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

Runoff Treatment Control Measure Fact SheetConstructed Wetland BasinStep 6 – Design Basin ForebayThe forebay provides a location for sedimentation of larger particles and has a solid bottom surface tofacilitate mechanical removal of accumulated sediment. The forebay is part of the permanent pool and hasa water surface area comprising 5 to 10% of the permanent pool water surface area and a volumecomprising 5 to 10% of the WQV. The depth of permanent pool in the forebay should be a minimum of 4feet. Provide the forebay inlet with an energy dissipation structure and/or erosion protection. Trashscreens at the inlet are recommended to keep trash out of the basin.Step 7 – Design Outlet WorksProvide outlet works that limit the maximum water surface elevation to WS Elev WQV . The outlet worksare to be designed to release the WQV over a 48-hour period. Protect the outlet from clogging with a trashrack and a skimmer shield that extends below the outlet and above the maximum WQV depth. A singleorifice outlet control is shown in Figure CWB-1.a. For single orifice outlet control or single row of orifices at the permanent pool elevation (WSElev pp ) (see Figure CWB-1), use the orifice equation based on the WQV (ft 3 ) and depth of waterabove orifice centerline D (ft) to determine orifice area (ft 2 ):Orifice EquationQ = C × A ×2gDwhereQ = Flow rate, (cfs)C = Orifice coefficient (use 0.61)A = Area of orifice, (ft 2 )g = Acceleration due to gravity (32.2 ft/sec 2 )D = Depth of water above orifice centerline (D WQV )The equation can be solved for A based on the WQV and using a design drawdown time (t) of 48hours.b. For perforated pipe outlets or vertical plates with multiple orifices, use the following equation todetermine required area per row of perforations, based on the WQV (acre-ft) and depth of waterabove centerline of the bottom perforation D (ft).whereArea/row (in 2 ) = WQV/K 48K 48 = 0.012D 2 + 0.14D – 0.06 (from Denver UDFCD, 1999)Select appropriate perforation diameter and number of perforations per row (columns) with theobjective of minimizing the number of columns and using a maximum perforation diameter of 2inches. Rows are spaced at 4 inches on center from the bottom perforation. Thus, there will be 3rows for each foot of depth plus the top row. The number of rows (nr) may be determined asfollows:nr = 1 + (D × 3)Calculate total outlet area by multiplying the area per row by number of rows.Total Orifice Area = area/row × nrStormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007CWB-5