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

Runoff Treatment Control Measure Fact SheetWater Quality Detention BasinsDesign CriteriaDesign criteria for stormwater detention basins are listed in Table DB-1.Table DB-1. Water Quality Detention Basin Design CriteriaDesignParameterDrainage areaCriteria for Basin TypeDry Wet Combo NotesTypically >20 acresDesign volume WQV 1.25 WQV 1.125 WQV See Figure DB-3 and Appendix E in this manualfor details.Max DrawdownTimeDepth of Basin/Permanent Pond48 hrs See Appendix E.NA 4 ft. – 8 ft 4 ft. – 8 ft A 4 ft pond depth ensures an open water area,retards cattail growth, reduces stagnation, & allowsfor mosquito fish. Water deeper than 4 ft increasesthe residence time and results in lessheating/stagnation in summer.BasinShapeSide slopes (H:V)Embankment sideslope (H:V)Length 3X width (min.)3:1 or flatter3:1 or steeper4:1 or flatter3:1 or flatterAlways maximize the distance between the inletand outlet. Whenever possible, shape the basin togradually expand from the inlet then graduallycontract toward the outlet (e.g., teardrop.)Basin side slopesPermanent ponds side slopesInsideOutside (without retaining wall)Longitudinal slope/bottom surfaceSlope atleast 2%Undulatethe bottomdepthUndulatethe bottomdepthBasin freeboard1 ft.Treatment for dryweatherflowssubmergedgravel bedpermanentpondpermanentpondOther dry weather treatment features for drybasins may be approved by the permitting agencyon a case by case basis.Sediment ForebayVolume5% to 10% max of the total designvolumeSediment forebays may not be required; checkwith the local permitting agency.Step 1 – Calculate Water Quality Volume (WQV)Using Appendix E in this manual, determine the stormwater quality design volume, WQV, for thecontributing area. See Figure DB-3 and table DB-1 for the volume requirements for dry,combination and wet basins.Step 2 – Design the BasinDesign the basin to:• Provide the required volume as determined in step 1.• Meet criteria on Table DB-1 regarding depth, shape, side slope and longitudinal slope• Incorporate a sediment forebay if required by the local agency. See step 3.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsDB-4 May 2007