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Line 159: − + − + − + − + − Note that narrow, linear bioretention features (or [[bioswales]]) drain faster than round or blocky footprint geometries. + − + − + − + − + − + + + + − When you wish 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). +
→Calculating infiltration practice drainage time assuming three dimensional (3D) drainageWoods Ballard, B., S. Wilson, H. Udale-Clarke, S. Illman, T. Scott, R. Ahsley, and R. Kellagher. 2016. The SuDS Manual. 5th ed. CIRIA, London.
[[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"
| 1
| 1
|-
|-
| [[Mulch]]
| [[Mulch]] (''d<sub>m</sub>'')
| colspan="2" |75 ± 25 mm
| colspan="2" |75 ± 25 mm
|
|
* 0.4 for [[stone]]
* 0.4 for [[stone]]
|-
|-
| [[Bioretention: Filter media|Filter media]] (''d<sub>m</sub>'')
| [[Bioretention: Filter media|Filter media]] (''d<sub>f</sub>'')
| colspan="2" |
| colspan="2" |
* 300 mm to support turf grass and accept only roof runoff
* 300 mm to support turf grass and accept only roof runoff
*d<sub>f</sub> = Depth of filter media (m)
*d<sub>f</sub> = Depth of filter media (m)
*n<sub>f</sub> = Porosity of filter media}}<br>
*n<sub>f</sub> = Porosity of filter media}}<br>
For practices with the underdrain perforated pipe elevated off the bottom, infiltration water storage is provided by the storage reservoir, d<sub>r</sub> (i.e. depth below the invert of the underdrain perforated pipe), and only the portion that can reliably drain by infiltration within the specified inter-event drainage time, t. So the infiltration water storage depth of the practice can be calculated as:
For practices with an underdrain where the perforated pipe is elevated off the bottom, infiltration water storage is provided by the depth of the storage reservoir below the invert of the underdrain perforated pipe, d<sub>r</sub>, and only the portion that can reliably drain by infiltration within the specified inter-event drainage time, t. So the infiltration water storage depth of the practice can be calculated as:
<math>d_{i}= f' t </math>
<math>d_{i}= f' t </math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*t = [[Drainage time]] (h), time required to fully drain the active 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}}<br>
*t = [[Drainage time]] (h), time required to fully drain the active 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}}<br>
For practices with the underdrain perforated pipe installed on the bottom and connected to a riser (e.g., standpipe and 90 degree coupling), infiltration water storage is provided by the storage reservoir depth between the inverts of the riser outlet (i.e invert elevation of the 90 degree coupling) and reservoir bottom, and is calculated the same way as above.<br>
For practices with an underdrain where the perforated pipe is installed on the bottom and connected to a riser (e.g., standpipe and two 90 degree couplings), infiltration water storage is provided by the storage reservoir depth between the inverts of the riser outlet (i.e invert elevation of the 90 degree coupling) and reservoir bottom, and is calculated the same way as above.<br>
To boost drainage performance on fine-textured, low permeability soils, consider designing storage reservoirs even deeper than those calculated using the above approach, that many not fully drain between storm events (i.e. includes inactive water storage), which increases hydraulic head and thereby, infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.
To boost drainage performance on fine-textured, low permeability soils, consider designing storage reservoirs even deeper than those calculated using the above approach, that many not fully drain between storm events (i.e. includes inactive water storage), which increases hydraulic head and thereby, infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.
*D = Design storm duration (h)}}<br>
*D = Design storm duration (h)}}<br>
* Step 6: Compare required surface area of the practice to available space.<br>
* Step 6: Compare required surface area of the practice to available space.<br>
To decrease A<sub>p</sub>, consider increasing ponding depth if feasible and would not create a safety hazard (recommended maximum of 0.45 m), using Darcy 3D drainage equation or tool (see below) to optimize the infiltration water storage depth, assuming both vertical and horizontal drainage, or decreasing the catchment area.
To decrease A<sub>p</sub>, consider increasing ponding depth if feasible and would not create a safety hazard (recommended maximum of 0.45 m), using the Darcy 3D drainage equation or tool (see below) to optimize the infiltration water storage depth assuming both vertical and horizontal drainage, or decreasing the catchment area.
* 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>
*d<sub>i</sub> = Infiltration water storage depth of the practice (m), see equations above to calculate for with and without underdrain designs.
*d<sub>i</sub> = Infiltration water storage depth of the practice (m), see equations above to calculate for with and without underdrain designs.
*f' = Design infiltration rate of the underlying native soil (m/h)
*f' = Design infiltration rate of the underlying native soil (m/h)
*D= Duration of design storm (h)<br>
*D= Duration of design storm (h)}}<br>
For practices where inflow is directed to a surface ponding area, if A<sub>r</sub> is greater than A<sub>p</sub>, use the value for A<sub>r</sub> as the required footprint area of the practice, A<sub>p</sub>.
For practices where inflow is directed to a surface ponding area, if A<sub>r</sub> is greater than A<sub>p</sub>, use the value for A<sub>r</sub> as the required footprint area of the practice and adjust the ponding depth down to limit surface storage to what is required to fully capture the design storm runoff volume.
<br>
<br>
*Step 9: Calculate the required storage reservoir depth, d<sub>r</sub> (m):
*Step 9: Calculate the required storage reservoir depth, d<sub>r</sub> (m):
{{Plainlist|1= Where:
{{Plainlist|1= Where:
*d<sub>i</sub>' = Design infiltration water storage depth (m)
*d<sub>i</sub>' = Design infiltration water storage depth (m)
*n<sub>r</sub>' = Effective porosity of the storage reservoir fill material(s).<br>
*n<sub>r</sub>' = Effective porosity of the storage reservoir fill material(s).}}<br>
To minimize the total depth of the practice, d<sub>T</sub>, and save aggregate, consider installing void-forming structures (e.g. Permavoid, D-Raintank, low profile stormwater chamber, large diameter perforated pipes etc.) embedded in clear stone aggregate in the storage reservoir instead of aggregate alone, which will provide a greater effective porosity (n').
To minimize the total depth of the practice, d<sub>T</sub>, and save aggregate, consider installing void-forming structures (e.g. Permavoid, D-Raintank, low profile stormwater chamber, large diameter perforated pipes etc.) embedded in clear stone aggregate in the storage reservoir instead of aggregate alone, which will provide a greater effective porosity (n<sub>r</sub>').
==Calculate the total depth of the practice, d<sub>T</sub>==
==Calculate the total depth of the practice, d<sub>T</sub>==
* Step 10: Determine what the planting needs are and assign an appropriate depth of filter media, using the table above.
* Step 10: Determine what the planting needs are and assign an appropriate depth of filter media (d<sub>f</sub>), using the table above.
* Step 11: Select an underdrain perforated pipe diameter (typically 100 or 200 mm), assign this as an 'embedded' depth equal to the pipe diameter. The perforated pipe depth can be made part of the infiltration water storage of the practice when a riser (standpipe and 90 degree coupling) are used to design the underdrain.
* Step 11: Select an underdrain perforated pipe diameter (typically 100 or 200 mm), assign this as an 'embedded' depth equal to the pipe diameter. The perforated pipe depth can be made part of the infiltration water storage of the practice when a riser (standpipe and 90 degree coupling) are used to design the underdrain.
* Step 12: Sum total depth of bioretention components, and compare to available space (i.e. depth) between the elevations of the proposed surface grade and one (1) metre above the seasonally high water table or top of bedrock in the practice location.
* Step 12: Sum total depth of bioretention components, and compare to available space (i.e. depth) between the elevations of the proposed surface grade and one (1) metre above the seasonally high water table or top of bedrock in the practice location.
* 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), ponding depth and/or catchment impervious area.
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.
==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 in 1 dimension===
===Calculating infiltration practice drainage time assuming one dimensional (1D) drainage===
This spreadsheet compares drainage in a single dimension under zero head conditions, mean head conditions and falling head conditions. It provides a more conservative measurement of the drainage time for the purposes of groundwater mounding (where a shorter drainage time causes 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 drainage time calculator(.xlsx)]]}}
===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 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'')
*To estimate the time (''t'') to fully drain the facility
*To estimate the time (''t'') to fully drain the facility
t = [n<sub>r</sub>' * A<sub>p</sub>/f'*x] * ln[(d<sub>r</sub> + (A<sub>p</sub>/x))/(A<sub>p</sub>/x)]
''t = [n<sub>r</sub>' * A<sub>p</sub>/f'*x] * ln[(d<sub>r</sub> + (A<sub>p</sub>/x)/A<sub>p</sub>/x]''
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''n'' is the porosity of the filter media,
*''n<sub>r</sub>''' is the effective porosity of the storage reservoir fill material(s),
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>),
*''A<sub>p</sub>'' is the area of the practice (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),
*''d<sub>T</sub>'' is the total depth of the practice, including the surface ponding depth (m).}}
*''ln'' is the natural logarithm and,
*''d<sub>r</sub>'' is the depth of the storage reservoir (m).}}
Note that narrow, linear bioretention features (or [[bioswales]]) drain faster than round or blocky footprint geometries because they have larger perimeters (see above right figure).
===Groundwater mounding===
===Groundwater mounding===
<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).
{{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].
</poem>
</poem>
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