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HomeMy WebLinkAbout2009-12-14 - Agendas - FinalPlanning Commission Planning Commissioners Officers Sean Trumbo, Chair Audy Lack, Vice -Chair Matthew Cabe, Secretary aye evtlle ARKANSAS Tentative Agenda City of Fayetteville, Arkansas Planning Commission Meeting December 14, 2009 James Graves Craig Honchell Jeremy Kennedy Christine Myres Porter Winston Jim Zant A meeting of the Fayetteville Planning Commission will be held on December 14, 2009 at 5:30 PM in Room 219 of the City Administration Building located at 113 West Mountain Street, Fayetteville, Arkansas. Call to Order Roll Call Agenda Session Presentations, Reports and Discussion Items: 1. Retention Pond Safety Policy Report 2. Design Standards for Administrative Approval (following agenda) Consent: 1. Approval of the minutes from the Monday, November 23, 2009 meeting. 2. PPL 09-3290: (TWIN CREEKS VILLAGE LOT 19A, CMN II PH III, 172-211): Submitted by MCCLELLAND CONSULTING ENGINEERS for property located NW OF THE INTERSECTION OF E VAN ASCHE DRIVE AND N STEELE BOULEVARD. The property is zoned C-1, NEIGHBORHOOD COMMERCIAL and contains approximately 9.15 acres. The request is for a commercial subdivision with 8 lots. Planner: Jesse Fulcher Unfinished Business: 3. ADM 09-3466; (UDC AMENDMENTS, CHAPTER 151 AND 164): Submitted by City Planning Staff. The request is to amend several chapters of the Unified Development Code (UDC) to clarify vehicle related businesses, including vehicle repair, towing, and salvage. Planner: Jesse Fulcher New Business: 4. ADM 09-3490: (THE DICKSON STREET INN, 484): Submitted by RICHARD ALEXANDER for property located at 301 WEST DICKSON STREET. The request is for a variance from the outdoor lighting requirement of the Unified Development Code. Planner: Dara Sanders 5. LSD 09-3469: (PEDIATRIC DENTAL ASSOCIATES, 175): Submitted by BLAKE JORGENSEN for property located at THE SOUTH END OF VANTAGE DRIVE, S OF ARVEST BANK. The property is zoned C-1, NEIGHBORHOOD COMMERCIAL AND C- 2, THOROUGHFARE COMMERCIAL and contains approximately 1.79 acres. The request is for a children's dental facility with associated infrastructure and parking. Planner: Jesse Fulcher 6. CUP 09-3476: (PEDIATRIC DENTAL ASSOCIATES, 175): Submitted by BLAKE JORGENSEN for property located at THE SOUTH END OF VANTAGE DRIVE, S OF ARVEST BANK. The property is zoned C-1, NEIGHBORHOOD COMMERCIAL AND C- 2, THOROUGHFARE COMMERCIAL and contains approximately 1.79 acres. The request is to provide more parking than allowed by ordinance. Planner: Jesse Fulcher 7. CUP 09-3474: (CITY OF FAYETTEVILLE /BRYCE DAVIS DOG PARK, 401): Submitted by CITY OF FAYETTEVILLE for property located at 1595 DARTMOUTH. The property is zoned P-1, INSTITUTIONAL and R-PZD, RESIDENTIAL PLANNED ZONING DISTRICT LINKS AT FAYETTEVILLE, and contains approximately 26 acres. The request is to provide less parking than required by ordinance for a new dog park Planner: Andrew Garner NOTICE TO MEMBERS OF THE AUDIENCE All interested parties may appear and be heard at the public hearings. If you wish to address the Planning Commission on an agenda item please queue behind the podium when the Chair asks for public comment. Once the Chair recognizes you, go to the podium and give your name and address. Address your comments to the Chair, who is the presiding officer. The Chair will direct your comments to the appropriate appointed official, staff, or others for response. Please keep your comments brief, to the point, and relevant to the agenda item being considered so that everyone has a chance to speak. Interpreters or TDD, Telecommunication Device for the Deaf, are available for all public hearings; 72 hour notice is required. For further information or to request an interpreter, please call 575-8330. As a courtesy please turn off all cell phones and pagers. A copy of the Planning Commission agenda and other pertinent data are open and available for inspection in the office of City Planning (575-8267), 125 West Mountain Street, Fayetteville, Arkansas. All interested parties are invited to review the petitions. a' teville y ARKANSAS THE CITY OF FAYETTEVILLE, ARKANSAS PC Meeting of December 14, 2009 PLANNING DIVISION CORRESPONDENCE 125 W. Mountain St. Fayetteville, AR 72701 Telephone: (479) 575-8267 TO: Fayetteville Planning Commission FROM: Andrew Garner, Senior Planner Matt Casey, Assistant City Engineer THRU: Jeremy Pate, Development Services Director DATE: December 3, 2009 ATT: Retention Pond Safety Policy Report At the September 14, 2009 Planning Commission, the Happy Hollow Elementary School Large Scale Development (LSD 09-3402) was approved with a wet retention pond for stormwater detention. During the approval process for that item the commission discussed safety concerns with a wet pond being in close proximity to school children. However, there is notan official City policy or ordinance requiring safety measures for retention ponds. Based on input from the commission at that meeting, the Strategic Planning and Internal Consulting Department conducted research on retention pond safety. This information and staff recommendations have been compiled in a report that is attached to this memorandum. This report is provided for informational purposes should the commission direct staff to proceed with further recommendations or direction on this topic. K:IReports120091PC Reports126-December 141Retention Pond Safety Info ltem.doc December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 1 of 44 Retention Pond Safety Policy Report Prepared by the City of Fayetteville Strategic Planning and Internal Consulting Department 125 W. Mountain St Fayetteville, Ar. 72701 479-575-8267 October 2009 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 2 of 44 Summary Strategic Planning staff has researched the policy alternatives for increasing the safety of retention ponds in the City of Fayetteville. Three alternatives were developed and researched that range from keeping the existing standards and regulations to requiring specific pond design and security fencing for all retention ponds. Strategic Planning recommends: • Amending or rewriting the Fayetteville Drainage to incorporate stormwater retention pond design into the regulations. • Allowing developers to install security fencing around retention ponds if it is warranted or desired by the developer or end user. December 14, 2009 Planning Commission - Agenda Session Presentation Retention Pond Safety Policy Report Page 3 of 44 Issue The Planning Division has requested that Strategic Planning staff research requiring retention pond fencing for safety purposes. Members of the Planning Commission expressed concern about the proposed expansion of Happy Hollow Elementary School that includes a wet retention pond on-site. The proposed retention pond would serve the dual purposes of capturing stormwater runoff and being utilized as an outdoor learning lab for children. The primary concern is that children might play in and around the retention pond unsupervised by adults. Background Retention ponds are designed to hold a specific amount of water indefinitely. Usually, the pond is designed to have drainage leading to another location when the water level gets above the pond capacity. Water levels may fluctuate but the pond will normally have water in it. A detention pond is a low lying area that is designed to temporarily hold a set amount of water while slowly draining to another location. They are designed to capture large amounts of water following a rain event and then release it slowly. Water levels will drop through release, infiltration and evaporation, and the pond will quickly return to a dry state. Analysis Staff began by reviewing the City's Drainage Manual and looking at the existing fencing regulations. Currently, the City does not require fencing around stormwater retention ponds. Additionally, the City's Drainage Manual does not regulate the overall physical design of retention ponds. Next staff did an internet search for policy papers related to retention pond safety and design. Because these types of stormwater facilities are in use all over the country, other municipalities have had the same safety concerns. A white paper analysis titled "Essential Safety Considerations for Urban Stormwater Retention and Detention Ponds" was published in the January/February 2006 issue of Stormzvater Magazine. The primary purpose of this article was to; 1. Review safety hazards associated with retention/detention ponds 2. Discuss techniques that can be utilized to minimize safety hazards 3. Review policy recommendations utilized by government and engineers December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 4 of 44 The authors of this study cited numerous pond safety deficiencies found with existing retention ponds. These included; • Outlet pipes that are open and lacking a debris screen. • General poor quality design in ponds that are in high pedestrian volume areas such as parks, schools and open spaces. This includes steeply sloped sides, tall retaining walls without safety rails, etc. • Lack of posted signage detailing the risk of rapidly rising flood waters. • Lack of shallow safety benches around the perimeter of the pond. • Pond inflow and outflow pipes placed close together which can create a dangerous situation for anyone in the pond. • Undersized spillways. • Inadequate maintenance. Strategic Planning staff then analyzed local, regional, and national peer cities to determine the best practices used by local governments to address retention pond safety. The following table shows the prevalence of fencing requirements or retention pond design specifications in local peer cities: City Retention Pond Fencing Required Retention Pond Design Specifications Fort Smith, AR Not required None Springdale, AR Not required None Rogers, AR Not required None Bentonville, AR Not required None This table shows the prevalence of fencing and retention pond design in regional peer cities: City Retention Pond Fencing Required Retention Pond Design Specifications Little Rock, AR Not required None specific to retention Tulsa, OK Not required Screened outflow pipes, Springfield, MO Not required The facility shall be designed using the most currently accepted methods of the soil conservation service, or the corps of engineers. Columbia, MO Not required Minimum depth of 4 ft. Requires a 15 ft. access area December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 5 of 44 This table illustrates some of the best practices used by cities nationwide: City Retention Pond Fencing Required adjacent to the pond for maintenance Lawrence, KS Not required None Champaign, IL Not required None Ames, IA Not required None Denton, TX Security fencing with a minimum height of 6 feet shall encompass the detention storage area if the water velocity, depth, or slopes create a potentially dangerous condition. None Iowa City, IA Not required None Urbana, IL Not required None This table illustrates some of the best practices used by cities nationwide: City Retention Pond Fencing Required Retention Pond Design Specifications Raleigh, NC Not required, if proposed fencing shall meet color and material specifications Depth, width and length specifications. May also require inclusion of pedestrian amenities such as trails, benches, picnic tables Greensborough, NC Not required Depth, width and length specifications De Pere, WI Required around all ponds None Lexington- Fayette County, KY Not required Provide a 10' wide shallow bench around the perimeter of the pond Fairfax County, VA Fencing required around facilities that have a drop of more than 3' 10 to 15 foot wide bench required, screened outflow pipe Asheville, NC A 6 foot tall chain link fence with locked entrance required Screened outflow pipe Kansas City, MO Not required Screened outflow pipe Criteria Based upon the research from other cities staff developed the following criteria for analyzing regulations or standards: December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 6 of 44 • The proposed regulation should address the valid safety concerns regarding small children in close proximity to retention ponds. • The proposed regulation should be as cost neutral as possible for the developer to construct and maintain. • The proposed regulation should be as aesthetically pleasing as possible. Policy Alternatives A total of three possible alternatives were explored by staff; 1. Keep existing regulations/status quo • Purpose: Keep current policy regarding stormwater detention/retention ponds. • Background: Currently the City does not require the fencing of retention pond perimeters or regulate retention pond construction design. None of the major NWA cities regulates stormwater retention/detention pond designs for safety purposes. The majority of stormwater ponds that have been constructed in Fayetteville have been dry detention ponds that periodically hold water. Staff analysis: The issue of stormwater retention pond safety arises periodically with development projects that have a large usage by children such as elementary schools, after school centers, etc. The depth and the shape of the proposed Happy Hollow School retention pond can be designed in such a way as to increase safety. • Cost: No increased cost. 2. Require fencing around retention ponds • Purpose: To ensure that retention/detention ponds are safe by making them inaccessible to the public. • Staff analysis: This policy is used in numerous municipalities and it has been shown to be effective. However, there are a couple of issues that would make this a difficult proposal for staff to support. First, it would require a significant expense for the developer/ development/ POA to build and maintain. Second, in order to provide a secure barrier, proposed fencing would not be aesthetically pleasing. In most cases it would consist of tall board fences or chain link. December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 7 of 44 Retention Pond Fencing Pro's and Con's Pro Con Safe and effective Additional significant expense for the developer Not aesthetically pleasing Increased maintenance costs • Cost: An effective fencing policy will require a significant investment by the developer and significant cost over time to maintain. Staff looked at the material cost of the 6 -foot chain link fencing that has been used by the Trails Department along Scull Creek Trail. The City paid $16 per linear foot, and it would be reasonable to expect private developers to pay at least $20 per linear foot. 3. Develop retention pond design standards and allow appropriate fencing • Purpose: To ensure the retention/detention pond safety through proven design methods and best practices. • Staff. analysis: This policy would regulate the construction design of stormwater retention ponds. Ponds can be designed with a shallow shelf around the perimeter of the pond. This shallow shelf extends for a distance of 10 to 15 feet into the center of the pond. When full of water the shelf would be designed to have a maximum depth of approximately one foot. The center of the pond would be deeper in order to have sufficient holding capacity. The shallow shelf reduces the probability of accidental drowning for someone who falls or wades into the pond. Staff would also propose developing standards for: • Inflow and outflow facilities including debris screens and separation requirements • Appropriate side slopes and grading • Signage • The voluntary fencing of ponds Implementing this policy would require redrafting and readopting the City of Fayetteville Drainage Manual. The policy could be drafted to either require the proposed design for all retention ponds, or it could be drafted so that it applies only to retention ponds in close proximity to large numbers of children. • Cost: There would be an undetermined administrative cost associated with drafting and implementing this type of policy. December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 8 of 44 Chapter Revised 5/15/09 SECTION 10: WET DETENTION BASIN 10.1 General Characteristics and Purpose In Greensboro, the wet detention pond is currently the most commonly used BMP for meeting water -supply watershed protection requirements. This stormwatcr BMP improves stormwater quality primarily by detaining stormwater runoff for an extended period of time (usually 2 to 5 days) to allow pollutants that are suspended in the runoff to settle out. During a storm event, runoff enters the pond and replaces the "treated" water in the permanent pool that has been detained in the permanent pool from the previous storm event(s). As runoff enters the pond, its velocity is significantly reduced allowing suspended pollutants to begin settling. Many pollutant particles found in stormwater runoff are very small in size and, because smaller particles settle slower than larger particles, the pond is designed to provide adequate detention time so smaller particles have a chance to settle out. The components of the wet detention pond that help increase the pond's pollutant removal efficiency are the permanent pool, temporary pool, and forebay (see Figure 10-1). The permanent pool helps reduce particles that have settled to the pond bottom from re -suspending when runoff flows into the pond. The temporary pool is storage above the permanent pool that is designed to control the WQV. To increase the detention time of the runoff, the temporary pool is slowly released through low flow orifice(s). A separate smaller pond, called a forebay, is placed upstream of the main pond to trap a majority of the coarser fractions of suspended solids in the runoff before it enters the main pond. Figure 10-1 illustrates the main features of a' wet detention basin. Wet detention ponds, based on findings from the City's periodic BMP inspection program, seem to function better when the pond is larger and receives flow from a larger drainage area. This may be attributed to several factors, such as, in larger drainage areas there is usually a better chance for seasonal or permanent surface or groundwater flow into the pond as opposed to smaller drainage areas. This flow may help the permanent pool to be "flushed" more often (as opposed to only during storm events), which may help to prevent undesirable conditions from developing (for example, stagnate water, fluctuating permanent pool elevation, etc.). It is suggested that owners of smaller properties cooperate to construct and maintain one relatively large pond to serve several properties, rather than several individual smaller ponds. Although the City generally recommends a drainage area of 10 acres or more, wet detention ponds may be used for smaller drainage areas. Wet detention ponds have a higher tolerance for runoff with significant sediment concentration than the other BMPs. The wet detention pond BMP is most applicable for large industrial and commercial facilities and residential subdivisions where construction will take place in phases or in residential development where site disturbance will occur for a significant period after the BMP is installed. Also, properties where the land may remain fully or partially unstabilized or where sources of sediment remain on the property (e.g. stock yards, gravel/dirt areas, construction equipment storage, etc.), the wet detention pond is a good choice. City of Greensboro 84 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 9 of 44 Chapter Revised 5/15/09 Figure 10-1 Example of Wet Detention Basin PLAN Permanent Pool PROFILE City of Greensboro Stormwater Management Manual 85 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 10 of 44 Chapter Revised 5/15/09 10.3.2 Siting Issues Wet detention basins shall not be placed in jurisdictional wetlands, on intermittent or perennial streams. Also, wet detention ponds used to meet water -supply watershed regulations will be required to be designed to treat the total drainage area to the pond, on-site and off-site, per the City's water -supply watershed ordinance. When designing the dam and spillways, existing and potential future downstream development should be considered. Avoid placing the dam upstream of highly developed or traffic areas whenever possible. The discharge from the spillways should be directed to a conveyance system that can adequately handle the flow or if no conveyance is present, the discharge should be directed away from existing development. 10.3.3 Pretreatment and Inflow Inverts for inlet pipes should be at the elevation of the normal (permanent) pool to allow the pool to dissipate the energy of the inflow to prevent erosion along the embankment slope. Inlets should be designed to discharge to the pond perpendicular to the pool surface to minimize potential erosion problems to the side embankment. Riprap pads should be underlain with a gravel/sand filter or geotextile fabric and extend from the pipe invert to the pond bottom. For pond inlets that carry public runoff the invert must be designed to discharge above normal pool elevation to prevent water from backing up within the public storm sewer system. The top of the forebay baffle should be one foot below the normal (permanent) pool elevation. The baffle material may be earth, rip rap, etc. If earth baffles are used, provisions should be included to allow the forebay to drain out with the rest of the pond to facilitate sediment removal from the forebay. This could include a rip rap check dam in a section of the earth baffle or a perforated riser pipe connecting the forebay to the main pond. 10.3.4 Length, Width (Area), Depth, Geometry For ease of maintenance and safety, the pond embankments should be sloped to a maximum of 3H:1V with flatter slopes preferred. The engineering design of a wet detention basin must include a 10' wide vegetated bench (max. slope 10%) placed around the perimeter of the basin pond at the normal pool surface. The inside edge of the shelf shall be no deeper than 6" below the permanent pool elevation, and the outside edge shall be 6" above the permanent pool elevation. The vegetated shelf provides a location for a diverse population of emergent wetland vegetation that enhances biological pollutant removal, provides a habitat for wildlife, protects the shoreline from erosion, and improves sediment trap efficiency. The pond surface area shall have a minimum 1.5:1 length to width ratio, with 3:1 preferred. The distance between the inlets and outlets should be maximized to increase the pollutant removal capability of the pond. It is encouraged to create a pond that fits within the natural contours of the land but care should be taken to prevent "dead storage zones" (areas outside the flow path between the inlet and outlet) within the pond. Generally, the pond should be narrower at the inlet forebay area and City of Greensboro 86 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 11 of 44 Chapter Revised 5/15/09 become wider at the outlet. Whenever possible one forebay should be created and all pond inlet pipes discharge to the forebay area. The bottom of the pond should be slightly sloped from the upstream end to the downstream end. This will allow the pond and captured sediments to drain better when cleanouts are necessary. It is encouraged where appropriate to use educational signs at the pond describing the function of the pond and the purpose it serves. To meet NCDWQ requirements, if the pond outlet ties into an existing storm drainage system then the pond surface area must be designed for 90% TSS. In this case, a filter strip would not be required or practical. 10.3.6 Sediment Accumulation The engineering design of a wet detention basin must allocate additional volume for sediment accumulation between cleanouts. For design purposes, a sediment storage depth of a minimum of 0.125 inches of depth over the drainage area should be provided with approximately 75% of this volume allocated to the forebay. In no event should the depth of sediment storage be less than 1 foot in both the forebay and main pool. An access to the forebay for future sediment cleanouts is to be provided. The forebay access shall have a maximum slope of 15-20%, extending from the top of the embankment to the toe. This access will allow construction equipment to get down in the forebay and minimize disturbance to the vegetation. 10.3.7 Plant and Landscape Requirements Earthen embankments for wet detention basins can be surfaced with turf grass, riprap or other protective measures. The use (or growth) of trees and shrubs is not permitted on earthen embankments due to their capability to undermine the embankment structural stability. 10.3.8 Surrounding Soils and Liners When a wet detention basin is to be located in sandy or gravelly soils or in fractured bedrock, the BMP design shall incorporate an impervious clay or synthetic liner to sustain the permanent pool of water or alternatively incorporate designed features to cutoff seepage through the foundation and the abutments. The liner shall extend over the entire bottom surface of the permanent pool and water quality volume pool. If, at the time of BMP certification in accordance with Section 30-7-1-6 of the Greensboro Development Ordinance, the pond does not have a permanent pool established per the approved plans, one of the following requirements must be met: • Submit a performance bond to the City to cover all costs to upgrade the pond in the event it is determined that a permanent pool of water cannot be established. • For non residential development, obtain a letter from the owner stating all necessary corrective measures will be undertaken to upgrade the pond in the event it is determined that a permanent pool of water cannot be established. City of Greensboro 87 Stormwater Management Manual December 14, 2009. Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 12 of 44 Chapter Revised 5/15/09 • Submit supporting technical analysis / soil investigative reports confirming the geotechnical composition of the pond bottom and embankments is conducive to holding a permanent pool. 10.3.9 Outlet Design The principal spillway should be a riser/barrel, concrete free overfall weir, or concrete chute, with capacity to handle the 10 -year, 24-hour rain storm event at a minimum. The riser/barrel material of construction shall be either reinforced concrete, ductile iron, PVC, HDPE, or corrugated aluminum piping. The use of aluminized steel or other type of corrugated metal is not allowed. Reinforced concrete (where the joints are sealed and specified watertight) and ductile iron pipes (being rigid pipes) are preferred for this type of application. To prevent distortion of flexible conduit, such as corrugated aluminum piping, special care (including construction oversight) must be taken during compaction of the soil around it and result in internal soil erosion problems potentially leading to failure of the embankment around and above the conduit. The minimum diameter of the principal spillway barrel section (horizontal pipe) shall be twelve inches (12") to allow outflow to be limited to pre -developed peak for small sites. For wet detention basin design purposes, the capacity of the principal spillway is the 10 -year, 24- hour storm event. The 10 -year, 24-hour storm event shall not activate the emergency spillway unless the spillway structure is integrallydesigned to handle both the 10 -year, 24-hour and 100 - year, 24-hour storm events. Riser/barrel assemblies shallbe properly anchored to resist buoyancy forces. The low flow orifices are _to be protected from clogging due to floating debris using a trash guard. The trash guard should be durable and secure and should extend at least six inches below the normal pool surface. A common method when using a riser/barrel is to extend the principal spillway trash rack assembly below the normal pool. When the low flow orifice will be placed in a concrete dam or spillway, an inverted or submerged orifice can be used or a half aluminum pipe bolted to the concrete (see Section 5.8.8 for example trash protection devices). Refer to Section 3.5.2 for the preferred method for calculating the orifice drawdown time. The capacity of the emergency spillway (in conjunction with the principal spillway) shall be based on the spillway design flood storm event per N.C. Dam Safety regulations and at a minimum be the 100 -year, 24-hour rain storm event. The spillway should be located where it will not adversely affect the integrity of the dam and downstream properties such as roadways and building structures. As noted above, the emergency spillway may be incorporated into the principal spillway where accommodating the emergency spillway elsewhere is not feasible for the given site characteristics. The emergency spillway shall be cut into existing soils outside the fill section of the dam. Where this placement requirement cannot be met, the emergency spillway shall be designed to meet the chute or free overfall spillway requirements as listed in Section 5.8.6 of this supplement. The emergency spillway may be grass lined, when velocities permit, or lined with rip rap, concrete, or other erosion resistant materials. Grass lined spillways are to be planted with a dense cover of erosion resistant grasses, preferably incorporating turf reinforcement matting (TRM). The City of Greensboro strongly recommends a minimum eight foot (8') width emergency spillway be used for the 100 -year flow if it can be placed in in-situ soil regardless of whether the City of Greensboro 88 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 13 of 44 Chapter Revised 5/15/09 calculations indicated the principal spillway will pass the flow. The separate spillway provides an additional factor of safety that can be incorporated in the grading of the pond at a small cost and allows storm flow if a problem occurs with the riser/barrel assembly. To meet NC Dam Safety requirements, a minimum freeboard of 1 foot above the maximum stage associated with the spillway design storm event is required. The City of Greensboro encourages BMP designers to maximize the freeboard to the maximum extent practicable. The Engineer's Statement of Pond and Dam Safety as listed in Section 5.6 of this Supplement shall be listed along with the BMP design elements. 10.3.11 Safety Considerations Although not required by State Regulations or Local Ordinance, the use of fencing around wet detention basins is encouraged where safety risks are posed by children. The designer is encouraged to incorporate engineering design features (gentle contours, inlet and outlet screens, etc.) that minimize safety risks. 10.4 Construction A concrete retaining wall may be used for the pond's dam provided the requirements listed under Section 10.3.2 of the State's Stormwater Best Management Practices Manual are met. For earth dams, the top width of the dam shall be 10 feet minimum. A key trench shall be excavated under the, entire length of the dam and located at or upstream of the centerline of the dam. The key trench should be filled with highly impervious and well compacted clay material. The key trench should have a bottom width adequate to accommodate the equipment used for excavation, backfill, and compaction operations, with La minimum width of 4 feet. Side slopes? should be no steeper than one horizontal to one vertical. The minimum recommended depth of fill material is 3 feet below the stripped grade (or shallower should bedrock be encountered). The fill material should be placed in lifts not to exceed 8 inches in loose thickness and compacted to at least 95% standard proctor. The fill material and compaction specification for the key trench shall be specified on the BMP design plans. Incorporating a drainage blanket (preferably with a chimney drain) should be considered to reduce the potential for seepage problems. Also refer to Section 5.6.1 of this manual. A "filter and drainage diaphragm" shall be used to prevent piping along the barrel withinthe earth fill. Refer to Section 5.6.1 of. the BMP Design Supplement for seepage control requirements for conduits that extend through an embankment dam. For large ponds that have a considerable fetch, wind -generated wave action during storms shall be considered in the freeboard. Also protection (stone riprap layer laid on a gravel or geotextile bedding) against wave induced erosion should be provided covering the range of pool elevation above the minimum pool. Appropriate energy dissipation shall be used at the spillway exit to prevent erosive velocities for up to the 10 -year peak discharge rate, at a minimum. The basin discharge shall be evenly distributed across a minimum 30 foot long vegetated filter strip. Every effort should be made to discharge in defined conveyances and parallel to the existing flow to prevent bank erosion. City of Greensboro 89 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 14 of 44 Chapter Revised 5/15/09 Downstream channels may need to be modified and lined with rip rap to prevent erosion of the channel. Modifications to the downstream channel should be minimized as much as possible to prevent excessive disturbance in the channel. A pond drain is to be provided to drain the pond for routine maintenance or structural repairs in an emergency situation. The pond drain is to have the discharge capability to completely drain the pond in 24 hours, in the event of an emergency posed by impending failure of the pond dam. The upstream slope of the pond dam should be designed to be flat enough to prevent slope failure due to "quick drawdown" of the pond (NC Dam Safety Code specifies a factor of safety of 1.25) in an emergency. Care should be taken to minimize transport of settled sediment from the pond during draining. Eliminating a conduit through the embankment eliminates the requirement for seepage control. 10.5 Maintenance 10.5.1 Common Maintenance Issues The wet detention basin should be easily accessible for maintenance. A 20' wide access easement shall be provided from the public street right-of-way to the basin's DMTJE. A drainage maintenance and utility easement(DMUE) shall be placed over the basin and extend 15' beyond the toe of embankments and outside edges of erosion protection structures (energy dissipators concrete or riprap pads, etc) so as to include all key components of the basin. The access easement should be kept easily accessible for maintenance. The presence of some vegetation in ponds is desirable for aesthetic appearance and enhancement of fish: habitat. Excessive growth of aquatic vegetation is, however, detrimental to the functional sustainability of ponds. Therefore, a good balance between aquatic vegetation and other aquatic life in ponds is desirable. Recommendations for the management of aquatic weeds are beyond the scope of this supplement. However, the owner of a pond experiencing excessive aquatic weed problems is advised to contact the NC State Fisheries &. Pond Management Extension for assistance with the identification of the aquatic weeds and recommendations for appropriate management strategy. The website for the NC State Fisheries & Pond Management Extension is http://www.ces.ncsu.edu/nreos/wild/fisheries/index.html. Pond owners can also contact the local Guilford County Cooperative Extension Service at (336)375-5876 for advice and guidance. 10.5.10 Dam Safety Preserving the structural integrity of the dam of a pond BMP is important in protecting downstream life and property. There are at least four aspects of the dam that require specific attention: (1) assessment of hazard potential due to changes in downstream development, (2) seepage; (3) dam material problems; and (4) vegetation growth on the dam embankments (1) Assessment of Hazard Potential Before any dam is constructed, the design engineer is responsible for notifying the NC State Dam Safety Office of the proposed dam. If the dam falls under State Dam Safety jurisdiction, the dam must be constructed, maintained and operated according to their design and construction guidelines. Even if the dam does not fall under the NC Dam Safety Office's jurisdiction, the City of Greensboro 90 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 15 of 44 Chapter Revised 5/15/09 dam should be designed and constructed in accordance with current good engineering practice. The City has requirements concerning the maintenance of dams associated with required BMPs. As new development occurs downstream of the BMP, the chance of significant property damage or danger to human life may increase if catastrophic failure of the dam occurs. Although the dam may be initially exempt from regulation by the State, the owner is responsible for reporting to the State Dam Safety Office downstream development that may affect the hazard classification of the dam. (2) Seepage The downstream side of the dam should be inspected regularly for evidence of significant seepage. Seepage can emerge anywhere below the normal pool elevation, including the downstream slope of earth dams, areas beyond the toe of the dam, and around the spillway or pond outlet conduit. Indications of significant seepage include areas where the soil is saturated or where there is a flowing "spring" or leak. If "sinkholes" in the dam embankment are noticed, or if constant flowing water is noticed on the downstream side of the dam, then seepage has become excessive and professional engineering advice should be sought immediately to avert a major structural problem or a catastrophic failure of the dam. (3) Dam Material Problems For earth dams, pronounced cracks on the embankment surface indicate the first stages of potential dam failure. Transverse cracks (running perpendicular to the embankment face) generally indicating differential settlement of the dam, can provide pathways for excessive seepage. Longitudinal cracks (running parallel to the embankment face) may be due to inadequate compaction of the dam during construction or shrinkage of the clay (desiccation) in the top of the embankment during prolonged dry conditions. These cracks may eventuallylead to slope failure such as sliding or sloughing. For reinforced concrete dams, the concrete should be checked for pronounced cracking, leakage from the joints, and displacement (noticeable leaning or bulging). Also, excessive seepage, leakage, or springs just downstream of the concrete dam could be indicative of potential seepage - related "piping" problems under the dam. If such problems or other structural problems are observed, professional engineering advice should be sought. (4) Vegetative Growth on Dam Embankment Trees and other woody vegetation are not permitted on the top slopes of dam embankments. Large root systems from woody vegetation can weaken the dam structure and provide seepage pathways. Thick vegetative cover can also provide a haven for burrowing animals such as muskrats and/or groundhogs. These animals can create a network of burrows in the dam embankments that can significantly weaken the dam, by creating seepage paths, which may eventually lead to darn failure. Mowing of the dam embankments should occur, at a minimum, once every 6 months to prevent woody vegetation growth and cover for burrowing animals. The exception to this rule is for existing woody vegetation that already exceeds six inches (6") in diameter and is located on the upstream or downstream face of the dam. Removing this large woody vegetation may cause the already established root system to decay, thus allowing seepage City of Greensboro 91 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 16 of 44 Chapter Revised 5/15/09 to occur where the root system once existed. In this case, the least potentially hazardous situation is to just leave the root system in place and intact. 10.5.11 Reduction of Pollutants Entering BMPs Stormwater BMPs are not 100% efficient in removing pollutants; therefore, when the amount of pollutants into the BMP is higher, the amount of pollutants discharged from the BMP will be higher. Also, increased amounts of pollutants entering the BMP will increase the maintenance required to keep the BMP functioning properly. To assist the stormwater BMP in removing pollutants, every effort should be made to reduce the initial pollutant load entering the BMP. Pollution prevention activities described elsewhere in this manual should be implemented along with the following efforts: • Outside trash dumpsters should be kept covered, and the area around the dumpster should be kept neat and clean. * Chemicals, petroleum products and other pollution sources (such as machinery) should be stored in a covered area away from possible stormwater contact. Spent chemicals areto be properly disposed or recycled. i -Fertilizers and pesticides should be used conservatively on the property grounds. Excessive amounts of these chemicals can be washed away with stormwater runoff increasing the nutrient load to the pond. • b. Chemicals such as copper sulfate used to inhibit algae growth in the water quality pond degrade water quality. Since the pond's main function is to enhance water quality,these, chemicals should not be used. Rather, reducingthe amount of fertilizer application and ensuring that the pond outlets are properly functioning so thepool is flushedperiodically will help to deter algae growth. m Trash and vegetative floatables (grass clippings, leaves, limbs, etc.) should be cleaned from the pond surface and surroundings periodically to promote a healthy, aesthetically pleasing environment, and to prevent blockage of the pond outlets. Studies have shown that people are less likely to litter ponds that are aesthetically pleasing and support wildlife. 10.5.12 Stabilization of BMP drainage area The area draining to the BMP pond should remain stabilized to prevent excessive sediment from entering the BMP facility. When the bare soil is directly exposed to precipitation the sediment concentration in runoff is much higher than for soil that is stabilized. A stabilized area is covered by impervious surfaces (pavement, buildings), grass cover, landscaped cover (mulch, pine straw), etc. For filtration practices such as sand filtration facilities and bioretention, maintaining a stabilized drainage area isespecially important. Eroded sediment can quickly "seal" the filtration bed, drastically decreasing its filtration capacity. Erosion Concerns The inlet and outlet areas, side slopes (swales), and the rest of the conveyance area should be inspected for erosion problems. City of Greensboro 92 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 17 of 44 Chapter Revised 5/15/09 Where water discharges from a pipe and where the stormwater runs off impervious area onto pervious area, there may be erosion problems. The BMP should have riprap protection at the end of pipes and a gravel trench at the edge of impervious areas to help prevent erosion. These devices should be inspected to ensure they are functioning properly. If erosion is noticed in within the rip rap pad or along the edges of the pad, more rock may be needed or it may have been improperly placed (no geotextile liner or improper placement of liner, rip rap not well graded, etc.) If the rock or gravel is displaced downstream, a larger size rock or gravel should be used. Rill erosion (small channels or gulleys in the ground) is a common problem found in these control devices where the water runoff is naturally trying to channelize. Rill erosion can be repaired by filling in the rills with suitable (clayey) soils and reseeding. It may be necessary to use a temporary erosion resistant matting or to use sod to repair these areas. 10.5.13 Blockage of Outlets Wet extended detention ponds are designed for the water to exit the pond through the low flow orifice(s), the principal spillway, and the emergency spillway. It is important to check all three outlets for blockage that would impair the pond's water quality and hydraulic functionality. Low Flow Orifice(s) Unless an inverted orifice is used, some type of trash guard is to be maintainedover the low flow orifice(s) to prevent clogging. When the orifice becomes clogged the water level rises to the principal spillway elevation and the benefits associated with temporary storage and its gradual release are lost. To preserve "extended detention" the low flow orifice should be inspected for blockage twice a month and after Large storms. 10.5.14 Principal and Emergency Spillway Principal and emergency spillways are designed to safely convey larger than one inch storms that produce runoff which exceed the water quality volume of the BMP. If these spillways are blocked so they do not operate at full capacity, the risk of dam overtopping or other uncontrolled releases may result. To ensure the hydraulic capacity of the spillways, the spillways should be inspected for blockage twice a month and after large storms. If a riser/barrel is used for the principal spillway, a trash rack is to be maintained on the riser. Vegetative growth in the riser should be removed promptly so that the design capacity of the spillway is maintained. Also, the outlet area where the barrel projects from the fill should be clear of tree limbs, sediment accumulation, etc. City of Greensboro 93 Stormwater Management Manual December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 18 of 44 (Corr. 5/11/04) ORDINANCE (2004) 627 TC 246 TC -12-03 AN ORDINANCE TO REQUIRE THAT PERMANENT STORMWATER RETENTION PONDS AND DETENTION BASINS BE INCORPORATED INTO THE DEVELOPMENT OF A SITE AS AMENITY FEATURES OR THAT SUCH FACILITIES BE SCREENED FROM ON-SITE AND OFF-SITE VIEWS. BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF RALEIGH, NORTH CAROLINA that: Section 1. That the Raleigh City Code be amended to include the following new subsection 10- 2082.13: Sec. 10-2082.13. SCREENING REQUIREMENTS FOR PERMANENT STORMWATER RETENTION PONDS AND DETENTION BASINS (a) Purpose. Due to their nature as service features, some stormwater control facilities can have a negative visual impact upon properties where they are located as well as upon adjacent properties and the adjacent public realm. As a result of these factors, the purpose of this Section is to: (1) Mitigate the negative visual impact from certain stormwater control facilities by requiring screening from on-site and off-site views; (2) Recognize that a stormwater control facility may include elements which allow it to function as a site amenity; (3) Explain the conditions under which a stormwater control facility could be considered, as a site amenity, and therefore not require screening; and (4) Offer incentives for development of stormwater control facilities as site amenities. (b) Applicability. The provisions of this Section shall apply to all off-site and on-site permanent stormwater retention ponds and detention basins permitted, constructed, and/or modified to a cumulative extent greater than fifty (50) percent of their original size after application of this ordinance. December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 19 of 44 Ordinance (2004) 627 TC 246 TC -12-03 May 4, 2004 Editor's Note: Section 10-2082.13 first became applicable on June 3, 2004 (Ord. No. 627 TC 246). (c) Screening Requirements. (1) All permanent on-site and off-site stormwater retention ponds and detention basins subject to the provisions of this Section shall provide vegetative screening from on-site and off-site views consistent with the following requirements: a. All vegetative screening shall be 75% locally -adapted evergreen species; and b. All vegetative material shall be planted so as to attain a screen occupying at least seventy-five (75) percent of a vertical plane around the perimeter of the facility to an average height of six (6) feet above grade within three (3) years of planting; and. c. Screening shall be required around the base of the dam structure (as applicable), but not on the dam structure, with those plant materials in immediate proximity to the dam characterized by shallow, non-invasive root systems; and d. Screening shall not be required within required facility inlets or facility outlets, nor within a maintenance access path provided that such path does not exceed twelve (12) feet in width; in all other instances, within three (3) years of planting the maximum open horizontal space between vegetative screening materials shall not be more than two (2) feet in width; and e. Vegetative material composing the screen shall be selected and installed so as to exhibit variety in texture, color, spread and height by using ornamental and/or deciduous shade trees in combination with evergreen materials; and f. In some cases, vegetative material may be placed in clusters or groups to add additional visual interest as well as to achieve intermittent levels of vertical height; and g• In areas where the required screening is immediately adjacent to other required yard or landscaping areas, the trees or shrubs required for screening the facility may be relocated to elsewhere within the facility perimeter of the facility provided that the screen requirements in (1)b. above are met; and h. In situations where the stormwater control facility utilizes a fence, all vegetative material associated with screening the facility shall be located outside the fence; and i. In situations where a fence and gate are proposed, the fence and gate shall be colored black, forest green, dark brown or similar dark color so as to recede from view, unless the fence is constructed of masonry, wood, or similar natural material, in which case it may be left to weather naturally. 2 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 20 of 44 Ordinance (2004) 627 TC 246 TC -12-03 May 4, 2004 (2) Permanent stormwater retention ponds and detention basins and their related facilities, including associated amenity features shall not be placed within required tree protection areas nor landscaping areas, including but not limited to: street protective yards, vehicular surface area landscaping, utility service area landscaping, loading/display area landscaping, or transitional protective yards. (3) Stormwater control facility vegetative screening material shall not be credited towards other required landscaping, including but not limited to: street protective yards, vehicular surface area landscaping, utility service area landscaping, loading/display area landscaping or transitional protective yards except as provided for within this Section. (4) Stormwater permanent retention ponds and detention basins which contain amenity features as set forth in Section (d) below shall be exempted from these screening requirements. (d) Amenity Approval Criteria. Permanent stormwater retention ponds and detention basins may be supplemented with certain features which enable the facility to functionas an amenity to the site or development in addition to its primary function as a stormwater device. In situations where such amenity features are provided consistentwith these provisions, no screening of the stormwater control facility from on-site or off-site views shall be required. In the event a permanent stormwater retention pond or detention basin loses a feature by which the facility qualifies as an amenity, the feature shall be restored within 30 days, or the facility shall be subject to the screening provisions listed in the Section above. (1) Basic Amenity Features. To qualify as an amenity to the site or development where it is located, the following basic amenity features shall be provided within any stormwater control facility: a. Integration of the permanent stormwater retention pond or detention basin into the design of the public areas within the site or development through the use of appropriate placement, common building materials, textures, features, or other treatments intended to lend architectural significance to the stormwater control facility. For the purposes of this Section, "integration" shall mean at least two (2) of the following three (3) elements: 1. Proximate placement of the stormwater facility to the principal structure(s), or 2. Provision of pedestrian access to the facility through installation of a delineated walk or trail from the principal structure(s), or 3. Utilization of similar planting materials and building materials as used in the principal structure(s). 3 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 21 of 44 Ordinance (2004) 627 TC 246 TC -12-03 May 4, 2004 b. No fence shall be installed around the permanent stormwater retention pond or detention basin which acts to prevent pedestrian access to the facility from the site or development where it is located; in cases where a fence is needed to restrict access, such fence shall be screened in accordance with the screening provisions described in Subsection (c). c. Rock riprap or other "hard armoring" shall be limited to ten (10) percent or less of the entire surface area of the stormwater control facility above the low pool line. d. Permanent stormwater retention ponds and detention basins shall incorporate features designed to reduce mosquito populations through provision of non -chemical mosquito mitigation measures, including but not limited to: cyclical alteration of the pond level, installation of aeration/agitation features to disrupt larval growth, providing nesting boxes for mosquito -predacious birds or bats, or stocking ponds with mosquito -predacious fish (e.g. Gambusia of finis holbrooki). (2) Supplemental Amenity Features. In addition to the basic requirements described in subsection (1) above, a permanent stormwater retention pond or detention basin must also provide supplemental features to be considered an amenity. The types of supplemental amenity features differ based upon the type of stormwater control facility. a. Permanent retention (wet) ponds. To be considered as an amenity feature, permanent retention ponds shall: 1. Retain water within a permanent pool, and 2. Maintain slopes of 4:1 or shallower to a distance of ten feet below the median pool line (beyond which slopes may become steeper); alternatively, a retaining wall may be incorporated into a portion of the facility, provided the design of the wall is in accordance with §10-2082.13 (d)(1)a. above, and 3. Include horizontal curves or other sculptural elements within the shape of the facility so as to avoid a simple square or round shape. 4. Provide at least two (2) of the following four (4) features: a. Inclusion of an active water feature (i e , aerator / fountain / waterfall) within or adjacent to a permanent pool(s), or b. Pedestrian access trails to and/or around the stormwater control facility from the existing and/or proposed pedestrian network associated with the site or development. Such access shall not be required to meet the minimum specifications for sidewalks or multipurpose trails as described within the 4 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 22 of 44 Ordinance (2004) 627 TC 246 TC -12-03 May 4, 2004 City's standard specifications and details, and may be constructed of any discernible, leveled, and stable surface material, including but not limited to: brick or masonry, gravel, mulch, wood chips, mowed grass or low groundcover, or c. Provision of riparian plant materials throughout the stormwater control facility to prevent erosion and add visual interest, and additional perimeter plantings consisting of at minimum three (3) ornamental trees or two (2) shade trees totaling eight (8) inches in caliper at time of planting, and four (4) shrubs for every one hundred (100) linear feet equivalent to the pond's maximum pool circumference and located no more than seventy-five (75) feet from the pond's maximum pool line, or d. Inclusion of other permanent, pedestrian -oriented features, including but not limited to: seating, dining tables, and mounted trash cans, in areas around or proximate to the stormwater control facility. b. Detention (Dry) Basins. To be considered as an amenity feature, detention ponds shall: 1. Not contain water on a permanent basis, and 2. Provide a one hundred. (100) percent vegetative cover capable of withstanding extended periods of inundation, except in spaces designed for specific recreational uses (e.g., baseball fields), and 3. Maintain slopes of 4:1 or shallower; alternatively, a retaining wall may be incorporated into a portion of the facility, provided the design of the wall is in accordance with §10-2082.13 (d)(1)a. above, and 4. Provide at least two (2) of the following three (3) features: a. Pedestrian access trails to the stormwater control facility from the existing and/or proposed pedestrian network associated with the site or development. Such access trails shall not be required to meet the minimum specifications for sidewalks or multipurpose trails as described within the City's standard specifications and details, and may be constructed of any discernible, leveled, and stable surface material, including but not limited to: brick or masonry, gravel, mulch, wood chips, mowed grass, or b. Inclusion of other permanent, pedestrian -oriented features, including but not limited to: seating, dining tables, and mounted trash cans, in areas around or proximate to the stormwater control facility, or 5 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 23 of 44 Ordinance (2004) 627 TC 246 TC -12-03 May 4, 2004 c. Grading and slopes of 8:1 or shallower which will allow utilization of the facility as an area for recreation when not in use as a detention facility (except on the dam structure and as is necessary to tie the dam back to existing grades); alternatively, a retaining wall may be incorporated into a portion of the facility, provided the design of the wall is in accordance with §10-2082.13 (d)(1)a. above. (e) Incentives. When a permanent stormwater retention pond or detention basin provides amenityfeatures in compliance with the minimum requirements of Subsection (d), then the following provisions may also apply: (I) The permanent stormwater retention pond and detention basin and their related facilities, including amenity features, may be placed within required landscaping areas, including but not limited to: street protective yards, vehicular surface area landscaping, utility service area landscaping, loading/display area landscaping, or transitional protective yards (except in tree conservation areas adjoining thoroughfares, within areas zoned for Resource Management, tree protection areas, and in natural protective yards). (2) When a permanent stormwater retention pond or detention basin is located within a required landscaping area, the portion of land associated with or adjacentto the stormwater control facility shall only be required to meet eighty (80) percent of the 'r . minimum planting requirements of the required landscaping area. Section 2. That Raleigh City Code Section 10-2146.5(a) be amended to include a new entry in the Cross reference which shall read: "Permanent stormwater retention ponds and dry detention basins §10-2082.13(b)." Section 3. That Raleigh City Code Section 10-3073(a)(6)d. be repealed and rewritten to read as follows: "d. The retention pond is an amenity complying with Section 10-2082.13(d). Cross reference: "Permanent stormwater retention ponds and dry detention basins §10- 2082.13." Section 4. All laws and clauses of laws in conflict herewith are hereby repealed to the extent of said conflict. Section 5. If this ordinance or application thereof to any person or circumstance is held invalid, such invalidity shall not affect other provisions or applications of the ordinance which can be given separate effect and to the end the provisions of this ordinance are declared to be severable. 6 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 24 of 44 Ordinance (2004) 627 TC 246 TC -12-03 May 4, 2004 Section 6. This ordinance has been adopted following a duly advertised joint public hearing of the Raleigh City Council and the City Planning Commission following a recommendation of the Planning Commission. Section 7. This ordinance has been provided to the North Carolina Capital Commission as required by law. Section 8. This ordinance shall be enforced by law as provided in G.S.N.C. 160A-75 or as provided in the Raleigh City Code. All criminal sanctions shall be the maximum allowed by law notwithstanding the fifty dollar limit in G.S. 14-4(a) or similar limitations. Section 9. This ordinance shall become effective thirty (30) days following its adoption, and it shall not apply to stormwater retention ponds and detention basins submitted to the City prior to the effective date of this ordinance. Adopted: Effective: 5/4/04 DISTRIBUTION:. Planning -Chapman, Hallam, Powell, Sumpter, Brandon, Grudzinski Inspections —Ellis, Strickland, McMillan Tew Teachey, Yost Taylor 7 December 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 25 of 44 This paper was published in the January/February 2006 issue of Stormwater magazine,. Essential Safety Considerations for Urban Stormwater Retention and Detention Ponds By: Jonathan E. Jones, P.E.1, James Guo, Ph.D., P.E.2, Ben Urbonas, P.E.3, and Rachel Pittinger4 INTRODUCTION Urban stormwater retention and detention ponds are widely used in the United States. Retention ponds, also commonly called "wet ponds" refer to facilities that maintain a permanent pool, while detention ponds, often called "dry ponds" contain water only in the aftermath of runoff events. Although retention and detention ponds can be effective for stormwater management and flood control, they can also pose risks to public health, safety, and welfare. Urban storm drainage system planners, designers, facility owners, maintenance staff, and municipalities, including their elected officials and governing bodies, must be aware of such risks and insist on the use of recommended techniques to minimize them. Licensed professional engineers should be especially concerned about the risks that their designs may pose and be knowledgeable of design approaches that reduce such risks, given that their paramount responsibility as licensed professionals is to protect public health, safety, and welfare. The purposes of this paper are to: Review safety hazards that can be associated with retention/detention ponds. • Discuss techniques that can be used to reduce the risk of such hazards. Review representative pond safety recommendations and guidance from municipalities, state and federal governments, professional societies, and the general stormwater literature. Although the focus of this paper is on wet and dry ponds, many of the issues and recommendations presented herein apply to other stormwater facilities, such as best management practices (BMPs), long underground pipes, and culverts (see discussion of this topic near end of paper). Conceptual designs of a typical wet pond and dry pond are provided in Figures 1 and 2, respectively. Photographs 1 and 2 show examples of typical facilities. Jonathan E. Jones, P.E., Chief Executive Officer, Wright Water Engineers, Inc., 2490 W. 26th Ave., Ste. IOOA, Denver, CO 80211;jonjones(aiiwrightwater.com. 2 James Guo, Ph.D., P.E., Professor, University of Colorado, Civil Engineering Department, Campus Box 113, P.O. Box 173364, Denver, CO 80217-3364. iames.guo@,cudenver.edu. 3 Ben Urbonas, P.E., Manager, Master Planning Program, Urban Drainage & Flood Control District, 2480 W. 26th Avenue, Ste. 156B, Denver, CO 80211. burbonas(a)udfcd.org. 4 Rachel Pittinger, Project Engineer, Wright Water Engineers, Inc., 2490 W. 26th Ave., Ste. 100A, Denver, CO 80211; rpittinger@wrightwater.com. C:\Documents and Settings\BUrbonas\My Documents \CONFERENCES\2006-04 UDFCD Seminary-iandout Submittals\Jones - TextdoDecember 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 26 of 44 This paper was published in the January/February 2006 issue of Stormwater magazine,. TYPICAL SAFETY CONCERNS Overview Unsafe conditions can occur under both dry and wet weather conditions. These range from readily apparent problems such as outlet pipes that are open (unprotected with trash/safety racks) to less obvious concerns such as outflow pipes that are subject to overwhelming hydrostatic forces due to high headwater depths. A complicating factor is that children are often attracted to stormwater facilities, and this poses special design challenges and risks. For instance, the authors are aware of a case where a dry pond in an office park had a rapid rise, and a child playing in the pond was apparently knocked down by jet flows from an inlet pipe, tumbled by vortex flows, and ultimately, dragged into an unprotected outlet pipe by suction forces. These forces were not visible (apparent) when the water depth in the pond covered the pipe entrances and outlet. Although it is not feasible to anticipate every public safety risk, many scenarios are foreseeable and can be accounted for during design. Pond safety issues that do not involve drowning must also be considered. For example, embankment slopes that are too steep can be hazardous to the public and maintenance staff (such as those operating lawn mowers). Another example is high wingwalls or other vertical structures. In the past few years, a serious public health concern that has emerged is related to ponds that create mosquito -breeding habitat due to shallow and stagnant standing water, thus increasing the risk of West Nile virus to the adjacent community. Specific Safety Deficiencies The authors have observed the following safety deficiencies in storage facilities. • 'Outlets are open and unprotected, they lack trash/safetyracks, the racks have openings large enough to pose a danger to the public, and/or the racks are too close to the outlet to provide sufficiently slow flow velocities that will not impinge a person against them. • Adjacent land uses are incompatible with storage facilities and few, if any, steps have been taken to minimize obvious risks. For example, a nursery school playground without a fence was observed immediately next to a retention pond that had a high concrete wall along one side without a suitable railing. • The public is effectively "invited" to spend time near storage facilities because they are located in parks, along bike trails, next to playgrounds, etc., yet the designers fail to recognize that frequent use will occur, and public safety has clearly not been a specific design objective. • Education of community residents, office and industrial park employees, users of multi- purpose recreational facilities, etc. regarding pond hazards is not provided. Signs warning the public of rapidly rising floodwaters and associated danger are not posted. • Side slopes of the facility are excessively steep or vertical without suitable safety rails. As a result, it would be very difficult for someone to get out of the pond when water levels are rising. • Side slopes within the pond's permanent pool are too steep, and/or ponds lack "safety benches" around their perimeter. 2 C:\Documents and Settings\BUrbonas\My Documents \ CONFERENCES \2006-04 UDFCD Seminar\Handout SubmittaisUones - Text.dopecember 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page 27 of 44 This paper was published in the JanuarylFebruary 2006 issue of Stormwater magazine,. • Pond inflow and outflow pipes are directly across from and in close proximity to one another. In this case, a person can be knocked over by the impulse forces (momentum) of inflows and then sucked into and/or pinned against the outlet structure. • Pond depths increase very rapidly, and inflow/outflow pipes are quickly inundated and not visible. • Hydraulic structures are designed and constructed in a manner that makes them hazardous. For example, steel bars on grates are not beveled, rounded, or covered, but have sharp ends. Bolts have jagged, exposed ends. Gaps between steel bars and concrete walls are too wide. Railings are either not used where they should be or are improperly designed. • Ponds with a "hard edge" appearance, such as a block or cobblestone vertical wall, immediately adjacent to the water surface drop into a pond that has steep side slopes, so a person who falls in cannot get out without having to swim. • A variety of problems with spillways have been observed. For example, spillways are undersized. Dams and embankments are not designed to withstand overtopping forces during floods larger than they were designed to detain, despite the presence of homes and businesses in the "dam break" floodplain downstream. This is often the result of designers assessing embankment behavior for the design event, such as the 50 -year storm or the 100 -year storm, but failing to recognize that larger events can and do occur, and that the consequences of such events have to be considered. • Inadequate maintenance and monitoring occur, thus leaving the facilities unable to function as designed or intended. For example, when pond outlet structures are fully or partially blocked with debris, the risk of embankment overtopping and failure increases. Orifices in riser pipes that are used to gradually "bleed down" a water quality design storm typically have small diameters, which leaves them vulnerable to plugging by trash, debris, sediment, algae, etc. unless frequently inspected and maintained. This can lead to prolonged pooling of shallow, stagnant water, which sets the stage for mosquitoes and, potentially, West Nile virus. • Other drainage facilities adjacent to the pond (designed when the pond was) are unsafe, such as channels, drop structures, energy dissipaters, and culverts. The key to reducing the observations described above is careful consideration of risks in the design phase, coupled with regular inspection and maintenance of the pond to insure that the facility is functioning as intended and that unforeseen hazards have not been created. DESIGN AND OPERATIONAL TECHNIQUES TO REDUCE SAFETY RISKS The following risk reduction techniques are recommended (see Photographs 3 — 10 for examples): 1) Inform members of the pond design team that promoting public safety is an essential design objective. Raise the subject regularly while the design is progressing. Educate designers to understand that safety can be addressed without significantly increasing costs, or disrupting hydraulic function. 3 C:\Documents and Settings\BUrbonas\My Documents \CONFERENCES\ 2006-04 UDFCD Seminar\iandout SubmittalsUones - TextdoDecember 14, 2009 Planning Commission Agenda Session Presentation Retention Pond Safety Policy Report Page28 of 44