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

Runoff Treatment Control Measure Fact SheetConstructed Wetland BasinTable CWB-2. Distribution of Wetland ComponentsComponents% Permanent PoolSurface Area Design Water DepthForebay 5-10% 4 feet (minimum)Open-water zone 10-50% 4 feet (minimum)Wetland zones withemergent vegetation50-70% 6 to 12 inches (30% to 50% of this area should be 1foot deep with bottom slope ≤ 10%)Outlet zone 5-10% 3 feet (minimum)Step 4 – Determine Surcharge Depth of WQV above Permanent Pool and Maximum WaterSurface ElevationThe surcharge depth of the WQV above the permanent pool’s water surface (D WQV ) should be ≤ 2.0 feet.a. Estimate WQV surcharge depth (D WQV ) based on A pp .D WQV = WQV/A ppb. If D WQV > 2.0 feet, adjust value of V pp and/or D avg to increase A pp and yield D WQV ≤ 2.0.The water surface of the basin will be greater than A pp when the WQV is added to the permanentpool.c. Estimate maximum water surface area (A WQV ) with WQV based on basin shape.d. Recalculate Final D WQV based on A WQV and A pp . Note: V pp and/or D avg can be adjusted to yieldFinal D WQV ≤ 2.0 feet.Final D WQV = WQV/((A WQV + A pp )/2)e. Calculate maximum water surface elevation in basin with WQV.WS Elev WQV = WS Elev pp + Final D WQVStep 5 – Determine inflow requirementA net inflow of water must be available at all times through a perennial base flow or supplemental watersource. Use the following equation and parameters to estimate the quantity of monthly inflow required atvarious times of the year. The maximum monthly requirement will govern the design requirement.Q inflow = Q E-P + Q seepage + Q ETwhereQ inflow = Estimated base flow (acre-ft/mo.) (Estimate by seasonal measurementsand/or comparison to similar watersheds)Q E-P = Loss due to evaporation minus the gain due to precipitation (acre-ft/mo.)Q seepage = Loss or gain due to seepage to groundwater (acre-ft/mo.)Q ET = Loss due to evapotranspiration (additional loss through plant area abovewater surface not including the water surface) (acre-ft/mo.)Note that an impermeable liner may be required to maintain permanent pool level in areas with extremelypermeable soils.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsCWB-4 May 2007