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→Determine the infiltration water storage depth of the practice
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*t = [[Drainage time]] (h), time required to fully drain the internal water storage reservoir of the practice, based on local criteria or long term average inter-event period for the location}}<br>
*t = [[Drainage time]] (h), time required to fully drain the internal water storage reservoir of the practice, based on local criteria or long term average inter-event period for the location}}<br>
For practices with an underdrain where the perforated pipe is installed on the bottom and connected to a riser (e.g., standpipe and two 90 degree couplings), infiltration water storage is provided by the storage reservoir depth between the inverts of the riser outlet (i.e invert elevation of the top 90 degree coupling) and reservoir bottom, and is calculated the same way as above. See [[Bioretention: Internal water storage]] page for guidance on water quality treatment benefits of internal water storage reservoirs or zones in partial infiltration bioretention designs.<br>
For practices with an underdrain where the perforated pipe is installed on the bottom and connected to a riser (e.g., standpipe and two 90 degree couplings), infiltration water storage is provided by the storage reservoir depth between the inverts of the riser outlet (i.e invert elevation of the top 90 degree coupling) and reservoir bottom, and is calculated the same way as above. See [[Bioretention: Internal water storage]] page for further design guidance and information on water quality treatment benefits of internal water storage reservoirs or zones in partial infiltration bioretention designs.<br>
To boost drainage performance on fine-textured, low permeability soils, consider designing storage reservoirs even deeper than those calculated using the above approach, that many not fully drain between storm events (i.e. includes inactive water storage), which increases hydraulic head and thereby, infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.
To boost drainage performance on fine-textured, low permeability soils, consider designing storage reservoirs even deeper than those calculated using the above approach, that many not fully drain between storm events (i.e. includes inactive water storage), which increases hydraulic head and thereby, infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.