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

Technical RequirementsSand FilterA trash rack shall be provided for the outlet. Opening in the rack should not exceed 1/3 the diameter ofthe vertical riser pipe. The rack should be made of durable material, resistant to rust and ultraviolet rays.The bottom rows of perforations of the riser pipe should be protected from clogging. To prevent cloggingof the bottom perforations, it is recommended that geotextile fabric be wrapped over the pipe’s bottomrows and that a cone of one (1) to three (3) inch diameter gravel be placed around the pipe. If a geotextilefabric wrap is not used, the gravel must be large enough not to enter the riser piper perforations. Analternative design, such as geocomposite drain, may also be approved by the local permitting agency.Step 7 – Design the Basin to Avoid Short-CircuitingDesign the sedimentation basin with baffles as needed to avoid short-circuiting (i.e., flow reaching theoutlet before it passes through the sedimentation basin volume).Step 8 – Design the Sediment Trap (Optional)A sediment trap is a storage area that captures sediment and removes it from the basin flow regime,thereby inhibiting resuspension of solids during subsequent runoff events and improving long-termremoval efficiency. The trap also helps the basin maintain adequate storage volume by reducing sedimentthat would otherwise accumulate within it; this, in turn, can reduce maintenance needs. If a sediment trapis provided, size the volume to be equal to 10 percent of the sedimentation basin volume and design it tocompletely drain within 48 hours. Place the invert of the drain pipe above the surface of the sand bed ofthe filtration basin and make sure the grading of the piping to the filtration basin is at least 1/4 inch perfoot (two percent slope). Provide access for cleaning the sediment trap drain system.Figure SF-2.Example SedimentTrap DetailsStormwater Quality Design Manual for the Sacramento and South Placer RegionsSF-6 May 2007