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→Calculate drawdown time
[[file:Hydraulic radius.png|thumb|Two footprint areas of 9 m<sup>2</sup>.<br>
[[file:Hydraulic radius.png|thumb|Two footprint areas of 9 m<sup>2</sup>.<br>
Perimeter = 12 m (left) Perimeter = 20 m (right)]]
Perimeter = 12 m (left) Perimeter = 20 m (right)]]
In some situations, it may be possible to reduce the size of the bioretention required, by accounting for rapid drainage. Typically, this is only worth exploring over sandy soils with rapid infiltration.
Note that narrow, linear bioretention features 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'').
*To estimate the time (''t'') to fully drain the facility:
:<math>t=\frac{V_{R}A_{p}}{f'P}ln\left [ \frac{\left (d+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math>
{{Plainlist|1=Where:
*''V<sub>R</sub>'' is the void ratio of the media,
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>),
*''f''' is the design infiltration rate (mm/hr),
*''P'' is the perimeter of the practice (m), and
*''d'' is the total depth of the practice, including the ponding zone (m).}}
To calculate the time (''t'') to fully drain the facility:
To calculate the time (''t'') to fully drain the facility:
:<math>t=\frac{V_{R}A_{p}} {q'P}ln\left [ \frac{\left (d+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math>
:<math>t=\frac{V_{R}A_{p}} {q'P}ln\left [ \frac{\left (d+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math>