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

Runoff Treatment Control Measure Fact SheetVegetated Filter StripHow does a Vegetated Filter Strip work?A Vegetated Filter Strip is designed to create shallow sheet flow ofrunoff over a gently sloping, densely vegetated surface. Treatment ofthe runoff occurs through a variety of natural mechanisms as therunoff flows through the vegetation and over the soil surface. Toensure adequate treatment, the vegetation must be dense and strong.Greater surface area and contact time promote greater runofftreatment efficiencies. The volume of runoff can be reduced throughinfiltration into underlying soils. See Figure VFS-1 for a typicalVegetated Filter Strip configuration.Other Names: Grass filter strips, BiofilterPlanning and Siting Considerations• Select location where site topography allows for the designof filter strips with proper slopes in flow direction.Do not confuse aVegetated Filter Stripwith a Vegetated Swale,described elsewhere inthis manual, or a grassbuffer strip, which is usedas a low impact designstrategy or forpretreatment. The latterprovides only limitedpollutant removalbecause of higherapplication rates, and itrequires downstreamtreatment controls.• Integrate Vegetated Filter Strips into open space buffers, undisturbed natural areas, and otherlandscape areas when possible.• For parking lot design, stalls can be shortened if tire curbs are provided around the perimeter ofthe filter strip and cars are allowed to overhang the filter strip.• Irrigation is typically required to maintain viability of the filter strip vegetation. Coordinatedesign of general landscape irrigation system with that of the Vegetated Filter Strip, as applicable.Design CriteriaDesign criteria for Vegetated Filter Strips are listed in Table VFS-1. Use the design data summary sheet(Table VFS-3) provided at the end of this fact sheet to record design information for review by the agencyplan reviewer.Table VFS-1. Vegetated Filter Strip Design CriteriaDesign Parameter Criteria NotesDrainage area ≤ 5 acres For larger areas, break up into sub-sheds of 5 acres or less with a filter strip foreach.Design flow WQF See Appendix E in this Design Manual.Maximum linearapplication rate (q a )0.005 cfs/ftof widthRate at which runoff is applied across the top width of filter strip. This rate,combined with the design flow, will define the design width of filter strip.Minimum slope inflow directionMaximum slope inflow directionMinimum length inflow directionVegetation height(typical)1% Gentler slopes are prone to ponding of water on surface4% Steeper slopes are prone to channeling25 ft Most treatment occurs in the first 25 feet of flow. Longer lengths will typicallyprovide somewhat higher levels of treatment2 – 4 in. Vegetation should be maintained at a height greater than the depth of flow atdesign flow rate but sufficiently low to prevent lodging or shading.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsVFS-2 May 2007