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| [[choker layer|Choker layer]]
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* Step 5: Calculate the surface area of the practice (A<sub>p</sub>) needed to capture the volume of runoff produced from the catchment by the design storm event.<br>
* Step 5: Calculate the surface area of the practice (A<sub>p</sub>) needed to capture the volume of runoff produced from the catchment by the design storm event.<br>
For practices where flow is delivered to a surface ponding area:
For practices where flow is delivered to a surface ponding area:
<math>A_{p}=i\times D\times A_{i}/[d_{p}' + (f_{f, min} \times D)]</math>
<math>A_{p}=i\times D\times A_{i}/[d_{p}' + (K_{f} \times D)]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*i = Design storm intensity (m/h)
*i = Design storm intensity (m/h)
*A<sub>i</sub> = Catchment impervious area (m<sup>2</sup>)
*A<sub>i</sub> = Catchment impervious area (m<sup>2</sup>)
*d<sub>p</sub>' = Design surface ponding depth (m)
*d<sub>p</sub>' = Design surface ponding depth (m)
*f<sub>f, min</sub> = minimum acceptable infiltration rate (i.e. saturated hydraulic conductivity) of the filter media (m/h), see [[Bioretention: Filter media|Filter media]] for guidance}}<br>
*K<sub>f</sub> = minimum acceptable saturated hydraulic conductivity of the filter media (m/h), see [[Bioretention: Filter media|Filter media]] for guidance}}<br>
<br>
<br>
For practices where flow is delivered directly to the storage reservoir:
For practices where flow is delivered directly to the storage reservoir:
This spreadsheet allows calculation of drainage time assuming one dimensional drainage under zero head conditions, mean head conditions and falling head conditions. It provides a conservative estimate of drainage time for the purposes of groundwater mounding analysis, where shorter drainage times cause a greater impact.
This spreadsheet allows calculation of drainage time assuming one dimensional drainage under zero head conditions, mean head conditions and falling head conditions. It provides a conservative estimate of drainage time for the purposes of groundwater mounding analysis, where shorter drainage times cause a greater impact.
{{Clickable button|[[Media:Darcy drainage.xlsx|Download drainage time calculator(.xlsx)]]}}
{{Clickable button|[[Media:Darcy drainage.xlsx|Download the Darcy drainage time calculator tool]]}}
===Calculating infiltration practice drainage time assuming three dimensional (3D) drainage<ref>Woods Ballard, B., S. Wilson, H. Udale-Clarke, S. Illman, T. Scott, R. Ahsley, and R. Kellagher. 2016. The SuDS Manual. 5th ed. CIRIA, London.</ref>===
===Calculating infiltration practice drainage time assuming three dimensional (3D) drainage<ref>Woods Ballard, B., S. Wilson, H. Udale-Clarke, S. Illman, T. Scott, R. Ahsley, and R. Kellagher. 2016. The SuDS Manual. 5th ed. CIRIA, London.</ref>===
<poem>
<poem>
To model the extent of groundwater mounding beneath an infiltration facility use the tool below to determine if there is potential for interaction between groundwater levels and the base of the practice during drainage. This tool uses Hantush's derivation (1967).
To model the extent of groundwater mounding beneath an infiltration facility use the tool below to determine if there is potential for interaction between groundwater levels and the base of the practice during drainage. This tool uses Hantush's derivation (1967).
{{Clickable button|[[Media:Hantush.xlsm|Download groundwater mounding calculator(.xlsm)]]}}
{{Clickable button|[[Media:Hantush.xlsm|Download the Hantush groundwater mounding tool]]}}
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and [https://pubs.usgs.gov/sir/2010/5102/ hosted by the USGS].
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and [https://pubs.usgs.gov/sir/2010/5102/ hosted by the USGS].
</poem>
</poem>
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