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

Runoff Treatment Control Measure Fact SheetInfiltration TrenchHow does an Infiltration Trench work?An infiltration trench is designed to retain the stormwater quality designvolume (or WQV) in the trench and allow that volume to infiltrate intothe native soil profile over the design drawdown period. Infiltrated watertypically reaches and can recharge the underlying groundwater.Treatment of the runoff occurs through a variety of natural mechanismsas the water flows through the trench media and the soil profile. Toensure adequate treatment and protect groundwater, the depth ofunsaturated soil between the trench bottom and the highest seasonalgroundwater surface level should be at least 10 feet. See Figure IT-1 fora typical infiltration trench configuration.Infiltration vaults (Figure IT-2) and infiltration leach fields (Figure IT-3)are similar to infiltration trenches except they are entirely below ground;runoff is conveyed to the upper zone of the gravel bed media viaperforated pipes.Other Names: Percolation trenchPrevent Clogging!Infiltration trenchesneed to be protectedfrom sediment loadsto prevent clogging;a grass buffer is required.If sedimentdeposition significantlyreduces soilinfiltration rates, thecost of restoring thetrench can be high.Planning and Siting Considerations• Conduct an on-site permeability test to confirm suitable infiltration rate prior to beginning design.At least one soil boring under in proposed trench location is recommended. Local permittingagency will require results before accepting design.• Integrate infiltration trenches into open space buffers, undisturbed natural areas, and otherlandscape areas when possible.• Plan for setback requirements as listed in Table IT-1.• Do not locate trenches under tree drip lines.• Install a pretreatment grass buffer strip to filter out sediment and protect the trench from highsediment loads (see Figure IT-1).Design CriteriaDesign criteria for infiltration trenches are listed in Table IT-1. Use the Design Data Summary Sheetprovided at the end of this fact sheet (Table IT-3) to record design information for the permitting agency’sreview.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsIT-2 May 2007