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[[File:Sizing bio1.png|thumb|The three storage areas 'bowl', 'filter media' and 'reservoir' are sized together.]] The bioretention facility will comprise three vertical zones: #a 'bowl' permitting water to pond occasionally (maximum depth, ''d<sub>p</sub>'' = 0.5 m)#a layer of [[filter media]] to support plant growth (minimum depth, ''d<sub>m</sub>'' = 0.6 m) , and #a reservoir of [[stone]] to store and facilitate [[infiltration]] of additional water (depth, ''d<sub>R</sub>'', includes 0.1 m choking layer). Firstly, all three zones will be sized together within the limited depth to estimate the footprint area of the whole [[bioretention]] facility. To do this the mean void ratio (''V<sub>R</sub>'') will estimated as ~ 0.4.
To calculate the required practice area (''A<sub>p</sub>'') or footprint where the depth is constrained::<math>A_{p}=\frac{V}{V_{R}d}</math>{{Plainlist|1=Where:*''V'' = Required volume of storage in m<sup>3</sup>*''V<sub>R</sub>'' = Void ratio (porosity), as measured (or default to 0.4) *''A<sub>p</sub>'' = Area of the infiltration practice in m<sup>2</sup>*''d<sub>T</sub>'' = Total depth of infiltration practice in m (= ''d<sub>b</sub>'' + ''d<sub>m</sub>'' + ''d<sub>R</sub>'')}}===Step 1 alternative, accounts for infiltration during a design storm event===This may be desirable where the practice is located on particularly freely draining soils and/or where the design storm is of longer duration. To calculate the required practice area (''A<sub>p</sub>'') or footprint where the depth is constrained::<math>A_{p}=\frac{A_{c}iD}{V_{R}d+f'D}</math>{{Plainlist|1=Where:*''D'' = Duration of design storm in hrs*''i'' = Intensity of design storm in mm/hr*''f''' = Design infiltration rate in mm/hr*''V<sub>R</sub>'' = Void ratio (porosity), as measured (or default to 0.35 for all aggregates) *''A<sub>p</sub>'' = Area of the infiltration practice in m<sup>2</sup>*''A<sub>c</sub>'' = Area of the catchment in m<sup>2</sup>*''d<sub>T</sub>'' = Total depth of infiltration practice in m (= ''d<sub>b</sub>'' + ''d<sub>m</sub>'' + ''d<sub>R</sub>'')}}==Step 2, geometry and perimeter==Line 132:
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Check the mounding of groundwater beneath the facility to ensure no incompatibility with nearby sensitive receptors.
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==Step 1, total volume==
==Additional calculations==
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br>
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br>
P = 12 m (left), P = 20 m (right)]]
P = 12 m (left), P = 20 m (right)]]
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==Final step==
==Groundwater mounding==
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When you wish to model the extent of groundwater mounding beneath an infiltration facility. This tool uses Hantush's derivation (1967).
{{Clickable button|[[Media:Hantush.xlsm|Download groundwater mounding calculator(.xlsm)]]}}
{{Clickable button|[[Media:Hantush.xlsm|Download groundwater mounding calculator(.xlsm)]]}}
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].
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