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→Calculate drawdown time
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.
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.
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 (''x'').
*Begin the drainage time calculation by dividing the storage reservoir area of the practice (''A<sub>r</sub>'') by the perimeter (''x'').
*Use the following equation to estimate the time (''t'') to fully drain the facility:
*Use the following equation to estimate the time (''t'') to fully drain the facility:
:<math>t=\frac{nA_{p}}{f'x}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{x} \right )}{\left(\frac{A_{p}}{x}\right)}\right]</math>
:<math>t=\frac{nA_{r}}{f'x}ln\left [ \frac{\left (d_{r}+ \frac{A_{r}}{x} \right )}{\left(\frac{A_{r}}{x}\right)}\right]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''n'' is the porosity of the fill materials within the practice, depth weighted mean
*''n'' is the porosity of the storage reservoir fill materials
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>),
*''A<sub>r</sub>'' is the storage reservoir footprint area (m<sup>2</sup>),
*''f''' is the design infiltration rate (mm/hr),
*''f''' is the design infiltration rate of the native soil (mm/h),
*''x'' is the perimeter of the practice (m), and
*''x'' is the perimeter of the practice (m), and
*''d<sub>T</sub>'' is the total depth of the practice, including the ponding zone (m).}}
*''d<sub>r</sub>'' is the depth of the storage reservoir (m).}}
This 3 dimensional equation makes use of the hydraulic radius (''A<sub>p</sub>''/''x''), where ''x'' is the perimeter (m) of the facility. <br>
This 3 dimensional equation makes use of the hydraulic radius (''A<sub>r</sub>''/''x''), where ''x'' is the perimeter (m) of the facility. <br>
Maximizing the perimeter of the facility directs designers towards longer, linear shapes such as [[bioswales]].
Maximizing the perimeter of the facility directs designers towards longer, linear shapes such as [[bioswales]].
[[category: modeling]]
[[category: modeling]]
[[category: infiltration]]
[[category: infiltration]]