eq. 1.12) Where: Hg = head loss through grate (ft) Kg1= bar shape factor: 2.42 - sharp edged rectangular 1.83 - rectangular bars with semicircular upstream faces 1.79 - circular bars 1.67 - rectangular bars with semicircular up- and downstream faces w = maximum cross-sectional bar width facing the flow (in) x = minimum clear spacing between bars (in) V„ = approach velocity (ft/s) Og = angle of the grate with respect to the horizontal (degrees) The Corps of Engineers (HDC, 1988) has developed curves for trash racks based on similar and additional tests. These curves are for vertical racks but presumably they can be adjusted, in a manner similar to the previous equation, through multiplication by the sine of the angle of the grate with respect to the horizontal. Hg = KgzVuz 2g Where: Kgz is defined from a series of fit curves as: • sharp edged rectangular (length/thickness = 10) Kgz = 0.00158 - 0.03217 Ar + 7.1786 Arz • sharp edged rectangular (length/thickness = 5) Kgz = -0.00731 + 0.69453 Ar + 7.0856 Are • round edged rectangular (length/thickness = 10.9) Kgz = -0.00101 + 0.02520 Ar + 6.0000 Are • circular cross section Kgz = 0.00866 + 0.13589 Ar + 6.0357 Are and Ar is the ratio of the area of the bars to the area of the grate section. (1.13) 100 0 m kx R a� 0 10 < U c4 cc Z ro F- 1 0 10 20 30 40 50 60 Outlet Diameter in Inches Figure 1.19. Minimum Rack Size vs. Outlet Diameter (Source: UDCFD, 1992). SECTION 7: SECONDARY OUTLETS Introduction The purpose of a secondary outlet (emergency spillway) is to provide a controlled overflow for flows in excess of the maximum design storm for a storage facility. Figure 1.20 shows an example of an emergency spillway. In many cases, on-site stormwater storage facilities do not warrant elaborate studies to determine spillway capacity. While the risk of damage due to failure is a real one, it normally does not approach the catastrophic risk involved in the overtopping or breaching of a major reservoir. By contrast, regional facilities with homes immediately downstream could pose a significant hazard if failure were to occur, in which case emergency spillway considerations are a major design factor. Emergency Spillway Deaign Emergency spillway designs are open channels, usually trapezoidal in cross section, and consist of an inlet channel, a control section, and an exit channel (see Figure 1.20). The emergency spillway is proportioned to pass flows in excess of the design flood (typically the 100 -year flood or greater) without allowing excessive velocities and without overtopping of the embankment. Flow in the emergency spillway is open channel flow s I i k i - k _ k k s 1 i k ' I I Figure 1.19. Minimum Rack Size vs. Outlet Diameter (Source: UDCFD, 1992). SECTION 7: SECONDARY OUTLETS Introduction The purpose of a secondary outlet (emergency spillway) is to provide a controlled overflow for flows in excess of the maximum design storm for a storage facility. Figure 1.20 shows an example of an emergency spillway. In many cases, on-site stormwater storage facilities do not warrant elaborate studies to determine spillway capacity. While the risk of damage due to failure is a real one, it normally does not approach the catastrophic risk involved in the overtopping or breaching of a major reservoir. By contrast, regional facilities with homes immediately downstream could pose a significant hazard if failure were to occur, in which case emergency spillway considerations are a major design factor. Emergency Spillway Deaign Emergency spillway designs are open channels, usually trapezoidal in cross section, and consist of an inlet channel, a control section, and an exit channel (see Figure 1.20). The emergency spillway is proportioned to pass flows in excess of the design flood (typically the 100 -year flood or greater) without allowing excessive velocities and without overtopping of the embankment. Flow in the emergency spillway is open channel flow (see Chapter 6 for more information on open channel design). Normally, it is assumed that critical depth occurs at the control section. NRCS (SCS) manuals provide guidance for the selection of emergency spillway characteristics for different soil conditions and different types of vegetation. The selection of degree of retardance for a given spillway depends on the vegetation. Knowing the retardance factor and the estimated discharge rate, the emergency spillway bottom width can be determined. For erosion protection during the first year, assume minimum retardance. Both the inlet and exit channels should have a straight alignment and grade. Spillway side slopes should be no steeper the 3:1 horizontal to vertical. The most common type of emergency spillway used is a broad -crested overflow weir cut through original ground next to the embankment. The transverse cross section of the weir cut is typically trapezoidal in shape for ease of construction. Such an excavated emergency spillway is illustrated below. �Q Inlet Channel Cut Slope PLAN VIEW Excavated Earth Spillway Hp Critical Depth Inlet L Channel I Level I Channel Section PROFILE Centedirne Spilhway 3 .ti CROSS SECTION Level Section Embankment NOTE: Neither the location nor the alignment of the level section has to coincide with the centerrhe of embankment. DEFINITION OF TERMS: Hp— Depth of water in impow d wt above crest L — Length of level section b — bothm width of spillway So — slope of e)at channel Sc — critical slope Se — diope of inlet charnel Figure 1.20. Emergency Spillway (Source: VADCR, 1999). SECTION 8: REFERENCES Atlanta Regional Commission. 2001. Georgia Stormwater Management Manual, Volume 2: Technical Handbook Atlanta, GA. Brater, E. F. and H. W. King, 1976. Handbook of Hydraulics. 6th ed. New York: McGraw Hill Book Company. Chow, C. N., 1959. Open Channel Hydraulics. New York: McGraw Hill Book Company. Debo, Thomas N and Andrew J. Reese, 1995. Municipal Storm Water Management. Lewis Publishers: CRC Press, Inc., Boca Raton, Florida. McEnroe, B.M., J.M. Steichen and R. M. Schweiger, 1988. Hydraulics of Perforated Riser Inlets for Underground Outlet Terraces. Trans ASAE, Vol. 31, No. 4, 1988. NRCS, 1984. Engineering Field Manual for Conservation Practices, Soil Conservation Service, Engineering Division, Washington, D.C. Virginia Department of Conservation and Recreation, 1999. Virginia Stormwater Management Handbook. Sandvik, A., 1985. Proportional Weirs for Stormwater Pond Outlets. Civil Engineering, March 1985, ASCE pp. 54-56. Metropolitan Government of Nashville and Davidson County, 1988. Stormwater Management Manual - Volume 2 Procedures. Prepared by AMEC, Inc. (formerly The Edge Group) and CH2M Hill. United States Bureau of Reclamation, Water Measurement Manual. http://www.usbr.gov/wrrl/fmt/wmm/ Urban Drainage and Flood Control District, 1999. Criteria Manual, Denver, CO. Wycuff, R. L. and U. P. Singh, 1976. Preliminary Hydrologic Design of Small Flood Detention Reservoirs. Water Resources Bulletin. Vol. 12, No. 2, pp 337-49. APPENDIX H STORMWATER MANAGEMENT SOFTWARE Software STORMWATER MANAGEMENT SOFTWARE SECTION 1: INTRODUCTION Disclaimer. The mention of software herein does not constitute endorsement, nor does it excuse the licensed professional from the exercise of appropriate judgment. The list of approved software may be modified by the City Engineer at his or her discretion. For the purposes of this manual, the approved list of software includes hydrologic and hydraulic software for modeling of major and minor systems. Table 2.1 lists several widely used computer programs and modeling packages. Software use for stormwater management calculations is not limited to this list. However, if a different proprietary software program is to be used, it must be approved by the City Engineer during the technical plat review process. The design engineer shall include a description of the software used for analysis and design along with the formulas and methods that the software uses in its calculations for it to be considered for approval. The software must also be capable of producing an output report that provides information consistent with the requirements as listed in Chapter 6 of the Drainage Criteria Manual. For the purposes of Table 2.1 below, major drainage systems are defined as those draining to FEMA - regulated streams, or lakes or reservoirs. Minor drainage systems are smaller natural and man-made systems that drain to the larger streams. Minor drainage systems can have both closed and open -channel components and can include, but are not limited to, neighborhood storm sewers, culverts, ditches, and tributaries. SECTION 2: APPROVED SOFTWARE 2.1. Stormwater Software ModelingTable Major System Modeling . •Tools. Minor System Modeling Hydrologic Features Hydraulic Features HEC -HMS X X Hydraflow Hydrographs X X Pond Pack X X X Hydraflow Express X X HEC -RAS X X CulvertMaster X X FlowMaster X X Hydraflow Storm Sewers X X X StormCad X Appendix H Exhibit 1 0 SEE Outfall 1 40 Inlet A 2 4 3 Inlet C Inlet D Inlet B Schematic should match Drainage Area Inlet Map and Construction Drawing Labels Project File: Drainage Example.stm Number of lines: 4 Date: 117/2013 Storm Sewers v10.00 nA.__— Pagel _�rr�rrn _rrwrrr �nyrrnrrr�r� rcrrrrr�rr„ Storm Sewers vl0.00 Page 1 %JLVI III %7CWC1 1 QLJUIQLIVII Station Len Drng Area Rnoff Area x C Tc Rain Total Cap Vel Pipe Invert Elev HGL Elev Grnd ! Rim Elev Line ID coeff (I) flow full Line To Incr Total Incr Total Inlet Syst Size Slope Dn Up Dn Up Dn Up Line (ft) (ac) (ac) (C) (min) (min) (in/hr) (cfs) (cfs) (ftls) (in) N (ft) (ft) (ft) (ft) (ft) (ft) 1 End 42.000 0.73 3.83 0.75 0.55 2.10 5.0 18.7 4.9 11.79 13.16 4.21 18 1.57 1283.00 1283.66 1284.31 1284.77 1285.00 1290.02 2 1 44.000 1.92 3.10 0.50 0.96 1.55 18.2 18.5 4.9 7.78 10. 18 1.00 1283.72 1284.16 1286.09 1286.33 1290.02 1290.02 3 2 134.094 0.70 0.70 0.50 0.35 0.35 17.5 17.5 5.0 1.75 .25 2.17 18 2.70 1284.70 1288.32 1287.07 1288.83 1290.02 1293.39 4 2 68.000 0.48 0.48 0.50 0.24 0.24 16.2 16.2 5.2 1.2 18.19 1.84 18 3.00 1284.70 1286.74 1287.08 1287.16 1290.02 1290.94 1 1 1 J X Evaluate outlet velocity Compare HGL and Rim Elevation - Minimum 6 inch clearance Project File: Drainage Example.stm Number of lines: 4 Run Date: 1/7/2013 NOTES: Intensity = 141.10 1(Inlet time + 19.90) " 0.92 ; Return period =Yrs. 10 ; Total flows limited to inlet captured flows. c = cir e = ellip b = box Storm Sewers v10.00 Inlet Reno rt 20% Clogging Factor at Sag Max spread for pavement Standard 4 inches Page 1 - I- based on street classification Line No Inlet ID Q = CIA (cfs) Q carry (cfs) Q capt (cfs) Q Byp (cfs) Junc Type Curb Inle i Grate Inlet Gutter Inlet Byp Line No Ht (in) L (ft Area (sgft) L (ft) W (ft) So (ftlft) W (ft) SW (ftlft) Sx (ftlft) n Depth (ft) Spread (ft) Depth (ft) Spread (ft) pr (in) 1 2 3 4 InletA Inlet B Inlet C Inlet D 4.02 4.76 1.77 1.25 0.00 0.05 0.00 0.00 4.02 4.81 1.75 1.22 0.00 0.00 0.01 0.04 Curb Curb Curb Curb (2.0 2.0 2.0 2.0 6.00 .00 11.00 7.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sag Sag .052 .032 1.00 1.00 1.00 1.00 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.000 0.000 0.013 0.013 0.37 0.42 0.12 .12 D42 6.11 5.86 0.70 0.75 0.36 0.34 1.76 4.12 1.19 0.97 4.0 4.0 4.0 4.0 Off Off Spread for Sag must be calculated separately based on drainage area and gutter slope on each side of inlet Standard 6 inch curb with 4 inch depression 1 If gutter spread is exceeded, add inlet upstream and recalculate drainage area Max inches depth for parking lot is 6 Standard extension box with Project File: Drainage Example.stm Number of lines: 4 Run Date: 1/7/2013 NOTES Inlet N ,Valdes = U.0 I6. Iritenslty = 14 1. Ill , , In ICt Mlle T 19.91.1 ^ 11.92. Re urn oer1od = 1U t rs. Indicates Known 0 added. All curb inlets are Horiz throat. Storm Sewers v10.00 111 rO►A%Of' 0 r11aT T1111B T%A %A I7T1�lIII V ■iV■ ■■ ■ VV ■VV■ ■■ ■■L4 ■ ■■ ■ ■L ■ f�ANM■f�A V■V■ ■ Page 1 Line Line ID Tc Sheet Flow Shallow Concentrated Flow Channel Flow Total No. Method n- Value flow Length (ft) 2 -yr 24h P (in) Land Slope (%) Travel Time (min) flow Length (ft) Water Slope N Surf Descr Ave Vel (Ws) Travel Time (min) X -sec Area (sgft) Wetted Perim (ft) Chan Slope N n- Value Vel flow Length (ft) Travel Time (min) Travel Time (min) 1 2 3 4 User TR55 TR55 TR55 0.240 0.240 0.400 50.00 150.00 100.00 4.08 .08 4.08 0.20 2.00 3.00 18.23 17.48 16.17 5.00 18.20 17.50 16.20 Constant Show path and label on Drainage Area Inlet Map Project File: Drainage Example.stm Min. Tc used for intensity calculations = 5 min Number of lines: 4 Date: 117/2013 Storm Sewers 00 APPENDIX I CONSTRUCTION BEST MANAGEMENT PRACTICES Management Practices CONSTRUCTION BEST MANAGEMENT PRACTICES SECTION 1. TYPICAL DETAILS 10' MAXIMUM SPACING WITH WIRE -- SUPPORT FENCE 6' MAXIMUM SPACING WITHOUT WIRE SUPPORT FENCE r] EXTRA STRENGTH FILTER FABRIC NEEDED WITHOUT WIRE MESH SUPPORT FLOW .4 -- STEEL OR WOOD POST ATTACH FILTER FABRIC SECURELY TO UPSTREAM SIDE OF POST STEEL OR WOOD POST �36" HIGH MAX. PONDING HEIGHT FLOW 12" MIN. 4"x6" TRENCH WITH COMPACTED BACKFILL TRENCH DETAIL NOTES: 1. SILT FENCE SHALL BE PLACED ON SLOPE CONTOURS TO MAXIMIZE PONDING EFFICIENCY. 2. INSPECT AND REPAIR FENCE AFTER EACH STORM EVENT AND REMOVE SEDIMENT WHEN NECESSARY. 9" MAXIMUM RECOMMENDED STORAGE HEIGHT. 3. REMOVED SEDIMENT SHALL BE DEPOSED TO AN AREA THAT WILL NOT CONTRIBUTE SEDIMENT OFF-SITE AND CAN BE PERMANENTLY STABILIZED. Management Practices 1T N. ,CK a INSTALLATION WITHOUT TRENCHING Figure 1. Silt Fence. NOTES 1. INSTALL WATTLES ALONG CONTOURS PER MANUFACTURER'S SPECIFICATIONS. 2. WATTLES SHALL BE INSPECTED REGULARLY AND IMMEDIATELY AFTER A RUNOFF PRODUCING RAINFALL, TO ENSURE THEY REMAIN THOROUGHLY ENTRENCHED AND IN CONTACT WITH THE SOIL. 3. LIVE STAKES MAY BE USED FOR PERMANENT INSTALLATIONS. 4. PERFORM MAINTENANCE IN ACCORDANCE WITH MANUFACTURER'S SPECIFICATIONS. 5. INSTALL WATTLES SNUGLY INTO THE TRENCH. ABUT ADJACENT WATTLES TIGHTLY, END TO END, WITHOUT OVERLAPPING THE ENDS. S. PILOT HOLES MAY BE DRIVEN THROUGH THE WATTLE AND INTO THE SOIL, WHEN SOIL CONDITIONS REQUIRE. w" sir WMM ARF/► CM.) drm r wlliRL 9PMCNO (UK TANO ELEVATION VIEW WATTLE SPACING TABLE SLOPE MAXIMUM SPACING 1:1 20 FEET 2:1 30 FEET 3:1 40 FEET 4:1 50 FEET Figure 2. Wattle. Management Practices EACH M WL OC. MAN FIORI A 18" (0.5m) (150mm) 24" (0.8m) NOTE.• KEY STONE INTO CHANNEL 54NKS AND L A EXTEND IT BEYOND THE ABUTMENTS A MINIMUM OF 18" (0.5m) TO PREVENT FLOW AROUND DAM. VIM LOOKING UPSTREAM SECTION A - A THE DISTANCE SUCH THAT POINTS ;4 ' AND `B' ARE OF EQUAL £LEGATION. SPACING BETWEEN CHECK DAMS NOT TO SCALE Figure 3a. Rock Check Dam. Management Practices *POINT "A" MUST BE HIGHER THAN POINT "B" TO ENSURE THAT WATER FLOWS OVER THE DIKE AND NOT AROUND THE ENDS. *"STAPLES SHALL BE PLACED WHERE THE UNITS OVERLAP AND IN THE CENTER OF THE T UNIT AS SHOWN ON THE DIAGRAMS. 3" TO 6" TRENCH SECTION 'B -B' SECTION 'A -A' DIKE STAPLES C611111NI7ij0610 Figure 3b. Silt Dike Check Dam. Management Practices 2 OR FL4TTER I 1 SMOOTH FOUNDATION UNDER FILTER FILTER FABRIC OR SAND AND GRAVEL FILTER 6" (150mm) MINIMUM THICK CLASS 1 OR CLASS 2 DESIGNED STONE SIZE KEYWAY AT TOE OF SLOPE TYPICAL SECTION NOTE.• 'r = THICKNESS.• THICKNESS SHALL BE DETERMINED BY THE ENGINEER. MINIMUM THICKNESS SHALL BE 1.5x THE MAXIMUM STONE D14METER, NEVER LESS THAN 6'" (150mm). Figure 4. Riprap Protection. Management Practices J��PG� A "N c�pPp ori i of 8" MIMIMUM 3"-6" CLEAN STONE GEOTEXTILE UNDERLINER DIVERSION RIDGE REQUIRED WHERE GRADE EXCEEDS 2% CONSTRUCTION EXIT NOTES OK 0 0 0 0 ti 2% or Gf FILTER FABRIC SECTION A -A 1. REPLACE CONTAMINATED STONE AS REQUIRED TO PREVENT TRACKING OF SEDIMENT OR MUD ON PUBLIC STREETS. 2. CLEAN STREETS DAILY WITH BROOM AND SHOVEL. THE USE OF WATER IS PROHIBITED. 3. ALL VEHICLES MUST USE CONSTRUCTION EXIT. 4. WHEN WHEEL WASHING IS REQUIRED, IT SHALL BE DONE ON AN AREA STABILIZED WITH CRUSHED STONE THAT DRAINS INTO AN APPROVED SEDIMENT TRAP OR SEDIMENT BASIN. Figure S. Construction Entrance/Exit. Management Practices INSTALL SILT FENCE ALONG DOWN GRADIENT SIDE OF WASH AREA VEGETATED / Q4v AR EA 7 `'vf NATURAL SLOPE CONSTRUCT HAY BALE J s DIKE ALONG LOW POINT <o� OF FENCE OR USE \� WIRE REINFORCED SILT FENCE NOTES: 1. MODIFY CONFIGURATION OF WASH AREA AS NECESSARY TO FIT FIELD CONDITIONS. 2. GRADE WASH AREA AS NECESSARY TO DRAIN TO OUTLET. 3. DO NOT USE DETERGENTS OR SOAPS. 4. DO NOT DISCHARGE DIRECTLY TO A STORM DRAIN OR WATER COURSE IF POSSIBLE. CONSTRUCT GRAVEL PAD FOR WASH AREA OR MAINTAIN NATURAL VEGETATION IF UNDISTURBED LENGTH AS REQUIRED FOR PROJECT EQUIPMENT WIDTH AS REQUIRED FOR PROJECT EQUIPMENT Figure 6. Equipment Wash Area. Management Practices 2' (0.6m) ALL SLOPES 2.• 1 OR FLATTER 18" (0.5m) MINIMUM ---w— FLOW VEGETATION OR RIPR4P STABILIZATION TYPICAL TEMPORARY DIVERSION DIKE NOTES. - 1. THE DIKE SHALL BE CONSTRUCTED ON CONTOUR. Figure 7. Rock Dike. Management Practices DESIGN HEIGHT (H), WIDTH AND STONE SIZE SHALL BE DETERMINED BY THE ENGINEER „r* -1C0%.. , ,.,,., 1101. , rte, MINIMUM 6" (150mm) THICK LAYER OF 2" (50mm) MINIMUM DIAMETER DRAIN ROCK. LARGER SAME SHALL BE USED DEPENDENT UPON GRADIENT, SOIL TYPE, AND DESIGN FLOW. TYPICAL SECTION Figure 8. Rock Lined Channel. Management Practices 2' (0.6m) COMPACTED PROTECTED SO/L RILL SLOPE ALL SLOPES 2.• OR FLATTER 18" (0.5m) MIN/MUM I/EGETA77ON OR R/PRAP STABIL /Z4770N TYPICAL FILL DIVERSION 2' COMPACTED SOIL f. J. ALL SLOPES 2.• OR FLATTER 18" (0.5m) MIN/MUM --*— f7 OW VEGETA77ON OR R/PRAP STABIL /ZA770N TYPICAL TEMPORARY DIVERSION DIKE NOTES:• 1. THE CHANNEL BEH/ND THE DIKE SHALL HAVE P05177VE GRADE TO A STABILIZED OUTLET. 2. THE DIKE SHALL BE ADEOU.ATEL Y COMPACTED TO PREVENT FA/LURE. J. 7XF DIKE SHALL BE STABILIZED W/7N 7EMPORARY OR PERMANENT SEED/NG OR WRAP. Figure 9. Earth Dike. Management Practices FLARED END OVERFLOW ELEVATION SECTION RECEIVING MATERIAL ELEVATION THICKNESS ('d') =1.5 x MAX. ROCK DIAMETER - 6" (150mm) MIN. PROFILE La=4.5x'D' MIN. 'D'= PIPE DIAMETER o� o ROCK d50 - 50% HALL BE LARGER THAN 6" (150mm) MIN. 4.0 x'D' D, A. AND SIZED BASED ON MIN. MAXIMUM DISCHARGE VELOCITY 77777777777777777777 PLAN NOTES: 1. 'La' = LENGTH OF APRON. DISTANCE'La' SHALL BE OF SUFFICIENT LENGTH TO DISSIPATE ENERGY. 2. APRON SHALL BESET AT A ZERO GRADE AND ALIGNED STRAIGHT. 3. FILTER MATERIAL SHALL BE FILTER FABRIC OR 6"(150 mm) THICK MINIMUM GRADED GRAVEL LAYER. Figure 10. Storm Drain Outlet Protection. Management Practices RUNOFFDo- RUNOFF RUNG - 84CK OF SIDEWALK CATCH BASIN BURLAP SACKS TO OVERLAP ONTO CURB CURB INLET GRAVEL FILLED SANDBAGS STACKED 77GH71 Y PLAN VIEW BACK OF NOTES. 1. PLACE CURB TYPE SEDIMENT BARRIERS ON GENTLY SLOPING STREET SEGMENTS, WHERE WATER CAN POND AND ALLOW SEDIMENT TO SEPARATE FROM RUNOFF. 2. SANDBAGS OF EITHER BURLAP OR WOKEN 'GEOTEXNLE' FABRIC, ARE FILLED WITH GRAVEL, LAYERED AND PACKED T/GHTL Y. J. LEAVE A ONE SANDBAG GAP IN THE TOP ROW TO PROVIDE A SPILLWAY FOR OVERFLOW. 4. INSPECT BARRIERS AND REMOVE SEDIMENT AFTER EACH STORM EVENT. SEDIMENT AND GRAVEL MUST BE REMOVED fROM THE TRAVELED WAY IMMED14TEL Y. Figure 11. Curb Gutter Protection. Management Practices MATS/BLANKET SHOULD BE INSTALLED VERTICALLY DOWNSLOPE w w w w �✓ w w w w w w w .V I APRON NOTES: 1. SLOPE SURFACE SHALL BE FREE OF ROCKS, CLODS, STICKS, AND GRASS. MATS/BLANKETS SHALL HAVE GOOD SOIL CONTACT. 2. APPLY PERMANENT SEEDING BEFORE PLACING BLANKETS. 3. LAY BLANKETS LOOSELY AND STAKE OR STAPLE TO MAINTAIN DIRECT CONTACT WITH THE SOIL. DO NOT STRETCH. 4. REFER TO MANUFACTURER'S RECOMMENDED STAPLING PATTERN FOR STEEPNESS AND LENGTH OF SLOPE TO BE BLANKETED. Figure 12. Erosion Blanket Installation. Management Practices BACK OF SIDEWALK 7-A CATCH BASIN BACK OF CURB CURB INLET r— --------------- L - 2X4 WOOD STUD r-- CONCRETE BLOCK yeo a -v W�•-•o-•ru Wo•-•-=ry WIRE SCREEN OR CONCRETE BLOCK FILTER FABRIC LA 3/4" 0 DRAIN GRAVEL PLAN VIEW PONDING OVERFLOW CONCRETE BLOCK HEIGHT--- -- WIRE SCREEN OR FILTER FABRIC 2X4 WOOD STUD (100X50 TIMBER STUD) NOTES: QECURB INLET CATCH BASIN SECTION A — A USE BLOCK AND GRAVEL TYPE SEDIMENT BARRIER WHEN CURB INLET IS LOCATED IN GENTLY SLOPING STREET SEGMENT, WHERE WATER CAN POND AND ALLOW SEDIMENT TO SEPARATE FROM RUNOFF. 2. BARRIER SHALL ALLOW FOR OVERFLOW FROM SEVERE STORM EVENT. 3. INSPECT BARRIERS AND REMOVE SEDIMENT AFTER EACH STORM EVENT. SEDIMENT AND GRAVEL MUST BE REMOVED FROM THE TRAVELED WAY IMMEDIATELY. Figure 13. Curb Inlet Protection. Management Practices DRAIN GRATE PLAN VIEW CONCRETE BLOCK GRAVEL BACKFILL OVERFLOW WATER WATER DROP INLET SECTION A — A NOTES: 1. DROP INLET SEDIMENT BARRIERS ARE TO BE USED FOR SMALL, NEARLY LEVEL DRAINAGE AREAS. (LESS THAN 5%) 2. EXCAVATE A BASIN OF SUFFICIENT SIZE ADJACENT TO THE DROP INLET. 3. THE TOP OF THE STRUCTURE (PONDING HEIGHT) MUST BE WELL BELOW THE GROUND ELEVATION DOWNSLOPE TO PREVENT RUNOFF FROM BYPASSING THE INLET. A TEMPORARY DIKE MAY BE NECESSARY ON THE DOWNSLOPE SIDE OF THE STRUCTURE. CONCRETE BLOCK VEL BACKFILL - (20mm) MIN -WIRE SCREEN OR FILTER FABRIC _PONDING HT_ Figure 14. Block and Gravel Drop Inlet Sediment Barrier. Management Practices TAPERED INLET W O L LU O 10' (3m) MAX r•F7f1flAl ANCHORS AT 10' (-m) WITH — MIN. OF 36' (1m) EMBEDMENT OPEN TOP CHUTE ANCHORS PLAN VIEW DISCHARGE TO STABILIZED WATER COURSE OPEN TOP CHUTE Figure 15. Flexible Down Drain. Management Practices `TRACKING' WITH MACHINERY ON SANDY SOIL PROVIDES ROUGHENING WITHOUT UNDUE COMPACTION. STRAW ANCHORING NOTES: 1. ROUGHEN SLOPE WITH BULLDOZER 2. BRa4DCAST SEED AND FER77LIZER. 3. SPREAD SM4W MULCH 3" (76mm) THICK. (2 1/2 TONS PER ACRE) 4. PUNCH STR4W MULCH INTO SLOPE BY RUNNING BULLDOZER UP AND DOWN SLOPE. Figure 16. Straw Anchoring. Management Practices APPENDIX J Appendix J — Intrinsic GSP Specifications Erosion Control Appendix J - Intrinsic GSP Specifications tormwater Best Management Practice Y Minimum Measure Construction Site Stormwater Runoff Control Subcategory Erosion Control Purpose and Description A compost blanket is a layer of loosely applied composted material placed on the soil in disturbed areas to reduce stormwater runoff and erosion. This material fills in small rills and voids to limit channelized flow, provides a more permeable surface to facilitate stormwater infiltration, and promotes revegetation. Seeds can be mixed into the compost before it is applied. Composts are made from a variety of feedstocks, including yard trimmings, food residuals, separated municipal solid waste, and municipal sewage sludge (biosolids). Controlling erosion protects water quality in surface waters, such as streams, rivers, ponds, lakes, and estuaries; and increasing stormwater infiltration replenishes groundwater aquifers. Applying a compost blanket also works well as a stormwater best management practice (BMP) because it: • Retains a large volume of water, which aids in establishing vegetation growth within the blanket, • Acts as a cushion to absorb the impact energy of rainfall, which reduces erosion, • Stimulates microbial activity that increases the decomposition of organic matter, which increases nutrient availability and improves the soil structure, • Provides a suitable microclimate with the available nutrients for seed germination and plant growth, and • Removes pollutants such as heavy metals, nitrogen, phosphorus, fuels, grease and oil from stormwater runoff, thus improving downstream water quality (USEPA 1998). Applicability and Limitations Compost blankets can be placed on any soil surface: flat, steep, rocky, or frozen. The blankets are most effective when applied on slopes between 4:1 and 1:1 (horizontal run:vertical rise); such as construction sites, road embankments, and stream Figure 1. Applying a compost blanket on a bare and eroding slope Figure 2. Same slope after revegetation banks; where stormwater runoff can occur as sheet flow. On the steeper slopes (1:1) the compost blanket should be used in conjunction with netting or other confinement systems to further stabilize the compost and slope, or the compost particle size and depth should be specially designed for this application. Compost blankets should not be placed in locations that receive concentrated or channeled flows either as runoff or a point source discharge. If compost blankets are placed adjacent to highways and receive concentrated runoff from the traffic lanes, they should be protected by compost berms, or a similar BMP that diffuses or diverts the concentrated runoff before it reaches the blanket (Glanville, Richard, and Persyn 2003). Because a compost blanket can be applied to the ground surface without having to be incorporated into the soil, it provides excellent erosion and sediment control on difficult terrain, such as steep or rocky slopes (Figures 3, 4). Projects where the cost of transporting and applying composts is most easily justified are situations that demand both immediate erosion control and growth of vegetative cover, such as projects completed too late in the growing season to establish natural vegetation before winter or areas with poor quality soils that don't readily support vegetative growth (Glanville, Richard, and Persyn 2003). Figure 3. Applying a compost blanket on a steep, rocky slope Figure 4. Same slope after revegetation Stormwater Best Management Practice: Compost Blankets What Is Compost? Compost is the product of controlled biological decomposition of organic material that has been sanitized through the generation of heat and stabilized to the point that it is beneficial to plant growth. It is an organic matter resource that has the unique ability to improve the biological, chemical, and physical characteristics of soils or growing media. Compost contains plant nutrients but is typically not characterized as fertilizer (USCC 2008). This decomposition of organic material is produced by metabolic processes of microorganisms. These microbes require oxygen, moisture, and food in order to grow and multiply. When these three factors are maintained at optimal levels, the natural process of decomposition is greatly accelerated. The microbes generate heat, water vapor, and carbon dioxide as they transform the raw materials into a stable soil conditioner. Compost can be produced from many raw organic materials, such as leaves, food scraps, manure, and biosolids. However, the mature compost product bears little physical resemblance to the raw material from which it originated. Figure 5. Mature compostproduct How Is Compost Beneficial? Biological Benefits Provides an excellent substrate for soil biota. The activity of soil microorganisms is essential for productive soils and healthy plants. Their activity is largely based on the presence of organic matter. Soil microorganisms include bacteria, protozoa, and fungi. They are not only found within compost, but will also proliferate within the soil under a compost blanket. These microorganisms play an important role in organic matter decomposition, which leads to humus formation and nutrient availability. Some microorganisms also promote root activity; specific fungi work symbiotically with plant roots, assisting them in extracting nutrients from the soils. Suppresses plant diseases. The incidence of plant diseases may be influenced by the level and type of organic matter and microorganism present in soils. Research has shown that increased populations of certain microorganisms may suppress specific plant diseases, such as pythium blight and fusarium wilt. Chemical Benefits Provides nutrients. Compost blankets contain a considerable variety of macro- and micronutrients essential for plant growth. Since compost contains relatively stable sources of organic matter, these nutrients are supplied in a slow-release form. Modifies and stabilizes pH. The pH of composts differ. When necessary, a compost may be chosen that is most appropriate for revegetating a particular construction site. Physical Benefits Improved soil structure and moisture management. In fine -textured soils (i.e., clay or clay loam), the addition of compost will increase permeability, and reduce stormwater runoff and erosion. The soil -binding properties of compost are due to its humus content. Humus is a stable residue resulting from a high degree of organic matter decomposition. The constituents of humus hold soil particles together, making them more resistant to erosion and improving the soil's ability to hold moisture. Effectiveness of Compost, Topsoil, and Mulch Because of the biological, chemical, and physical benefits it can provide, compost makes a more effective erosion control blanket than topsoil. An Iowa State University study (Glanville, Richard, and Persyn 2003), sponsored by the Iowa Department of Natural Resources and Iowa Department of Transportation (DOT), compared the quantity of runoff from road embankments treated with topsoil and with compost blankets. The test plots were exposed to simulated, high intensity rainfall (3.7 inches/hour) lasting for 30 minutes. Results showed that the amount of runoff from the embankment treated with a compost blanket was far less than the runoff from the embankment treated with topsoil. Mulch is a protective covering placed around plants for controlling weeds, reducing evaporation, and preventing roots from freezing. It is made of various substances usually organic, such as hardwood or pine bark. A compost blanket is a much more effective BMP for erosion control and revegetation than mulch. A University of Georgia research study (Faucette and Risse 2002) reported that correctly applied compost blankets provide almost 100 percent soil surface coverage, while other Stormwater Best Management Practice: Compost Blankets methods (e.g., straw mats and mulches) provide only 70 to 75 percent coverage. Uniform soil coverage is a key factor in effective erosion and sediment control because it helps maintain sheet flow and prevents stormwater from forming rills under the compost blanket. Compost Qualit Compost Properties Maturity. Maturity indicates how well the compost will support plant growth. One maturity test measures the percent of seeds that germinate in the compost compared to the number of seeds that germinate in peat based potting soil. For example, if the same number of seeds was planted in the potting soil (control) and in a marketed compost product, and 100 of them germinate in the potting soil and 90 germinate in the compost, the compost's maturity would be 90 percent. Another maturity test compares the growth and vigor of seedlings after they have been growing in both compost and potting soil. Stability. Stability determines how "nice" the compost is. While microbial decay is actively transforming the feedstocks into compost, the unstable mixture may have unpleasant characteristics such as odors. However, after the decay process is completed, the stable compost product no longer resembles the feedstock or has offensive characteristics. During the composting process, CO2 is produced because the microbes are actively respiring. So the microbial respiration (CO2 evolution) rates can be measured and used to determine when the microbial decay is completed and the compost product has stabilized. Presence of Pathogens. The pathogen count indicates how sanitary the compost is. EPA has defined processes for composting biosolids that reduce the number of pathogenic organisms to nondetectible levels and ensure the resulting compost will be sufficiently heat treated and sanitary. These processes to further reduce pathogens (PFRP) are defined in 40 CFR, Part 503, Appendix B, Section B. Compost quality specifications often require compost to be treated by a PFRP process, so there are no measurable pathogenic microorganisms present. Other compost properties that may be found in compost quality specifications are plant nutrients and heavy metal concentrations, pH, moisture content, organic matter content, soluble salts, and particle size. Compost Quality Testing A compost testing, labeling, and information disclosure program, the Seal of Testing Assurance Program, has been established by the United States Composting Council (USCC), a private, nonprofit organization. Under this program testing protocols for determining the quality and condition of compost products at the point of sale have been jointly approved and published by the USCC and U.S. Department of Agriculture. These Test Methods for Evaluating Compost and Composting, the TMECC Testing Protocols are conducted by independent laboratories to help compost producers determine if their compost is safe and suitable for its intended uses, and to help users compare various compost products and verify the product safety and market claims. The goal of the program is to certify the compost products have been sampled and tested in accordance with these approved protocols. Compost producers who participate in this program have committed to having their products tested by an approved laboratory according to the prescribed testing frequency and protocols and to providing the test results to anyone upon request. The products of participating compost producers carry the USCC certification logo and product information label. Compost Quality Specifications The Federal Highway Administration supported developing specifications for compost used in erosion and sediment control through a cooperative agreement with the Recycled Materials Resource Center at the University of New Hampshire. The original compost blanket specifications (Alexander 2003) were developed under this grant. Working with the USCC and Ron Alexander (Alexander 2003), the American Association of State Highway and Transportation Officials finalized and approved these specifications (AASHTO 2010), which include: narrative criteria (e.g., no objectionable odors or substances toxic to plants), numerical specifications [e.g., pH, soluble salts, moisture content, organic matter content, particle size, stability, and physical contaminants (e.g., metal, glass, plastics)], and pathogen reduction using the EPA processes to further reduce pathogens. These AASHTO specifications also recommend the TMECC testing protocols. A number of states have now developed specifications for the compost they use in erosion and sediment control. Examples are the California DOT specifications and Texas DOT specifications. Stormwater Best Management Practice: Compost Blankets Compost Blanket Installation Once any trash and debris have been removed from a site, a compost blanket can be uniformly applied usually between 1 and 3 inches thick using a bulldozer, skid steer, manure spreader, or hand shovel. Application rates (thickness) are often included in compost blanket specifications. The compost blanket should extend at least 3 feet over the shoulder of the slope to ensure that stormwater runoff does not flow under the blanket (Alexander 2003). On very rocky terrain or if the slope is too steep for heavy equipment, a pneumatic blower truck is needed to apply the compost (Figure 6). If the slope is steep, a compost blanket may work best in conjunction with other BMPs, such as compost socks placed across the slope to Figure 6. Using a pneumatic blower truck to apply a compost blanket on a rocky 1:1 slope Figure 8. Using a compost berm to divert or defuse highway runoff before it reaches the compost blanket reduce the runoff velocity (Figure 7) or compost berms placed at the top of the slope to divert or diffuse concentrated runoff before it reaches the compost blanket (Figure 8). Figure 7. Using compost socks to reduce the runoff velocity Fabric netting can also be used to hold the compost blanket on steep slopes (Figure 9). The netting is usually stapled to the slope (Figure 10), and then the compost is blown on the slope and into the netting. Mature compost for erosion control on moderate slopes is shown in Figure 11, with a red pen for size comparison. The compost in ling stabilizing �nket Figure 10. Stapling netting to the slope Figure 5 is too fine for erosion control. Coarser compost should be avoided on slopes that will be landscaped or seeded, as it will make planting and crop establishment more difficult. But coarse and/or thicker compost is recommended for areas with rigure ii. uompostrorerosion higher annual precipitation or control on moderate slopes rainfall intensity, and even coarser compost is recommended for areas subject to wind erosion (Alexander 2003). Grass, wildflower, or native plant seeds appropriate for the soil and climate can be mixed into the compost. Although seed can be broadcast on the compost blanket after installation, it is typically incorporated into the compost before it is applied, to ensure even distribution of the seed throughout the compost and to reduce the risk of the seed being washed from the surface of the compost blanket by stormwater. Wood chips may also be added to reduce the erosive effect of rainfall's impact energy. Figure 12. Impact of rainfall Inspection and Maintenance The compost blanket should be inspected periodically and after each major rainfall. If areas of the compost blanket have washed out, another layer of compost should be applied. In some cases, it may be necessary to add another BMP to control the stormwater, such as a compost filter sock or silt fence. On slopes greater than 2:1, establishing thick, permanent vegetation as soon as possible is the key to successful erosion and sediment control. Restricting or eliminating pedestrian traffic on such areas is essential (Faucette and Ruhlman 2004). Stormwater Best Management Practice: Compost Blankets Climate Chanqe Mitiqation In 2005 an estimated 246 million tons of municipal solid wastes were generated in the United States. Organic materials including yard trimmings, food scraps, wood waste, paper and paper products are the largest component of our trash and make up about two-thirds of the solid waste stream. When this organic matter decomposes in landfills, the carbon is converted to methane (CH 4) and other volatile organic compounds, which are released into the atmosphere and contribute to global warming. EPA has identified landfills as the single largest source of methane, a potent greenhouse gas that is 23 times more efficient at trapping heat than carbon dioxide (CO2). Landfills contribute approximately 34 percent of all man-made methane released into the atmosphere in the United States (USEPA 2007). Two approaches for mitigating climate change are reducing carbon emissions and sequestering carbon in the atmosphere. Reducing carbon emissions. When organic materials are composted and then recycled, the composting feedstocks are diverted from already burdened municipal landfills, and landfill -generated methane gas emissions are reduced. Figure 13. As compost like this is recycled, green house gasses are reduced Sequestering Carbon. Carbon sequestration is the act of removing carbon dioxide from the atmosphere and storing it in carbons sinks, such as oceans, plants and other organisms that use photosynthesis to convert carbon from the atmosphere into biomass. Forest ecosystems and permanent grasslands are prime examples of terrestrial carbon sinks that sequester carbon. We no longer have the vast expanses of prairies and eastern forests, but we are using compost blankets to revegetate construction sites, road banks, and green roofs; Figure 14. Compost blankets will nurture and this vegetation revegetation, which sequesters carbon and sequesters carbon. prevents erosion 5 References AASHTO 2010. Standard Practice for Compost for Erosion/ Sediment Control (Compost Blankets), R 52-10. Washington, DC: American Association of State Highway and Transportation Officials. www.epa.gov/npdes/pubs/aashto. pdf Alexander, R. 2003. Standard Specifications for Compost for Erosion/Sediment Control, based on work supported by the Federal Highway Administration under a Cooperative Agreement with the Recycled Materials Resource Center at the University of New Hampshire, Durham, New Hampshire. www.alexassoc.net/organic_recylcing_composting_documents/ standard compost erosion sediment control specs.pdf Faucette, Britt, and Mark Risse 2002. "Controlling Erosion with Compost and Mulch." BioCycle June: 26-28. www.epa.gov/npdes/pubs/biocycle2002.pdf Faucette, Britt, and Melanie Ruhlman 2004. "Stream Bank Stabilization Utilizing Compost." BioCycle January: 27. www.epa.gov/npdes/pubs/biocycle2004.pdf Faucette, L.B., C.F. Jordan, L.M. Risse, M. Cabrera. D.C. Coleman, L.T. West 2005. "Evaluation of Stormwater from Compost and Conventional Erosion Control Practices in Construction Activities." Journal of Soil and Water Conservation 60: 288-297. Available from J. Soil & Water Con. abstract free and full text for a fee. Faucette, L.B., L.M. Risse, C.F. Jordan, M.L. Cabrera, D.C. Coleman, L.T. West 2006. "Vegetation and Soil Quality Effects from Hydroseed and Compost Blankets Used for Erosion Control in Construction Activities." Journal of Soil and Water Conservation 61: 355-362. Available from J. Soil & Water Con. abstract free and full text for a fee. Faucette, L.B., J. Governo, C.F. Jordan, B.G. Lockaby, H.F. Carino, R. Governo 2007. "Erosion Control and Storm Water Quality from Straw with PAM, Mulch, and Compost Blankets of Varying Particle Sizes." Journal of Soil and Water Conservation 62: 404-413. Available from J. Soil & Water Con. abstract free and full text for a fee. Stormwater Best Management Practice: Compost Blankets Faucette, L.B. 2008. "Performance and Design for Compost Erosion Control and Compost Storm Water Blankets." Proceedings of the International Erosion Control Association Annual Conference. Orlando, Florida: International Erosion Control Association. Abstract available from IECA. Faucette, L.B., B. Scholl, R. E. Bieghley, J. Governo. 2009. "Large -Scale Performance and Design for Construction Activity Erosion Control Best Management Practices." Journal of Environmental Quality 38: 1248-1254. Available from J. Env. Qual. Abstract and full text free. www.soils.org/publications/jeq/abstracts/38/3/1248 Filtrexx 2009. Filtrexx International's Carbon Reduction & Climate Change Mitigation Efforts, Item # 3324. Grafton, OH: Filtration International, LLC. Glanville, Tomas D., Tom L. Richard, Russell A. Persyn 2003. Impacts of Compost Blankets on Erosion Control, Revegetation, and Water Quality at Highway Construction Sites in Iowa. Ames: Iowa State University of Science and Technology, Agricultural and Biosystems Engineering Department. www.eng.iastate.edu/compost/papers/FinalReport_April2003_ ExecSummary.pdf Risse, M., L.B. Faucette. 2009. Compost Utilization for Erosion Control, Bulletin No. 1200. Athens: University of Georgia, Cooperative Agriculture Extension Service. USCC 2001. Compost Use on State Highway Applications. This is a series of case studies as well as model specifications developed by state DOTs for using compost in highway construction projects. Ronkonkoma, NY: U.S. Composting Council. Available from USCC for a fee. http://compostingcouncil.org/publications/ USCC 2008. USCC Factsheet: Compost and Its Benefits. Ronkonkoma, NY: U.S. Composting Council. http://compostingcouncil.org/admin/wp-content/ uploads/2010/09/Compost-and- Its- Benefits. pdf USEPA 1998. An Analysis of Composting as an Environmental Remediation Technology, EPA 530-R-98-008. Washington, DC: U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response. www.epa.gov/osw/conserve/rrr/composting/pubs/ USEPA 2007. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2005, USEPA 430-R-07-002. Washington, DC: U.S. Environmental Protection Agency, Office of Atmospheric Programs. www.epa.gov/climatechange/emissions/downloads06/07CR.pdf Websites Caltrans 2010. Compost Blanket. California Department of Transportation. www.dot.ca.gov/hq/LandArch/ec/organics/compost blanket.htm USEPA 2010. Compost Based Stormwater Best Management Practices Webinars. U.S. Environmental Protection Agency, Region 5, Chicago. www.epa.gov/region5/waste/solidwaste/compost/webinars.html Photograph Credits Figures 1, 2. Barrie Cogburn, Texas DOT Figures 3, 4. Dwayne Stenlund, CPESC Minnesota DOT Figure 5. Larry Strong, affiliation unknown Figure 6. Scott McCoy, KSS Consulting, LLC Figure 7. Tom Glanville, Iowa State University Figure 8. Jason Giles, CPESC, Rexius Figures 9, 10. Britt Faucette, CPESC, Filtrexx International, LLC Figure 11. Jason Giles, CPESC, Rexius Figure 12. Larry Beran, Texas A&M University Figures 13, 14. Jami Burke, CESCL, Cedar Grove Landscaping and Construction Services Disclaimer Please note that EPA has provided external links because they provide additional information that may be useful or interesting. EPA cannot attest to the accuracy of non -EPA information provided by these third -party websites and does not endorse any non-government organizations or their products or services. 6 Dust Control EPA NPDES Fact Sheet http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factshect_results&view=specific&bmp=52&minmeasure=4 Accessed 10-18-2013 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Dust control measures can be used to prevent dust from being transported by wind (Source: Dust Pro, Inc., no date) Description Dust control BMPs reduce surface activities and air movement that causes dust to be generated from disturbed soil surfaces. Construction sites can generate large areas of soil disturbance and open space for wind to pick up dust particles. Limited research at construction sites has established an average dust emission rate of 1.2 tons/acre/month for active construction (WA Dept. of Ecology, 1992). Airborne particles pose a dual threat to the environment and human health. First, dust can be carried offsite, thereby increasing soil loss from the construction area and increasing the likelihood of sedimentation and water pollution. Second, blowing dust particles can contribute to respiratory health problems and create an inhospitable working environment. Applicability Dust control measures are applicable to any construction site where there is the potential for air and water pollution from dust traveling across the landscape or through the air. Dust control measures are especially important in and or semiarid regions, where soil can become extremely dry and vulnerable to transport by high winds. Implement dust control measures on all construction sites where there will be major soil disturbances or heavy equipment construction activity such as clearing, excavation, demolition, or excessive vehicle traffic. Earthmoving activities are the major source of dust from construction sites, but traffic and general disturbances can also be major contributors (WA Dept. of Ecology, 1992). The dust control measures that are implemented at a site will depend on the topography and land cover of the site and its soil characteristics and expected rainfall. Siting and Design Considerations When designing a dust control plan for a site, the amount of soil exposed will dictate the quantity of dust generation and transport. Therefore, construction sequencing and disturbing only small areas at a time can greatly reduce problematic dust from a site. If land must be disturbed, consider using temporary stabilization measures before disturbance. A number of methods can be used to control dust from a site; not all will be applicable to a site. The owner, operator, and contractors responsible for dust control at a site will have to determine which practices accommodate their needs according to specific site and weather conditions. The following is a brief list of some control measures and design criteria. SprinklingArrigation. Sprinkling the ground surface with water until it is moist is an effective dust control method for haul roads and other traffic routes (Smolen et al., 1988). This practice can be applied to almost any site. Vegetative Cover. In areas not expected to handle vehicle traffic, vegetative stabilization of disturbed soil is often desirable. Vegetative cover provides coverage to surface soils and slows wind velocity at the ground surface, thus reducing the potential for dust to become airborne. Mulch. Mulching can be a quick and effective means of dust control for a recently disturbed area (Smolen et al., 1988). Wind Breaks. Wind breaks are barriers (either natural or constructed) that reduce wind velocity through a site and, therefore, reduce the possibility of suspended particles. Wind breaks can be trees or shrubs left in place during site clearing or constructed barriers such as a wind fence, snow fence, tarp curtain, hay bale, crate wall, or sediment wall (USEPA, 1992). Tillage. Deep tillage in large open areas brings soil clods to the surface where they rest on top of dust, preventing it from becoming airborne. Stone. Stone can be an effective dust deterrent for construction roads and entrances or as a mulch in areas where vegetation cannot be established. Spray -on Chemical Soil Treatments (palliatives). Examples of chemical adhesives include anionic asphalt emulsion, latex emulsion, resin -water emulsions, and calcium chloride. Chemical palliatives should be used only on mineral soils. When considering chemical application to suppress dust, determine whether the chemical is biodegradable or water-soluble and what effect its application could have on the surrounding environment, including waterbodies and wildlife. Table 1 shows application rates for some common spray -on adhesives, as recommended by Smolen et al. (1988). Table 1. Application rates for spray -on adhesives (Source: Smolen et al., 1988) Spray -on adhesive Anionic asphalt emulsion Latex emulsion Water dilution Type of nozzle Application (gal/acre) 7:1 Coarse spray 1,200 12.5:1 Fine spray 235 Resin in water 4:1 Fine spray 300 Limitations Applying water to exposed soils can be time intensive, and if done to excess, could result in excess runoff from the site or vehicles tracking mud onto public roads. Use chemical applications sparingly and only on mineral soils (not muck soils) because their misuse can create additional surface water pollution from runoff or contaminate ground water. Chemical applications might also present a health risk if excessive amounts are used. Maintenance Considerations Because dust controls are dependent on specific site and weather conditions, inspection and maintenance requirements are unique for each site. Generally, however, dust control measures involving application of either water or chemicals require more monitoring than structural or vegetative controls to remain effective. If structural controls are used, inspect them regularly for deterioration to ensure that they are still achieving their intended purpose. Effectiveness Mulch. Can reduce wind erosion by up to 80 percent. Wind Breaks/Barriers. For each foot of vertical height, an 8- to 10 -foot deposition zone develops on the leeward side of the barrier. The permeability of the barrier will change its effectiveness at capturing windborne sediment. Tillage. Roughening the soil can reduce soil losses by approximately 80 percent in some situations. Stone. The size of the stones can affect the amount of erosion to take place. In areas of high wind, small stones are not as effective as 20 cm stones. Spray -on Chemical Soil Treatments (palliatives). Effectiveness of polymer stabilization methods range from 70 percent to 90 percent, according to limited research. Cost Considerations Costs for chemical dust control measures can vary widely depending on specific needs of the site and the level of dust control desired. References Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R- 92-006. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Washington State Department of Ecology. 1992. Stormwater Management Manual for the Puget Sound Basin. Washington State Department of Ecology, Olympia, WA. Geotextiles EPA NPDES Fact Sheet http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factshect_results&view=specific&bmp=45&minmeasure=4 Accessed 10-18-2013 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Geotextlle maks not only protect ground surfaces from wind and stormwater erosion but also allow vegetative growth (Source: Rolanka International, 2000) Description Geotextiles are porous fabrics also known as filter fabrics, road rugs, synthetic fabrics, construction fabrics, or simply fabrics. Geotextiles are manufactured by weaving or bonding fibers that are often made of synthetic materials such as polypropylene, polyester, polyethylene, nylon, polyvinyl chloride, glass, and various mixtures of these materials. As a synthetic construction material, geotextiles are used for a variety of purposes such as separators, reinforcement, filtration and drainage, and erosion control (USEPA, 1992). Some geotextiles are made of biodegradable materials such as mulch matting and netting. Mulch mattings are jute or other wood fibers that have been formed into sheets and are more stable than normal mulch. Netting is typically made from jute, wood fiber, plastic, paper, or cotton and can be used to hold the mulching and matting to the ground. Netting can also be used alone to stabilize soils while the plants are growing; however, it does not retain moisture or temperature well. Mulch binders (either asphalt or synthetic) are sometimes used instead of netting to hold loose mulches together. Geotextiles can aid in plant growth by holding seeds, fertilizers, and topsoil in place. Fabrics come in a wide variety to match the specific needs of the site and are relatively inexpensive for certain applications. Applicability Geotextiles can be used in various ways for erosion control on construction sites. Use them as matting to stabilize the flow of channels or swales or to protect seedlings on recently planted slopes until they become established. Use matting on tidal or stream banks, where moving water is likely to wash out new plantings. Geotextiles can be used to protect exposed soils immediately and temporarily, such as when active piles of soil are left overnight. They can also be used as a separator between riprap and soil, which prevents the soil from being eroded from beneath the riprap and maintains the riprap's base. Siting and Design Considerations There are many types of geotextiles available; therefore, the selected fabric should match its purpose. To ensure the effective use of geotextiles, keep firm, continuous contact between the materials and the soil. If there is no contact, the material will not hold the soil, and erosion will occur underneath the material. Limitations Geotextiles (primarily synthetic types) have the potential disadvantage of disintegrating when exposed to light. Consider this before installing them. Some geotextiles might increase runoff or blow away if not firmly anchored. Depending on the type of material used, geotextiles might need to be disposed of in a landfill, making them less desirable than vegetative stabilization. If the geotextile fabric is not properly selected, designed, or installed, its effectiveness may be reduced drastically. Maintenance Considerations Inspect geotextiles regularly to determine if cracks, tears, or breaches have formed in the fabric; if so, repair or replace the fabric immediately. It is necessary to maintain contact between the ground and the geotextile at all times. Remove trapped sediment after each storm event. Effectiveness Geotextiles' effectiveness depends on the strength of the fabric and proper installation. For example, when protecting a cut slope with a geotextile, it is important to properly anchor the fabric. This will ensure that it will not be undermined by a storm event. Cost Considerations Costs for geotextiles range from $0.50 to $10.00 per square yard, depending on the type chosen (SWRCP, 1991). References Rolanka International. 2000. Bio -D Mesh. [http://www.rolanka.com I EXIT oisolaimer ]. Accessed November 10, 2005. SWRPC (Southeast Wisconsin Regional Planning Commission). 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures. Technical Report No. 31. Southeast Wisconsin Regional Planning Commission, Waukesha, WI. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Washington, DC. Gradient Terraces EPA NPDES Fact Sheet http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factshect_results&view=specific&bmp=46&minmeasure=4 Accessed 10-18-2013 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Terraces can be Incorporated Into the grading plan to shorten the length of the slope and reduce the velocity of stem water flows (Source: Boase eL al, 2001}) Description Gradient terraces are earthen embankments or ridge and channel systems that reduce erosion by slowing, collecting and redistributing surface runoff to stable outlets that increase the distance of overland runoff flow. Terraces hold moisture and help trap sediments, minimizing sediment - laden runoff. Applicability Gradient terraces perform most effectively in barren areas with an existing or expected water erosion problem. Gradient terraces are effective only if suitable runoff outlets are available. Do not build terraces on slopes comprised of rocky or sandy soil because these soil types may not adequately redirect flows. Siting and Design Considerations Gradient terraces should be properly spaced and constructed with an adequate grade, and they should have adequate and appropriate outlets toward areas not susceptible to erosion or other damage. Acceptable outlets include grassed waterways, vegetated areas, or tile outlets. General specifications require that: Whenever possible, use vegetative cover in the outlet. At the junction of the terrace and the outlet, make the terrace's water surface design - elevation no lower than the outlet's water surface design -elevation when both are performing at design flow. When constructing the terrace system, follow dust control procedures. When constructing the terrace system, follow proper vegetation/stabilization practices. Limitations Gradient terraces are inappropriate for use on sandy or shallow soils, or on steep slopes. If too much water permeates a terrace system's soils, sloughing could occur, potentially increasing cut and fill costs. Maintenance Considerations Inspect the terraces after major storms and at least once annually to ensure that they are structurally sound and have not eroded. References Boaze, P., and B. Wiggins. Building a Major Highway in Mountainous East Tennessee: Environmental Impacts. Land and Water. July/August 2000: 20-23. USEPA (U.S. Environmental Protection Agency). 1992. Storm Water Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R- 92-006. U.S. Environmental Protection Agency, Office of Water, Washington, DC Mulching EPA NPDES Fact Sheet http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factshect_results&view=specific&bmp=41&minmeasure=4 Accessed 10-18-2013 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Grass mulching is applied to stabilize exposed sails and to reduce stormwater runoff velocity Description Mulching is an erosion control practice that uses materials such as grass, hay, wood chips, wood fibers, straw, or gravel to stabilize exposed or recently planted soil surfaces. Mulching is highly recommended and is most effective when used in conjunction with vegetation. In addition to stabilizing soils, mulching can reduce stormwater velocity and improve the infiltration of runoff. Mulching can also aid plant growth by holding seeds, fertilizers, and topsoil in place, preventing birds from eating seeds, retaining moisture, and insulating plant roots against extreme temperatures. Mulch matting is made from materials such as jute or other wood fibers that are formed into sheets and are more stable than loose mulch. Use jute and other wood fibers, plastic, paper, or cotton individually or combine them into mats to hold mulch to the ground. Use netting to stabilize soils while plants are growing; although, netting does not retain moisture or insulate against extreme temperatures. Mulch tackifiers made of asphalt or synthetic materials are sometimes used instead of netting to bind loose mulches. Applicability Mulching is often used in areas where vegetation cannot be established. Mulching can provide immediate and inexpensive erosion control. On steep slopes and critical areas, such as those near waterways, use mulch matting with netting or anchoring to hold it in place. Use mulches on seeded and planted areas where slopes are steeper than 2:1 or where sensitive seedlings require insulation from extreme temperatures or moisture retention. Siting and Design Considerations When possible, natural mulches should be used for erosion control and plant material establishment. Suggested materials include loose straw, netting, wood cellulose, or agricultural silage. All materials should be free of seed. Anchor loose hay or straw by applying tackifier, stapling netting over the top, or crimping with a mulch crimping tool. Materials that are heavy enough to stay in place (for example, gravel or bark or wood chips on flat slopes) do not need anchoring. Other examples of organic mulches include hydraulic mulch products with 100 percent post -consumer paper content, yard trimming composts, and wood mulch from recycled stumps and tree parts. Use inorganic mulches such as pea gravel or crushed granite in unvegetated areas. Mulches may or may not require a binder, netting, or tacking. To ensure effective use of netting and matting material, keep firm, continuous contact between the materials and the soil. If there is no contact, the material will not hold the soil and erosion will occur underneath the material. Grading is not necessary before mulching. Use biodegradable netting, if possible. There must be adequate coverage to prevent erosion, washout, and poor plant establishment. If an appropriate tacking agent is not applied, or is applied in insufficient amounts, mulch will be lost to wind and runoff. The channel grade and liner must be appropriate for the amount of runoff, or the channel bottom will erode. Also, apply hydromulch in spring, summer, or fall to prevent deterioration of mulch before plants can become established. Table 1 presents guidelines for installing mulches. Table 1. Typical mulching materials and application rates Material Organic Mulches Straw Wood fiber or wood cellulose Wood chips Rate per acre Requirements Notes 1 - 2 tons /z - 1 ton 5-6tons Dry, unchopped, Spread by hand or unweathered; avoid machine; must be weeds tacked or tied down Air dry; add fertilizer N, 12 lb/ton Air dry, shredded, or Bark 35 yd hammermilled, or chips Use with hydroseeder; may be used to tack straw; do not use in hot, dry weather Apply with blower, chip handler, or by hand; not for fine turf areas Apply with mulch blower, chip handler, or by hand; do not use asphalt Limitations Mulching, matting, and netting might delay seed germination because the cover changes soil surface temperatures. The mulches themselves are subject to erosion and may be washed away in a large storm. Maintenance is necessary to ensure that mulches provide effective erosion control. Maintenance Considerations Anchor mulches to resist wind displacement. When protection is no longer needed, remove netting and compost it or dispose of it in a landfill. Inspect mulched areas frequently to identify areas where it has loosened or been removed, especially after rainstorms. Reseed these areas, if necessary, and replace the mulch cover immediately. Apply mulch binders at rates recommended by the manufacturer. If washout, breakage, or erosion occurs, repair, reseed and remulch surfaces, and install new netting. Continue inspections until vegetation is firmly established. Effectiveness Mulching effectiveness varies according to the type of mulch used. Soil loss reduction for different mulches ranges from 53 to 99.8 percent. Water velocity reductions range from 24 to 78 percent. Table 2 shows soil loss and water velocity reductions for different mulch treatments. Table 2. Measured reductions in soil loss for different mulch treatments (Source: Harding, 1990, as cited in USEPA, 1993) Mulch characteristics 100% wheat straw/top net 100% wheat straw/two nets 70% wheat straw/30% coconut fiber 70% wheat straw/30% coconut fiber 100% coconut fiber Soil loss reduction (%) 97.5 98.6 98.7 99.5 98.4 Water velocity reduction (% relative to bare soil) 73 56 71 78 77 tack Nets and mats Heavy, uniform; Jute net Cover area Woven of single jute Withstands water yarn; use with flow organic mulch Excelsior (wood Cover area fiber) mat Apply with Continuous fibers of compressed air Fiberglass roving /2 - 1 ton drawn glass bound ejector; tack with together with a non- emulsified asphalt at toxic agent a rate of 25 - 35 gal/1000 ft2 Limitations Mulching, matting, and netting might delay seed germination because the cover changes soil surface temperatures. The mulches themselves are subject to erosion and may be washed away in a large storm. Maintenance is necessary to ensure that mulches provide effective erosion control. Maintenance Considerations Anchor mulches to resist wind displacement. When protection is no longer needed, remove netting and compost it or dispose of it in a landfill. Inspect mulched areas frequently to identify areas where it has loosened or been removed, especially after rainstorms. Reseed these areas, if necessary, and replace the mulch cover immediately. Apply mulch binders at rates recommended by the manufacturer. If washout, breakage, or erosion occurs, repair, reseed and remulch surfaces, and install new netting. Continue inspections until vegetation is firmly established. Effectiveness Mulching effectiveness varies according to the type of mulch used. Soil loss reduction for different mulches ranges from 53 to 99.8 percent. Water velocity reductions range from 24 to 78 percent. Table 2 shows soil loss and water velocity reductions for different mulch treatments. Table 2. Measured reductions in soil loss for different mulch treatments (Source: Harding, 1990, as cited in USEPA, 1993) Mulch characteristics 100% wheat straw/top net 100% wheat straw/two nets 70% wheat straw/30% coconut fiber 70% wheat straw/30% coconut fiber 100% coconut fiber Soil loss reduction (%) 97.5 98.6 98.7 99.5 98.4 Water velocity reduction (% relative to bare soil) 73 56 71 78 77 Nylon monofilament/two nets 99.8 74 Nylon 53.0 24 monofilament/rigid/bonded Vinyl 89.6 32 monofilament/flexible/bonded Curled wood fibers/top net 90.4 47 Curled wood fibers/two nets 93.5 59 Antiwash nettingoute) 91.8 59 Interwoven paper and thread 93.0 53 Uncrimped wheat straw, 84.0 45 2,242 kg/ha Uncrimped wheat straw, 89.3 59 4,484 kg/ha Cost Considerations Costs of seed and mulch average $1,500 per acre and range from $800 to $3,500 per acre (USEPA, 1993). References Harding, M.V. 1990. Erosion Control Effectiveness: Comparative Studies of Alternative Mulching Techniques. Environmental Restoration, pp. 149-156, as cited in USEPA. 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Riprap EPA NPDES Fact Sheet Date accessed 10-18-2013 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=39&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Riprap can be used to stabilize drainageways and outlets to prevent erosion Description Riprap is a layer of large stones used to protect soil from erosion in areas of concentrated runoff. Riprap can also be used on slopes that are unstable because of seepage problems. Applicability Use riprap to stabilize cut -and -fill slopes; channel side slopes and bottoms; inlets and outlets for culverts, bridges, slope drains, grade stabilization structures, and storm drains; and streambanks and grades. Siting and Design Considerations Riprap can be unstable on very steep slopes, especially when rounded rock is used. For slopes steeper than 2:1, consider using materials other than riprap for erosion protection. Consider the following design recommendations for riprap installation (Smolen et al., 1988): Gradation. Use a well -graded mixture of rock sizes instead of one uniform size. Quality of stone. Use riprap material that is durable so that freeze and thaw cycles do not decompose it in a short time; most igneous stones, such as granite, have suitable durability. Riprap depth. Make the riprap layer at least two times as thick as the maximum stone diameter. Filter material. Apply a filter material --usually a synthetic cloth or a layer of gravel -- before applying the riprap. This prevents the underlying soil from moving through the riprap. Riprap Limits. Place riprap so it extends to the maximum flow depth, or to a point where vegetation will be satisfactory to control erosion. Curves. Ensure that riprap extends to five times the bottom width upstream and downstream of the beginning and ending of the curve and the entire curved section. Riprap Size. The size of the riprap material depends on the shear stress of the flows the riprap will be subject to, but it ranges from an average size of 2 inches to 24 inches in diameter (Idaho Department of Environmental Quality, no date). Wire Riprap Enclosures. Consider using chain link fencing or wire mesh to secure riprap installations, especially on steep slopes or in high flow areas. Limitations The steepness of the slope limits the applicability of riprap, because slopes greater than 2:1 can cause riprap loss due to erosion and sliding. If used improperly, riprap can actually increase erosion. In addition, riprap can be more expensive than other stabilization options. Maintenance Considerations Inspect riprap areas annually and after major storms. If riprap has been damaged, repair it promptly to prevent a progressive failure. If repairs are needed repeatedly at a location, evaluate the site to determine if the original design conditions have changed. Also, you might need to control weed and brush growth in some locations. Effectiveness When properly designed and installed, riprap can prevent erosion from the protected area. Cost Considerations The cost of riprap varies depending on location and the type of material selected. A cost of $35 to $50 per square yard of nongrouted riprap has been reported, while grouted riprap ranges from $45 to $60 per square yard (1993 dollars; Mayo et al., 1993). References FHWA (Federal Highway Administration). 1995. Best Management Practices for Erosion and Sediment Control. FHWA-SLP-94-005. Federal Highway Administration, Sterling, VA. Idaho Department of Environmental Quality. No date. Catalog of Stormwater BMPs for Cities and Counties: BMP #20 - Riprap Slope and Outlet Protection. http://www.deq.state.id.us/water/data_reports/storm_water/catalog/sec_2/bmps/5.pd£ Accessed May 10, 2006. Mayo, L., D. Lehman, L. Olinger, B. Donavan, and P. Mangarella. 1993. Urban BMP Cost and Effectiveness Summary Data for 6217(g) Guidance: Erosion and Sediment Control During Construction. Woodward -Clyde Consultants. MPCA (Minnesota Pollution Control Agency). 1998. Protecting Water Quality in Urban Areas. Minnesota Pollution Control Agency, Division of Water Quality, St. Paul, MN. Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources Land Quality Section, Raleigh, NC. SWRPC (Southeast Wisconsin Regional Planning Commission). 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures. Technical Report No. 31. Southeast Wisconsin Regional Planning Commission, Waukesha, WI. Seeding EPA NPDES Fact Sheet Date Accessed 10-24-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=42&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Hydroseeding is a popular technique for applying seeds, fertilizer, and chemical stabilizers in a single application (Source: Terra Firma Industries, 2000) Description Seeding is used to control runoff and erosion on disturbed areas by establishing perennial vegetative cover from seed. It reduces erosion and sediment loss and provides permanent stabilization. This practice is economical, adaptable to different site conditions, and allows selection of a variety of plant materials. Applicability Seeding is well-suited in areas where permanent, long-lived vegetative cover is the most practical or most effective method of stabilizing the soil. Use seeding on roughly graded areas that will not be regraded for at least a year. Vegetation controls erosion by protecting bare soil surfaces from displacement by raindrop impacts and by reducing the velocity and quantity of overland flow. Seeding's advantages over other means of establishing plants include lower initial costs and labor needs. Siting and Design Considerations Seed or plant permanent vegetation in areas I to 4 months after the final grade is achieved unless temporary stabilization measures are in place. Maximize successful plant establishment with planning; considering soil characteristics; selecting plant materials that are suitable for the site; preparing, liming, and fertilizing the seedbed adequately; planting timely; and maintaining regularly. Major factors that dictate the suitability of plants for a site include climate, soils, and topography. Prepare and amend the soil on a disturbed site to provide sufficient nutrients for seed germination and seedling growth. Loosen the soil surface enough for water infiltration and root penetration. If soils are too acidic, increase the pH to between 6.0 and 6.5 with liming or choose plants that are appropriate for the soil characteristics at your site. Protect seeds with mulch to retain moisture, regulate soil temperatures, and prevent erosion during seedling establishment. Limitations The effectiveness of seeding can be limited by high erosion during establishment, the need to reseed areas that fail to establish, limited seeding times, or unstable soil temperature and soil moisture content during germination and early growth. Seeding does not immediately stabilize soils; therefore, use temporary erosion and sediment control measures to prevent pollutants from disturbed areas from being transported off the site. Maintenance Considerations Maintenance for seeded areas will vary depending on the level of use expected. Use long-lived grass perennials that form a tight sod and are fine -leaved for areas that receive extensive use, such as homes, industrial parks, schools, churches, and recreational areas. Whenever possible, choose native species that are adapted to local weather and soil conditions to reduce water and fertilizer inputs and lower maintenance overall. In and areas, consider seeding with non -grass species that are adapted to drought conditions, called xeriscaping, to reduce the need for watering. Low -maintenance areas are mowed infrequently or not at all and do not receive lime or fertilizer regularly. Plants must be able to persist with minimal maintenance over long periods of time. Use grass and legume mixtures for these sites because legumes fix nitrogen from the atmosphere. Sites suitable for low -maintenance vegetation include steep slopes, stream or channel banks, some commercial properties, and "utility" turf areas such as road banks. Grasses should emerge within 4-28 days and legumes 5-28 days after seeding, with legumes following grasses. A successful stand has the following characteristics: Vigorous dark green or bluish green (not yellow) seedlings Uniform density, with nurse plants, legumes, and grasses well intermixed Green leaves that remain green throughout the summer --at least at the plant bases Inspect seeded areas for failure and, if needed, reseed and repair them as soon as possible. If a stand has inadequate cover, reevaluate the choice of plant materials and quantities of lime and fertilizer. Depending on the condition of the stand, repair by overseeding or reseeding after complete seedbed preparation. If timing is bad, overseed with rye grain or German millet to thicken the stand until a suitable time for seeding perennials. Consider seeding temporary, annual species if the season is not appropriate for permanent seeding. If vegetation fails to grow, test the soil to determine if low pH or nutrient imbalances are responsible. On a typical disturbed site, full plant establishment usually requires refertilization in the second growing season. Use soil tests to determine if more fertilizer needs to be added. Do not fertilize cool season grasses in late May through July. Grass that looks yellow might be nitrogen deficient. Do not use nitrogen fertilizer if the stand contains more than 20 percent legumes. Effectiveness Perennial vegetative cover from seeding has been shown to remove between 50 and 100 percent of total suspended solids from stormwater runoff, with an average removal of 90 percent (USEPA, 1993). Cost Considerations Seeding costs range from $200 to $1,000 per acre and average $400 per acre. Maintenance costs range from 15 to 25 percent of initial costs and average 20 percent (USEPA, 1993). References FHWA (Federal Highway Administration). 1995. Best Management Practices for Erosion and Sediment Control. FHWA-SLP-94-005. Federal Highway Administration, Sterling, VA. Smolen, M.D., D.W. Miller, L.C. Wyall, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. Terra Firma Industries. 2000. Hydroseeding. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-13-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Sodding EPA NPDES Fact Sheet Date accessed 10-24-13. http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=43&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Crass sad Is laid on exposed soli to stabilize the soil and to reduce the velocity of store water runoff (Source: Landscape USA, no date) Description Sodding is a permanent erosion control practice and involves laying a continuous cover of grass sod on exposed soils. Sodding can stabilize disturbed areas and reduce the velocity of stormwater runoff. Sodding can provide immediate vegetative cover for critical areas and stabilize areas that cannot be readily vegetated by seed. It also can stabilize channels or swales that convey concentrated flows and reduce flow velocities. Applicability Sodding is appropriate for any graded or cleared area that might erode, requiring immediate vegetative cover. Locations that are well-suited to sod stabilization include: Residential or commercial lawns and golf courses where prompt use and aesthetics are important Steeply sloped areas Waterways and channels carrying intermittent flow Areas around drop inlets that require stabilization Siting and Design Considerations Sodding eliminates the need for seeding and mulching. Sod can be laid during times of the year when seeded grasses are likely to fail. Water the sod frequently within the first few weeks of installation. Select a type of sod that is composed of plants adapted to the site conditions. Sod composition should reflect environmental conditions and the function of the area where it will be laid. Know the genetic origin of the sod, and make sure it is free of noxious weeds, diseases, and insects. Ensure that the sod is machine cut at a uniform soil thickness of 15 to 25 mm (not including top growth or thatch) at the time of establishment. If a soil test determines the need, prepare the soil and add lime and fertilizer. Lay the sod in strips perpendicular to the direction of waterflow and stagger it in a brick -like pattern. Staple the corners and middle of each strip firmly. Peg jute or plastic netting over the sod to protect against washout during establishment. In the area to be sodded, clear all trash, debris, roots, branches, stones and clods larger than 2 inches in diameter. Ensure that sod is harvested, delivered, and installed within a period of 36 hours. If it is not transplanted within this period, inspect and approve the sod before its installation. Limitations Compared to seed, sod is more expensive and more difficult to obtain, transport, and store. To ensure successful establishment, prepare the soil and provide adequate moisture before, during, and after installation. If sod is laid on poorly prepared soil or an unsuitable surface, the grass will die quickly because it is unable to root. After installation, inadequate irrigation can cause root dieback or cause the sod to dry out. Maintenance Considerations To maintain adequate moisture in the root zone and to prevent dormancy, water the sod, especially within the first few weeks of installation. When mowing, do not remove more than one-third of the shoot. Maintain grass height between 2 and 3 inches. After the first growing season, determine if additional fertilization or liming is needed. Permanent, fine turf areas require yearly maintenance fertilization. Fertilize warm -season grass in late spring to early summer; fertilize cool -season grass in late winter and again in early fall. Effectiveness Sod removes up to 99 percent of total suspended solids in runoff, but its sediment trapping efficiency is highly variable depending on hydrologic, hydraulic, vegetation, and sediment characteristics. Cost Considerations Average construction costs of sod average $0.20 per square foot and range from $0.10 to $1.10 per square foot; maintenance costs are approximately 5 percent of installation costs (USEPA, 1993). References FHWA (Federal Highway Administration). 1995. Best Management Practices for Erosion and Sediment Control. FHWA-SLP-94-005. Federal Highway Administration, Sterling, VA. Landscape USA. No date. Installing Sod for an Instant Lawn. [www.landscgpeusa.com/til2s/turfhtmlEXIT disclaimer ]. Accessed November 10, 2005. Smolen, M.D., D.W. Miller, L.C. Wyall, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-13-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Soil Retention EPA NPDES Fact Sheet Date accessed 10-24-13. http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=47&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control A permanent retaining wall prevents slope failure Description Soil retention measures are structures or practices that hold soil in place or keep it contained within a site boundary. They include grading or reshaping the ground to lessen steep slopes or shoring excavated areas with wood, concrete, or steel structures. Some soil -retaining measures are used only for erosion control, while others are also used to protect workers during excavation projects. Applicability Assess site conditions before breaking ground and, where possible, reduce steep slopes by grading. When sites have very steep slopes or loose, highly erodible soils that cause other methods, such as chemical or vegetative stabilization or regrading, to be ineffective, use reinforced soil -retaining structures. As much as possible, maintain the preconstruction drainage pattern. Siting and Design Considerations Examples of reinforced soil retaining structures include: Skeleton sheeting. An inexpensive soil bracing system that consists of construction grade lumber used to support the excavated face of a slope. This method requires the soil to be cohesive. Continuous sheeting. Involves using a material, such as face -steel, concrete, or wood, to cover the entire slope continuously, with struts and boards placed along the slope to support it. Permanent retaining walls. Walls of concrete masonry or wood that are left in place after construction is complete to provide continued support of the slope. The proper design of reinforced soil -retaining structures is crucial for erosion control and safety. To ensure safety of the retaining structure, have a qualified engineer design it --one who understands all the design considerations, such as the nature of the soil, location of the ground water table, and the expected loads. Ensure that hydraulic pressure does not build up behind the retaining structure and cause it to fail. Limitations To be effective, design soil -retention structures to handle expected loads. Heavy rains can damage or destroy these structures and result in sediment inputs to waterbodies. The structures must be properly installed and maintained to avoid failure. Maintenance Considerations Inspect soil -stabilization structures periodically, especially after rainstorms, to check for erosion, damage, or other signs of deterioration. Repair any damage to the actual slope or ditch, such as washouts or breakage, before reinstalling materials for the soil -stabilization structure. Effectiveness Soil -retention structures, if properly designed and installed, can effectively prevent erosion in areas with steep slopes and erodible soils. The potential for failure depends on the design, installation, and maintenance of the structures, and the likelihood of catastrophic events such as heavy rains, earthquakes, and landslides. Cost Considerations If planned appropriately, slope reduction can be accomplished during site development with minimal additional cost. Soil stabilization structures can be expensive because they require a professional engineer to develop a design (estimated to be 25 to 30 percent of construction costs [Ferguson et al., 1997]). Depending on the size of the proposed structure and the relief of the surrounding area, excavation and installation costs can be high. Capital costs include mobilization, grading, grooving, tracking and compacting fill, and installing the structures. Labor costs for regular inspection and repairs are also a consideration. References Fergusen, T., R. Gignac, M. Stoffan, A. Ibrahim, and J. Aldrich. 1997. Rouge River National Wet Weather Demonstration Project: Cost Estimating Guidelines Best Management Practices and Engineered Controls. Rouge River National Wet Weather Demonstration Project, Wayne County, MI. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Virginia Department of Conservation and Recreation. 1995. Virginia Erosion & Sediment Control Field Manual. 2nd ed. Virginia Department of Conservation and Recreation, Division of Soil and Water Conservation, Richmond, VA. Soil Roughening EPA NPDES Fact Sheet Date accessed 10-24-13. http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=44&minmeasure=4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Exposed soils can be temporarily stabilized by driving a tractor over the surface Description Soil roughening is a temporary erosion control practice often used in conjunction with grading. Soil roughening involves increasing the relief of a bare soil surface with horizontal grooves by either stair -stepping (running parallel to the contour of the land) or using construction equipment to track the surface. Slopes that are not fine graded and left in a roughened condition can also reduce erosion. Soil roughening reduces runoff velocity, increases infiltration, reduces erosion, traps sediment, and prepares the soil for seeding and planting by giving seed an opportunity to take hold and grow. Applicability Soil roughening is appropriate for all slopes, but works especially well on slopes greater than 3:1, on piles of excavated soil, and in areas with highly erodible soils. This technique is especially appropriate for soils that are frequently disturbed, because roughening is relatively easy. To slow erosion, roughen the soil as soon as possible after the vegetation has been removed from the slope or immediately after grading activities have ceased (temporarily or permanently). Use this practice in conjunction with seeding, planting, and temporary mulching to stabilize an area. A combination of surface roughening and vegetation is appropriate for steeper slopes and slopes that will be left bare for longer periods of time. Siting and Design Considerations Roughened slope surfaces help establish vegetation, improve infiltration, and decrease runoff velocity. A rough soil surface allows surface ponding that protects lime, fertilizer, and seed and decreases erosion potential. Grooves in the soil are cooler and provide more favorable moisture conditions than hard, smooth surfaces. These conditions promote seed germination and vegetative growth. Avoid excessive soil compacting, because this inhibits vegetation growth and causes higher runoff velocity. Limit roughening with tracked machinery to sandy soils that do not compact easily; also, avoid tracking on heavy clay soils, especially when wet. Seed roughened areas as quickly as possible, and follow proper dust control procedures. Depending on the type of slope and the available equipment, use different methods for roughening soil on a slope. These include stair -step grading, grooving, and tracking. When choosing a method, consider factors such as slope steepness, mowing requirements, whether the slope is formed by cutting or filling, and available equipment. Choose from the following methods for surface roughening: Cut slope roughening for areas that will not be mowed. Use stair -step grades or groove -cut slopes for gradients steeper than 3:1. Use stair -step grading on any erodible material that is soft enough to be ripped with a bulldozer. Also, it is well suited for slopes consisting of soft rock with some subsoil. Make the vertical cut distance less than the horizontal distance, and slope the horizontal portion of the step slightly toward the vertical wall. Keep individual vertical cuts less than 2 feet deep in soft materials and less than 3 feet deep in rocky materials. Grooving. This technique uses machinery to create a series of ridges and depressions that run across the slope along the contour. Make grooves using any appropriate implement that can be safely operated on the slope, such as disks, tillers, spring harrows, or the teeth on a front-end loader bucket. Make the grooves less than 3 inches deep and less than 15 inches apart. Fill slope roughening for areas that will not be mowed. Fill slopes with a gradient steeper than 3:1 should be placed in lifts less than 9 inches, and properly compact each lift. The face of the slope should consist of loose, uncompacted fill 4 to 6 inches deep. If necessary, roughen the face of the slopes by grooving the surface as described above. Do not blade or scrape the final slope face. Cuts, fills, and graded areas that will be mowed. Make mowed slopes no steeper than 3:1. Roughen these areas with shallow grooves less than 10 inches apart and deeper than 1 inch using normal tilling, disking, or harrowing equipment (a cultipacker- seeder can also be used). Excessive roughness is undesirable where mowing is planned. Roughening with tracked machinery. To avoid undue compaction of the soil surface, limit roughening with tracked machinery only to sandy soils. Operate tracked machinery perpendicularly to the slope to leave horizontal depressions in the soil. Tracking is generally not as effective as other roughening methods. Limitations Soil roughening is not appropriate for rocky slopes. Tracked machinery can excessively compact the soil. Typically, soil roughening is effective only for gentle or shallow depth rains. If roughening is washed away in a heavy storm, re -roughen the surface and reseed. Maintenance Considerations Inspect roughened areas after storms to see if re -roughening is needed. Regular inspection should indicate where additional erosion and sediment control measures are needed. If rills (small watercourses that have steep sides and are usually only a few inches deep) appear, fill, regrade, and reseed them immediately. Use proper dust control methods. Effectiveness Soil roughening provides moderate erosion protection for bare soils while vegetative cover is being established. It is inexpensive and simple for short-term erosion control when used with other erosion and sediment controls. Cost Considerations Soil roughening requires minimal materials but requires using heavy equipment. References Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-13-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Temporary Slope Drain EPA NPDES Fact Sheet Data accessed 10-24-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=48&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Description A temporary slope drain is a flexible conduit for stormwater that extends the length of a disturbed slope to divert the flow and serve as a temporary outlet. Temporary slope drains, also called pipe slope drains, convey runoff without causing erosion on or at the bottom of the slope. This practice is a temporary measure, typically used for less than 2 years. It is used during grading operations until permanent drainage structures are installed and until slopes are permanently stabilized. Applicability Temporary slope drains can be used on most disturbed slopes to eliminate gully erosion from concentrated flows. Siting and Design Considerations A temporary slope drain used with a diversion conveys stormwater flows and reduces erosion until permanent drainage structures are installed. The following are design recommendations for temporary slope drains: The drain pipe should consist of heavy-duty material manufactured for the purpose and have grommets for anchoring at a spacing of 10 feet or less. Observe the minimum slope drain diameters for varying drainage areas. The entrance to the pipe should consist of a standard flared section of corrugated metal. The corrugated metal pipe should have watertight joints at the ends. The rest of the pipe is typically corrugated plastic or flexible tubing. For flatter, shorter slopes, a polyethylene -lined channel is sometimes used. Make sure the height of the diversion at the pipe is the diameter of the pipe plus 0.5 foot. Place the outlet at a reinforced or erosion -resistant location. Limitations The area drained by a temporary slope drain should not exceed 5 acres. Physical obstructions substantially reduce the drain's effectiveness. Other concerns are failures from overtopping because of inadequate pipe inlet capacity, and reduced diversion channel capacity and ridge height. Prawa un Camaga�d JAeial DIt� Minn O011ar iorpUr S&OX �AiL- SIZE OF SLOPE DRAIN! Dmkmr Area PWO SQe {,acres} {in�as} X15 �P' oob 24' Drains can be Installed along a steep exposed slope to divert runoff and prevent erosion (Source: Urban Drainage and Flood Control District, 7999) Maintenance Considerations Inspect the slope drain after each rainfall to determine whether capacity was exceeded or blockages occurred. Make needed repairs promptly. Reroute construction equipment and vehicular traffic around slope drains to avoid damage. References FHWA (Federal Highway Administration). 1995. Best Management Practices for Erosion and Sediment Control. FHWA-SLP-94-005. Federal Highway Administration, Sterling, VA. MPCA (Minnesota Pollution Control Agency). 1998. Protecting Water Quality in Urban Areas. Minnesota Pollution Control Agency, Division of Water Quality, St. Paul, MN. Smolen, M.D., D.W. Miller, L.C. Wyall, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. Urban Drainage and Flood Control District. 1999. Urban Storm Drainage: Criteria Manual. Denver, CO. Temporary Stream Crossings EPA NPDES Fact Sheet Data accessed 10-24-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=49&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Erosion Control Description A temporary steam crossing is used to provide a safe, stable way for construction vehicle traffic to cross a watercourse. Temporary stream crossings provide streambank stabilization, reduce the risk of damage to the streambed or channel, and minimize sediment loading from construction traffic. The crossing might be a bridge, a culvert, or a ford. Applicability Temporary stream crossings are appropriate where heavy construction equipment must be moved from one side of a stream channel to the other. They can also be used where lighter construction vehicles will cross the stream repeatedly during construction. A bridge or culvert is the best choice for most temporary stream crossings because each can support heavy loads. The materials used to construct most bridges and culverts can be salvaged after they are removed. A ford is a shallow area in a stream that can be crossed safely. Fords are appropriate in steep areas where flash flooding might occur and where normal flow is shallow or intermittent across a wide channel. Fords should be used only where stream crossings are expected to be infrequent. Siting and Design Considerations Because of the potential for stream degradation, flooding, and safety hazards, avoid stream crossings whenever possible. Consider alternative routes to accessing a site before planning to erect a temporary stream crossing. If a stream crossing is necessary, select an area where the potential for erosion is low. If possible, select the stream crossing structure during a dry period to reduce sediment transport into the stream. If over -stream bridges are needed, construct them only under the supervision and approval of a qualified engineer. Gravel deck := - 400 mm s4cb- N! X94 H.WL. s � FxusLng grade bi�iare mmiructlon Mbdulm type Timber dec* with =w lieu Campactad gravel �1,540 - 2,400 m* m P.w. L. normally cedar Debris clearance subjeo to cngineoring Judgment Properly installed stream crossings can prevent destruction of stream habitat (Source: British Columbia Ministry of Forests, no date) When constructing a culvert, use filter cloth to cover the streambed and streambanks to reduce settlement and make the culvert structure more stable. The filter cloth should extend at least 6 inches and no more than 1 foot beyond the end of the culvert and bedding material. The culvert piping should not exceed 40 feet in length and should be of sufficient diameter to allow flow to pass completely during peak flow periods. Cover the culvert pipes with at least 1 foot of aggregate. If multiple culverts are used, separate the pipes with at least 1 foot of aggregate. Construct fords of stabilizing material such as large rocks. Limitations Bridges can be a safety hazard if not properly designed and constructed. Bridges might also be costly in terms of repairs and lost construction time if they are washed out or collapse (Smolen et al., 1988). Construction and removing culverts usually disturb the surrounding area, and erosion and downstream soil movement often occur. Culverts can create obstructions to flow in a stream and get in the way of migrating fish. Depending on their size, culverts can be blocked by large debris in a stream and are vulnerable to frequent washout. The approaches to fords are likely to erode. In addition, excavating the streambed and approach to lay riprap or other stabilization material causes major stream disturbance. Mud and other debris are transported directly into the stream unless the crossing is used only during periods of low flow. Take care to obtain all necessary permits for work in and around streams. Review local, state, and federal regulations before starting any stream -related work. Maintenance Considerations Inspect temporary stream crossings at least once a week and after all significant rainfall events. If any structural damage to a bridge or culvert is reported, stop using the structure until it is repaired. Repair streambank erosion immediately. Inspect fords closely after major storm events to make sure stabilization materials remain in place. If material has moved downstream during periods of peak flow, replace the lost material immediately. Effectiveness The effectiveness of a temporary stream crossing depends on the applicability of the crossing type, proper design and installation, and long-term maintenance needs. Cost Considerations Implementation costs for a temporary stream crossing depend on the site needs, crossing type, maintenance needs, and other site-specific factors. Typically, temporary bridges are more expensive to design and construct than culverts. Bridges also have higher maintenance and repair costs if they fail. References British Columbia Ministry of Forests. No date. Forest Practices Code Stream Crossing for Fish Streams Guidebook. [http://www.for.gov.bc.ca/tasb/legsregs/fpc/FPCGUIDE/Guidetoc.htm IEXIT disclaimer Accessed December 2012. Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources Land Quality Section, Raleigh, NC. VDCR (Virginia Department of Conservation and Recreation). 1995. Virginia Erosion & Sediment Control Field Manual. 2nd ed. Virginia Department of Conservation and Recreation, Division of Soil and Water Conservation, Richmond, VA. Runoff Control Appendix J - Intrinsic GSP Specifications Check Dams EPA NPDES Fact Sheet Date Accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=36&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Runoff Control Check dams are used to reduce the energy of stormwater to prevent erosion Description Check dams are relatively small, temporary structures constructed across a swale or channel. They are used to slow the velocity of concentrated water flows, a practice that helps reduce erosion. As stormwater runoff flows through the structure, the check dam catches sediment from the channel itself or from the contributing drainage area. However, check dams should not be used as a substitute for other sediment -trapping and erosion -control measures. Check dams are typically constructed out of gravel, rock, sandbags, logs or treated lumber, or straw bales. They are most effective when used with other stormwater, erosion, and sediment -control measures. Applicability Check dams are temporary measures used in swales or channels where it is impractical to implement other flow -control practices (such as lining the channel) (USEPA, 1993). Check dams are effective in small channels with a contributing drainage area of two to 10 acres. Multiple check dams, spaced at appropriate intervals, can be effective. Dams used in a series should be spaced so that the base of the upstream dam is at the same elevation as the top of the next downstream dam (VDCR, 1995). Siting and Design Considerations Check dams can be made of a variety of materials. They are most commonly made of rock, logs, or sandbags. When using rock, the material diameter should be two to 15 -inches. Logs should have a diameter of six to eight -inches. Regardless of the material used, build the check dam carefully to ensure its effectiveness. That is, do not simply dump the material into the channel. That would be inappropriate, and it might actually increase erosion. A check dam should not be more than three -feet high, and the center of the dam should be at least six -inches lower than its edges. This design creates a weir effect that helps to channel flows away from the banks and prevent further erosion. Dams can be made more stable by implanting the material approximately six -inches into the sides and bottom of the channel (VDCR, 1995). When installing a series of check dams in a channel, install outlet stabilization measures below the final dam in the series. Because this area is likely to be vulnerable to further erosion, the use of other stabilization measures like riprap or geotextile lining is highly recommended. Limitations Do not build check dams in live, flowing streams unless approved by an appropriate regulatory agency (USEPA, 1992; VDCR, 1995). The primary function of check dams is to slow runoff in a channel. Do not use them as a standalone substitute for other sediment -trapping devices. Also, fallen leaves can clog check dams, so in the fall it may be necessary to increase inspections and maintenance. Maintenance Considerations Inspect check dams after each storm event to ensure their structural integrity. The center of a check dam should always be lower than its edges. Additional stone may have to be added to maintain the correct height. During inspection, remove large debris, trash, and leaves. When the sediment has reached a height of approximately one-half the original height of the dam (measured at the center), remove accumulated sediment from the upstream side of the dam. When check dams are removed, care must be taken to remove all dam materials to ensure proper flow within the channel. If erosion or heavy flows cause the edges of a dam to fall to a height equal to or below the height of the center, repair it immediately. In addition, before removing a check dam, remove all accumulated sediment. Remove a check dam only after the contributing drainage area has been completely stabilized. Use permanent vegetation to stabilize the area from which the dam material is removed. Effectiveness Field experience has shown that rock check dams are more effective than silt fences or straw bales to stabilize wet -weather ditches (VDCR, 1995). For long channels, check dams are most effective when used in a series, creating multiple barriers to sediment -laden runoff. Cost Considerations The cost of check dams varies according to the material they are made of and the width of the channel to be dammed. EPA (1992) estimated that check dams constructed of rock cost about $100 per dam, although Brown and Schueler (CWP, 1997) estimated that rock check dams cost approximately $62 per installation, including the cost for filter fabric bedding. Logs and sandbags may be less expensive alternatives to install, but their use may result in higher maintenance costs. References Brown and Schueler, 1997. The Economics of Stormwater BMPs in the Mid -Atlantic Region. Prepared for the Chesapeake Research Consortium. Edgewater, MD by the Center for Watershed protection, Ellicott City, MD. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. VDCR (Virginia Department of Conservation and Recreation). 1995. Virginia Erosion & Sediment Control Field Manual. 2nd ed. Virginia Department of Conservation and Recreation, Division of Soil and Water Conservation, Richmond, VA. Washington State Department of Ecology. 2005. 2005 Stormwater Management Manual for Western Washington: Volume II -- Construction Stormwater Pollution Prevention Stormwater Management Manual for the Puget Sound Basin. Technical Manual. Washington State Department of Ecology, Olympia, WA. Grass -Lined Channels EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=38&minmeasure=4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Runoff Control A grass -lined channel can be used to filter and convey runoff Description A grass -lined channel conveys stormwater runoff through a stable conduit. Vegetation lining the channel slows down concentrated runoff. Because grassed channels are not usually designed to control peak runoff loads by themselves, they are often used with other BMPs, such as subsurface drains and riprap stabilization. Where moderately steep slopes require drainage, grassed channels can include excavated depressions or check dams to enhance runoff storage, decrease flow rates, and improve pollutant removal. Peak discharges can be reduced by temporarily holding them in the channel. Pollutants can be removed from stormwater by filtration through vegetation, by deposition, or in some cases by infiltration of soluble nutrients into the soil. The degree of pollutant removal in a channel depends on how long the water stays in the channel and the amount of contact with vegetation and the soil surface. Local conditions affect the removal efficiency. Applicability The first choice of lining should be grass or sod because this reduces runoff velocity and provides water quality benefits through filtration and infiltration. If the velocity in the channel would erode the grass or sod, riprap, concrete, or gabions can be used (USEPA, 2004). Geotextile materials can be used in conjunction with either grass or riprap linings to provide additional protection at the soil -lining interface. Use grassed channels in areas where erosion - resistant conveyances are needed, including areas with highly erodible soils and moderately steep slopes (though less than 5 percent). Install them only where space is available for a relatively large cross section. Grassed channels have a limited ability to control runoff from large storms, so do not use them in areas where flow rates exceed 5 feet per second. Siting and Design Considerations Site grass -lined channels in accordance with the natural drainage system. They should not cross ridges. The channel design should not have sharp curves or significant changes in slope. The channel should not receive direct sedimentation from disturbed areas and should be sited only on the perimeter of a construction site to convey relatively clean stormwater runoff. To reduce sediment loads, separate channels from disturbed areas by using a vegetated buffer or another BMP. Basic design recommendations for grassed channels include the following: Construct and vegetate the channel before grading and paving activities begin. Make sure design velocities are less than 5 feet per second. Consider using geotextiles to stabilize vegetation until it is fully established. Consider covering the bare soil with sod, mulches with netting, or geotextiles to provide reinforced stormwater conveyance immediately. Use triangular channels with low velocities and small quantities of runoff; use parabolic grass channels for larger flows and where space is available; use trapezoidal channels with large, low-velocity flows (low slope). Install outlet stabilization structures if the runoff volume or velocity might exceed the capacity of the receiving area. Slope the sides of the channel less than 2:1; slope triangular channels along roads 2:1 or less for safety. Remove all trees, brushes, stumps, and other debris during construction. Effectiveness Grass -lined channels can effectively transport stormwater from construction areas if they are designed for expected flow rates and velocities and if they do not receive sediment directly from disturbed areas. Limitations If grassed channels are not properly installed, they can change the natural flow of surface water and adversely affect downstream waters. And if the design capacity is exceeded by a large storm event, the vegetation might not be adequate to prevent erosion and the channel might be destroyed. Clogging with sediment and debris reduces the effectiveness of grass -lined channels for stormwater conveyance. Maintenance Considerations The maintenance requirements for grass channels are relatively minimal. While vegetation is being established, inspect the channels after every rainfall. After vegetation is established, mow it, remove litter, and perform spot vegetation repair. The most important objective in grassed channel maintenance is to maintain a dense and vigorous growth of turf. Periodically clean the vegetation and soil buildup in curb cuts so that water flow into the channel is unobstructed. During the growing season, cut the channel grass no shorter than the level of the design flow. Cost Considerations Costs of grassed channels range according to depth. The cost of a 1.5 -foot -deep grassed channel with 3:1 side slopes and a 2 -foot -wide channel bottom is estimated to cost between $202 and $625 per 100 feet of channel length. The cost of a 3 -foot -deep grassed channel with 3:1 side slope adn a 2 -foot -wide bottom is expected to cost between $397 and $1,198 for 100 feet of channel (SEWRPC, 1991). Grassed channels can be left in place permanently after the construction site is stabilized to contribute to long-term stormwater management. The channels, in combination with other practices that detain, filter, and infiltrate runoff, can substantially reduce the size of permanent detention facilities like stormwater ponds and wetlands, thereby reducing the overall cost of stormwater management. References FHWA (Federal Highway Administration). 1995. Best Management Practices for Erosion and Sediment Control. FHWA-SLP-94-005. Federal Highway Administration, Sterling, VA. MPCA (Minnesota Pollution Control Agency). 1998. Protecting Water Quality in Urban Areas. Minnesota Pollution Control Agency, Division of Water Quality, St. Paul, MN. Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. SEWRPC (Southeast Wisconsin Regional Planning Commission). 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures. Technical Report No. 31. Southeast Wisconsin Regional Planning Commission, Waukesha, WI. USEPA (U.S. Environmental Protection Agency). 2004. Development Document for Final Action for Effluent Guidelines and Standards for the Construction and Development Category. EPA -821-13-04-001. Washington, DC. Permanent Slope Diversions EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=33&minmeasure=4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Runoff Control Description Permanent slope diversions are designed to transport runoff down a slope in a manner that minimizes the potential for erosion. Diversions can be constructed by creating channels laterally across slopes to intercept the down-slope flow of runoff. The channels have a supporting earthen ridge on the bottom sides to reduce slope length, collect stormwater runoff, and deflect the runoff to outlets that convey it without causing erosion. Applicability Diversions should be considered for use on slopes where uncontrolled runoff might cause property damage due to erosion or resulting sedimentation. They can also be used to promote the growth of vegetation by redirecting flows while the vegetation is becoming established. Siting and Design Considerations A properly designed earthen ridge typically has side slopes no steeper than 2:1, a width at the design water elevation of at least 4 feet, a minimum freeboard of 0.3 foot, and a 10 percent settlement factor included in the design (reference?). A stormwater conveyance channel can be vegetated or hardened (e.g., with rock or concrete). Both types should be sufficient in shape and size to carry stormwater runoff away from developing areas without any erosion damage. Paved flumes are not recommended unless very high flows with excessive erosive power are expected because faster runoff might exacerbate erosion at the flume's outfall. Paved flumes also prevent surface runoff from infiltrating, which can cause increased volumes and erosive forces of the runoff that leaves the site. Adequate outfall protection should be provided to prevent damage from the discharge of high -velocity flows. Where possible, vegetated channels should be used to minimize flow velocity and to enhance pollutant removal. Riprap, gabions, or turf reinforcement mats can provide additional channel stabilization. The following are general specifications required for channel construction: Remove all obstructions and unsuitable material, such as trees, roots, brush, and stumps, and any excess soil from the channel area and dispose of them properly. Make sure the channel meets grade and cross section specifications, and compact any fill used to ensure equal settlement. Parabolic and triangular, grass -lined channels should not have a top width of more than 30 feet. Trapezoidal, grass -lined channels may not have a bottom width of more than 15 feet unless there are multiple or divided waterways, they have a riprap center, or other methods of controlling the meandering of low flows are provided. If grass -lined channels have a base flow, provide a stone center or subsurface drain or another method for managing the base flow. ,icy * _2' i o srnam +9o�v na�i�,r,�n 2.1 side �_ _ AdOM _ __ �; o rug'• _ � love ill��I—��ti=lJf�l it JIM II1=fffll=iR1iz—v1fI� I I W11==lisrr—_r,�ir air Site planners incorporate diversions into the overall grading plan to direct clean runoff away from exposed areas All channels must have outlets that are protected from erosion. Locate structurally lined aprons or other appropriate energy -dissipating devices at channel outlets to slow stormwater flows and prevent scouring at stormwater outlets, protect the outlet structure, and minimize the erosion potential downstream. Construction specifications for outlet protection practices require the following: No bends occur in the horizontal alignment. There is no slope along the length of the apron, and the invert elevations are equal at the receiving channel and the apron's downstream end. No overfall at the end of the apron is allowed. If a pipe discharges into a well-defined channel, the channel's side slopes may not be steeper than 2:1. The apron is lined with riprap, grouted riprap, concrete, or gabion baskets; all riprap conforms to standards and specifications; and the median -sized stone for riprap is specified in the plan. Filter cloth, conforming to standards and specifications, must be placed between riprap and the underlying soil to prevent any soil movement through the riprap. All grout for grouted riprap must be one part Portland cement for every three parts sand, mixed thoroughly with water. Once stones are in place, the spaces between them are to be filled with grout to a minimum depth of 6 inches, with the deeper portions choked with fine material. All concrete aprons must be installed as specified in the plan. The end of the paved channel in a paved channel outlet must be smoothly joined with the receiving channel section, with no overfall at the end of the paved section. Limitations Immediately after constructing a vegetated ridge and channel, seed and mulch them along with any disturbed areas that drain into the diversion. To prevent soil from moving into the diversion, sediment -trapping measures must remain in place in case the upslope area is not stabilized. Remove all obstructions and unsuitable material, such as trees, brush, and stumps, from the channel area and dispose of them so the diversion can function properly. The channel must meet grade and cross section specifications. Make sure any fill used is free from excessive organic debris, rocks, or other unsuitable material. Compact the fill to ensure equal settlement. Permanently stabilize disturbed areas according to applicable local standards and specifications. Stabilize the area around the channel that is disturbed by channel construction so that it is not subject to erosion similar to that of the slope the channel is built to protect. Maintenance Considerations Inspect diversions after every rainfall and at least once every 2 weeks before final stabilization. Clear channels of sediment, make repairs when necessary, and reseed seeded areas if a vegetative cover is not established. Costs Costs of slope drains vary based on pipe (material) selection, length, and the outlet protection that is used. Supplied and installed costs (not inlcuding trenching) for corregated steel pipe ranges from less than $20 per linear foot for 12" pipe to more than $50 per linaer foot for 30" pipe and from less than $25 per linear foot to $130 per linear foot (also supplied and installed, exclusing trenching) for PVC pipe (CASQA Handbook) References CASQA, 2003.California Constrcution BMP HandbookSection 3; EC -11 -Slope Drain Fact Sheet. http://www.cabmphandbooks.org/Construction.asp Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1992a. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992b. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-006. U.S. Environmental Protection Agency, Office of Water, Washington, DC. VDCR (Virginia Department of Conservation and Recreation). 1995. Virginia Erosion & Sediment Control Field Manual. 2nd ed. Virginia Department of Conservation and Recreation, Division of Soil and Water Conservation, Richmond, VA. Temporary Diversion Dikes EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=53&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Runoff Control Diversion dikes can be used to contain stormwater onske Description An earthen perimeter control usually consists of a dike or a combination dike and channel constructed along the perimeter of and within the disturbed part of a site. An earthen perimeter control is a ridge of compacted soil, often accompanied by a ditch or swale with a vegetated lining, at the top or base of a sloping disturbed area. Depending on its location and the topography of the landscape, an earthen perimeter control can achieve one of two goals. When on the upslope side of a site, earthen perimeter controls help to prevent surface runoff from entering a disturbed construction site. An earthen structure located upslope can improve working conditions on a construction site. It can prevent an increase in the total amount of sheet flow runoff traveling across the disturbed area and thereby lessen erosion on the site. Earthen perimeter control structures also can be located on the downslope side of a site. They divert sediment -laden runoff created onsite to onsite sediment -trapping devices, preventing soil loss from the disturbed area. These control practices are called temporary diversion dikes, earth dikes, and interceptor dikes. No matter what they are called„ all earthen perimeter controls are constructed in a similar way with a similar objective --to control the velocity or route (or both) of sediment -laden stormwater runoff. Applicability Temporary diversion dikes apply where it is desirable to divert flows away from disturbed areas such as cut or fill slopes and to divert runoff to a stabilized outlet (USEPA, 1992). The dikes can be erected at the top of a sloping area or in the middle of a slope to divert stormwater runoff around a disturbed construction site. In this way, earth dikes can be used to reduce the length of the slope across which runoff travels, reducing the erosion potential of the flow. If diversion dikes are placed at the bottom of a sloping disturbed area, they can divert flow to a sediment - trapping device. Temporary diversion dikes are usually appropriate for drainage basins smaller than 5 acres. With modifications they can service areas as large as 10 acres. With regular maintenance, earthen diversion dikes have a useful life span of about 18 months. To prevent stormwater runoff from entering a site, earthen perimeter controls can be used to divert runoff from areas upslope around the disturbed construction site. A continuous, compacted earthen mound is constructed along the upslope perimeter of the site. As an additional control measure, a shallow ditch can accompany the earthen mound. Siting and Design Considerations The siting of earthen perimeter controls depends on the topography of the area surrounding the construction site. Another factor is whether the goal is to prevent sediment -laden runoff from entering the site or to keep stormwater runoff from leaving the site. When determining the appropriate size and design of earthen perimeter controls, consider the shape and drainage patterns of the surrounding landscape. Also consider the amount of runoff to be diverted, the velocity of runoff in the diversion, and the erodibility of soils on the slope and in the diversion channel or swales (WA State Dept. of Ecology, 2005). Construct diversion dikes and fully stabilize them before any major land disturbance begins. This approach makes the diversion measure effective as an erosion and sediment control device. The top of earthen perimeter controls designed as temporary flow diversion measures should be at least 2 feet wide. The bottom width at ground level is typically 6 feet. The minimum height for earth dikes should be 18 inches, with side slopes no steeper than 2:1. At points where vehicles will cross the dike, make sure the slope is no steeper than 3:1 and make the mound gravel rather than soil. This design makes the dike last longer and strengthens the point of vehicle crossing. If a channel is excavated along the dike, its shape can be parabolic, trapezoidal, or V-shaped. Before any excavating or mound -building, remove all trees, brush, stumps, and other objects in the path of the diversion structure. Till the base of the dike before laying the fill. The maximum design flow velocity should range from 1.5 to 5.0 feet per second, depending on the vegetative cover and soil texture. Most earthen perimeter structures are designed for short-term, temporary use. If the expected life span of the structure is more than 15 days, seed the earthen dike and the accompanying ditchwith vegetation immediately after construction. This increases the stability of the perimeter control and can decrease the need for frequent repairs and maintenance. Limitations Earth dikes are an effective means of diverting sediment -laden stormwater runoff around a disturbed area. But the concentrated runoff in the channel or ditch has increased erosion potential. Direct diversion dikes to sediment -trapping devices, where sediment can settle out of the runoff before it is discharged to surface waters. Sediment -trapping devices that work with temporary diversion structures include sediment basins, sediment chambers/filters, and any other structures designed to allow sediment to be collected for proper disposal. If a diversion dike crosses a vehicle roadway or entrance, its effectiveness can be reduced. When possible, design diversion dikes to avoid crossing vehicle pathways. Maintenance Considerations Inspect earthen diversion dikes after each rainfall to ensure continued effectiveness. Maintain dikes at their original height. Repair any decrease in height due to settling or erosion immediately. To remain effective, earth dikes must be compacted at all times. Regardless of rainfall frequency, inspect dikes at least once every 2 weeks for evidence of erosion or deterioration. Effectiveness When properly placed and maintained, earth dikes used as temporary diversions can control the velocity and direction of stormwater runoff. Used by themselves, they do not have any pollutant removal capability. They must be used with an appropriate sediment -trapping device at the outfall of the diversion channel. Cost Considerations The cost of constructing an earth dike can be broken down into two components: (1) site preparation (including excavation, placement, and compacting of fill) and grading, and (2) site development, including topsoiling and seeding for vegetative cover. The Southeastern Wisconsin Regional Planning Commission (1991) estimated the total cost of site preparation to be $46.33 to $124.81 for a 100 -foot dike with 1.5 -foot -deep, 3:1 side slopes. The cost of site development was estimated at $115.52 to $375.44. The total cost was between $162 and $500. The cost for constructinig diversion berms range from $15 to $55 per ft for both earthwork and stabilization and depends on the availability of suitable material, site location, and access. Small dikes range from $2.50 to $6.50 per linear ft and large dikes cost about $2.50 per cubic yard of earth (CASQA, 2003). References CASQA, 2003. California Constrcution BMP HandbookSection 3; EC -9 Earth Dikes and Drainage Swales Fact Sheet. http://www.cabmphandbooks.org/Construction.asp Smolen, M.D., D.W. Miller, L.C. Wyall, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1992. Storm Water Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. VDCR (Virginia Department of Conservation and Recreation). 1995. Virginia Erosion & Sediment Control Field Manual. 2nd ed. Virginia Department of Conservation, Division of Soil and Water Conservation, Richmond, VA. Walker, J., G. Jennings, and J. Arnold. 1996. Water Quality and Waste Management, Erosion and Sediment Control in North Carolina. North Carolina Cooperative Extension. Washington State Department of Ecology. 2005. 2005 Stormwater Management Manual for Western Washington: Volume II -- Construction Stormwater Pollution Prevention. Washington Department of Ecology, Olympia, WA. http://www.ecy.wa.gov/biblio/0510030.html Sediment Control Appendix J - Intrinsic GSP Specifications Compost Filter Berms EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=119&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Description A compost filter berm is a dike of compost or a compost product that is placed perpendicular to sheet flow runoff to control erosion in disturbed areas and retain sediment. It can be used in place of a traditional sediment and erosion control tool such as a silt fence. The compost filter berm, which is trapezoidal in cross section, provides a three-dimensional filter that retains sediment and other pollutants (e.g., suspended solids, metals, oil and grease) while allowing the cleaned water to flow through the berm. Composts used in filter berms are made from a variety of feedstocks, including municipal yard trimmings, food residuals, separated municipal solid waste, biosolids, and manure. Compost filter berms are generally placed along the perimeter of a site, or at intervals along a slope, to capture and treat stormwater that runs off as sheet flow. A filter berm also can be used as a check dam in small drainage ditches. Vegetated compost filter berm. Note sediment on upstream side of berm and clear water on downstream side. Source: S. McCoy, Texas Commission on Environmental Quality. The berms can be vegetated or unvegetated. Vegetated filter berms are normally left in place and provide long-term filtration of stormwater as a post -construction best management practice (BMP). Unvegetated berms are often broken down once construction is complete and the compost is spread around the site as a soil amendment or mulch. Filter berms, in general, provide an effective physical barrier in sheet flow conditions; however, the use of compost in the filter berm provides additional benefits. These benefits include the following: The compost retains a large volume of water, which helps prevent or reduce rill erosion and aids in establishing vegetation on the berm. The mix of particle sizes in the compost filter material retains as much or more sediment than traditional perimeter controls, such as silt fences or hay bale barriers, while allowing a larger volume of clear water to pass through the berm. Silt fences often become clogged with sediment and form a dam that retains stormwater, rather than letting the filtered stormwater pass through. In addition to retaining sediment, compost can retain pollutants, such as heavy metals, nitrogen, phosphorus, oil and grease, fuel, herbicides, pesticides, and other potentially hazardous substances, from stormwater. improving water quality downstream of the berm (USEPA, 1998). Nutrients and hydrocarbons adsorbed and/or trapped by the compost filter can be naturally cycled and decomposed through bioremediation by microorganisms commonly found in the compost matrix (USEPA, 1998). Applicability Compost filter berms are applicable to construction sites with relatively small drainage areas, where stormwater runoff occurs as sheet flow. Common industry practice is to use compost filter berms in drainage areas that do not exceed 0.25 acre per 100 feet of berm length and where flow does not typically exceed 1 cubic foot per second (see Siting and Design Considerations discussion for more detail). Compost filter berms can be used on steeper slopes with faster flows if they are spaced more closely or used in combination with other stormwater BMPs such as compost blankets or silt fences. Siting and Design Considerations Compost Quality: Compost quality is an important consideration when designing a compost filter berm. Use of sanitized, mature compost will ensure that the compost filter berm performs as designed and has no identifiable feedstock constituents or offensive odors. The compost used in filter berms should meet all local, state, and Federal quality requirements. Biosolids compost must meet the Standards for Class A biosolids outlined in 40 Code of Federal Regulations (CFR) Part 503. The U.S. Composting Council (USCC) certifies compost products under its Seal of Testing Assurance (STA) Program. Compost producers whose products have been certified through the STA Program provide customers with a standard product label that allows comparison between compost products. The current STA Program requirements and testing methods are posted on the USCG IEXIT oisriaimerwebsite. The nutrient and metal content of some composts are higher than some topsoils. This, however, does not necessarily translate into higher metals and nutrient concentrations or loads in stormwater runoff. A recent study by Glanville, et al. (2003) compared the stormwater runoff water quality from compost- and topsoil -treated plots. They found that although the composts used in the study contained statistically higher metal and nutrient concentrations than the topsoils used, the total masses of nutrients and metals in the runoff from the compost -treated plots were significantly less than plots treated with topsoil. Likewise, Faucette et al. (2005) found that nitrogen and phosphorus loads from hydroseed and silt fence treated plots were significantly greater than plots treated with compost blankets and filter berms. In areas where the receiving waters contain high nutrient levels, the site operator should choose a mature, stable compost that is compatible with the nutrient and pH requirements of the selected vegetation. This will ensure that the nutrients in the composted material are in organic form and are therefore less soluble and less likely to migrate into receiving waters. The American Association of State Highway Transportation Officials (AASHTO) and many individual state Departments of Transportation (DOTS) have issued specifications for filter berms (AASHTO, 2003; USCC, 2001). These specifications describe the quality and particle size distribution of compost to be used in filter berms, as well as the size and shape of the berm for different scenarios. The filter berm media parameters developed for AASHTO specification MP 9-03 are shown in Table 1 as an example (Alexander, 2003). Research on these parameters continues to evolve; therefore, the DOT or Department of Environmental Quality (or similar designation) for the state where the filter berm will be installed should be contacted to obtain any applicable specifications or compost testing recommendations. Design: Filter berms installed to control erosion and sediment on a slope or near the base of a slope are trapezoidal in cross section, with the base generally twice the height of the berm. The height and width of the berm will vary depending upon the precipitation and the rainfall erosivity index (EPA, 200 1) of the site. Example compost filter berm dimensions for various rainfall scenarios developed for AASHTO specification MP 9-03 are shown in Table 2 ( Alexander, 2003). Example filter berm dimensions based on the site slope and slope length developed by the Oregon Department of Environmental Quality (ODEQ) are shown in Table 3 (ODEQ, 2004). The compost filter berm dimensions should be modified based on site-specific conditions, such as soil characteristics, existing vegetation, site slope, and climate, as well as project -specific requirements. Coarser compost products are generally used in regions subject to high rainfall or wind erosion. Table 1. Example Filter Berm Media Parameters Parameters1'4 Units of Measure Berm to be Vegetated Berm to be left Unvegetated pHZ JpH units 5.0.8.5 Not applicable Soluble salt concentration (electrical dS/m (mmhos/cm) Maximum 5 Not applicable conductivity) Moisture content %, wet weight 30.60 30.60 basis Organic matter content %, dry weight 25.65 25.100 basis - 3 in. (75 mm), 100% - 3 in. (75 mm), 100% % passing a passing passing Particle size selected mesh size, - 1 in. (25 mm), 90. - 1 in. (25 mm), 90. dry weight basis 100% passing 100% passing - 0.75 in. (19 mm), 70. - 0.75 in. (19 mm), 70. 100% passing 100% passim Source: Alexander, 2003 1 Recommended test methodologies are provided in [Test Methods for the Evaluation of Composting and Compost EXIT d isclaime r Z Each plant species requires a specific pH range and has a salinity tolerance rating. s Stability/maturity rating is an area of compost science that is still evolving, and other test methods should be considered. Compost quality decisions should be based on more than one stability/maturity test. 4 Landscape architects and project engineers may modify the above compost specification ranges based on specific field conditions and plant requirements. Table 2. Example Compost Filter Berm Dimensions for Various Rainfall Scenarios Annual Rainfall/ Flow Rate Precipitation/year (Rainfall Erosivity Index) - 0.25 in. (6.4 mm), 30. - 0.25 in. (6.4 mm), 30. 1 .25 in. 1 ftx2ftto 1.5ftx3ft 75% passing 75% passing (30 cm x 60 cm to 45 cm x 90 Maximum particle size Maximum particle size 26.50 in. length of 6 in (152 mm) length of 6 in (152 mm) (30 cm x 60 cm to 45 cm x 90 Avoid compost with less Avoid compost with less cm) than 30% fine particle than 30% fine particle (1 mm) to achieve (1 mm) to achieve optimum reduction of optimum reduction of total suspended solids total suspended solids No more than 60% No more than 60% passing 0.25 in (6.4 mm) passing 0.25 in (6.4 mm) in high rainfall/flow rate in high rainfall/flow rate situations situations Stability3 mg CO2.0 per Carbon dioxide gram of organic <8 Not applicable evolution rate matter per day Physical contaminants %, dry weight <1 <1 (manmade inerts) basis Source: Alexander, 2003 1 Recommended test methodologies are provided in [Test Methods for the Evaluation of Composting and Compost EXIT d isclaime r Z Each plant species requires a specific pH range and has a salinity tolerance rating. s Stability/maturity rating is an area of compost science that is still evolving, and other test methods should be considered. Compost quality decisions should be based on more than one stability/maturity test. 4 Landscape architects and project engineers may modify the above compost specification ranges based on specific field conditions and plant requirements. Table 2. Example Compost Filter Berm Dimensions for Various Rainfall Scenarios Annual Rainfall/ Flow Rate Precipitation/year (Rainfall Erosivity Index) Berm Dimensions (height x width) 1 .25 in. 1 ftx2ftto 1.5ftx3ft Low (20.90) (30 cm x 60 cm to 45 cm x 90 cm) 26.50 in. 1 ftx2ftto 1.5ftx3ft Average (30 cm x 60 cm to 45 cm x 90 (91.200) cm) Source: Alexander, 2003 Table 3. Example Compost Filter Berm Dimensions Based on Slope and Slope Length Slope Slope Length 1.5 ft x 3 ft to 2 ft x 4 ft <50:1 e 51 in. 1 ft x 2 ft High (e 201) (45 cm x 90 cm to 60 cm x 10:1 .5:1 100 ft 120 cm) Source: Alexander, 2003 Table 3. Example Compost Filter Berm Dimensions Based on Slope and Slope Length Slope Slope Length Berm Dimensions (height x width) <50:1 250 ft 1 ft x 2 ft 50:1 .10:1 125 ft 1 ft x 2 ft 10:1 .5:1 100 ft 1 ft x 2 ft 3:1.2:1 50 ft 1.3ftx2.6ft >2:1 25 ft 1.5ftx3ft Source: ODEQ, 2004 Siting: For sites in high rainfall areas or where there are severe grades or long slopes, larger dimension berms should be used. The project engineer may also consider placing berms at the top and base of the slope, constructing a series of berms down the profile of the slope (15 to 25 feet apart), or using filter berms in conjunction with a compost blanket. Installation: The compost berm can be installed by hand; by using a backhoe, bulldozer, or grading blade; or by using specialized equipment such as a pneumatic blower or side discharge spreader with a berm attachment. The compost should be uniformly applied to the soil surface, compacted, and shaped to into a trapezoid. Compost filter berms can be installed on frozen or rocky ground. The filter berm may be vegetated by hand, by incorporating seed into the compost prior to installation (usually done when the compost is installed using a pneumatic blower or mixing truck with a side discharge), or by hydraulic seeding following berm construction. Proper installation of a compost filter berm is the key to effective sediment control. Limitations Compost filter berms can be installed on any type of soil surface; however, heavy vegetation should be cut down or removed to ensure that the compost contacts the ground surface. Filter berms are not suitable for areas where large amounts of concentrated runoff are likely, such as streams, ditches, or waterways, unless the drainage is small and the flow rate is relatively low. Maintenance Considerations Compost filter berms should be inspected regularly, as well as after each rainfall event, to ensure that they are intact and the area behind the berm is not filled with silt. Accumulated sediments should be removed from behind the berm when the sediments reach approximately one third the height of the berm. Any areas that have been washed away should be replaced. If the berm has experienced significant washout, a filter berm alone may not be the appropriate BMP for this area. Depending upon the site-specific conditions, the site operator could remedy the problem by increasing the size of the filter berm or adding another BMP in this area, such as an additional compost filter berm or compost filter sock, a compost blanket, or a silt fence. Effectiveness Numerous qualitative studies have reported the effectiveness of compost filter berms in removing settleable solids, total suspended solids, and various organic and inorganic contaminants from stormwater. These studies have consistently shown that compost filter berms are at least as effective as other traditional erosion and sediment control BMPs in controlling sediment; however, the results of the studies varied depending upon the site conditions. One quantitative study conducted in Portland, Oregon (W&H Pacific, 1993) compared the effectiveness of a silt fence and a mixed yard debris compost filter berm to a control plot during five storm events. The study found that the filter berm was over 90 percent effective in removing settleable and total suspended solids when compared to the control plot and was approximately 66 percent more effective than the silt fence. Another quantitative study performed by the Snohomish County, Washington, Department of Planning and Development Services (Caine, 200 1) showed no decrease in turbidity with a silt fence but a 67 percent reduction in turbidity using a compost filter berm. Cost Considerations The TCEQ reports that compost filter berms cost $1.90 to $3.00 per linear foot when used as a perimeter control and $3 to $6 per linear foot when used as a check dam (McCoy, 2005). The ODEQ reports that compost filter berms cost approximately 30 percent less to install than silt fences (Juries, 2004). These costs do not include the cost of removal and disposal of the silt fence or the cost of dispersing the compost berm once construction activities are completed. The cost to install a compost filter berm will vary, depending upon the availability of the required quality of compost in an area. References Alexander, R. 2003. Standard Specifications for Compost for Erosion/Sediment Control, developed for the Recycled Materials Resource Center, University of New Hampshire, Durham, New Hampshire. Available at [ www.alexassoc.net IEXIT oisdaimer ]. Alexander, R. 2001. Compost Use on State Highway Applications, Composting Council Research and Education Fund and U.S. Composting Council, Harrisburg, Pennsylvania. AASHTO. 2003 Standard Specifications for Transportation materials and Methods of Sampling and Testing, Designation MP -9, Compost for Erosion/Sediment Control (Filter Berms), Provisional, American Association of State Highway Transportation Officials, Washington, D.C. Caine, E. 2001. Quilceda-Allen Watershed Erosion Control Program, Water Quality Monitoring Report, Snohomish County, Washington, Department of Planning and Development Services, Building Division. EPA. 2001. Stormwater Phase II Final Rule, Fact Sheet 3. 1, Construction Rainfall Erosivity Waiver, EPA 833-F-00-014, U.S. Environmental Protection Agency, Office of Water, Washington, D.C. Faucette, et al. 2005. Evaluation of Stormwater from Compost and Conventional Erosion Control Practices in Construction Activities, Journal of Soil and Water Conservation, 60:6, 288- 297. Glanville et al. 2003. Impacts of Compost Blankets on Erosion Control, Revegetation, and Water Quality at Highway Construction Sites in Iowa, T. Glanville, T. Richard, and R. Persyn, Agricultural and Biosystems Engineering Department, Iowa State University of Science and Technology, Ames, Iowa. Juries, D. 2004. Environmental Protection and Enhancement with Compost, Oregon Department of Environmental Quality, Northwest Region. McCoy, S. 2005. Presentation at Erosion, Sediment Control and Stormwater Management with Compost BMPs Workshop, U.S. Composting Council 13 th Annual Conference and Trade Show, January 2005, San Antonio, Texas. ODEQ. 2004. Best Management Practices for Stormwater Discharges Associated with Construction Activity, Guidance for Eliminating or Reducing Pollutants in Stormwater Discharges, Oregon Department of Environmental Quality, Northwest Region. Risse, M. and B. Faucette. 2001. Compost Utilization for Erosion Control, University of Georgia, Cooperative Extension Service, Athens, Georgia. USCC, 2001. Compost Use on State Highway Applications, U.S. Composting Council, Washington, D.C. USEPA. 1998. An Analysis of Composting as an Environmental Remediation Technology. U.S. Environmental Protection Agency, Solid Waste and Emergency Response (5305W), EPA530-R- 98-008, April 1998. W&H Pacific. 1993. Demonstration Project Using Yard Debris Compost for Erosion Control, Final Report, presented to Metropolitan Service District, Portland, Oregon. Compost Filter Socks EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=120&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Description A compost filter sock is a type of contained compost filter berm. It is a mesh tube filled with composted material that is placed perpendicular to sheet -flow runoff to control erosion and retain sediment in disturbed areas. The compost filter sock, which is oval to round in cross section, provides a three-dimensional filter that retains sediment and other pollutants (e.g., suspended solids, nutrients, and motor oil) while allowing the cleaned water to flow through (Tyler and Faucette, 2005). The filter sock can be used in place of a traditional sediment and erosion control tool such as a silt fence or straw bale barrier. Composts used in filter socks are made from a variety of feedstocks, including municipal yard trimmings, food residuals, separated municipal solid waste, biosolids, and manure. Compost filter socks are generally placed along the perimeter of a site, or at intervals along a slope, to capture and treat stormwater that runs off as sheet flow. Filter socks r Installation of filter socks in a road ditch by Earth Corps for Indiana Department of Transportation. The filter socks will be staked through the center. Source: Filtrexx International, LLC. are flexible and can be filled in place or filled and moved into position, making them especially useful on steep or rocky slopes where installation of other erosion control tools is not feasible. There is greater surface area contact with soil than typical sediment control devices, thereby reducing the potential for runoff to create rills under the device and/or create channels carrying unfiltered sediment. Additionally, they can be laid adjacent to each other, perpendicular to stormwater flow, to reduce flow velocity and soil erosion. Filter socks can also be used on pavement as inlet protection for storm drains and to slow water flow in small ditches. Filter socks used for erosion control are usually 12 inches in diameter, although 8 inch, 18 inch, and 24 inch— diameter socks are used in some applications. The smaller, 8 inch—diameter filter socks are commonly used as stormwater inlet protection. Compost filter socks can be vegetated or unvegetated. Vegetated filter socks can be left in place to provide long-term filtration of stormwater as a post -construction best management practice (BMP). The vegetation grows into the slope, further anchoring the filter sock. Unvegetated filter socks are often cut open when the project is completed, and the compost is spread around the site as soil amendment or mulch. The mesh sock is then disposed of unless it is biodegradable. Three advantages the filter sock has over traditional sediment control tools, such as a silt fence, are: Installation does not require disturbing the soil surface, which reduces erosion It is easily removed The operator must dispose of only a relatively small volume of material (the mesh) These advantages lead to cost savings, either through reduced labor or disposal costs. The use of compost in this BMP provides additional benefits, include the following: o The compost retains a large volume of water, which helps prevent or reduce rill erosion and aids in establishing vegetation on the filter sock. o The mix of particle sizes in the compost filter material retains as much or more sediment than traditional perimeter controls, such as silt fences or hay bale barriers, while allowing a larger volume of clear water to pass through. Silt fences often become clogged with sediment and form a dam that retains stormwater, rather than letting the filtered stormwater pass through. o In addition to retaining sediment, compost can retain pollutants such as heavy metals, nitrogen, phosphorus, oil and grease, fuels, herbicides, pesticides, and other potentially hazardous substances—improving the downstream water quality (USEPA, 1998). o Nutrients and hydrocarbons adsorbed and/or trapped by the compost filter can be naturally cycled and decomposed through bioremediation by microorganisms commonly found in the compost matrix (USEPA, 1998). Applicability Compost filter socks are applicable to construction sites or other disturbed areas where stormwater runoff occurs as sheet flow. Common industry practice for compost filter devices is that drainage areas do not exceed 0.25 acre per 100 feet of device length and flow does not exceed one cubic foot per second (see Siting and Design Considerations). Compost filter socks can be used on steeper slopes with faster flows if they are spaced more closely, stacked beside and/or on top of each other, made in larger diameters, or used in combination with other stormwater BMPs such as compost blankets. Siting and Design Considerations Compost Quality: Compost quality is an important consideration when designing a compost filter sock. Use of sanitized, mature compost will ensure that the compost filter sock performs as designed and has no identifiable feedstock constituents or offensive odors. The compost used in filter socks should meet all local, state, and Federal quality requirements. Biosolids compost must meet the Standards for Class A biosolids outlined in 40 Code of Federal Regulations (CFR) Part 503. The U.S. Composting Council (USCC) certifies compost products under its Seal of Testing Assurance (STA) Program. Compost producers whose products have been certified through the STA Program provide customers with a standard product label that allows comparison between compost products. The current STA Program requirements and testing methods are posted on the USCG I ENIT oisriaimerwebsite. The nutrient and metal content of some composts are higher than some topsoils. This, however, does not necessarily translate into higher metals and nutrient concentrations or loads in stormwater runoff. A recent study by Glanville, et al. (2003) compared the stormwater runoff water quality from compost- and topsoil -treated plots. They found that although the composts used in the study contained statistically higher metal and nutrient concentrations than the topsoils used, the total masses of nutrients and metals in the runoff from the compost -treated plots were significantly less than plots treated with topsoil. Likewise, Faucette et al. (2005) found that nitrogen and phosphorus loads from hydroseed and silt fence treated plots were significantly greater than plots treated with compost blankets and filter berms. In areas where the receiving waters contain high nutrient levels, the site operator should choose a mature, stable compost that is compatible with the nutrient and pH requirements of the selected vegetation. This will ensure that the nutrients in the composted material are in organic form and are therefore less soluble and less likely to migrate into receiving waters. The American Association of State Highway Transportation Officers (AASHTO) and many individual State Departments of Transportation (DOTs) have issued quality and particle size specifications for the compost to be used in filter berms (USCC, 2001; AASHTO, 2003). The compost specifications for vegetated filter berms developed for AASHTO Specification MP 9-03 (Alexander, 2003) are also applicable to vegetated compost filter socks (personal communication, B. Faucette, R. Tyler, and N. Goldstein, 2005). These specifications are provided as an example in Table 1. Installations of unvegetated compost filter socks, however, have shown that they require a coarser compost than unvegetated filter berms. The Minnesota DOT erosion control compost specifications for "compost logs" recommend 30 to 40 percent weed -free compost and 60 to 70 percent partially decomposed wood chips. They recommend that 100 percent of the compost passes the 2 -inch (51 mm) sieve and 30 percent passes the 3/8 -inch (10 mm) sieve. Research on these parameters continues to evolve; therefore, the unvegetated filter sock parameters shown in Table 1 are a compilation of those that are currently in use by industry practitioners (personal communication, B. Faucette, R. Tyler, R. Alexander, and N. Goldstein, 2005). The DOT or Department of Environmental Quality (or similar designation) for the state where the filter sock will be installed should be contacted to obtain any applicable specifications or compost testing recommendations. Design: Filter socks are round to oval in cross section; they are assembled by tying a knot in one end of the mesh sock, filling the sock with the composted material (usually using a pneumatic blower), then knotting the other end once the desired length is reached. A filter sock the length of the slope is normally used to ensure that stormwater does not break through at the intersection of socks placed end-to-end. In cases where this is not possible, the socks are placed end-to-end along a slope and the ends are interlocked. The diameter of the filter sock used will vary depending upon the steepness and length of the slope; example slopes and slope lengths used with different diameter filter socks are presented in Table 2. Siting: Although compost filter socks are usually placed along a contour perpendicular to sheet flow, in areas of concentrated flow they are sometimes placed in an inverted V going up the slope, to reduce the velocity of water running down the slope. The project engineer may also consider placing compost filter socks at the top and base of the slope or placing a series of filter socks every 15 to 25 feet along the vertical profile of the slope. These slope interruption devices slow down sheet flow on a slope or in a watershed. Larger diameter filter socks are recommended for areas prone to high rainfall or sites with severe grades or long slopes. Coarser compost products are generally used in regions subject to high rainfall and runoff conditions. Table 1. Example Compost Filter Parameters Parameters a,l,a Units of Vegetated Filter Unvegetated Filter Measurea Berm/Socka Sock pHZ IpH units 5.0-8.5 6 — 8 Soluble salt concentration dS/m Maximum 5 Not applicable (electrical (mmhos/cm) conductivity) Moisture content %, wet weight 30-60 30-60 basis Organic matter content %, dry weight 25 —65 25 —65 basis - 3 in. (75 mm), 100% - 2 in. (51 mm), 100% passing passing - 1 in. (25 mm), 90 — - 0.375 in. (10 mm), 10% 100% passing — 30% passing - 0.75 in. (19 mm), 70 — 100% passing - 0.25 in. (6.4 mm), 30 — 75% passing % passing a Particle size selected mesh Maximum particle size size, dry weight length of 6 in. (152 mm) basis Avoid compost with less than 30% fine particle (1 mm) to achieve optimum reduction of total suspended solids No more than 60% passing 0.25 in. (6.4 mm) in high rainfall/flow rate Sources: 'Alexander, 2003; bPersonal communication, B. Faucette, R. Tyler, N. Goldstein, R. Alexander, 2005 Notes: Recommended test methodologies are provided in [Test Methods for the Evaluation of Composting and Compost EXIT o isclairne r Z Each plant species requires a specific pH range and has a salinity tolerance rating. 3 Stability/maturity rating is an area of compost science that is still evolving, and other test methods should be considered. Compost quality decisions should be based on more than one stability/maturity test. 4 Landscape architects and project engineers may modify the above compost specification ranges based on specific field conditions and plant requirements. Table 2. Example Compost Filter Sock Slopes, Slope Lengths, and Sock Diameters Slope Slope Length (feet) situations <50:1 Stability, 50:1-10:1 125 12 10:1-5:1 mg CO2 -C per 12 3:1-2:1 Carbon dioxide gram of organic <8 (same as vegetated) evolution rate matter per day Physical contaminants %, dry weight <1 <1 (manmade inerts) basis Sources: 'Alexander, 2003; bPersonal communication, B. Faucette, R. Tyler, N. Goldstein, R. Alexander, 2005 Notes: Recommended test methodologies are provided in [Test Methods for the Evaluation of Composting and Compost EXIT o isclairne r Z Each plant species requires a specific pH range and has a salinity tolerance rating. 3 Stability/maturity rating is an area of compost science that is still evolving, and other test methods should be considered. Compost quality decisions should be based on more than one stability/maturity test. 4 Landscape architects and project engineers may modify the above compost specification ranges based on specific field conditions and plant requirements. Table 2. Example Compost Filter Sock Slopes, Slope Lengths, and Sock Diameters Slope Slope Length (feet) Sock Diameter (inches) <50:1 250 12 50:1-10:1 125 12 10:1-5:1 100 12 3:1-2:1 50 18 >2:1 25 18 Source: Oregon Department of Environmental Quality (ODEQ), 2004 Installation: No trenching is required; therefore, soil is not disturbed upon installation. Once the filter sock is filled and put in place, it should be anchored to the slope. The preferred anchoring method is to drive stakes through the center of the sock at regular intervals; alternatively, stakes can be placed on the downstream side of the sock. The ends of the filter sock should be directed upslope, to prevent stormwater from running around the end of the sock. The filter sock may be vegetated by incorporating seed into the compost prior to placement in the filter sock. Since compost filter socks do not have to be trenched into the ground, they can be installed on frozen ground or even cement. Limitations Compost filter socks offer a large degree of flexibility for various applications. To ensure optimum performance, It eavy vegetation should be cut down or removed, and extremely uneven surfaces should be leveled to ensure that the compost filter sock uniformly contacts the ground surface. Filter socks can be installed perpendicular to flow in areas where a large volume of stormwater runoff is likely, but should not be installed perpendicular to flow in perennial waterways and large streams. Maintenance Considerations Compost filter socks should be inspected regularly, as well as after each rainfall event, to ensure that they are intact and the area behind the sock is not filled with sediment. If there is excessive ponding behind the filter sock or accumulated sediments reach the top of the sock, an additional sock should be added on top or in front of the existing filter sock in these areas, without disturbing the soil or accumulated sediment. If the filter sock was overtopped during a storm event, the operator should consider installing an additional filter sock on top of the original, placing an additional filter sock further up the slope, or using an additional BMP, such as a compost blanket in conjunction with the sock(s). Effectiveness A large number of qualitative studies have reported the effectiveness of compost filter socks in removing settleable solids and total suspended solids from stormwater (McCoy, 2005; Tyler and Faucette, 2005). These studies have consistently shown that compost filter socks are at least as effective as traditional erosion and sediment control BMPs and often are more effective. Compost filter socks are often used in conjunction with compost blankets to form a stormwater management system. Together, these two BMPs retain a very high volume of stormwater, sediment, and other pollutants. The compost in the filter sock can also improve water quality by absorbing various organic and inorganic contaminants from stormwater, including motor oil. Tyler and Faucette (2005) conducted a laboratory test using 13 types of compost in filter socks. They found that half of the compost filter socks removed 100 percent of the motor oil introduced into the simulated stormwater (at concentrations of 1,000 — 10,000 milligrams per liter [mg/L]) and the remaining compost filter socks removed over 85 percent of the motor oil from the stormwater. Cost Considerations The Texas Commission on Environmental Quality reports that the cost of a 12 -inch diameter compost filter sock ranges from $1.40 to $1.75 per linear foot when used as a perimeter control (McCoy, 2005). The costs for an 18 -inch diameter sock used as a check dam range from $2.75 to $4.75 per linear foot (McCoy, 2005). These costs do not include the cost of removing the compost filter sock and disposing of the mesh once construction activities are completed; however, filter socks are often left on site to provide slope stability and post -construction stormwater control. The cost to install a compost filter sock will vary, depending upon the availability of the required quality and quantity of compost and the availability of an experienced installer. References Alexander, R. 2003. Standard Specifications for Compostfor Erosion/Sediment Control, developed for the Recycled Materials Resource Center, University of New Hampshire, Durham, New Hampshire. Available at [www.alexassoc.net I EXIT Disclaimer ]. Alexander, R. 2001. Compost Use on State Highway Applications, Composting Council Research and Education Fund and U.S. Composting Council, Harrisburg, Pennsylvania. AASHTO. 2003 Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Designation MP -9, Compost for Erosion/Sediment Control (Filter Berms), Provisional, American Association of State Highway Officials, Washington, D.C. Faucette, et al. 2005. Evaluation of'Stormwaterfrom Compost and Conventional Erosion Control Practices in Construction Activities, Journal of Soil and Water Conservation, 60:6, 288-297. Glanville et al. 2003. Impacts of Compost Blankets on Erosion Control, Revegetation, and Water Quality at Highway Construction Sites in Iowa, T. Glanville, T. Richard, and R. Persyn, Agricultural and Biosystems Engineering Department, Iowa State University of Science and Technology, Ames, Iowa. Juries, D. 2004. Environmental Protection and Enhancement with Compost, Oregon Department of Environmental Quality, Northwest Region. McCoy, S. 2005. Filter Sock Presentation provided at Erosion, Sediment Control and Stormwater Management with Compost BMPs Workshop, U.S. Composting Council 13th Annual Conference and Trade Show, January 2005, San Antonio, Texas. MnDOT. 2005. Storm Drain Inlet Protection Provisions, S-5.5 Materials, B. Compost Log, Minnesota Department of Transportation, Engineering Services Division, Technical Memorandum No. 05 -05 -ENV -03, January 18, 2005. ODEQ. 2004. Best Management Practices. for Stormwater Discharges Associated with Construction Activity, Guidance for Eliminating or Reducing Pollutants in Stormwater Discharges, Oregon Department of Environmental Quality, Northwest Region. Personal communications, 2005. Industry representatives were interviewed regarding the particle size and composition of composts currently used in vegetated and unvegetated filter socks. These representatives included Britt Faucette and Rod Tyler of Filtrexx, International, LLC; Nora Goldstein of BioCycle, Journal of Composting & Organics Recycling; and Ron Alexander of R. Alexander Associates, Inc. Tyler, R. and B. Faucette. 2005. Organic BMPs used for Stormwater Management Filter Media Test Results from Private Certification Program Yield Predictable Performance, U.S. Composting Council 13 th Annual Conference and Trade Show, January 2005, San Antonio, Texas. USCC. 2001. Compost Use on State Highway Applications, U.S. Composting Council, Washington, D.C. USEPA. 1998. An Analysis of Composting as an Environmental Remediation Technology. U.S. Environmental Protection Agency, Solid Waste and Emergency Response (5305W), EPA530-R-98-008, April 1998. W&H Pacific. 1993. Demonstration Project Using Yard Debris Compost.for Erosion Control, Final Report, presented to Metropolitan Service District, Portland, OR Construction Entrances EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.cpa.eov/npdes/stormwater/menuofbmps/index.cfm?action=factshect results&view=specific&bmp=35&minmeasure 4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Stabilized construction entrances allow dirt to be removed from ti re treads and collected as tru cks leave co n stru cti on site & Description The purpose of stabilizing entrances to a construction site is to minimize the amount of sediment leaving the area as mud and sediment attached to vehicles. Installing a pad of gravel over filter cloth where construction traffic leaves a site can help stabilize a construction entrance. As a vehicle drives over the pad, the pad removes mud and sediment from the wheels and reduces soil transport off the site. The filter cloth separates the gravel from the soil below, keeping the gravel from being ground into the soil. The fabric also reduces the amount of rutting caused by vehicle tires. It spreads the vehicle's weight over a soil area larger than the tire width. In addition to using a gravel pad, a vehicle washing station can be established at the site entrance. Using wash stations routinely can remove a lot of sediment from vehicles before they leave the site. Diverting runoff from vehicle washing stations into a sediment trap helps to make sure the sediment from vehicles stays onsite and is handled properly. Applicability Typically, stabilized construction entrances are installed where construction traffic leaves or enters an existing paved road. But site entrance stabilization should be extended to any roadway or entrance where vehicles enter or leave the site. From a public relations point of view, stabilizing construction site entrances can be worth the effort. If the site entrance is the most noticeable part of a construction site, stabilizing the entrance can improve both the appearance and the public perception of the construction project. Siting and Design Considerations Stabilize all entrances to a site before construction and further site disturbance begin. Make sure the stabilized site entrances are long and wide enough to allow the largest construction vehicle that will enter the site to fit through with room to spare. If many vehicles are expected to use an entrance in any one day, make the site entrance wide enough for two vehicles to pass at the same time with room on either side of each vehicle. If a site entrance leads to a paved road, make the end of the entrance flared so that long vehicles do not leave the stabilized area when they turn onto or off the paved roadway. If a construction site entrance crosses a stream, swale, or other depression, provide a bridge or culvert to prevent erosion from unprotected banks. Make sure stone and gravel used to stabilize the construction site entrance are large enough so that they are not carried offsite by vehicles. Avoid sharp -edged stone to reduce the possibility of puncturing tires. Install stone or gravel at a depth of at least 6 inches for the entire length and width of the stabilized construction entrance. Limitations Although stabilizing a construction entrance reduces the amount of sediment leaving a site, some soil might still be deposited from vehicle tires onto paved surfaces. To further reduce the chance of these sediments polluting stormwater runoff, sweep the paved area adjacent to the stabilized site entrance. For sites that use wash stations, a reliable water source to wash vehicles before leaving the site might not be initially available. Water might have to be trucked to the site at additional cost. Maintenance Considerations Maintain stabilization of the site entrances until the rest of the construction site has been fully stabilized. You might need to add stone and gravel periodically to each stabilized construction site entrance to keep the entrance effective. Sweep up soil tracked offsite immediately for proper disposal. For sites with wash racks at each site entrance, construct sediment traps and maintain them for the life of the project. Periodically remove sediment from the traps to make sure they keep working. Effectiveness Stabilizing construction entrances to prevent sediment transport offsite is effective only if all the entrances to the site are stabilized and maintained. Stabilizing the site entrances might not be very effective unless a wash rack is installed and routinely used (Corish, 1995). This can be problematic for sites with multiple entrances and high vehicle traffic. Cost Considerations Without a wash rack, construction site entrance stabilization costs range from $1,000 to $4,000. On average, the initial construction cost is around $2,000 per entrance. Including maintenance costs for a 2 -year period, the average total annual cost is approximately $1,500. If a wash rack is included in the construction site entrance stabilization, the initial construction costs range from $1,000 to $5,000, and the average initial cost is $3,000 per entrance. The total cost, including maintenance for an estimated 2 -year life span, is approximately $2,200 per year (USEPA, 1993). References Corish, K. 1995. Clearing and Grading Strategies for Urban Watersheds. Metropolitan Washington Council of Governments, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-13- 92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Fiber Rolls EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=121&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Description Fiber rolls (also called fiber logs or straw wattles) are tube -shaped erosion -control devices filled with straw, flax, rice, coconut fiber material, or composted material. Each roll is wrapped with UV -degradable polypropylene netting for longevity or with 100 percent biodegradable materials like burlap, jute, or coir. Fiber rolls complement permanent best management practices used for source control and revegetation. When installed in combination with straw mulch, erosion control blankets, hydraulic mulches, or bounded fiber matrices for slope stabilization, these devices reduce the effects of long or steep slopes (Earth Saver Erosion Control Products, 2005). Fiber rolls also help to slow, filter, and spread overland flows. This helps to prevent erosion and minimizes rill and gully development. Fiber rolls also help reduce sediment loads to receiving waters by filtering runoff and capturing sediments. Applicability Fiber rolls can be used in areas of low shear stress. Avoid using them in channels that are actively incising or in reaches with large debris loads or potential for significant ice buildup (Maryland Department of the Environment, 2000). Fiber rolls have been used to control erosion in a variety of areas --along highways and at construction sites, golf courses, ski areas, vineyards, and reclaimed mines. According to the California Stormwater Quality Association (CASQA, 2003), fiber rolls can be suitable in the following settings: Along the toe, top, face, and at -grade breaks of exposed and erodible slopes to shorten slope length and spread runoff as sheet flow At the end of a downward slope where it transitions to a steeper slope Along the perimeter of a project As check dams in unlined ditches Downslope of exposed soil areas Around temporary stockpiles Siting and Design Considerations Fiber rolls should be prefabricated rolls or rolled tubes of geotextiles fabric. When rolling the tubes, make sure each tube is at least 8 inches in diameter. Bind the rolls at each end and every 4 feet along the length of the roll with jute -type twine (California Stormwater Quality Association, 2003). Slope ground projects On slopes, install fiber rolls along the contour with a slight downward angle at the end of each row to prevent ponding at the midsection (California Straw Works, 2005). Turn the ends of each fiber roll upslope to prevent runoff from flowing around the roll. Install fiber rolls in shallow trenches dug 3 to 5 inches deep for soft, loamy soils and 2 to 3 inches deep for hard, rocky soils. Determine the vertical spacing for slope installations on the basis of the slope gradient and soil type. According to California Straw Works (2005), a good rule of thumb is: 1:1 slopes = 10 feet apart 2:1 slopes = 20 feet apart 3:1 slopes = 30 feet apart 4:1 slopes = 40 feet apart For soft, loamy soils, place the rows closer together. For hard, rocky soils, place the rows farther apart. Stake fiber rolls securely into the ground and orient them perpendicular to the slope. Biodegradable wood stakes or willow cuttings are recommended. Drive the stakes through the middle of the fiber roll and deep enough into the ground to anchor the roll in place. About 3 to 5 inches of the stake should stick out above the roll, and the stakes should be spaced 3 to 4 feet apart. A 24 -inch stake is recommended for use on soft, loamy soils. An 18 -inch stake is recommended for use on hard, rocky soils. Projects without slopes Fiber rolls can also be used at projects with minimal slopes. Typically, the rolls are installed along sidewalks, on the bare lot side, to keep sediment from washing onto sidewalks and streets and into gutters and storm drains. For installations along sidewalks and behind street curbs, it might not be necessary to stake the fiber rolls, but trenches must still be dug. Fiber rolls placed around storm drains and inlets must be staked into the ground. These rolls should direct the flow of runoff toward a designated drainage area. Place them 1 to 1'/z feet back from the storm drain or inlet. Limitations The installation and overall performance of fiber rolls have several limitations, including the following (California Stormwater Quality Association, 2003): Fiber rolls are not effective unless trenched. Fiber rolls can be difficult to move once saturated. To be effective, fiber rolls at the toe of slopes greater than 5:1 must be at least 20 inches in diameter. An equivalent installation, such as stacked smaller -diameter fiber rolls, can be used to achieve a similar level of protection. If not properly staked and entrenched, fiber rolls can be transported by high flows. Fiber rolls have a very limited sediment capture zone. Fiber rolls should not be used on slopes subject to creep, slumping, or landslide. Maintenance Considerations The maintenance requirements of fiber rolls are minimal, but short-term inspection is recommended to ensure that the rolls remain firmly anchored in place and are not crushed or damaged by equipment traffic (Murphy and Dreher, 1996). Monitor fiber rolls daily during prolonged rain events. Repair or replace split, torn, unraveled, or slumping fiber rolls. Fiber rolls are typically left in place on slopes. If they are removed, collect and dispose of the accumulated sediment. Fill and compact holes, trenches, depressions, or any other ground disturbance to blend with the surrounding landscape. Effectiveness Unlike other BMPs that could cause water to back up and flow around the edges, fiber rolls allow water to flow through while capturing runoff sediments. Fiber rolls placed along the shorelines of lakes and ponds provide immediate protection by dissipating the erosive force of small waves. As an alternative to silt fences, fiber rolls have some distinct advantages, including the following (Earth Saver, 2005): They install more easily, particularly in shallow soils and rocky material. They are more adaptable to slope applications and contour installations than other erosion and sediment control practices. They are readily molded to fit the bank line. They blend in with the landscape and are less obtrusive than other erosion and sediment controls such as silt fence. They do not obstruct hydraulic mulch and seed applications. They can be removed or left in place after vegetation is established. Fiber rolls can provide slope protection for 3 to 5 years (California Straw Works, 2005). They slowly decompose into mulch, and the netting breaks down into small pieces. The San Diego State University Soil Erosion Research Laboratory reported that the use of fiber roll products reduced offsite sediment delivery by 58 percent (International Erosion Control Association, 2005). The Flint Creek watershed, which covers approximately 28 square miles of Lake and Cook counties in northeastern Illinois, was listed in the Illinois Water Quality Report (1994-1995) as being impaired due to nonpoint source pollution from land development, channelization, and urban runoff. Along with other bioengineering techniques, fiber rolls were installed along the shorelines of the creek to reduce the effects of wave action. Native plants were installed in the fiber rolls. As a result, the growth of vegetative cover increased and helped to stabilize the slopes along the banks of the creek. Ultimately, the water quality of Flint Creek was improved (USEPA, 2002). Cost Considerations Material costs for fiber rolls range from $20 to $30 per 25 -foot roll (CASQA, 2003). Labor hours should also be allocated for installation, monitoring, and maintenance. Because fiber rolls are usually left along slopes and are biodegradable, labor costs for removing them are avoided. However, sediment removal and disposal are still necessary in areas where sediment accumulates to at least one-half the distance between the top of the fiber roll and the ground surface. References CASQA (California Stormwater Quality Association). 2003. California Stormwater BMP Handbook. (http://www.cabmphandbooks.com/Documents/Construction/SE-5.pdf [PDF - 115 KB - 4 pp] IEXIT Disclaimer ). Accessed May 19, 2005. California Straw Works. 2005. Straw Wattles. (http://www.publlcworks.com/doc.mvc/Straw- Wattles-0001 I EXIT disclaimer ). Accessed May 19, 2005. Earth Saver Erosion Control Products. 2005. Rice Straw Wattles. (http://www.earth- savers.com/ProductsAndSpecs.html I EXIT Disclaimer >). Accessed November 28, 2012. Maryland Department of the Environment. 2011. Slope Protection and Stabilization Techniques. (http://www.mde.mar_ l�gov/programs/Water/StormwaterManagementProgram/SoilErosionan dSedimentControl/Pages/2011 ESC details.aspx I EXIT Disclaimer ). Accessed July 12, 2012. Murphy, M., and D. Dreher. 1996. Northeastern Illinois Planning Commission, Chicago. LakeNotes: Shoreline Stabilization Bioengineering Alternatives. (http://www.lakesprin fie�g/files/shoreline stabilization.pdf [PDF - 228 KB - 4 pp] EXIT disclaimer ) USEPA (U.S. Environmental Protection Agency). 2002. Restoration of the Flint Creek Watershed: Restoration Partnership Completes Multiple Projects. Section 319 Success Stories, Vol. III. (http://www.epa.gov/nps/Section3l9III/IL.htm I EXIT Disclaimer >). Accessed May 20, 2005. Filter Berms EPA NPDES Fact Sheet Date accessed 10-24-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=37&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Description A gravel or stone filter berm is a temporary ridge made up of loose gravel, stone, or crushed rock. It slows and filters flow and diverts it from an open traffic area. It acts as an efficient form of sediment control. One type of filter berm is the continuous berm, a geosynthetic fabric berm that captures sand, rock, and soil. Applicability Gravel or stone filter berms are most suitable in areas where traffic needs to be rerouted because roads are under construction, or in traffic areas within a construction site. Siting and Design Considerations Consider the following guidelines when building a berm: Use well -graded gravel or crushed rock to build the berm, with rock size ranging from 3/4 inches to 3 inches in diameter containing less than 5 percent fines (Massachusetts DEP, 2003). Space berms according to the steepness of the slope. Space them closer together as the slope increases. Remove and dispose of sediment that builds up, and replace the filter material. Regular inspection should indicate how often sediment needs to be removed. Limitations Berms are intended to be used only in gently sloping areas (less than 10 percent). They do not last very long unless they are maintained regularly because they are prone to clogging with mud and soil from vehicle tires. Maintenance Considerations Inspect the berm after every rainfall to make sure sediment has not built up and that vehicles have not damaged it. It is important to make repairs at the first sign of deterioration to keep the berm functioning properly. Effectiveness The effectiveness of a rock filter berm depends on rock size, slope, soil and rainfall amount. The continuous berm is not staked into the ground, and no trenching is required. Effectiveness has been rated at up to 95 percent for sediment removal. Effectiveness depends on local conditions such as hydrologic, hydraulic, topographic, and sediment characteristics. Cost Considerations Construction materials for filter berms (mainly gravel) are relatively low in cost. Installing a berm and regularly cleaning and maintaining it can result in substantial labor costs. Costs are lower in areas of less traffic, gentler slopes, and low rainfall. References Fifield, S.J. 1997. Field Manual for Effective Sediment and Erosion Control Methods. Hydrodynamics, Inc., Parker, CO. Massachusetts Department of Environmental Protection. 2003. Massachusetts Erosion and Sediment Control Guidelines for Urban and Suburban Areas: A Guide for Planners, Designers and Municipal Officials. http://www.mass. og v/dep/water/esfull.pdf EXIT aisrlaimer. Accessed May 17, 2006. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Sediment Basins and Rock Dams EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.cpa.eov/npdes/stormwater/menuofbmps/index.cfm?action=factshect results&view=specific&bmp=57&minmeasure 4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Sediment basins are used to trap sediments and temporarily detain runoff on larger construction sites Description Sediment basins and rock dams can be used to capture sediment from stormwater runoff before it leaves a construction site. Both structures allow a pool to form in an excavated or natural depression, where sediment can settle. The pool is dewatered through a single riser and drainage hole leading to a suitable outlet on the downstream side of the embankment or through the gravel of the rock dam. The water is released more slowly than it would be without the control structure. A sediment basin is constructed by excavation or by erecting an earthen embankment across a low area or drainage swale. The basin can be temporary (up to 3 years) or permanent. Some sediment basins are designed to drain completely during dry periods. Others are constructed so that a shallow pool of water remains between storm events. Rock dams are similar to sediment basins with earthen embankments. These damming structures are constructed of rock and gravel. They release water from the settling pool gradually through the spaces between the rocks. Applicability Sediment basins are usually used for drainage areas of 5 to 100 acres. They can be temporary or permanent. Sediment basins designed to be used for up to 3 years are usually described as temporary. Those designed for longer service are considered permanent. Temporary sediment basins can be converted into permanent stormwater runoff management ponds, but they must meet all regulatory requirements for wet ponds. For EPA Construction General Permit permittees, a sediment basin or its equivalent should accomplish the following for drainage areas of different sizes: 10 or more acres of disturbed area: For common drainage locations that serve an area with 10 or more acres disturbed at one time, a temporary (or permanent) sediment basin that provides storage for a calculated volume of runoff from the drainage area from a 2 -year, 24-hour storm, or equivalent control measures, must be provided where attainable until final stabilization of the site. Where no such calculation has been performed, a temporary (or permanent) sediment basin providing 3,600 cubic feet of storage per acre drained, or equivalent control measures, must be provided where attainable until final stabilization of the site. When computing the number of acres draining into a common location, it is not necessary to include flows from offsite areas and flows from on-site areas that are either undisturbed or have undergone final stabilization where such flows are diverted around both the disturbed area and the sediment basin. In determining whether installing a sediment basin is attainable, the operator may consider factors such as site soils, slope, available area on-site, etc. In any event, the operator must consider public safety, especially as it relates to children, as a design factor for the sediment basin, and alternative sediment controls must be used where site limitations would preclude a safe design. For drainage locations which serve 10 or more disturbed acres at one time and where a temporary sediment basin or equivalent controls is not attainable, smaller sediment basins and/or sediment traps should be used. At a minimum, silt fences, vegetative buffer strips, or equivalent sediment controls are required for all down slope boundaries (and for those side slope boundaries deemed appropriate as dictated by individual site conditions). Less than 10 acres of disturbed area: For drainage locations serving less than 10 acres, smaller sediment basins and/or sediment traps should be used. At a minimum, silt fences, vegetative buffer strips, or equivalent sediment controls are required for all down slope boundaries (and for those side slope boundaries deemed appropriate as dictated by individual site conditions) of the construction area unless a sediment basin providing storage for a calculated volume of runoff from a 2 -year, 24-hour storm or 3,600 cubic feet of storage per acre drained is provided. Sediment basins are applicable in drainage areas where it is expected that other erosion controls, such as sediment traps, will not adequately prevent offsite transport of sediment. Whether to construct a sediment basin or a rock dam depends on the materials available, the location of the basin, and the desired capacity for holding stormwater runoff and settling sediment. Rock dams are suitable where earthen embankments would be difficult to construct and where rocks for the dams are readily available. They are also desirable where the top of the dam structure is to be used as an overflow outlet. Rock dams are best for drainage areas of less than 50 acres. Earthen damming structures are appropriate where failure of the dam will not result in substantial damage or loss of property or life. If sediment basins with earthen dams are properly constructed, they can handle runoff from drainage basins as large as 100 acres. Siting and Design Considerations Investigate potential sites for sediment basins during the initial site evaluation. Construct the basins before any grading takes place in the drainage area. For permanent structures, a qualified professional engineer experienced in designing dams should complete the basin design. Limit sediment basins with rock dams to a drainage area of 50 acres. Limit the rock dam height to 8 feet with a top width of at least 5 feet. Side slopes for rock dams should be no steeper than 2:1 on the basin side of the structure and 3:1 on the outlet side. Cover the basin side of the rock dam with fine gravel from top to bottom for at least 1 foot. This slows the drainage rate from the pool that forms and gives sediments time to settle. The detention time should be at least 8 hours. Outfit sediment basins with earthen embankments with a dewatering pipe and riser set just above the sediment removal cutoff level. Place the riser pipe at the deepest point of the basin and make sure it extends no farther than I foot below the level of the earthen dam. Place a water -permeable cover over the primary dewatering riser pipe to prevent trash and debris from entering and clogging the spillway. To provide an additional path for water to enter the primary spillway, you can drill secondary dewatering holes near the base of the riser pipe, but make sure you protect the holes with gravel to keep sediment out of the spillway piping. To ensure adequate drainage, use the following equation to approximate the total area of dewatering holes for a particular basin (Smolen et al., 1988): Ao = (A, x (2h) i (T x Cd x 20,428) where A,, = total surface area of dewatering holes, ft2; A, = surface area of the basin, ft2; h = head of water above the hole, ft; Cd = coefficient of contraction for an orifice, approximately 0.6; and T = detention time or time needed to dewater the basin, hours. In all cases, an appropriate professional should design such structures. The designer should consider local hydrologic, hydraulic, topographic, and sediment conditions. Limitations Do not use a sediment basin with an earthen embankment or a rock dam in an area of continuously running water (live streams). Do not use a sediment basin in an area where failure of the earthen or rock dam will result in loss of life or damage to homes or other buildings. Do not use sediment basins in areas where failure will prevent the use of public roads or utilities. Maintenance Considerations Routine inspection and maintenance of sediment basins is essential to their continued effectiveness. Inspect basins after each storm event to ensure proper drainage from the collection pool and determine the need for structural repairs. Replace material eroded from earthen embankments or stones moved from rock dams immediately. Locate sediment basins in an area that is easily accessible to maintenance crews for removal of accumulated sediment. Remove sediment from the basin when the storage capacity has reached approximately 50 percent. Remove trash and debris from around dewatering devices promptly after rainfall events. Effectiveness The effectiveness of a sediment basin depends primarily on the sediment particle size and the ratio of basin surface area to inflow rate (Smolen et al., 1988). Basins with a large surface area -to -volume ratio are the most effective. Studies have shown that the following equation relating surface area and peak inflow rate gives a trapping efficiency greater than 75 percent for most sediment in the Coastal Plain and Piedmont regions of the southeastern United States (Barfield and Clar, in Smolen et al., 1988): A = 0.01q where A is the basin surface area in acres and q is the peak inflow rate in cubic feet per second. USEPA (1993) estimates an average total suspended solids removal rate for all sediment basins of 55 percent to 100 percent. The average effectiveness is 70 percent. Cost Considerations For a sediment basin with less than 50,000 ft3 of storage space, the cost of installing the basin ranges from $0.20 to $1.30 per cubic foot of storage (about $1,100 per acre of drainage). The average cost for basins with less than 50,000 ft3 of storage is approximately $0.60 per cubic foot of storage (USEPA, 1993). If constructing a sediment basin with more than 50,000 ft3 of storage space, the cost of installing the basin ranges from $0.10 to $0.40 per cubic foot of storage (about $550 per acre of drainage). The average cost for basins with greater than 50,000 ft3 of storage is approximately $0.30 per cubic foot of storage (USEPA, 1993). References Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840- B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Sediment Filters and Sediment Chambers EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=58&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Description Sediment filters are sediment -trapping devices typically used to remove pollutants (mainly particulates) from stormwater runoff. Sediment filters have four components: (1) inflow regulation, (2) pretreatment, (3) filter bed, and (4) outflow mechanism. Sediment chambers are one component of a sediment filter system. Inflow regulation is diverting stormwater runoff into the sediment -trapping device. After runoff enters the filter system, it enters a pretreatment sedimentation chamber. This chamber is used as a preliminary settling area for large debris and sediments. It is usually no more than a wet detention basin. As water reaches a predetermined level, it flows over a weir into a bed of some filter medium. The medium is typically sand, but it can consist of sand, soil, gravel, peat, compost, or a combination. The filter bed removes small sediments and other pollutants from the stormwater as it percolates through the filter medium. Finally, treated flow exits the sediment filter system via an outflow mechanism. It returns to the stormwater conveyance system. Sediment filter systems can be confined or unconfined, on-line or off-line, and aboveground or belowground. Confined sediment filters are constructed with the filter medium contained in a structure, often a concrete vault. Unconfined sediment filters are made without a confining structure. For example, sand might be placed on the banks of a permanent wet pond detention system to create an unconfined filter. On-line systems retain stormwater in its original stream channel or storm drain system. Off-line systems divert stormwater. Applicability Sediment filters might be a good alternative for small construction sites where a wet pond is being considered as a sediment -trapping device. They are widely applicable, and they can be used in urban areas with large amounts of highly impervious area. Confined sand filters are man- made systems, so they can be applied to most development sites and have few constraining factors (MWCOG, 1992). However, for all sediment filter systems, the drainage area to be serviced should be no more than 10 acres. LADDER& DtWATEAING ENTRANCE ¢3WN MI PICA L) ff L-WASHEDSAND ! CLEAN OUT PIPE WASNED ]" ACGRIVATE Rtur91.41Pt%f-01W17l19?13!kLI P TILr4R N.T .5 Schematic representation of a sed imentfilter The type of filter system chosen depends on the amount of land available and the desired location. The Austin sand filter and the Delaware sand filter are examples of sediment filter systems. The Austin sand filter is a surface filter system that can be used in areas with space restrictions. If space is at a premium, an underground filter might be the best choice. For effective stormwater sediment control at the perimeter of a site, consider the Delaware sand filter. It consists of two parallel, trench -like chambers installed at a site's perimeter. The first trench (sediment chamber) provides pretreatment sediment settling before the runoff spills into the second trench (filter medium). Siting and Design Considerations The available space is likely to be the most important siting and design consideration. Another important consideration when deciding to install sediment -filtering systems is the amount of available head. Head is the vertical distance available between the inflow of the system and the outflow point. Because most filtering systems depend on gravity to move water through the system, if enough head is not available, the system will not be effective. It might cause more harm than good. For surface and underground sand filters, a minimum head of 5 feet is suggested (Claytor and Schueler, 1996). Perimeter sand filters like the two -chambered Delaware sand filter should have a minimum available head of 2 to 3 feet (Claytor and Schueler, 1996). The depth of filter media will vary depending on media type. For sand filters it is recommended that the sand (0.04 -inch diameter or smaller) be at least 18 inches deep, with at least 4 to 6 inches of gravel for the bed of the filter. Throughout the life of a sediment filter system, there will be a need for frequent access to assess effectiveness and perform routine maintenance and emergency repairs. Because most maintenance requires manual rather than mechanical removal of sediments and debris, locate filter systems to allow easy access. Limitations Sediment filters are usually limited to removing pollutants from stormwater runoff. To provide flood protection, they have to be used with other stormwater management practices. Do not use sediment filters on fill sites or near steep slopes (Livingston, 1997). In addition, sediment filters are likely to lose effectiveness in cold regions because of freezing conditions. Maintenance Considerations Maintenance of stormwater sediment filters can be relatively high compared to other sediment - trapping devices. Routine maintenance includes raking the filter medium and removing surface sediment and trash. These chores will likely need to be done by hand rather than by mechanical means. Depending on the medium used in the structure, the filter material might have to be changed or replaced up to several times a year. How often depends on, among other things, rainfall intensity and the expected sediment load. Inspect sediment filters of all media types monthly and after each significant rainfall event to make sure they are filtering properly. Remove trash and debris during inspections. Remove sediment from the filter inlets and sediment chambers when 75 percent of the storage volume has been filled. Because filter media have the potential for high loadings of metals and petroleum hydrocarbons, have the filter medium analyzed periodically to prevent it from reaching levels that would classify it as a hazardous waste. This is especially true on sites where solvents or other potentially hazardous chemicals are used. Implement spill prevention measures as necessary. Replace the top 3 to 4 inches of the filter medium once a year, or more frequently if the water level does not go down within 36 hours of a storm event. Effectiveness Treatment effectiveness depends on factors like treatment volume; whether the filter is on-line or off-line, confined or unconfined; and the type of land use in the contributing drainage area. MWCOG (1992) states that sand filter removal rates are "high" for sediment and trace metals and "moderate" for nutrients, biochemical oxygen demand, and fecal coliform bacteria. Removal rates can be increased slightly by using a peat/sand mixture as the medium because peat has adsorptive properties (pollutants attach to it) (MWCOG, 1992). The estimated pollutant removal capabilities for various filter systems are shown in Table 1. Table 1. Pollutant removal efficiencies for sand filters Source Filter system TSS' (%) TP' (%) TN' (%) Other pollutants Surface sand 85 55 35 Bacteria: 40%-80% Claytor and filter Metals: 35%-90% Schueler, 1996 Perimeter sand 80 65 45 Hydrocarbons: 80% filter Livingston, 1997 Sand filter 60-85 30-75 30-60 Metals: 30%-80% (general) 'TSS=total suspended solids; TP=total phosphorus; TN=total nitrogen. Cost Considerations MWCOG (1992) estimates the cost of construction for sand filters at $3.00 to $10.00 per cubic foot of runoff treated. Annual costs are estimated at about 5 percent of construction costs. References Claytor, R., and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection, Silver Spring, MD. Livingston. 1997. Operation, Maintenance, and Management of Stormwater Management Systems. Watershed Management Institute, Ingleside, MD. MWCOG (Metropolitan Washington Council of Governments). 1992. A Current Assessment of Urban Best Management Practices: Techniques for Reducing Non -Point Source Pollution in the Coastal Zone. Metropolitan Washington Council of Governments, Department of Environmental Programs, Washington, DC. Sediment Traps EPA NPDES Fact Sheet Date accessed 10-25-13 http://cfpub.epa.�zov/npdes/stonnwater/menuothmps/index.cfm?action=factsheet results&view=specific&bmp=59&minmeasure 4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Sediment traps are used to collect sediment - laden runoff from disturbed areas on construction sites Description Sediment traps are small impoundments that allow sediment to settle out of construction runoff. They are usually installed in a drainageway or other point of discharge from a disturbed area. Temporary diversions can be used to direct runoff to the sediment trap (USEPA, 1993). Sediment traps detain sediments in stormwater runoff to protect receiving streams, lakes, drainage systems, and the surrounding area. The traps are formed by excavating an area or by placing an earthen embankment across a low area or drainage swale. An outlet or spillway is often constructed using large stones or aggregate to slow the release of runoff (USEPA, 1992). Applicability Sediment traps are commonly used at the outlets of stormwater diversion structures, channels, slope drains, construction site entrance wash racks, or any other runoff conveyance that discharges waters containing sediment and debris. Siting and Design Considerations Sediment traps can simplify stormwater management on a construction site by trapping small amounts of sediment at multiple spots (USEPA, 1992). Note the natural drainage patterns, and place the traps in areas with the highest erosion potential. Design alternative diversion pathways to accommodate potential overflows. Design a sediment trap to maximize the surface area for infiltration and sediment settling. This increases the effectiveness of the trap and decreases the likelihood of backup during and after periods of high runoff intensity. Site conditions dictate specific design criteria, but the minimum storage capacity should be 1,800 ft3 per acre of total drainage area (Smolen et al., 1988). The volume of a natural sediment trap can be approximated using the following equation (Smolen et al., 1988): Volume (ft) = 0.4 x surface area (ft2) x maximum pool depth (ft) In the siting and design phase, take care to situate sediment traps for easy access by maintenance crews. This allows for periodic inspection and maintenance. When excavating an area for a sediment trap, make sure the side slopes are no steeper than 2 and the embankment height no more than 5 feet from the original ground surface. Machine -compact all embankments to ensure stability. To reduce flow rate from the trap, line the outlet with well -graded stone. The spillway weir for each temporary sediment trap should be at least 4 feet long for a 1 -acre drainage area and increase by 2 feet for each additional drainage acre added, up to a maximum drainage area of 5 acres. Limitations Do not use sediment traps for drainage areas greater than 5 acres (USEPA, 1993). The effective life span of these structures is usually limited to 24 months (Smolen et al., 1988). Although sediment traps allow eroded soils to settle, their detention periods are too short for removing fine particles like silts and clays. Maintenance Considerations The primary maintenance consideration for temporary sediment traps is removing accumulated sediment. Do this periodically to ensure that the trap continues to operate effectively. Remove sediments when the basin reaches about 50 percent sediment capacity. Inspect the sediment trap after each rainfall event to ensure that the trap is draining properly. Also check the structure for damage from erosion. Check the depth of the spillway and maintain it at a minimum of 1.5 feet below the low point of the trap embankment. Effectiveness Sediment trapping efficiency is a function of surface area and peak inflow rate (Smolen et al., 1988). Traps that provide pools with large length -to -width ratios have a greater chance of success. Sediment traps have a useful life of about 18 to 24 months (USEPA, 1993), but their effectiveness depends on the amount and intensity of rainfall and erosion, and proper maintenance. USEPA (1993) estimates an average total suspended solids removal rate of 60 percent. An efficiency rate of 75 percent can be obtained for most Coastal Plain and Piedmont soils by using the following equation (Barfield and Clar, in Smolen et al., 1988): Surface area at design flow (acres) _ (0.01) peak inflow rate (cfs) Cost Considerations The cost of installing temporary sediment traps ranges from $0.20 to $2.00 per cubic foot of storage (about $1,100 per acre of drainage). The average cost is sbout $0.60 per cubic foot of storage (USEPA, 1993). References Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840- B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. tormwater Best Management Practice Y Minimum Measure Construction Site Stormwater Runoff Control Subcategory Sediment Control Purpose and Description The purpose of a silt fence is to retain the soil on disturbed land (Figure 1), such as a construction site, until the activities disturbing the land are sufficiently completed to allow revegetation and permanent soil stabilization Figure 1. Silt fence retaining sediment to begin. Keeping the soil on a construction site, rather than letting it be washed off into natural water bodies (e.g., streams, rivers, ponds, lakes, estuaries) prevents the degradation of aquatic habitats and siltation of harbor channels. And not letting soil wash off onto roads, which readily transport it to storm sewers, avoids having sewers clogged with sediment. The cost of installing silt fences on a watershed's construction sites is considerably less than the costs associated with losing aquatic species, dredging navigation channels, and cleaning sediment out of municipal storm sewers. A silt fence is a temporary sediment barrier made of porous fabric. It's held up by wooden or metal posts driven into the ground, so it's inexpensive and relatively easy to remove. The fabric ponds sediment -laden stormwater runoff, causing sediment to be retained by the settling processes. A single 100 foot (ft) run of silt fence may hold 50 tons of sediment in place. Most construction sites today do have silt fences. But many do not work effectively because they are not well designed, installed, or maintained. The focus of this fact sheet is—how to make silt fences work. Design The three principal aspects of silt fence design are: proper placement of fencing, adequate amount of fencing, and appropriate materials. Proper Placement of Fencing Placement is important because where a fence starts, runs, and ends is critical to its effectiveness. Improper placement can make the fence a complete waste of money. Analyze the construction site's contours to determine the proper placement. Segment the site into manageable sediment storage areas for using multiple silt fence runs. The drainage area above any fence should usually not exceed a quarter of an acre. Water flowing over the top of a fence during a normal rainfall indicates the Figure 2. Create manageable sedimern storage areas drainage area is too large. An equation for calculating the maximum drainage area length above a silt fence, measured perpendicular to the fence, is given in Fifield, 2011. Avoid long runs of silt fence because they concentrate the water in a Figure 3. Water should not flow over the small area where it will easily filter fabric during a normal rainfall overflow the fence. The lowest point of the fence in Figure 4 is indicated by a red arrow. Water is directed to this low point by both long runs of fence on either side of the arrow. Most of the water overflows the fence at this low point and little sediment is trapped for such a long fence. Figure 4. Avoid long runs of silt fence Stormwater Best Management Practice: Silt Fences Use J -hooks as shown in Figures 5 and 6, which have ends turning up the slope to break up long fence runs and provide multiple storage areas that work like mini -retention areas. If the fence doesn't create a ponding condition, it will not work well. The silt fence in Figure 7 doesn't pond water or retain sediment. Stormwater will run around the fence carrying sediment to the street, which will transport the water and its sediment load to the storm sewer inlet. Figure 5. Use J -hook fences to break up long fence runs Figure 6. J -hook silt fences provide multiple storage areas Figure 7. This silt fence doesn't work Water flowing around the ends of a silt fence will cause additional erosion and defeat its purpose. The bottom of each end of the fence should be higher than the top of the middle of of a gentle slope, if large (Figure 10), can be more important than its slope in determining sediment loss. A silt fence should not be placed in a channel with continuous flow (channels in Figures 8 and 9 don't Figure 10. Gentle slopes may require a have a continuous flow), nor silt fence across a narrow or steep - sided channel. But when necessary a silt fence can be placed parallel to the channel to retain sediment before it enters the watercourse. Paved streets are major conduits of stormwater and silt, and they drain to storm sewer inlets. The best solution is to retain as much sediment as possible before it reaches paved surfaces. Install a silt fence at the inlet side of a storm sewer or culvert, rather than at the discharge where there is greater velocity and less storage area. Streets cut in the grade, but not yet paved, are also prime erosion conduits. If the streets are not going to be paved right away, they need a containment barrier such as a silt fence. Finally as a construction site's dynamics change, the silt fence layout should be adjusted when necessary to maintain its effectiveness. Designers and contractors should also consider diverting the fence (Figure 8). This insures that during an unusually heavy sediment -laden runoff water to a sediment detention pond. If rain, water will flow over the top rather than around either end of the site can provide a large enough area, this is usually the the fence. Only fine suspended material will spill over the top, which is not as harmful as having erosion at the ends. When there is a long steep slope, install one fence near the head of the slope to reduce the volume and velocity of water flowing down the slope, and another fence 6-10 ft from the toe of the slope to create a sediment storage area near the bottom. A common misconception is that you only have to worry about water running off steep slopes. However, steep slopes may have a relatively small water collection area. The total drainage area Figure 8. Proper installation, bottom of both ends are above the top of the middle Figure 9. Poor installation, water can flow around the ends causing additional erosion most effective and economical best management practice for retaining sediments. Silt fences are needed when there is insufficient space for a detention pond or when roads and other structures are in the way. Adequate Amount of Fencing The amount of fencing means the total linear length of the silt fencing runs on the construction site. A reasonable rule -of -thumb for the proper amount of silt fence is -100 ft of silt fence per 10,000 square foot (sq ft) of disturbed area. Soil type, slope, slope length, rainfall, and site configuration are all important elements in determining the adequate silt fence protection for a site, and to what extent it fits the 100 ft per 10,000 sq ft rule -of -thumb. If the amount of fencing provides the volume of runoff storage needed, then over -flowing the silt fence runs will be minimized. This is the basic test; if fences are over -flowing after a moderate rainfall event, the amount of fencing probably needs to be increased to avoid undercutting, washouts, and fence failures. Stormwater Best Management Practice: Silt Fences Appropriate Materials There are different types of porous fabrics available, e.g., woven, non -woven, mono -filament, but all types tend to clog rapidly and don't provide lasting filtration. The support posts and installation method are more important than the fabric type for overall sediment retention. However, a lightweight fabric tends to tear where it is attached to the posts. Posts must hold the fabric up and support the horizontal load of retained water and sediment. Hardwood posts (2" x 2") are potentially strong enough to support the loads, but are difficult to drive into the ground more than 6-8". To hold 2 ft of sediment and water, the posts should be driven 2 ft into the ground. Steel posts are best because they can be driven into compacted soil to a depth of 2 ft. The support posts should be spaced 3-4 ft apart where water may run over the top of the fence, 5 ft in most other areas, and 6-7 ft where there isn't a considerable horizontal load. Improper post depth and spacing is often the cause of sagging fabric and falling posts. Some authorities believe a more robust wire or chain link supported silt fence is needed to withstand heavy rain events. However, this may double the cost of a silt fence installation and entails disposing of more material in a landfill when the fence is removed. Installing silt fencing having five interacting features: (1) proper placement based on the site's contours, (2) adequate amount of fencing without long runs, (3) heavy porous filter fabric, (4) metal posts with proper depth and spacing, and (5) tight soil compaction on both sides of the silt fence will usually obviate the need for wire or chain link reinforced fencing. Prefabricated silt fences, e.g., fabric attached to wooden posts in a 100 ft package, doesn't provide for posting after the Figure 11. Chain link supported silt fence ground is compacted or allow variable post spacing. Silt Fence Installation Two commonly used approaches for installing silt fences are the static slicing method and the trenching method. Static Slicing Method The static slicing machine pulls a narrow blade through the ground to create a slit 12" deep, and simultaneously inserts the silt fence fabric into this slit behind the blade. The blade is 3 designed to slightly disrupt soil upward next to the slit and to minimize horizontal compaction, thereby creating an optimum condition for compacting the soil vertically on both sides of the fabric. Compaction is achieved by rolling a tractor wheel along both sides of the slit in the ground 2 to 4 times to achieve nearly the same or greater compaction as the original undisturbed soil. This vertical compaction reduces the air spaces between soil particles, which minimizes infiltration. Without this compaction infiltration can saturate the soil, and water may find a pathway under the fence. When a silt fence is holding back several tons of accumulated water and sediment, it needs to be supported by posts that are driven 2 ft into well - compacted soil. Driving in the posts and attaching the fabric to them completes the installation. Trenching Method Figure 12. Static slicing machine Figure 13. Tractor wheel compacting the soil Ponding heigght max. 24" POSTSPACING: 7' max. on open runs 4' max. on pooling areas Attach fabric to upstream side of post FLOW— Drive over each side of i', POST DEPTH: silt fence 2 to 4 times with device exerting As much below ground as fabric above ground 6O P.O. or greater 100%compaction 1DO%compaction i Figure 14. Silt fence installation using the static slicing method Trenching machines have been used for over twenty-five years to dig a trench for burying part of the filter fabric underground. Usually the trench is about 6" wide with a 6" excavation. Its walls are often more curved than vertical, so they don't provide as much support for the posts and fabric. Turning the trencher is necessary to maneuver around obstacles, follow terrain contours or property lines, and install upturns Figure 15. Trenchers make a wider or J -hooks. But trenchers excavation at turns Stormwater Best Management Practice: Silt Fences Figure 16. Poor compaction has resulted in infiltration and water flowing under this silt fence causing retained sediment washout can't turn without making a wider excavation, and this results in poorer soil compaction, which allows infiltration along the underground portion of the fence. This infiltration leads to water seeking pathways under the fence, which causes subsequent soil erosion and retained sediment washout under the fence. The white line on the fence in Figure 16 and red arrow both mark the previous sediment level before the washout. Post setting and fabric installation often precede compaction, which make effective compaction more difficult to achieve. EPA supported an independent technology evaluation (ASCE 2001), which compared three progressively better variations of the trenching method with the static slicing method. The static slicing method performed better than the two lower performance levels of the trenching method, and was as good or better than the trenching method's highest performance level. The best trenching method typically required nearly triple the time and effort to achieve results comparable to the static slicing method. Proper Attachment Regardless of the installation method, proper attachment of the fabric to the posts is critical to combining the strength of the fabric and support posts into a unified structure. It must be able to support 24" of sediment and water. For steel posts use three plastic ties per post (50 Ib test strength), located in the top 8" of the fabric, with each tie hung on a post nipple, placed diagonally to attach as many vertical and horizontal threads as possible. For wooden posts use several staples per post, with a wood lath to overlay the fabric. Perimeter Silt Fences When silt fences are placed around the perimeter of a stock pile or a construction site, the conventional silt fence design and materials discussed previously may not be sufficient. Stock pile example. A stock pile of dirt and large rocks is shown in Figures 17 and 18 with a silt fence protecting a portion of its perimeter. Rocks that roll down the pile would likely Figure 17. Back of silt fence on part of the stock pile's perimeter damage a conventional silt fence. The bottom of the porous fabric is held firmly against both the ground and base of precast concrete, highway, barriers by light-colored stones. An alternative installation would be having the concrete barriers rest directly on the Figure 18. Front of silt fence on part of stock pile's perimeter bottom edge of the filter fabric, which would extend under the barriers about 10", so the barriers' weight will press the fabric against the ground to prevent washout. Water passing through the silt fence (red arrow in Figure 18) flows to a storm sewer culvert inlet, which is surrounded by a fabric silt fence (yellow arrows in Figures 17 and 18) that reduces the runoff's velocity and allows settling before the water is discharged to a creek. Bridge abutment example. During the construction of a bridge over a river between two lakes, an excavation on the river bank was needed to pour footings for the bridge abutment. The silt fence along the excavation's perimeter, composed of concrete highway barriers with orange filter fabric, was designed to prevent stormwater from washing excavated spoil into the river and to fend off the river during high flows. A portion of the orange filter fabric that has blown away from the concrete barriers shows the need to overlap and reinforce the joints where two sections of filter fabric are attached. Figure 19. Silt fence for bridge abutment excavation Highway example. Because of the proximity of a construction site to a highway, a concrete barrier was required by Minnesota's DOT to protect the highway and an underground fiber optic cable next to the highway from construction activities. The concrete barrier was used to support a silt fence along the perimeter of a large amount of dirt that was stock piled before being used for fill at a different location. Figure 20. Silt fence protecting a highway and underground fiber optics cable Stormwater Best Management Practice: Silt Fences Figure 21. Silt fence protecting a lake shore Lake shore example. The lake's shoreline is being restored with plant plugs and seeded with native plant species. A plywood, perimeter, silt fence is used to trap sediment from a construction site on the right -side of the picture, protect the lake shore from boat -wake erosion, and to prevent geese from eating the seeds and young plants. This fencing will be removed when 70% vegetative cover is achieved. Inspection and Maintenance Figure 22. A silt fence full of sediment that needs maintenance Permanent Soil Stabilization When the land disturbing activities are sufficiently completed to allow permanent soil stabilization on the site, the silt fences and sediment basins are removed. The fabric and damaged posts go to the landfill. Steel posts and some of the wooden posts can be reused. Then the sediment is spread over the site to provide fertile soil, and the area can be seeded and mulched to support revegetation. References ASCE 2001. Environmental Technology Verification Report for Installation of Silt Fence Using the Tommy Static Slicing Method, CERF Report #40565. Washington, DC: American Society of Civil Engineers. www.epa.gov/etv/pubs/08 vs_tommy.pdf ASTM 2003. Standard Practice for Silt Fence Installation, Silt fences should be D 6462-03(2008). West Conshohocken, PA: American Society inspected routinely and of Testing Materials International. after runoff events to www.astm.org/SEARCH/search-reskin.html?query-D6462- determine whether they 03&siteTypestore-standards&searchType-standards-full need maintenance because they are full (Figure 22) or damaged by construction equipment. The ASTM silt fence specification (ASTM 2003) recommends removing sediment deposits from behind the fence when they reach half the height of the fence or installing a second fence. However, there are several problems associated with cleaning out silt fences. Once the fabric is clogged with sediment, it can no longer drain slowly and function as originally designed. The result is normally a low volume sediment basin because the cleaning process doesn't unclog the fabric. The soil is normally very wet behind a silt fence, inhibiting the use of equipment needed to move it. A back hoe is commonly used, but, if the sediment is removed, what is to be done with it during construction? Another solution is to leave the sediment in place where it is stable and build a new silt fence above or below it to collect additional sediment as shown in Figure 23. The proper maintenance may be site specific, e.g. small construction sites might not have sufficient space for another silt fence. Adequate access to the sediment control devices should be provided so inspections and maintenance can be performed. Figure 23. New silt fence below the old fence Carpenter, Thomas 2000. Silt Fence That Works. Ankey, Iowa: Thomas Carpenter. www.tommy-sfm.com/pages/resources/ Silt%20Fence%2OThat%2OWorks%2OManual.pdf Fifield, Jerald S. 2011. Designing and Reviewing Effective Sediment and Erosion Control Plans, 3rd Edition. Santa Barbara, CA: Forester Press. U.S. Environmental Protection Agency 2007. Developing Your Stormwater Pollution Prevention Plan, EPA 833-R-06-004. Washington: EPA. Available from EPA hardcopy 800-490-9198 or www.epa.gov/npdes/pubs/sw swppp guide.pdf Photograph Credits Figures 1-10, 12-16, 22, 23. Thomas Carpenter, CPESC, Carpenter Erosion Control Figure 11. Pete Schumann, Fairfax County, Virginia, Department of Public Works and Environmental Services Figure 17-21. Dwayne Stenlund, CPESC, Minnesota Department of Transportation Disclaimer Please note that EPA has provided external links because they provide additional information that may be useful or interesting. EPA cannot attest to the accuracy of non -EPA information provided by these third -party websites and does not endorse any non-government organizations or their products or services. Storm Drain Inlet Protection EPA NPDES Fact Sheets Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=60&minmeasure=4# Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Description Storm drain inlet protection measures prevent soil and debris from entering storm drain drop inlets. These measures are usually temporary and are implemented before a site is disturbed. There are several types of inlet protection: Excavation around the perimeter of the drop inlet: Excavating a small area around an inlet creates a settling pool that removes sediments as water is released slowly into the inlet through small holes protected by gravel and filter fabric. are many different ways to it sediment from entering drains. Fabric barriers around inlet entrances: Erecting a barrier made of porous fabric around an inlet creates a shield against sediment while allowing water to flow into the drain. This barrier slows runoff while catching soil and other debris at the drain inlet. Block and gravel protection: Standard concrete blocks and gravel can be used to form a barrier to sediments that permits water runoff to flow through select blocks laid sideways. Sandbags can also be used to create temporary sediment barriers at inlets. For permanent inlet protection after the surrounding area has been stabilized, sod can be installed. This permanent measure is an aesthetically pleasing way to slow stormwater near drop inlet entrances and to remove sediments and other pollutants from runoff. Applicability All temporary inlet protection should have a drainage area no greater than 1 acre per inlet. Temporary controls should be constructed before the surrounding landscape is disturbed. Excavated drop inlet protection and block and gravel inlet protection are applicable to areas of high flow, where drain overflow is expected. Fabric barriers are recommended for smaller, flatter drainage areas (slopes less than 5 percent leading to the drain). Temporary drop inlet control measures are often used in sequence or with other erosion control techniques. Siting and Design Considerations With the exception of sod drop inlet protection, install these controls before any soil disturbance in the drainage area. Excavate around drop inlets at least 1 foot deep (2 feet maximum), excavating a volume of at least 35 yd per acre disturbed. Side slopes leading to the inlet should be no steeper than 2:1. Design the shape of the excavated area such that the dimensions fit the area from which stormwater is expected to drain. For example, the longest side of an excavated area should be along the side of the inlet expected to drain the largest area. Stake fabric inlet protection close to the inlet to prevent overflow onto unprotected soils. Stakes should be at least 3 feet long and spaced no more than 3 feet apart. Construct a frame for fabric support during overflow periods, and bury it at least 1 foot below the soil surface. It should rise to a height no greater than 1.5 feet above the ground. The top of the frame and fabric should be below the downslope ground elevation to keep runoff from bypassing the inlet. Block and gravel inlet barriers should be at least 1 foot high (2 feet maximum). Do not use mortar. Lay the bottom row of blocks at least 2 inches below the soil surface, flush against the drain for stability. Place one block in the bottom row on each side of the inlet on its side to allow drainage. Place 1/2 -inch wire mesh over all block openings to prevent gravel from entering the inlet. Place gravel (3/4 to 1/2 inch in diameter) outside the block structure at a slope no greater than 2:1. Do not consider sod inlet protection until the entire surrounding drainage area is stabilized. Lay the sod so that it extends at least 4 feet from the inlet in each direction to form a continuous mat around the inlet. Lay the sod strips perpendicular to the direction of flows. Stagger them so that the strip ends are not aligned. The slope of the sodded area should not be steeper than 4:1 approaching the drop inlet. Limitations To increase the effectiveness of these practices, use them with other measures, such as small impoundments or sediment traps (USEPA, 1992). In general, stormwater inlet protection measures are practical for areas receiving relatively clean runoff that is not heavily laden with sediment. They are designed to handle drainage from areas less than 1 acre (CASQA, 2003). To prevent clogging, storm drain control structures must be maintained frequently. If sediment and other debris clog the water intake, drop inlet control measures can actually cause erosion in unprotected areas. Maintenance Considerations Check all temporary control measures after each storm event. To maintain the capacity of the settling pools, remove accumulated sediment from the area around the drop inlet (excavated area, area around fabric barrier or block structure) when the capacity is reduced by half. Remove additional debris from the shallow pools periodically. The weep holes in excavated areas around inlets can become clogged, preventing water from draining out of the pools. If that happens, it might be difficult and costly to unclog the intake. Effectiveness Excavated drop inlet protection can be used to improve the effectiveness and reliability of other sediment traps and barriers, such as fabric or block and gravel inlet protection. The effectiveness of inlet protection alone is low for erosion and sediment control, long-term pollutant removal, and habitat and stream protection. Cost Considerations The cost of implementing storm drain inlet protection measures varies depending on the control measure used. Initial installation costs range from $50 to $150 per inlet depending on the materials used, with an average cost of $100 (USEPA, 1993). Maintenance costs can be high (up to 100 percent of the initial construction cost annually) because of the frequent inspection and repair needs. The Southeastern Wisconsin Regional Planning Commission has estimated the cost of installing inlet protection devices at $106 to $154 per inlet (SEWRPC, 1991). References California Stormwater Quality Association (CASQA). 2003. Stormwater Best Management Practice Handbook: Construction. [http://www.cabmphandbooks.com/ I EXIT oisriaimer]. Accessed May 8, 2006. SEWRPC (Southeastern Wisconsin Regional Planning Commission). 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures. Technical report no. 31. Southeastern Wisconsin Regional Planning Commission, Waukesha, WI. Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R- 92-006. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Vegetated Buffers EPA NPDES Fact Sheets Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=50&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Sediment Control Buffers at the perimeters of construction sites are similar to agricultural buffers in that they trap sediments and remove pollutants in runoff from exposed areas (Source: Noma Scotia Department of Agri culture and Fisheries, 2000) Description Vegetated buffers are areas of natural or established vegetation maintained to protect the water quality of neighboring areas. Buffer zones slow stormwater runoff, provide an area where runoff can permeate the soil, contribute to ground water recharge, and filter sediment. Slowing runoff also helps to prevent soil erosion and streambank collapse. Applicability Vegetated buffers can be used in any area able to support vegetation. They are most effective and beneficial on floodplains, near wetlands, along streambanks, and on unstable slopes. Siting and Design Considerations To establish an effective vegetative buffer, follow these guidelines: Make sure soils are not compacted. Make sure slopes are less than 5 percent unless temporary erosion control mats are also used. Determine buffer widths after carefully considering slope, vegetation, soils, depth to impermeable layers, runoff sediment characteristics, type and amount of pollutants, and annual rainfall. Make sure buffer widths increase as slope increases. Intermix zones of vegetation (native vegetation in particular), including grasses, deciduous and evergreen shrubs, and understory and overstory trees. In areas where flows are concentrated and fast, combine buffer zones with other practices such as level spreaders, infiltration areas, or diversions to prevent erosion and rilling. Limitations Adequate land must be available for a vegetated buffer. If land cost is high, buffer zones might not be cost-effective. In addition, adequate vegetative cover must be maintained in the buffer to keep it effective. Vegetated buffers work well with sheet flows, but they are not appropriate for mitigating concentrated stormwater flows. Maintenance Considerations Keeping vegetation healthy in vegetated buffers requires routine maintenance. Depending on species, soil types, and climatic conditions, maintenance can include weed and pest control, mowing, fertilizing, liming, irrigating, and pruning. Inspection and maintenance are most important when buffer areas are first installed. Once established, vegetated buffers do not require maintenance beyond the routine procedures and periodic inspections. Inspect them after heavy rainfall and at least once a year. Focus on encroachment, gully erosion, the density of the vegetation, evidence of concentrated flows through the areas, and any damage from foot or vehicular traffic. If more than 6 inches of sediment has accumulated, remove it. Effectiveness Several studies indicate greater than 90 percent reductions in sediment and nitrate concentrations when vegetated buffers are used. Buffer/filter strips do a reasonably good job of removing phosphorus attached to sediment, but they are not so effective at removing dissolved phosphorus (Gilliam, 1994). References Gilliam, J.W. 1994. Reparian Wetlands and Water Quality. Journal of Environmental Quality 23:896-900. Cited in Michigan Department of Environmental Quality. 1998. Guidebook of Best Management Practices for Michigan Watersheds. Michigan Department of Environmental Quality, Surface Water Quality Division, Lansing, MI. Nova Scotia Department of Agriculture and Fisheries. 2000. Awareness and Communication Project Reports, Appendix E: Photographs [http://gov.ns.ca/nsaf/]. Accessed December 1, 2005. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R- 92-006. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1996. Protecting Natural Wetlands: A Guide to Stormwater Best Management Practices. EPA 843-B-96-001. U.S. Environmental Protection Agency, Office of Water, Washington, DC. Good Housekeeping Appendix J - Intrinsic GSP Specifications tormwater Best Management Practice Y Minimum Measure Construction Site Stormwater Runoff Control Subcategory Good Housekeeping/Materials Management Description of Concrete Washout at Construction Sites Concrete and its ingredients Concrete is a mixture of cement, water, and aggregate material. Portland cement is made by heating a mixture of limestone and clay containing oxides of calcium, aluminum, silicon and other metals in a kiln and then pulverizing the resulting clinker. The fine aggregate particles are usually sand. Coarse aggregate is generally gravel or crushed stone. When cement is mixed with water, a chemical reaction called hydration occurs, which produces glue that binds the aggregates together to make concrete Concrete washout After concrete is poured at a construction site, the chutes of ready mixed concrete trucks and hoppers of concrete pump trucks must be washed out to remove the remaining concrete before it hardens. Equipment such as wheelbarrows and hand tools also need to be washed down. At the end of each work day, the drums of concrete trucks must be washed out. This is customarily done at the ready mixed batch plants, which are usually off-site facilities, however large or rural construction projects may have on-site batch plants. Cementitious (having the properties of cement) washwater and solids also come from using such construction materials as mortar, plaster, stucco, and grout. Environmental and Human Health Impacts Concrete washout water (or washwater) is a slurry containing toxic metals. It's also caustic and corrosive, having a pH near 12. In comparison, Drano liquid drain cleaner has a pH of 13.5. Caustic washwater can harm fish gills and eyes and interfere with reproduction. The safe pH ranges for aquatic life habitats are 6.5 — 9 for freshwater and 6.5 — 8.5 for saltwater. Construction workers should handle wet concrete and washout water with care because it may cause skin irritation and eye damage. If the washwater is dumped on the ground (Fig. 1), it can run off the construction site to adjoining roads and enter roadside storm drains, which discharge to surface waters such as rivers, lakes, or estuaries. The red arrow in Figure 2 points to a ready mixed truck chute that's being washed out into a roll -off bin, which isn't watertight. Leaking washwater, shown in the foreground, will likely follow similar Figure 1. Chute washwater being dumped on the ground Figure 2. Chute washwater leaking from a roll -off bin being used as a washout container paths to nearby surface waters. Rainfall may cause concrete washout containers that are uncovered to overflow and also transport the washwater to surface waters. Rainwater polluted with concrete washwater can percolate down through the soil and alter the soil chemistry, inhibit plant growth, and contaminate the groundwater. Its high pH can increase the toxicity of other substances in the surface waters and soils. Figures 1 and 2 illustrate the need for better washout management practices. Best Management Practice Objectives The best management practice objectives for concrete washout are to (a) collect and retain all the concrete washout water and solids in leak proof containers, so that this caustic material does not reach the soil surface and then migrate to surface waters or into the ground water, and (b) recycle 100 percent of the collected concrete washout water and solids. Another Stormwater Best Management Practice: Concrete Washout objective is to support the diversion of recyclable materials from landfills. Table 1 shows how concrete washout materials can be recycled and reused. Table 1— Recycling concrete washout materials a. Fine particles of cementitious material (e.g., Portland cement, slag cement, fly ash, silica fume) b. Recyclable, if allowed by the concrete quality specifications c. Treated to reduce the pH and remove metals, so it can be delivered to a municipal wastewater treatment plant, where it is treated further and then returned to a natural surface water Washwater recycling, treatment, disposal Washwater from concrete truck chutes, hand mixers, or other equipment can be passed through a system of weirs or filters to remove solids and then be reused to wash down more chutes and equipment at the construction site or as an ingredient for making additional concrete. A three chamber washout filter is shown in Figure 3. The first stage collects the coarse aggregate. The middle stage filters out the Figure 3. Concrete washout small grit and sand. The third stage titter has an array of tablets that filter out fines and reduces the pH. The filtered washwater is then discharged through a filter sock. An alternative is to pump the washout water out of the washout container (Fig 4) and treat the washwater off site to remove metals and reduce its pH, so it can be delivered to a publicly owned treatment works (POTW), also known as a municipal wastewater treatment plant, which provides additional treatment allowing the washwater to be discharged to a surface water. The POTW should be 2 contacted to inquire about any pretreatment requirements, i.e., the National Pretreatment Standards for Prohibited Dischargers (40CFR 403.5) before discharging the washwater to the POTW. The washwater can also Figure 4. Vacuuming washwater out of a be retained in the washout washout container for treatment and reuse container and allowed to evaporate, leaving only the hardened cementitious solids to be recycled. Solids recycling The course aggregate materials that are washed off concrete truck chutes into a washout container can be either separated by a screen and placed in aggregate bins to be reused at the construction site or returned to the ready mixed plant and washed into a reclaimer (Fig. 5). When washed out into a reclaimer, the fine and course aggregates are separated out and placed in different piles or bins to be reused in making fresh concrete. Reclaimers with settling tanks separate cement fines from the washwater, and these fines can also be used in new concrete unless prohibited by the user's concrete quality specifications. Figure 5. Ready mixed truck washing out into a reclaimer Hardened concrete recycling When the washwater in a construction site concrete washout container has been removed or allowed to evaporate, the hardened concrete that remains can be crushed (Fig. 6) and reused as a construction material. It makes an excellent aggregate for road base and can be used as fill at the Figure 6. Crushed concrete stockpile and crusher construction site or delivered to a recycler. Concrete recyclers can be found at municipal solid waste disposal facilities, private recycling plants, or large construction sites. Concrete Washout Materials N N N d QJ C1 C1 C C C C Uses of Recycled Materials � cl sJ �- CM 0 C) C _ C C> Reused to washout additional mixer truck chutes or drums x Reused as a ready mixed concrete ingredient x xb x x Reused as an ingredient of precast concrete products, e.g., highway x x x x x barriers, retaining wall blocks, riprap Reused as crushed concrete products, e.g., road base or fill x x x x Reused to pave the yards of ready mixed concrete plants x Returned back to a surface water, e.g., river, lake, or estuary x� a. Fine particles of cementitious material (e.g., Portland cement, slag cement, fly ash, silica fume) b. Recyclable, if allowed by the concrete quality specifications c. Treated to reduce the pH and remove metals, so it can be delivered to a municipal wastewater treatment plant, where it is treated further and then returned to a natural surface water Washwater recycling, treatment, disposal Washwater from concrete truck chutes, hand mixers, or other equipment can be passed through a system of weirs or filters to remove solids and then be reused to wash down more chutes and equipment at the construction site or as an ingredient for making additional concrete. A three chamber washout filter is shown in Figure 3. The first stage collects the coarse aggregate. The middle stage filters out the Figure 3. Concrete washout small grit and sand. The third stage titter has an array of tablets that filter out fines and reduces the pH. The filtered washwater is then discharged through a filter sock. An alternative is to pump the washout water out of the washout container (Fig 4) and treat the washwater off site to remove metals and reduce its pH, so it can be delivered to a publicly owned treatment works (POTW), also known as a municipal wastewater treatment plant, which provides additional treatment allowing the washwater to be discharged to a surface water. The POTW should be 2 contacted to inquire about any pretreatment requirements, i.e., the National Pretreatment Standards for Prohibited Dischargers (40CFR 403.5) before discharging the washwater to the POTW. The washwater can also Figure 4. Vacuuming washwater out of a be retained in the washout washout container for treatment and reuse container and allowed to evaporate, leaving only the hardened cementitious solids to be recycled. Solids recycling The course aggregate materials that are washed off concrete truck chutes into a washout container can be either separated by a screen and placed in aggregate bins to be reused at the construction site or returned to the ready mixed plant and washed into a reclaimer (Fig. 5). When washed out into a reclaimer, the fine and course aggregates are separated out and placed in different piles or bins to be reused in making fresh concrete. Reclaimers with settling tanks separate cement fines from the washwater, and these fines can also be used in new concrete unless prohibited by the user's concrete quality specifications. Figure 5. Ready mixed truck washing out into a reclaimer Hardened concrete recycling When the washwater in a construction site concrete washout container has been removed or allowed to evaporate, the hardened concrete that remains can be crushed (Fig. 6) and reused as a construction material. It makes an excellent aggregate for road base and can be used as fill at the Figure 6. Crushed concrete stockpile and crusher construction site or delivered to a recycler. Concrete recyclers can be found at municipal solid waste disposal facilities, private recycling plants, or large construction sites. Stormwater Best Management Practice: Concrete Washout Wet concrete recycling Builders often order a little more ready mixed concrete than they actually need, so it is common for concrete trucks to have wet concrete remaining in their drum after a delivery. This unused concrete can be returned to the ready mixed plant and either (1) used to pour precast concrete products (e.g., highway barriers, retaining wall blocks, riprap), (2) used to pave the ready mixed plant's yard, (3) washed into a reclaimer, or (4) dumped on an impervious surface and allowed to harden, so it can be crushed and recycled as aggregate. Unused wet concrete should not be dumped on bare ground to harden at construction sites because this can contribute to ground water and surface water contamination. Washout Containers Different types of washout containers are available for collecting, retaining, and recycling the washwater and solids from washing down mixed truck chutes and pump truck hoppers at construction sites. Chute washout box A chute washout box is mounted on the back of the ready mixed truck. If the truck has three chutes, the following procedure is used to perform the washout from the top down: (1) after the pour is completed, the driver attaches the extension chute to the washout box, (2) the driver then rotates the main chute over the extension chute (Fig. 7) and washes down the hopper first then the main chute, (3) finally the driver washes down the flop down chute and last the extension chute hanging on the box. All washwater and solids are captured in the box. Figure 7. Chute washout box After the wash down, washwater and solids are returned to the ready mixed plant for recycling. A filter basket near the top of the washout box separates out the coarse aggregates so they can be placed in a bin for reuse either at the construction site or back at the cement plant. Chute washout bucket and pump After delivering ready mixed concrete and scraping the last of the customer's concrete down the chute, the driver hangs a washout bucket shown in Figure 8 (see red arrow) on the end of the truck's chute and secures the hose to insure no leaks. The driver then washes down the chute into the bucket to remove any cementitious material before it hardens. After washing out the chute, the driver pumps (yellow arrow points to the pump) the washwater, sand, and other fine solids from the bucket up into the truck's drum to be returned to the Figure 8. Chute washout bucket and pump ready mixed plant, where it can be washed into a reclaimer. A removable screen at the bottom of the washout bucket prevents course aggregate from entering the pump. This course aggregate can also be returned to the plant and added to the coarse aggregate pile to be reused. All the materials are recycled. Hay bale and plastic washout pit A washout pit made with hay bales and a plastic lining is shown in Figure 9. Such pits can be dug into the ground or built above grade. The plastic lining should be free of tears or holes that would allow the washwater to escape (Fig. 10). After the pit is used to wash down the chutes of multiple ready mixed trucks and the washwater has evaporated or has been vacuumed off, the remaining hardened solids can be broken up and removed from the pit. This process may damage the hay bales and plastic lining. If damage occurs, the pit will need to be repaired and relined with new plastic. When the hardened solids are removed, they may be bound up with the plastic lining and have to be sent to a landfill, rather than recycled. Recyclers usually accept only unmixed material. If the pit is going to be emptied and repaired more than a few times, the hay bales and plastic will be generating additional solid waste. Ready mixed concrete bale and plastic Figure 10. Leaking washout pit that has not been well maintained Stormwater Best Management Practice: Concrete Washout trucks can use hay bale washout pits, but concrete pump trucks have a low hanging hopper in the back that may prevent their being washed out into bale -lined pits. Vinyl washout container The vinyl washout container (Fig. 11) is portable, reusable, and easier to install than a hay bale washout pit. The biodegradable filter Figure 11. Vinyl washout pit with filter bag bag (Fig. 12) assists in extracting the concrete solids and prolongs the life of the vinyl container. When the bag is lifted, the water is filtered out and the remaining concrete solids and the bag can be disposed of together in a landfill, or the hardened concrete can be delivered to a recycler. After the solids have been removed several times and the container is full of washwater, the washwater can be allowed to evaporate, so the container can be reused. The washwater can be removed more quickly by placing another filter bag in the container and spreading water gelling granules evenly across the water. In about five minutes, the water in the filter bag will turn into a gel that can be removed with the bag. Then the gel and filter bag can be disposed to together. Metal washout container Figure 12. Extracting the concrete solids or gelled washwater The metal roll -off bin (Fig. 13) is designed to securely contain concrete washwater and solids and is portable and reusable. It also has a ramp that allows concrete pump trucks to wash out their hoppers (Fig. 14). Roll -off providers offer recycling services, such as, picking up the roll -off bins after the washwater has evaporated and the solids have hardened, replacing them with empty washout bins, and delivering the hardened concrete to a recycler (Fig. 15), rather than a landfill. Some providers will vacuum off the washwater, treat it to remove metals and reduce the pH, deliver it to a wastewater treatment plant for additional treatment and Figure 13. Mixer truck being washed out into a roll -off bin Il subsequent discharge to a surface water. Everything is recycled or treated sufficiently to be returned to a natural surface water. ,np truck using the out into a roll -off bin Figure 15. Delivering hardened Concrete to a recycler Another metal, portable, washout container, which has a rain cover to prevent overflowing, is shown in Figure 16. It is accompanied by an onsite washwater treatment unit, which reduces the pH and uses a forced weir tank system to remove the coarse aggregate, fine aggregate, and cement fines. The washwater can then be reused at the construction site to wash out other mixer truck chutes ana equlpmem. Figure 16. Washout container with a rain cover and The solids are onsite washwater treatment allowed to harden together and can be taken to a concrete recycler (Fig. 17) to be crushed and used as road base or aggregate for making precast products, such as retaining wall blocks. All materials are recycled. Figure 17. Delivering hardened concrete to a recycler Siting Washout Facilities Concrete washout facilities, such as washout pits and vinyl or metal washout containers, should be placed in locations that provide convenient access to concrete trucks, preferably near the area where concrete is being poured. However they Stormwater Best Management Practice: Concrete Washout should not be placed within 50 feet of storm drains, open ditches, or waterbodies. Appropriate gravel or rock should cover approaches to concrete washout facilities when they are located on undeveloped property. On large sites with extensive concrete work, washouts should be placed at multiple locations for ease of use by ready mixed truck drivers. If the washout facility is not within view from the pour location, signage will be needed to direct the truck drivers. Operating and Inspecting Washout Facilities Concrete washout facilities should be inspected daily and after heavy rains to check for leaks, identify any plastic linings and sidewalls have been damaged by construction activities, and determine whether they have been filled to over 75 percent capacity. When the washout container is filled to over 75 percent of its capacity, the washwater should be vacuumed off or allowed to evaporate to avoid overflows. Then when the remaining cementitious solids have hardened, they should be removed and recycled. Damages to the container should be repaired promptly. Before heavy rains, the washout container's liquid level should be lowered or the container should be covered to avoid an overflow during the rain storm. Educatinq Concrete Subcontractors The construction site superintendent should make ready mixed truck drivers aware of washout facility locations and be watchful for improper dumping of cementitious material. In addition, concrete washout requirements should be included in contracts with concrete delivery companies. Reference NRMCA 2009. Environmental Management in the Ready Mixed Concrete Industry, 2PEMRM, 1st edition. By Gary M. Mullins. Silver Springs, MD: National Ready Mixed Concrete Association. Websites and Videos Construction Materials Recycling Association www.concreterecycling.org National Ready Mixed Concrete Association www.nrmca.org National Ready Mixed Concrete Research and Education Foundation www.rmc-foundation.org Additional information and videos on concrete washout containers and systems can be found by a web search for "concrete washout." PhotoaraDh Credits Figures 1, 2. Mark Jenkins, Concrete Washout Systems, Inc. Figure 3. Mark Shaw, Ultra Tech International, Inc. Figure 4. Mark Jenkins, Concrete Washout Systems, Inc. Figure 5. Christopher Crouch, CCI Consulting Figure 6. William Turley, Construction Materials Recycling Association Figure 7. Brad Burke, Innovative Concrete Solutions, LLC Figure 8. Ron Lankester, Enviroguard Figures 9, 10. Mark Jenkins, Concrete Washout Systems, Inc. Figures 11, 12. Tom Card, RTC Supply Figures 13, 14, 15. Mark Jenkins, Concrete Washout Systems, Inc. Figures 16, 17. Rick Abney Sr., Waste Crete Systems, LLP Disclaimer Please note that EPA has provided external links because they provide additional information that may be useful or interesting. EPA cannot attest to the accuracy of non -EPA information provided by these third -party websites and does not endorse any non-government organizations or their products or services. Spill Prevention and Control Plan EPA NPDES Fact Sheets Date accessed 10-25-13 http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=factsheet results&view=specific&bmp=62&minmeasure-4 Minimum Measure: Construction Site Stormwater Runoff Control Subcategory: Good Houskeeping/Materials Management Absorbent snakes can be used to protect storm drains from spills (Sp i11911, no date) Description Spill Prevention and Control Plans (SPCP) should clearly state measures to stop the source of a spill, contain the spill, clean up the spill, dispose of contaminated materials, and train personnel to prevent and control future spills. Applicability SPCPs are applicable to construction sites where hazardous wastes are stored or used. Hazardous wastes include pesticides, paints, cleaners, petroleum products, fertilizers, and solvents. Siting and Design Considerations When developing an SPCP, a construction site operator should identify potential spill or source areas, such as loading and unloading, storage, and processing areas; places where dust or particulate matter is generated; and areas designated for waste disposal. Also, evaluate spill potential for stationary facilities, including manufacturing areas, warehouses, service stations, parking lots, and access roads. Conduct this evaluation during the project planning phase, and reevaluate it during each phase of construction. The SPCP should define material handling procedures and storage requirements and outline actions necessary to reduce spill potential and impacts on stormwater quality. This can be achieved by: Recycling, reclaiming, or reusing process materials, thereby reducing the amount of process materials that are brought into the facility Installing leak detection devices, overflow controls, and diversion berms Disconnecting any drains from processing areas that lead to the storm sewer Performing preventative maintenance on storm tanks, valves, pumps, pipes, and other equipment Using material transfer procedures or filling procedures for tanks and other equipment that minimize spills Substituting less or non-toxic materials for toxic materials The SPCP should document the locations of spill response equipment and procedures to be used and ensure that procedures are clear and concise. The plan should include step-by-step instructions for the response to spills at a facility. In addition, the spill response plan should: Identify individuals responsible for implementing the plan Define safety measures to be taken with each kind of waste Specify how to notify appropriate authorities, such as police and fire departments, hospitals, or municipal sewage treatment facilities for assistance State procedures for containing, diverting, isolating, and cleaning up the spill Describe spill response equipment to be used, including safety and cleanup equipment The plan can be a procedural handbook or a poster to be placed in several locations at the site. Limitations Training is necessary to ensure that all workers are knowledgeable enough to follow procedures outlined in the SPCP. Make equipment and materials for cleanup readily accessible, and mark them clearly so workers can follow procedures quickly and effectively. Maintenance Considerations Update the SPCP regularly to accommodate any changes in the site, procedures, or responsible staff. Conduct regular inspections in areas where spills might occur to ensure that procedures are posted and cleanup equipment is readily available. Effectiveness An SPCP can be highly effective at reducing the risk of surface and ground water contamination; however, to ensure that procedures are followed, a construction site operator should provide worker training, appropriate materials and equipment for cleanup, and adequate staff time. Cost Considerations Spill prevention and control plans can be inexpensive to implement; however, adequate time and resources are needed to properly handle and dispose of spills. References Spill911. No date. Spill Containment: Oil and Sediment Curbguard. [www.spill91 Lcom EXIT disclaimer ]. Accessed November 10, 2005. USEPA (U.S. Environmental Protection Agency). 2006. Spill Prevention, Control and Countermeasure Guides. http://www.epa.gov/emergencies/content/spcc/index.htm. Accessed May 15, 2006. USEPA (U.S. Environmental Protection Agency). 1992a. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-005. U.S. Environmental Protection Agency, Office of Water, Washington, DC. USEPA (U.S. Environmental Protection Agency). 1992b. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-006. U.S. Environmental Protection Agency, Office of Water, Washington, DC. NORTHWEST ARKANSAS DEMOCRAT -GAZETTE NORTHWESTARKANSAS THE MORNING NEWS OF SPRINGDALE S_P_M_E:D.S THE MORNING NEWS OF ROGERS NORTHWEST ARKANSAS TIMES LLC BENTON COUNTY DAILY RECORD 212 NORTH EAST AVENUE, FAYETTEVILLE, ARKANSAS 72701 1 P.O. BOX 1607, 72702 479.442.1700 1 WWW.NWANEWS.COM AFFIDAVIT OF PUBLICATION I, Karen Caler, do solemnly swear that I am the Legal Clerk of the Northwest Arkansas Newspapers, LLC, printed and published in Washington and Benton County, Arkansas, bona fide circulation, that from my own personal knowledge and reference to the files of said publication, the advertisement of: City of Fayetteville - Ordinance 5702 Was inserted in the Regular Editions on: August 14, 2014 Publication Charges: $ 136.19 (-� ali-tj CC&Qk___, Karen Caler Subscribed and sworn to before me This L i;�- day of O , 2014. Notaryublic T /� MyCo! [T!f[I ssLLp Y1 pirea. li ■ CATHY WILES Arkansas • Benton County Notary Public • Comm# 12397118 My Commission Expires Feb 20, 2024 * *NOTE* * Please do not pay from Affidavit. Invoice will be sent. RECEIVED AUG 2 5 2014 CITY OF FAYETTEVILLE CITY CLERK'S OFFICE O ""INANCE AN ORDINANCE TOIAMEND § 156 04 STORAAWATERANAGE AND EROSION "� CONTROL CHAPTER . 169 PHYSICAL �� ALTERATION . Ol` R DLAND C,I APTER _ 'ARKANSs S, 170 `3TORMMINATER MANAGEMENT, w DRgINAGE �ANDROSION CONTROL AND CHAPTER 179. LOW IMPACT , REVELOPM�NT AF THE UNIFIED DEUELOPMENT.CODE, ,. ,' WHER)_AS�the Cityy�s4Dra�nage Gnter�a Ma7ival Lias been successful m mini mrzing floo 4ngyin large storjn everltsbgt does�raot address Juater quality does not pCotecpropyert�es�from damage stue�to smaller storm events and does not. ° consrdeNlow rmp�o�t davelo��pent�netl�o`ds and � INNERE�1�, in �ugu�t�2D��t�e G�sele4le�d antl�aWaed a oontract to %�i'N: Assoc�aas to corgptetely Ye�lee`�he�ily��Drarr��ge CrlteraatiM�artu�l to address WtE3EAS, adopfir.................................gthe r�ewsDwar ag'e,te�ilanualuviliv3�t�+a'ff adtli� t iobal filexrptlity to allow �es19�s t�a�mee�,the��Otent of`je�cR�e�aO��to ma1�e . kadgeeghe Drainage GptegMarSal astechnoaog,ar�d methbdsxevolue ` ,' N01NTHER�fOREBE IT. ORATNED�SYTFIE"CITY CbU1C)170 THEs C1T�YrOF"FYEITEI/ILLE, ARKANSAS E TION 1fltat3he Cdy CouOcil of the�C�fy of l ayetYyJlle; Afkanssrepeals § i5Fr045trfnvater Drainage antl Eros►�Ctrol and a ec#sep7acement„ - § 136„��Ohysrcal IAiteratwrt of Land aid Stormv�i a�e�r Di+a"inage aifidE�� soon Contfo 2sshowrr;on Exhrbrf A' attached herd ip L r r k z the Gty G4uncil he Ci yp� ett le Arkansasttepeals Gha er i6 Physrc�igiteratron of�Landantl enae 'A- a lacefnent Chaptere 169 Phreal Aiteratron of land as shown on Ezhrbit B attached hereto ETI 1i ,ThaY the�Grty Council�o� Ste Crty of Fayetteville Afkansas repeals Chapter X70 Stornwatec Manage ent, Drar�tage and Erosion Control and enacts a replace5ne%t ChapfeYl7 $to7mvfater M0'09 hent Drainage and' ErosionyControlasshown on Exhibit ;C� attached hereto $79bN 4tiate C Gpunc3l o the City of Fayettevillekitsa repaels -; Chapter:179 Low -Impact Development anderiacts areplab�ment Chapter 979 Lbw Im(Sact Development as shown ori Ezhrbil D' attaehetl hereto` SEOTION 5 Thathe City Cquncil o"the City ofayettevdle Arkansas hereby, approves antl adopts thereVised Drarra�e Gntena Manuals shownn Exhibit., attaeFied hereto, P.14SSED and APPROVED this 5th day'of August 2014 APPROUED ATTEST Y � LIONELD JORDAN Mayor- `- SONDRA E SMITH City ClerkJTreasurer� Exhibit"s for this ordinance may be vieWed in the office of thelCity Clerk% ' KTreasurer