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

Technical RequirementsVegetated Swale• In parking lots, plan areas for pedestrians to crossswales without damaging vegetation.• The required swale length to meet treatment criteriafor a 1-acre project site is typically in the range of 75to 100 feet. The length will vary depending on severalvariables, including the geometry of the swale and therunoff coefficient for the site.Design CriteriaEarly Design Is Critical!Vegetated swales must belocated on the site plan at theearliest possible design phasewhen laying out the buildingand parking footprints andbefore the site grading plan isprepared.Design criteria for vegetated swales are listed in Table VS-1.Use the Design Data Summary Sheet provided at the end ofthis fact sheet (Table VS-3) to record design information for the permitting agency’s review.Table VS-1. Vegetated Swale Design CriteriaDesign Parameter Criteria NotesTributary drainage area ≤ 10 acres For larger areas, break up into sub-sheds of 10 acres or less, with aswale for eachWater Quality design flow WQF See Appendix E in this Design Manual.Roughness coefficient (n) fortreatment designRoughness coefficient (n) forconveyance designMinimum contact time fortreatment of the WQFMinimum bottom width0.2 Reflects the roughness associated with shallow flow through densevegetation.0.1 Reflects the roughness of swale when depth of flow is above theheight of the grass. To be used to determine capacity of swale toconvey peak hydraulic flows7 minutes Provide sufficient length to yield minimum contact time for the WQF2 ftMaximum bottom width 10 ft Swales wider than 10 feet must meet additional flow spreadingrequirements.Maximum side slopes 3:1 Side slopes must allow for ease of mowing. Steeper slopes may beallowed with adequate slope stabilization.Longitudinal slope 0.5-2.5%Check dams As required For longitudinal slope > 2.5% but less than 5%, and as a means ofpromoting more infiltration. Spacing as required to maintainmaximum longitudinal bottom slope ≤ 2.5%.Underdrains As required For longitudinal slope < 1.0%Maximum depth of flow(treatment)Maximum flow velocity(treatment)3-5 in. 1 inch below top of vegetation1 ft/sec Based on Manning’s n = 0.20. Concentrated inlet flow must bespreadInlet Design/Curb cuts ≥ 12 in. wide To prevent clogging and promote flow spreading, pavement shouldbe slightly higher than swale. Include energy dissipaters/flowspreaders such as cobble, porous concrete, or gravel at inlet.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007VS-3