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

Technical RequirementsInfiltration BasinHow does an Infiltration Basin work?An infiltration basin is designed to retain the stormwater quality designvolume (WQV) within a basin and to allow that volume to infiltrate intothe native soil profile over the design drawdown period. Infiltrated watertypically reaches and recharges the underlying groundwater. Treatment ofthe runoff occurs through a variety of natural mechanisms as the waterflows through the soil profile. To ensure adequate treatment, the depth ofunsaturated soil between the infiltration basin bottom and the seasonalmaximum groundwater surface level should be at least 10 feet. See FigureIB-1 for a typical infiltration basin configuration.Other Names: retention basin, percolation basinDo not confuse anInfiltration Basin withan Extended DryDetention Basin, whichis designed to infiltratesome runoff andrelease the rest, asdescribed elsewhere inthis chapter.Planning and Siting Considerations• Soil permeability, depth to groundwater, and design safety factors should be determined by aqualified geotechnical engineer or geologist to ensure that conditions conform to the criteria listedin Table IB-1. A soil permeability test will be required by permitting agency to confirmacceptable saturated permeability. Number of soil borings will depend on size of facility.• Not suitable for areas with existing groundwater contamination.• Integrate infiltration basins into open space buffers, undisturbed natural areas, and otherlandscape areas when possible. Avoid placing features in open space and wetland preserveswhere future maintenance of the water quality facility will be restricted or prohibited.• Not suitable for active parkland/recreation use.• Irrigation may be required to maintain vegetation on the slopes and bottom of the basin. Ifirrigation is needed, coordinate its design with that of the general landscape irrigation system forthe project.• Plan for setback requirements (see Table IB-1).Design CriteriaDesign criteria for infiltration basins are listed in Table IB-1. Use the Design Data Summary Sheetprovided at the end of this fact sheet (Table IB-3) to record design information for the permittingagency’s review.Table IB-1. Infiltration Basin Design CriteriaDesign Parameter Criteria NotesTributary Drainage Area ≤ 50acresDesign volume WQV See Appendix E in this Design ManualSoil infiltration rate 0.5-2.0in/hrTo be confirmed by permeability test. Higher permeability allowed ifpretreatment provided.Maximum drawdown time 48 hrs. Based on WQV (see Appendix E)Minimum groundwaterseparationFreeboard (minimum)10 ft Between basin bottom and top of seasonally high groundwater1 ftStormwater Quality Design Manual for the Sacramento and South Placer RegionsIB-2 May 2007