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→Determine the required surface area of the practice
*f<sub>m, min</sub> = minimum infiltration rate (i.e. saturated hydraulic conductivity) of the filter media (m/h), see [[Bioretention: Filter media|Filter media]] for guidance}}<br>
*f<sub>m, min</sub> = minimum infiltration rate (i.e. saturated hydraulic conductivity) of the filter media (m/h), see [[Bioretention: Filter media|Filter media]] for guidance}}<br>
For practices where flow is delivered directly to the storage reservoir:
For practices where flow is delivered directly to the storage reservoir:
<math>A_{p}=i\times D\times A_{i}/[(d_{a}\times n') + (f' \times D)]</math>
<math>A_{p}=i\times D\times A_{i}/[d_{a} + (f' \times D)]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*d<sub>a</sub> = Active storage depth (m)
*d<sub>a</sub> = Active water storage depth (m)
*n' = effective porosity of the active storage fill material(s), (effective porosity of surface ponding depth and filter media depth combined for practices with no underdrain; porosity of storage reservoir aggregate or effective porosity of void-producing structure system for practices with an underdrain)
*n' = effective porosity of the active storage fill material(s), (effective porosity of surface ponding depth and filter media depth combined for practices with no underdrain; porosity of storage reservoir aggregate or effective porosity of void-producing structure system for practices with an underdrain)
*f' = design infiltration rate of the underlying native soil (m/h)
*f' = design infiltration rate of the underlying native soil (m/h)