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→Time required to drain surface ponded water (1D drainage)
==Time required to drain surface ponded water (1D drainage)==
==Time required to drain surface ponded water (1D drainage)==
The following equation assumes one dimensional drainage over the surface ponding area.
The following equations assume one dimensional drainage over the surface ponding area.
It is best applied to calculate the maximum duration of ponding on the surface of [[Bioretention |bioretention cells]] and [[Stormwater Tree Trenches |stormwater tree trenches]], and upstream of check dams of [[bioswales]] and [[enhanced grass swales]] to ensure all surface ponding drains within 24 hours.
It is best applied to calculate the maximum duration of ponding on the surface of [[Bioretention |bioretention cells]] and [[Stormwater Tree Trenches |stormwater tree trenches]], and upstream of check dams of [[bioswales]] and [[enhanced grass swales]] to ensure all surface ponding drains within 24 hours.<br>
To calculate the time (''t'') to fully drain surface ponded water through the filter media or growing medium:
To calculate the time (''t'') to fully drain surface ponded water through the filter media or growing medium:
<math>t=\frac{d_{p}'}{K_{f}}</math>
<math>t=\frac{d_{p}'}{K_{f}}</math> <br>
Where <br>
Where <br>
d<sub>p</sub>' is the effective or mean surface ponding depth (mm).<br>
d<sub>p</sub>' is the effective or mean surface ponding depth (mm).<br>
K<sub>f</sub> is the minimum acceptable saturated hydraulic conductivity of the filter media and growing medium when compacted to 85% maximum dry density (mm/h); minimum of 25 mm/hr is recommended for bioretention filter media; minimum of 15 mm/hr is recommended for enhanced grass swale and stormwater tree trench growing medium.
K<sub>f</sub> is the minimum acceptable saturated hydraulic conductivity of the filter media and growing medium when compacted to 85% maximum dry density (mm/h); minimum of 25 mm/hr is recommended for bioretention filter media; minimum of 15 mm/hr is recommended for enhanced grass swale and stormwater tree trench growing medium.<br>
<br>
For full infiltration design practices that do not feature an underdrain, once the internal water storage capacity has been filled, the length of time required to fully drain surface ponded water is limited by the saturated hydraulic conductivity of the underlying in-situ (native) subsoil.<br>
To calculate the time (''t'') to fully drain surface ponded water once filled to capacity:
<math>t=\frac{d_{p}'}{f'}</math> <br>
Where <br>
d<sub>p</sub>' is the effective or mean surface ponding depth (mm).<br>
f' is the design infiltration rate of the underlying native soil.
==Time to drain internal water storage reservoir==
==Time to drain internal water storage reservoir==