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

Technical RequirementsSand FilterStep 9 – Determine Sedimentation Basin Liner DesignIf the sedimentation basin is an earthen structure and an impermeable liner is required to protectgroundwater quality, the liner shall provide a maximum permeability of 1x10 -6 cm/sec (ASTM MethodD-2434). If an impermeable liner is not required, then install a geotextile fabric liner that meets thespecifications listed in Table SF-3 unless the basin has been excavated to bedrock.Design Procedure – Filtration BasinThe design procedure and application of design criteria for the Austin Sand Filter Filtration Basin areoutlined in the following steps.Step 1 – Determine Minimum Filtration Basin Storage VolumeThe storage capacity of the filtration basin above the surface of the sand filter bed should be greater thanor equal to 20 percent of the WQV. This capacity is necessary in order to account for backwater effectsresulting from partially clogged filter media.V fbs ≥ 0.2 x WQVStep 2 – Determine Filter Bed Surface AreaSurface area is the primary design parameter for the filtration basin and is a function of sand permeability,filter bed depth, hydraulic head, and filtration rate. The design filter bed area should be the larger of theminimum area required for storage and the minimum area required for flow.a. Determine minimum filter surface area required for storage (A fbs )A fbs = V fbs / d fbswhereV fbs = Storage volume above filter bedA fbs = Filter bed surface area based on storage, ft 2d fbs = Depth of storage above filter bed, ft (3 ft minimum)b. Determine minimum filter surface area required for flow (Aff)Aff(WQV)( df)=(k)(d + d )(tfbsff)whereWQV = Design Water Quality Volume, ft 3A ff = Filter surface area based on flow, ft 2d f = Filter bed depth, ftk = Coefficient of permeability for sand filter, ft/h (0.146 ft/h)d fbs = Depth of storage above filter bed, ftt f = Time required for runoff volume to pass through filter, hrs (48 hrs)c. Use the larger of Afbs and Aff as design value for sand filter bed areaStep 3 – Design Inlet StructureThe inlet structure should spread the flow uniformly across the surface of the sand filter. Flow spreaders,weirs, or multiple orifice openings are recommended. See Figure SF-1 for an example inlet structuredesign.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007SF-7