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→Determine the infiltration water storage depth of the practice
* Step 4: Calculate the infiltration water storage depth of the practice, d<sub>i</sub> which is the depth of water stored by the practice that can drain by infiltration alone.<br>
* Step 4: Calculate the infiltration water storage depth of the practice, d<sub>i</sub> which is the depth of water stored by the practice that can drain by infiltration alone.<br>
For practices without an underdrain, components contributing to infiltration water storage include the surface ponding, mulch and filter media depths (i.e. total depth of the practice). The infiltration water storage depth of the practice can be calculated as:
For practices without an underdrain, components contributing to infiltration water storage include the surface ponding, mulch and filter media depths (i.e. total depth of the practice). The infiltration water storage depth of the practice can be calculated as:
<math>d_{i}=d_{p}'+ (d_{m}\times n_{m}) + (d_{f}\times n_{f})</math>
<math>d_{i}=d_{p}'- (d_{m}\times (1-n_{m})) + (d_{f}\times n_{f})</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*d<sub>p</sub>' = Design surface ponding depth (m)
*d<sub>p</sub>' = Design surface ponding depth (m)
*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 active storage components of the practice (i.e. surface ponding and infiltration water storage depths), 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 active storage components of the practice (i.e. surface ponding and infiltration water storage depths), 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.<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>
See [[Bioretention: Internal water storage]] page for further guidance on water quality treatment benefits of internal water storage reservoirs or zones in bioretention.
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.