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→Determine the active water storage depth of the practice
*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, the active storage component is the depth of water in the storage reservoir below the invert of the underdrain perforated pipe that can reliably drained within the specified drainage time:<br>
For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, the active storage component is the depth of water in the storage reservoir below the invert of the underdrain perforated pipe that can reliably drained within the specified drainage time:<br>
<math>d_{a}= f'/times t </math>
<math>d_{a}= 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)