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* Design storm duration, D (h)
* Design storm duration, D (h)
* Infiltration volume target for the design storm event, V<sub>i</sub> (m<sup>3</sup>), based on average annual water budgets for the catchment for pre- and post-development scenarios and the infiltration volume deficit (pre-development minus post-development infiltration)
* Infiltration volume target for the design storm event, V<sub>i</sub> (m<sup>3</sup>), based on average annual water budgets for the catchment for pre- and post-development scenarios and the infiltration volume deficit (pre-development minus post-development infiltration)
* Drainage time t (h) to fully drain the active storage of the practice, based on provincial or municipal criteria or average inter-event period for the location
* Drainage time t (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
* Field infiltration rate of the underlying native soil f<sub>f</sub> (mm/h), median of field measurements or based on interpolation from median grain-size distribution results
* Field measured infiltration rate of the underlying native soil f<sub>f</sub> (mm/h), median of measurements or based on interpolation from median grain-size distribution results
* Design infiltration rate of the underlying native soil f' (mm/h), median field measured value divided by a safety factor (z)
* Design infiltration rate of the underlying native soil f' (mm/h), median field measured value divided by a safety factor (z)
* Safety factor, z (dimensionless value between 2 and 3) chosen by designer based on consideration of risk of failure factors
* Safety factor, z (dimensionless value between 2 and 3) chosen by designer based on consideration of risk factors
* Proposed surface grade elevation at the practice location (metres above sea level, masl)
* Proposed surface grade elevation at the practice location (metres above sea level, masl)
* Elevation of the seasonally high water table or bedrock surface (metres above sea level, masl), below the practice location
* Elevation of the seasonally high water table or bedrock surface (metres above sea level, masl), below the practice location
*d<sub>f</sub> = Depth of filter media (m)
*d<sub>f</sub> = Depth of filter media (m)
*n<sub>f</sub> = Porosity of filter media}}<br>
*n<sub>f</sub> = Porosity of filter media}}<br>
For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, infiltration water storage is only provided by the depth of storage reservoir below the invert of the underdrain perforated pipe, and only the portion that can reliably drain by infiltration within the specified drainage time. So the infiltration water storage depth of the practice can be calculated as:
For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, infiltration water storage is provided by the depth of storage reservoir below the invert of the underdrain perforated pipe, and only the portion that can reliably drain by infiltration within the specified drainage time, t. So the infiltration water storage depth of the practice can be calculated as:
<math>d_{i}= f' t </math>
<math>d_{i}= f' t </math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*t = [[Drainage time]] (h). Check provincial or local criteria for drainage time requirements}}<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 the underdrain perforated pipe installed on the bottom of the storage reservoir and connected to a riser (e.g., standpipe and 90 degree coupling), the infiltration water storage is only provided by the storage reservoir depth between the inverts of the reservoir bottom and riser outlet (i.e invert elevation of the 90 degree coupling) and is calculated the same way as above.<br>
For practices with the underdrain perforated pipe installed on the bottom of the storage reservoir and connected to a riser (e.g., standpipe and 90 degree coupling), the infiltration water storage is provided by the storage reservoir depth between the inverts of the reservoir bottom and riser outlet (i.e invert elevation of the 90 degree coupling) and is calculated the same way as above.<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, which increases hydraulic head and 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, which increases hydraulic head and infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.