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

Technical RequirementsInfiltration BasinStep 3 – Calculate minimum surface area of Infiltration Basin bottom (Amin)A min= WQV/D maxwhereA min = minimum area required (ft 2 )D max = maximum allowable depth (ft)Step 4 – Design forebay settling basinThe forebay provides a zone for removal of course sediment by sedimentation. Design the forebayevolumeto be five (5) to ten (10) percent of the WQV. Separate the forebay from the basin by a berm orsimilar feature. Provide an outlet pipe connecting the bottom of the forebay and the basin and size it toallow the forebay volume to drain within 45 minutes.Step 5 – Design embankmentsInterior slopes (H:V) should be no steeper than 4:1 and exterior slopes no steeper than 3:1. Flatter slopesare preferable.Step 6 – Design Maintenance AccessProvide for all-weather access for maintenance vehicles to the bottom and outlet works. Maximum gradesof access ramps should be ten (10) percent and minimum width will vary according to local permittingagency requirements, but usually between 15-20 feet. Pave ramps with concrete that is colored to blendwith surroundings.Step 7 – Design Security FencingTo protect habitat and for safety reasons, provide aesthetic security fencing approved by the permittingagency around the infiltration basin, except when specifically waived by the permitting agency.Step 8 – Design BypassProvide for bypass or overflow of runoff volumes in excess of the WQV. Provide spillway or overflowstructures, as applicable (see Figure IB-1).Step 9 – Design Relief DrainProvide 4-inch diameter perforated plastic relief underdrain with a valved outlet to allow removal ofstanding water in the event of loss of soil infiltration capacity. Cutoff collars are recommended alongdrain pipes running under the embankment at 10 to 20 feet intervals to prevent the water from pipingthrough the fill. The portion of the relief drain that is under the embankement should not be permeable.Step 10 – Select VegetationPlant basin bottoms, berms, and side slopes with native grasses or with irrigated turf. Vegetation provideserosion protection and filters sediment out of the runoff..Step 11 – Design irrigation systemProvide an irrigation system to maintain viability of vegetation (short-term establishment and long-term needs).Stormwater Quality Design Manual for the Sacramento and South Placer RegionsIB-4 May 2007