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

Runoff Treatment Control Measure Fact SheetInfiltration TrenchTable IT-1. Infiltration Trench Design CriteriaDesign Parameter Criteria NotesTributary Drainage Area≤ 5 acresDesign volume WQV See Appendix E in this Design Manual.Maximum drawdown time for WQV 48 hrs Based on WQVSoil infiltration rate 0.5-2 in./hr (soil permeability test required)Minimum groundwater separation 10 ft Between trench bottom and top of seasonallyhigh groundwater tableMaximum trench surcharge depth (D max ) 10 ftSetbacks100 ft20 ft100 ft–From wells, tanks, fields, springsDownslope from foundationsUpslope from foundationsDo not locate under tree drip-linesTrench media material size/type -3 in. diameter Washed gravel6-12 in. deep sand (if desired)Trench lining material – Geotextile fabric prevents cloggingObservation well size 4-6 in. Perforated PVC pipe with removable capPretreatment grass buffer striplength/slope10 ft/4% Minimum length/maximum slope in flowdirectionDesign ProcedureStep 1 – Calculate Water Quality Volume (WQV)Using the Appendix E in this Design Manual, determine thetributary drainage area and stormwater quality design volume(WQV) for 48-hour drawdown.Use the Design DataSummary Sheet (TableIT-3) to record designinformation for thepermitting agency’s review.Step 2 – Calculate design depth of water surcharge inInfiltration Trench (D max )Maximum depth should not exceed ten (10) feet.whereDmaxtmax× I=12 × s × Pt max = Maximum drawdown time = 48 hrsI = Site infiltration rate (soil permeability) (in/hr)s = Safety factorP = Porosity of Infiltration Trench gravel material (use 0.30)In 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 (e.g., vegetatedfilter strip, vegetated swale).Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007IT-3