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

Runoff Treatment Control Measure Fact SheetWater Quality Detention BasinsStep 3 – Sediment ForebayA sediment forebay may be required (check with local permitting agency) at each inflow point in order totrap sediment where it can be easily cleaned out. The forebay size should be about 5% to 10% maximumof the total design volume and designed to release in 3-5 minutes.For easy-maintenance, design the sediment forebay(s) to include:• An access road. See step 5 for detailed information.• Concrete lining to prevent equipment from sinking during cleaning.• A concrete wall to separate the forebay from the rest of the water quality basin. The concrete wallshould include overflow weir allowing flows to exit at non-erosive velocities during the 2-yearand 10-year frequency design storms.Step 4 - Design the Inlet and OutletInlet DesignDesign the inlet structure to:• Dissipate energy of incoming flows to prevent erosion and prevent resuspension of previouslydeposited sediment.• For permanent ponds, set pipe invert approximately two (2) feet from pond bottom abovesediment storage.• Install trash/access control rack. Check with the local agencies details.• Provide access per Step 5.Outlet DesignThe goal of the outlet design criteria is to detain flows for a sufficient period of time to permit the settlingof smaller sized sediments while meeting the release criteria so that storage is available for subsequentstorm events. Outlets should be designed in accordance to Figure DB-6 or equal design as approved bythe local agency and shall include trash racks to keep debris from clogging the outlet without interferingwith the hydraulic capacity. In addition, flap gates should be installed to avoid the effects of backwater inthe downstream creek or channel. The release criteria for the basins are as follows:• Dry Basin - A dry basin is required to release 75 percent of the water quality volume in aminimum of 24 hours and the total design volume over an additional 24 hours for a total releasetime of 48 hours. The WQV should not be discharged too quickly or pollutant removal will becompromised.• Wet basins - A wet basin is required to maintain the permanent pond volume while dischargingbased on the downstream drainage systems capacity.• Combination basins - A combination basin is required to maintain the permanent pond volumewhile releasing 75 percent of the 0.5 WQV in a minimum of 24 hours and the remainder 25percent of the 0.5 WQV over an additional 24 hours for a total release time of 48 hours.Step 5 – Access DesignProvide a way for maintenance vehicles to access all structures and cells within the basin such as thebasin bottom, sediment forebay, inlets and outlets, low flow channels and submerged gravel beds.Design access roads to have an all weather access surface, a width of 15’ to 20’ (check with the localpermitting agency for required width) and a minimum turning radius of 40 feet. Design access ramps tobe concrete or other impervious surface (check with local permitting agency) to the basin bottom with amaximum slope of 10% and a width of 15 feet. Place gates across all access ramps to discourage access.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsDB-6 May 2007