VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14 WET

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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14WET PONDVIRGINIA DEQ STORMWATERDESIGN SPECIFICATION No. 14WET PONDVERSION 1.9March 1, 2011Amended May 11, 2015SECTION 1: DESCRIPTIONWet ponds consist of a permanent pool of standing water that promotes a better environment forgravitational settling, biological uptake and microbial activity. Runoff from each new stormenters the pond and partially displaces pool water from previous storms. The pool also acts as abarrier to re-suspension of sediments and other pollutants deposited during prior storms. Whensized properly, wet ponds have a residence time that ranges from many days to several weeks,which allows numerous pollutant removal mechanisms to operate. Wet ponds can also provideextended detention (ED) above the permanent pool to help meet channel protection requirements(see Table 14.1).Designers should note that a wet pond is the final element in the roof-to-stream runoff reductionsequence, so one should be considered only if there is remaining Treatment Volume orChannel Protection Volume to manage after all other upland runoff reduction options havebeen considered and properly credited. Wet ponds may be allowed in certain coastal plainsituations where the water table is within 3 feet of the ground surface.Version 1.9, March 1, 2011Page 1 of 23

VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14WET PONDSECTION 2: PERFORMANCETable 14.1. Summary of Stormwater Functions Provided by Wet PondsStormwater FunctionAnnual Runoff Volume Reduction1(RR)Total Phosphorus (TP) EMC2Reduction by BMP TreatmentProcessTotal Phosphorus (TP) Mass LoadRemoval2Total Nitrogen (TN) EMC Reductionby BMP Treatment ProcessTotal Nitrogen (TN) Mass LoadRemovalLevel 1 DesignLevel 2 Design0%0%50% (45%)50% (45%)30% (20%)30% (20%)333375% (65%)75% (65%)40% (30%)40% (30%)3333Yes; detention storage can be provided above the permanentpool.Yes; flood control storage can be provided above theFlood Mitigationpermanent pool.1Runoff Reduction rates for ponds used for year round irrigation can be determined through a waterbudget computation.2Change in event mean concentration (EMC) through the practice.3Note that EMC removal rate is slightly lower in the coastal plain if the wet pond is influenced bygroundwater. See Section 6.2 of this design specification and CSN Technical Bulletin No. 2. (2009).Sources: CWP and CSN (2008), CWP (2007)Channel ProtectionSECTION 3: DESIGN TABLEThe major design goal for Wet Ponds in Virginia is to maximize 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 removal. The basic criteria for the two levels of wet ponddesign are shown in Table 14.2 below. At this point, there is no runoff volume reduction creditfor wet ponds.Version 1.8, April 13, 2010Page 2 of 23

VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14WET PONDTable 14.2. Level 1 and 2 Wet Pond Design Guidance155Level 1 Design (RR:0 ; TP: 50 ; TN:30 )Tv [(1.0)(Rv)(A)/12] – volume reduced byupstream BMPSingle Pond Cell (with forebay)Length/Width ratio OR Flow path 2:1 ormore4Length of shortest flow path / overall length 0.5 or moreStandard aquatic benchesTurf in pond buffersNo Internal Pond Mechanisms155Level 2 Design (RR:0 ; TP: 75 ; TN:40 )Tv [1.5 (Rv) (A) /12] – volume reduced by upstreamBMP23Wet ED (24 hr) and/or a Multiple Cell DesignLength/Width ratio OR Flow path 3:1 or moreLength of shortest flow path/overall length4 0.8 ormoreWetlands more than 10% of pond areaPond landscaping to discourage geeseAeration (preferably bubblers that extend to or nearthe bottom or floating islands1Runoff volume reduction can be computed for wet ponds designed for water reuse and uplandirrigation.2Extended Detention may be provided to meet a maximum of 50% of the Treatment Volume; Refer toDesign Specification 15 for ED design3At least three internal cells must be included, including t d detention volume associatedwith the Tv may not extend more than 12 inches above the wetland cell permanent pool (at least10% of the Level 2 surface area) at its maximum water surface elevation. The maximum ED andchannel protection detention levels can be up to 5 feet above the wet pond permanent pool.Stormwater Pond Benches. The perimeter of all pool areas greater than 4 feet in depth must besurrounded by two benches, as follows:xxA Safety Bench is a flat bench located just outside of the perimeter of the permanent pool toallow for maintenance access and reduce safety risks. Except when the stormwater pond sideslopes are 5H:1V or flatter, provide a safety bench that generally extends 8 to 15 feetoutward from the normal water edge to the toe of the stormwater pond side slope Themaximum slope of the safety bench is 5%.An Aquatic Bench is a shallow area just inside the perimeter of the normal pool thatpromotes growth of aquatic and wetland plants. The bench also serves as a safety feature,reduces shoreline erosion, and conceals floatable trash. Incorporate an aquatic bench thatgenerally extends up to 10 feet inward from the normal shoreline, has an irregularconfiguration, and extends a maximum depth of 18 inches below the normal pool watersurface elevation.Safety Features.xxxxxxThe principal spillway opening must be designed and constructed to prevent access by smallchildren.End walls above pipe outfalls greater than 48 inches in diameter must be fenced to prevent ahazard.An emergency spillway and associated freeboard must be provided in accordance withapplicable local or state dam safety requirements. The emergency spillway must be locatedso that downstream structures will not be impacted by spillway discharges.Both the safety bench and the aquatic bench should be landscaped with vegetation thathinders or prevents access to the pool.Warning signs prohibiting swimming should be posted.Local governments and homeowner associations may require fencing of wet ponds at theirdiscretion. Fencing is required at or above the maximum water surface elevation in the raresituations when the pond slope is a vertical wall.6.7. Landscaping and Planting PlanA landscaping plan must be provided that indicates the methods used to establish and maintainvegetative coverage in the pond and its buffer. Minimum elements of a plan include thefollowing:xxxxDelineation of pondscaping zones within both the pond and bufferSelection of corresponding plant speciesThe planting planThe sequence for preparing the wetland benches (including soil amendments, if needed)Version 1.8, April 13, 2010Page 11 of 23

VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14xxxxxxWET PONDSources of native plant materialThe landscaping plan should provide elements that promote diverse wildlife and waterfowluse within the stormwater wetland and buffers.Woody vegetation may not be planted or allowed to grow within 15 feet of the toe of theembankment nor within 25 feet from the principal spillway structure.A vegetated buffer should be provided that extends at least 25 feet outward from themaximum water surface elevation of the wet pond. Permanent structures (e.g., buildings)should not be constructed within the buffer area. Existing trees should be preserved in thebuffer area during construction.The soils in the stormwater buffer area are often severely compacted during the constructionprocess, to ensure stability. The density of these compacted soils can be so great that iteffectively prevents root penetration and, therefore, may lead to premature mortality or lossof vigor. As a rule of thumb, planting holes should be three times deeper and wider than thediameter of the root ball for ball-and-burlap stock, and five times deeper and wider forcontainer-grown stock.Avoid species that require full shade, or are prone to wind damage. Extra mulching aroundthe base of trees and shrubs is strongly recommended as a means of conserving moisture andsuppressing weeds.For more guidance on planting trees and shrubs in wet pond buffers, consult the following:xxCappiella et al (2006)Riparian Buffer Modification & Mitigation Guidance Manual, available online at:http://www.dcr.virginia.gov/chesapeake bay local assistance/ripbuffmanual.shtmlxAppendix E: Landscaping of the Introduction to the New Virginia Stormwater DesignSpecifications , as posted on the Virginia Stormwater BMP Clearinghouse web site.6.8. Maintenance Reduction FeaturesThe following wet pond maintenance issues can be addressed during the design, in order to makeon-going maintenance easier:xMaintenance Access. Good access is needed so crews can remove sediments, make repairsand preserve pond treatment capacity).o Adequate maintenance access must extend to the forebay, safety bench, riser, and outletstructure and must have sufficient area to allow vehicles to turn around.o The riser should be located within the embankment for maintenance access, safety andaesthetics. Access to the riser should be provided by lockable manhole covers andmanhole steps within easy reach of valves and other controls.o Access roads must (1) be constructed of load-bearing materials or be built to withstandthe expected frequency of use, (2) have a minimum width of 12 feet, and (3) have aprofile grade that does not exceed15%. Steeper grades are allowable if appropriatestabilization techniques are used, such as a gravel road.o A maintenance right-of-way or easement must extend to the stormwater pond from apublic or private road.Version 1.8, April 13, 2010Page 12 of 23

VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14xWET PONDLiners. When a stormwater pond is located over highly permeable soils or fractured bedrock,a liner may be needed to sustain a permanent pool of water. If geotechnical tests confirm theneed for a liner, acceptable options include the following: (1) a clay liner following thespecifications outlined in Table 14.4 below; (2) a 30 mil poly-liner; (3) bentonite; (4) use ofchemical additives; or (5) an engineering design, as approved on a case-by-case basis by thelocal review authority. A clay liner should have a minimum thickness of 12 inches with anadditional 12 inch layer of compacted soil above it, and it must meet the specificationsoutlined in Table 14.4. Other synthetic liners can be used if the designer can supplysupporting documentation that the material will achieve the required performance.Table 14.4. Clay Liner SpecificationsPropertyTest MethodUnitPermeabilityASTM D-2434Cm/secPlasticity Index of ClayASTM D-423/424%Liquid Limit of ClayASTM D-2216%Clay Particles PassingASTM D-422%Clay CompactionASTM D-2216%Source: Virginia Stormwater Management Handbook (1999)Specification61 x 10Not less than 15Not less than 30Not less than 3095% of standard proctor density6.9. Wet Pond Material SpecificationsWet ponds are generally constructed with materials obtained on-site, except for the plantmaterials, inflow and outflow devices (e.g., piping and riser materials), possibly stone for inletand outlet stabilization, and filter fabric for lining banks or berms.The basic material specifications for earthen embankments, principal spillways, vegetatedemergency spillways and sediment forebays shall be as specified in Appendices A through D ofthe Introduction to the New Virginia Stormwater Design Specifications, as posted on the VirginiaStormwater BMP Clearinghouse web site, at the following .htmlWhen reinforced concrete pipe is used for the principal spillway to increase its longevity, “O”ring gaskets (ASTM C-361) should be used to create watertight joints, and they should beinspected during installation.SECTION 7: REGIONAL & SPECIAL CASE DESIGN ADAPTATIONS7.1. Karst TerrainKarst regions are found in much of the Ridge and Valley province of Virginia. The presence ofkarst complicates both land development in general and stormwater design in particular.Designers should always conduct geotechnical investigations in karst terrain to assess this risk induring the project planning stage. Because of the risk of sinkhole formation, groundwatercontamination, and frequent facility failures, use of wet ponds is highly restricted in karst regions(see CSN Technical Bulletin No. 1, 2008, and Appendix 6-C of Chapter 6 of the VirginiaVersion 1.8, April 13, 2010Page 13 of 23

VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14WET PONDStormwater Management Handbook, 2010). At a minimum, designers must specify thefollowing:xxxxA minimum of 6 feet of unconsolidated soil material exists between the bottom of the basinand the top of the karst layer.Maximum temporary or permanent water elevations within the basin does not exceed 6 feet.Annual maintenance inspections must be conducted to detect sinkhole formation. Sinkholesthat develop should be reported immediately after they have been observed, and should berepaired, abandoned, adapted or observed over time following the guidance prescribed by theappropriate local or state groundwater protection authority (see Section 9.3)A liner is installed that meets the requirements outlined in Table 14.5.Table 14.5. Required Groundwater Protection Liners for Pondsin Karst Terrain (WVDEP, 2006 and Virginia Stormwater Management Handbook, 1999)SituationPond not excavated to bedrockPond excavated to or near bedrockCriteria24 inches of soil with a maximum hydraulic5conductivity of 1 x 10- cm/sec.124 inches of clay with a maximum hydraulic6conductivity of 1 x 10- cm/sec.Pond excavated to bedrock within a wellheadprotection area, in a recharge area for a domestic Synthetic liner with a minimum thickness of 60 mil.well or spring, or in a known faulted or folded area1Cl

VA DEQ STORMWATER DESIGN SPECIFICATION NO. 14 WET POND Version 1.9, March 1, 2011 Page 1 of 23 VIRGINIA DEQ STORMWATER DESIGN SPECIFICATION No. 14 WET POND VERSION 1.9 March 1, 2011 Amended May 11, 2015 SECTION 1: DESCRIPTION Wet ponds consist of a permanent pool

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