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

Technical RequirementsVegetated SwaleHow does a Vegetated Swale work?A vegetated swale differs from a conventional drainage channel or roadside ditch due to the incorporationof specific features that enhance stormwater pollutant removal. A vegetated swale is designed to controlflow velocities through the vegetation in the swale and to provide sufficient contact time to promotesettling and filtering of the runoff flowing through it. Greater surface area and contact time promotegreater runoff treatment efficiencies. The volume of runoff can also be reduced through infiltration intounderlying soils.Runoff is treated as it flows through, not over, the vegetation in avegetated swale. Vegetation can cause considerable turbulence,resulting in energy loss and retardance of flow. To provideadequate treatment, the vegetation must be dense and strong.Other Names: Grassy swale, bioswalePlanning and Siting Considerations• Select location where site topography allows for the designof a channel with sufficiently mild slope (unless small dropstructures are used) and enough surface area to maintainnon-erosive velocities in the channel.• Integrate swales into open space buffers, undisturbednatural areas, and other landscape areas when possible. Donot place in open space or wetland preserve areas wherefuture maintenance could be prohibited.• For parking lot design, stalls can be shortened if tire curbsare provided around the perimeter of the swale and cars areallowed to overhang the swale (see Farm Bureau photo).Alternative vegetation is encouraged but issubject to approval of local permittingagency. Parking lot swale at Elk GroveMarketplace, Elk Grove, CA. Photo: CKBEnvironmentalVegetated Swale Planted with Native Clump Grasses (note that treesas shown are now discouraged; locate outside of flowline), U.S. FarmBureau, Sacramento. Photo: City of SacramentoRoadside Swale. Photo: City of SpokaneStormwater Quality Design Manual for the Sacramento and South Placer RegionsVS-2 May 2007