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Line 85: ==Determine the required surface area of the practice==* Step 4: Calculate the surface (i.e footprint) area of the practice (A<sub>p</sub>) needed to capture the runoff volume produced from the catchment during the design storm event<br>For practices where flow is delivered to the filter media bed:<math>A_{p}=i\times D\times A_{i}/[d_{p}' + (f_{m, min} \times D)]</math>{{Plainlist|1=Where:*i = Design storm intensity (m/h)*D = Design storm duration (h)*A<sub>i</sub> = Catchment impervious area (m<sup>2</sup>)*d<sub>p</sub>' = Design surface ponding depth (m)*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 to the storage reservoir:<math>A_{p}=i\times D\times A_{i}/[(d_{a}\times n) + (f' \times D)]</math>{{Plainlist|1=Where:*d<sub>a</sub> = Active storage depth (m)*n = porosity of storage reservoir aggregate or effective porosity of void-producing structure system*f' = design infiltration rate of the underlying native soil (m/h)*D = Design storm duration (h)}}<br>
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For practices with hard edges and vertical-walled ponding areas (e.g., stormwater planter, soil cell) use the maximum ponding depth:
For practices with hard edges and vertical-walled ponding areas (e.g., stormwater planter, soil cell) use the maximum ponding depth:
<math>d_{p}'=d_{p, max}</math>
<math>d_{p}'=d_{p, max}</math>
==Determine the active storage depth of the practice==
==Determine the active storage depth of the practice==
* Step 5: Calculate the active storage depth of the storage reservoir (''d<sub>a'', mm):<br>
* Step 5: Calculate the active storage depth of the storage reservoir (''d<sub>a'', mm):<br>
For practices with no underdrain, this includes the combined surface ponding and storage reservoir depth that will reliably drain within the specified drainage time. <br>
For practices with no underdrain, this includes the combined surface ponding and storage reservoir depth that will reliably drain within the specified drainage time. <br>
For practices with an underdrain, this includes only the storage reservoir depth below the invert of the underdrain perforated pipe that will reliably drain within the specified drainage time:
For practices with an underdrain, this includes only the storage reservoir depth below the invert of the underdrain perforated pipe that will reliably drain within the specified drainage time:
<math>d_{a}=(f'\times t \times 1/n)</math>
<math>d_{a}=(f'\times t \times 1/n)</math>