Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Runoff Treatment Control Measure Fact SheetWater Quality Detention Basins• Wet basins: Permanent ponds can provide significant water quality improvement across a broadspectrum of constituents including dissolved nutrients.• Wet basins: Can treat dry weather flows without the need for additional features.• Dry basins: May be easier to maintain than wet basins.Limitations• May pose some safety concerns where there is public access.• Cannot be placed on steep, unstable slopes.• Wet basins: May need a supplemental water source to replenish and maintain the permanentpond.• Wet basins: Are typically not permitted if routine pumping of ground or surface water would beneeded to maintain the pond. Check with the local permitting agency.• Dry basins: May require more land than combination and wet ponds.How does a water quality detention basin work?Stormwater quality detention basins allow particles and associated pollutants to settle out. Permanentponds (micropools) may enhance pollutant removal through biological and chemical processes. Thevolume of runoff may also be reduced through infiltration and evaporation. Dry basins fill up during astorm event and detain the water quality volume for a period of 48 hours. Wet basins allow stormwaterrunoff to slowly pass through the pond displacing water from the permanent pond. Combination basinsinclude both a permanent pond and additional storage for detaining a portion of the water quality volumefor a period of 48 hours.Other Names: water quality basin, extended detention basin, dry extended basin, wet ponds, wetextended-detention basins, dry pondsPlanning and Siting Considerations• Plan water quality basins to be aesthetically-pleasing public amenities (see Figure DB-1).• Where possible, design water quality basins as a joint use with parks (passive recreation), openspace, wildlife habitat, aesthetic amenities and/or flood control detention facilities (see FiguresDB-1 and DB-2). Generally, the area within the water quality volume (WQV) is not well suitedfor recreation facilities such as ballparks, picnic areas and restrooms.• Ponds present special design considerations such as the selection of appropriate vegetation andnuisance abatement in order to function properly as both a water quality control measure and apublic amenity.• Use dry basins if dry weather flows are not sufficient to maintain the permanent pond of wet andcombination ponds. See Figure DB-4.• Wet and combination basins may require a supplemental water source to maintain the permanentpond until the entire drainage shed is built out.• Wet and combination basins require submitting water balancing calculations to ensure that thepermanent pond volume will be maintained in the dry season. Use an evaporation rate of 0.45in/day for the Sacramento area.• May be required to include aeration and/or fountains for permanent ponds with depths greaterthan 6 feet.• Place top soil within the top 12” of the basin to support plant growth.• Refer to the local agency drainage criteria for flood control design.• Consider re-circulating dry-weather flows in a water feature or as irrigation water to conservewater and benefit water quality.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsDB-2 May 2007