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

Runoff Treatment Control Measure Fact SheetInfiltration TrenchDescriptionPhoto credit: CASQA, 2003An infiltration trench is a long, narrowtrench constructed in naturallypervious soils (types A or B) and filledwith gravel (and sand if desired).Runoff is stored in the trench until itinfiltrates into the soil profile over aspecified drawdown period. Overflowdrains are often provided to allowdrainage if the infiltration trenchbecomes clogged. Infiltration vaultsand infiltration leach fields aresubsurface variations of the infiltrationtrench concept; runoff is distributed tothe upper zone of a subsurface gravelbed by means of perforated pipes.Siting Considerations• Tributary Drainage area: Up to 5 acres. Tributary areas shouldhave a low potential for erosion.• Soil Infiltration Rate: 0.5-2.0 in/hr (permeability test required).Soils with higher permeability will require pretreatment device.• Depth to groundwater: ≥10 ft from trench bottom required• Setback requirements: 100 ft from wells; 20 ft downslope and100 ft upslope from foundations.• Maximum contributing area slope: 5%, maximum downstreamslope: 20%Vector Considerations• Potential for mosquitoes due to standing water will be greatlyreduced or eliminated if the trench is properly designed,constructed, and operated to maintain its infiltration capacityand drawdown time.POLLUTANT REMOVALEFFECTIVENESSSedimentNutrientsTrashMetals*Bacteria*Oil and GreaseOrganics*HighHighHighHighHighHighHigh*The following are target pollutants forSacramento area: copper; lead; mercury;pathogens; diazinon; and chlorpyrifosSource: CASQA California StormwaterBMP Handbook, January 2003Advantages• Reduces or eliminates stormwater discharge to surface waters during most storm events.• Reduces peak flows during small storm events.• Can be incorporated into site landscaping.Limitations• Not appropriate for areas with slowly permeable soil or high groundwater.• Must be protected from high sediment loads; difficult to restore functionality when clogged.• Not appropriate for industrial sites or locations where spills may occur.General Maintenance Recommendations (Low to Moderate)• Repair/replace vegetation buffer as necessary to maintain full cover and prevent erosion.• Periodically remove debris from trench surface.• Check and record infiltration rate during and after major storm events to document infiltration rates.• Repair or replace trench material to restore infiltration capacity when infiltration rate falls belowdesign rate.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007IT-1