[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br>
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br> P = 12 m (left), P = 20 m (right)]]
P = 12 m (left), P = 20 m (right)]]
In some situations, it may be possible to reduce the size of the biortention, by accounting for rapid drainage. Typically, this is only worth exploring over sandy soils with rapid infiltration.
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Note that narrow, linear bioretention features drain faster than round or blocky footprint geometries.
Narrow, linear cells drain faster than round or blocky footprint geometries.
*Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
Divide the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').