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[[File:Sizing Bioretention.jpg|thumb|The vertical storage zones in a bioretention cell include: ponding, mulch, filter media, choker layer, embedded pipe diameter depth and the storage reservoir.]]
[[File:Sizing Bioretention.jpg|thumb|The vertical storage zones in a bioretention cell include: ponding, mulch, filter media, choker layer, embedded pipe diameter depth and the storage reservoir.]]
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Many of the dimensions in a bioretention system can be predetermined according to the function of the component. There is greatest flexibility in the ponding depth and the depth of the storage reservoir beneath the optional underdrain pipe. The table below describes some recommended values to use to begin the design process.
Many of the dimensions in a bioretention system can be predetermined according to the function of the component. There is greatest flexibility in the ponding depth, filter media depth and the depth of the storage reservoir beneath the optional underdrain pipe. The table below describes some recommended values to use to begin the design process.
{| class="wikitable"
{| class="wikitable"
|-
|-
| [[choker layer|Choker layer]]
| [[choker layer|Choker layer]]
| 100 mm
| Not applicable
| 0.4
| 0.4
|-
|-
* Design storm duration, D (h)
* Design storm duration, D (h)
* Infiltration volume target for the design storm event, V<sub>i</sub> (m<sup>3</sup>), based on average annual water budgets for the catchment for pre- and post-development scenarios and equal to the infiltration volume deficit (pre-development minus post-development annual infiltration)
* Infiltration volume target for the design storm event, V<sub>i</sub> (m<sup>3</sup>), based on average annual water budgets for the catchment for pre- and post-development scenarios and equal to the infiltration volume deficit (pre-development minus post-development annual infiltration)
* Drainage time, t (h), time required to fully drain the active water storage components of the practice (i.e surface ponding and infiltration water storage depths), based on local criteria or long term average inter-event period for the location
* Drainage time, t (h), time required to fully drain the active water storage components of the practice (i.e surface ponding and active storage reservoir depths), based on local criteria or long term average inter-event period for the location
* Field measured infiltration rate of the underlying native soil, f (mm/h), median of field measurements or based on interpolation from median grain-size distribution results
* Field measured infiltration rate of the underlying native soil, f (mm/h), median of field measurements or based on interpolation from median grain-size distribution results
* Design infiltration rate of the underlying native soil, f' (mm/h), median field measured value, f divided by a safety factor, z
* Design infiltration rate of the underlying native soil, f' (mm/h), median field measured value, f divided by a safety factor, z
* Proposed surface grade elevation at the practice location (metres above sea level, masl)
* Proposed surface grade elevation at the practice location (metres above sea level, masl)
* Elevation of the seasonally high water table or bedrock surface (metres above sea level, masl), below the practice location
* Elevation of the seasonally high water table or bedrock surface (metres above sea level, masl), below the practice location
* Effective porosity of the storage reservoir fill material(s), including any void-forming structures (e.g. pipes, chambers, tanks, soil cells, etc.) and surrounding aggregate, n<sub>r</sub>'
* Effective porosity of the storage reservoir fill material, including any void-forming structures (e.g. pipes, chambers, tanks, soil cells, etc.) and surrounding aggregate, n'
* Types of plants to be supported by the filter media bed (i.e. grasses vs. mix of grasses, plants and shrubs vs. trees)
* Types of plants to be supported by the filter media bed (i.e. grasses vs. mix of grasses, plants and shrubs vs. trees)
* How runoff will be delivered to the practice (i.e. to surface ponding area, or directly to storage reservoir)
* How runoff will be delivered to the practice (i.e. to surface ponding area, or directly to storage reservoir)
This spreadsheet tool has been set up to perform all of the bioretention sizing calculations shown below and allows side-by-side comparison of equation outputs for each potential design approach or constraint scenario.<br>
{{Clickable button|[[Media:Infiltration Sizing 20200525 locked.xlsx|Download the infiltration practice sizing tool]]}}
==Decide if an underdrain will be included==
==Decide if an underdrain will be included==
* 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:
* Step 7: Calculate catchment impervious area to practice permeable (footprint) area ratio, R, also referred to as I/P ratio:
* Step 7: Calculate catchment impervious area to practice permeable (footprint) area ratio, R, also referred to as I/P ratio:
<math>R=A_{i}/A_{p}</math><br>
<math>R=A_{i}/A_{p}</math><br>
You can use the infiltration practice sizing tool noted above to adjust d<sub>p</sub>, d<sub>i</sub>, A<sub>p</sub> or A<sub>i</sub> to find practice dimensions that provide the required storage volume and will drain within the specified drainage time, while keeping R between 5 and 20.
==Determine the required surface area of the storage reservoir==
==Determine the required surface area of the storage reservoir==
* Step 13: Adjust component depths to maintain a separation of one (1) metre between the base of the practice and the seasonally high water table or top of bedrock elevation, or a lesser or greater value based on groundwater mounding analysis. See below and [[Groundwater]] for more information.<br>
* Step 13: Adjust component depths to maintain a separation of one (1) metre between the base of the practice and the seasonally high water table or top of bedrock elevation, or a lesser or greater value based on groundwater mounding analysis. See below and [[Groundwater]] for more information.<br>
For practices without an underdrain in locations constrained in the vertical dimension, consider decreasing filter media depth (and adjusting supported plant types) and/or catchment impervious area.<br>
For practices without an underdrain in locations constrained in the vertical dimension, consider decreasing filter media depth (and adjusting supported plant types) and/or catchment impervious area.<br>
For practices with an underdrain in locations constrained in the vertical dimension, consider installing the perforated pipe on the bottom of the storage reservoir and including a riser (saves the depth of aggregate needed to embed the pipe), and/or decreasing filter media depth (and adjusting supported plant types), storage reservoir depth and/or catchment impervious area.
<br>
For practices with an underdrain in locations constrained in the vertical dimension, consider installing the perforated pipe on the bottom of the storage reservoir and including a riser (saves the depth of aggregate needed to embed the pipe), and/or decreasing filter media depth (and adjusting supported plant types), storage reservoir depth and/or catchment impervious area.<br>
<br>
For more on sizing bioretention practices on constrained sites see [[Bioretention: Sizing and modeling]].
==Calculate peak flow rates==
==Calculate peak flow rates==
* Step 16. Determine if downstream [[flow control]] is required to achieve hydrologic objectives.
* Step 16. Determine if downstream [[flow control]] is required to achieve hydrologic objectives.
===Calculating infiltration practice drainage time, t assuming 1 dimensional (1D) drainage===
===Calculating infiltration practice drainage time assuming one dimensional (1D) drainage===
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]]}}
===Drainage time (3D)<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>===
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br> x = 12 m (left), x = 20 m (right)]]
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br> x = 12 m (left), x = 20 m (right)]]
In some situations, it may be desirable to reduce the size of the bioretention required, by accounting for rapid drainage.
In some situations, it may be desirable to optimize the size of the bioretention practice, by accounting for drainage in three dimensions rather than one.
Typically, this is only worth exploring over sandy textured native subsoils with rapid infiltration.
Typically, this is only worth exploring over sandy textured native subsoils with rapid infiltration.
The drainage time calculator noted above can be used to calculate drainage time assuming three dimensional drainage as well.
The drainage time calculator noted above can be used to calculate drainage time assuming both one and three dimensional drainage and allows for comparison between the estimates.
*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 area of the practice (''A<sub>p</sub>'') by the perimeter (''x'')
<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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