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

Technical RequirementsSand FilterStep 4 – Design Filter BedThe sand filter bed may be either of the two configurations given below. Sand bed depths are final,consolidated depths, so consolidation effects must be taken into account.a. Sand Bed with Gravel Underdrain (Figure SF-3)The sand layer shall be a minimum depth of 18 inches and shall consist of 0.02 to 0.04-inchdiameter sand. Below the sand is a layer of 0.5 to 2-inch diameter gravel that provides aminimum of two (2) inches of cover over the top of the underdrain lateral pipes. No gravel isrequired under the lateral pipes. A layer of geotextile fabric meeting the specifications in TableSF-2 must separate the sand and gravel and must be wrapped around the lateral pipes.Drainage matting meeting the specifications in Table SF-2 should be placed under the laterals toprovide for adequate vertical and horizontal hydraulic conductivity to the laterals.In areas with expected high sediment loads (total suspended solids concentration ≥ 200 mg/L),the two-inch layer of gravel on top of the sand filter should be underlain with Enkadrain 9120filter fabric or equivalent meeting the specifications in Table SF-2.b. Sand Bed with Trench Underdrain (Figure SF-4)The top layer shall be 12-18 inches of 0.02 to 0.04-inch diameter sand. Laterals shall be placed intrenches with a covering of 0.5 to 2-inch gravel and geotextile fabric (see Table SF-2). Thelaterals shall be underlain by a layer of drainage matting (see Table SF-2).In areas with expected high sediment loads, see the note above about use of Enkadrain filterfabric or equivalent.Step 5 – Design Underdrain PipingThe underdrain piping consists of the main collector pipe(s) and perforated lateral branch pipes (see planview in Figure SF-1). The piping should be reinforced to withstand the weight of the overburden. Internaldiameters of lateral branch pipes should be six inches or greater and perforations should be 3/8 inch. Eachrow of perforations should contain at least six holes and the maximum spacing between rows ofperforations should not exceed six inches. All piping is to be Schedule 40 polyvinyl chloride (PVC) orgreater strength. The minimum slope of piping shall be 1/8 inch per foot (one (1) percent; slopes down toone-half (0.5) percent may be acceptable to the permitting agency). Access for cleaning all underdrainpiping is needed.Note: Unlike the sedimentation basin, the filtration basin does not require a drawdown time for release.Thus, it is not necessary to have a specifically-designed orifice for the filtration basin outlet structure.Step 6 – Design Filtration Basin LinerIf the filtration basin is an earthen structure and an impermeable liner is required to protect groundwaterquality, the liner shall provide a maximum permeability of 1x10 -6 cm/sec (ASTM Method D-2434). If animpermeable liner is not required, then install a geotextile fabric liner that meets the specifications listedin Table SF-3 unless the basin has been excavated to bedrock.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsSF-8 May 2007