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→Calculate drawdown time
*Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
*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:
*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>
:<math>t=\frac{V_{R}A_{p}}{f'P}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''V<sub>R</sub>'' is the void ratio of the media,
*''V<sub>R</sub>'' is the void ratio of the media,
*''f''' is the design infiltration rate (mm/hr),
*''f''' is the design infiltration rate (mm/hr),
*''P'' is the perimeter of the practice (m), and
*''P'' is the perimeter of the practice (m), and
*''d'' is the total depth of the practice, including the ponding zone (m).}}
*''d<sub>T</sub>'' is the total depth of the practice, including the ponding zone (m).}}
This 3 dimensional equation makes use of the hydraulic radius (''A<sub>p</sub>''/''P''), where ''P'' is the perimeter (m) of the facility. <br>
This 3 dimensional equation makes use of the hydraulic radius (''A<sub>p</sub>''/''P''), where ''P'' is the perimeter (m) of the facility. <br>