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

Technical RequirementsSand FilterDesign Procedure – Sedimentation BasinThe design procedure and application of design criteria for the Austin Sand Filter sedimentation basin areoutlined in the following steps. This procedure is for full sedimentation design. Partial sedimentationdevices are not recommended due to high maintenance concerns.Step 1 – Determine Water Quality Volume (WQV)Using the Appendix E in this Design Manual, determine thetributary drainage area and stormwater quality design volume(WQV) for 48-hour drawdown.Step 2 – Determine Sedimentation Basin Volume (Vsb)The volume of the sedimentation basin must be ≥ WQVV sb = WQV (minimum)Use the Design DataSummary Sheet (TableSF-5) to record designinformation for thepermitting agency’s review.Step 3 – Determine Sedimentation Basin Water Depth (d sb )The allowable water depth in the sedimentation basin will be governed by the available hydraulic head,which will be based on the difference in elevation between the sedimentation basin inlet and the filltrationbasin outlet. The design d sb value should be ≥ 3 ft and ≤ 10 ft. Select a design depth in the allowable rangethat yields the required V sb given any footprint area constraints.Step 4 – Determine Sedimentation Basin Area (Asb)A sb = V sb / d sbStep 5 – Determine Sedimentation Basin ShapeDetermine overall length (L sb ) and width (W sb ) dimensions to yield the A sb for the basin, given anyfootprint area constraints.A sb = L sb x W sbThe length-to-width ratio should be at least 2:1. If necessary, provide internal baffling to achieve this ratioand to mitigate short-circuiting and/or dead storage problemsIf the basin is not rectangular, shape the basin with a gradual expansion from the inlet and a gradualcontraction toward the outlet. Design the basin to maximize the distance from the inlet (near whereheavier sediment will be deposited) to the outlet structure. This configuration improves basin performanceand reduces maintenance requirements.Step 6 – Determine Inlet/Outlet DesignDesign the inlet structure to convey the water quality volume to the unit and bypass flows in excess ofthis volume directly to the downstream storm drain system. Provide energy dissipation at the inlet tomaintain quiescent conditions needed for effective sedimentation; keep inlet velocities at three (3) feet persecond or less.The outlet structure conveys the water quality volume from the sedimentation basin to the filtration basinand should be a perforated riser pipe equipped with a trash rack to prevent clogging. Trash racks alloweasy access for inspecting and cleaning outlet orifices. Size trash racks to prevent clogging of the outletwithout restricting the hydraulic capacity of the outlet control orifices.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007SF-5