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

Runoff Treatment Control Measure Fact SheetWater Quality Detention BasinsSource: County of Sacramento, Department of WaterResourcesDescriptionA stormwater quality detention basin (also called awater quality basin or extended detention basin) isdesigned to hold stormwater runoff from smallstorms and the initial runoff (“first flush”) fromlarger storms for a regulated downstream release.Pollutants are removed from stormwater throughgravitational settling and biological processesdepending on the type of basin. There are three typesof water quality detention basins:• Wet stormwater quality detention basins (wetbasins) that store a permanent pond of water• Dry extended stormwater quality detentionbasins (dry-extended basins) that temporarilystore stormwater runoff• Combination (wet/dry) stormwater qualitydetention basins (combination basins) thatcombine the wet and dry basin treatmentsystemsStormwater quality basins must incorporate featuresthat treat dry-weather flows (such as irrigationrunoff). Wet basins and combination basins treat thedry weather flows within the permanent pond (micropool); however, dry basin designs must include anadditional feature such as a submerged gravel bed or other agency approved feature.Siting Considerations• Drainage area: typically greater than 20 acres• Longitudinal bottom slope: At least 2% in dry basins.Can undulate in wet basins.• Side slopes: 3:1 or flatter for basin; 3:1 or steeper forpermanent ponds.• Impermeable liners may be required in areas withhigh groundwater.Vector Considerations• Potential for mosquitoes due to standing water willbe greatly reduced or eliminated if the basins andassociated features are properly designed,constructed, operated and maintained.• Permanent ponds shall be stocked with mosquito fishfor vector control.POLLUTANT REMOVAL EFFECTIVENESSDry basinWet basinSediment Medium HighNutrients Low MediumTrash High HighMetals* Medium HighBacteria* Medium HighOil & Grease Medium HighOrganics* Medium High*Target Pollutant for Sacramento AreaSource: CASQA California Stormwater BMP Handbook,January 2003Advantages• May provide both stormwater quality treatment and flood control if designed for flood flows.• Mitigate increased volume and flow impacts caused by development.• If properly designed, constructed, and maintained, have substantial aesthetic and recreationalvalue and provide wildlife habitat.• Viewed as a public amenity when integrated into a park or open space setting and properlydesigned and landscaped.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007DB-1