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

Technical RequirementsVegetated SwaleDesign ProcedureStep 1 – Determine the vegetated swale’s functionThe vegetated swale can be designed to function as both a treatment control measure for the stormwaterquality design flow and as a conveyance system to pass the peak hydraulic design flows, if the swale islocated “on-line”.Step 2 – Calculate Water Quality Flow (WQF)Using Appendix E in this Design Manual, determine the contributing area and stormwater quality designflow, WQF.Step 3 – Provide for peak hydraulic design flowsUsing local hydrologic design criteria, calculate flows greater than WQF to be diverted around or flowthrough the swale. Design the diversion structure, if needed.Step 4 – Design the vegetated swale using Manning’s EquationSwales can be trapezoidal or parabolic in shape, as illustrated in Figure VS-1. While trapezoidal channelsare the most efficient for conveying flows, parabolic configurations provide good water quality treatmentand may be easier to mow since they don’t have sharp breaks in slope.Use a roughness coefficient (n) of 0.20 with Manning’s Equation to design the treatment area of a swaleto account for the flow through the vegetation. To determine the capacity of the swale to convey peakhydraulic flows, use a roughness coefficient (n) of 0.10 with Manning’s Equation.Manning’s Equationwhere5 / 31.49 AQ = × s2 / 3n P1/ 2Q = WQF, (cfs)A = Cross sectional area of flow, (ft 2 )P = Wetted perimeter of flow, (ft)s = Bottom slope in flow direction, (ft/ft)n = Manning’s n (roughness coefficient)Record all of yourcalculations on theVegetated Swale DesignData Summary Sheet (TableVS-3) at the end of thissection. The data sheet willbe checked by the agencyplan review staff.For treatment design, solve Manning’s equation by trial and error to determine a bottom width that yieldsa flow depth of 3 to 5 inches at the design WQF and the swale geometry (i.e., side slope and s value) forthe site under design.Step 5 – Design Inlet Controls• For flow introduced along the length of the swale through curb cuts, provide minimum curb cutwidths of 12 inches and avoid short-circuiting the swale by providing the minimum contact timeof 7 minutes.• For swales that receive direct concentrated runoff at the upstream end, provide an energydissipater, as appropriate, and a flow spreader such as a pea gravel diaphragm flow spreader at theupstream end of the swale.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsVS-4 May 2007