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

Technical RequirementsInfiltration BasinDesign Parameter Criteria NotesSetbacks100 ft20 ft100 ftFrom wells, tanks, fields, springsDownslope from foundationsUp slope from foundationsInlet/outlet erosion control – Use energy dissipater to reduce inlet/outlet velocityForebay setting basinvolume/drain timeEmbankment side slope(H:V)Maintenance access rampslope (H:V)Maintenance access rampwidthRelief underdrain pipediameter5-10%/45min.≥ 4:1≥ 3:1Based on WQVInsideOutside (without retaining walls)10:1 Or flatter15 to 20ftCheck with permitting agency for their minimum width. Pave approachwith concrete or porous pavement materials, subject to approval ofpermitting agency.4 in. Perforated plastic pipeVegetation – Side slopes and bottom (may require irrigation)Design ProcedureStep 1 – Calculate Water Quality Volume (WQV)Using the Appendix E in this Design Manual, determine the tributarydrainage area and stormwater quality design volume (WQV) for 48-hourdrawdown.Step 2 – Calculate design depth of water surcharge ininfiltration basin (D max )Dmaxtmax× I=12 × sUse the Design DataSummary Sheet(Table IB-3) to recorddesign information forthe permitting agency’sreview.wheret max = Maximum drawdown time = 48 hrsI = Site infiltration rate (soil permeability) (in/hr)s = Safety factorIn the formula for maximum allowable depth, the safety factor accounts for the variability in soilpermeability at the site and the relative uncertainty in the infiltration rate measurements. The morevariable the soil conditions and the less certain the infiltration rate, the higher the safety factor should be.Safety factors typically range between two (2) and ten (10) and should be determined by a qualifiedgeotechnical engineer or geologist based on field measurements of saturated vertical permeability at theproposed site. Note that soils with permeability greater than two (2) inches per hour may be used if fullpretreatment is provided using one of the approved treatment controls from this manual.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007IB-3