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Line 1: − This article is specific to [[bioretention]], vegetated systems that infiltrate water to the native soil. <br>+ − If you are designing a planted system which does not infiltrate water, see advice on [[Planters: Sizing]]. − − {{TOClimit|2}} − 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. + + + + + + + Line 14:
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[[File:Bioretention sizing update.png|thumb|550px|The vertical storage zones in a bioretention cell include: ponding, mulch, filter media, choker layer, embedded pipe diameter depth and the internal water storage (IWS) 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.]]
This article is specific to [[bioretention]], vegetated systems that infiltrate water to the native soil.
If you are designing a planted system which does not infiltrate water, see advice on [[Planters: Sizing]].<br>
{{TOClimit|3}}<br>
<br>
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 internal water storage reservoir beneath the optional underdrain pipe. The table below describes some recommended values to use to begin the design process.
<br>
{| class="wikitable"
{| class="wikitable"
|-
|-
| Surface ponding, maximum (''d<sub>p, max</sub>'')
| Surface ponding, maximum (''d<sub>p, max</sub>'')
|
|
* 300 mm; filter media Blend A
* 125 to 400 mm (filter media Blend A or B)
* 350 mm; filter media Blend B
* Consult local design standards where available
| See below
| See below
| 1
| 1
| colspan="2" |75 ± 25 mm
| colspan="2" |75 ± 25 mm
|
|
* 0.7 for wood based
* 0.7 for shredded wood
* 0.4 for [[stone]]
* 0.4 for [[stone]]
|-
|-
|-
|-
| [[choker layer|Choker layer]]
| [[choker layer|Choker layer]]
| 100 mm
| Not applicable
| 0.4
| 0.4
|-
|-
| [[Pipes|Perforated pipe]] and surrounding [[Aggregates|aggregate]]
| [[Pipes|Perforated pipe]]
| 200 mm (i.e. pipe diameter)
| 200 mm diameter pipe
| Not applicable
| Not applicable
| 0.4
| 0.4 for clear stone aggregate in which the pipe is embedded
|-
|-
| Storage reservoir (''d<sub>r</sub>'')
| [[Bioretention: Internal water storage|Internal water storage reservoir]] (''d<sub>r</sub>'')
| See below
| See below
| See below
| See below
| 0.4
| 0.4 for clear stone aggregate below the perforated pipe; Higher effective porosity can be achieved by including void-forming structures
|}
|}
Before you begin you will need to know the following;<br>
Before you begin you will need to know the following;<br>
* 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 active storage reservoir 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 internal water storage reservoir of the practice, 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
* 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'
* 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 internal water 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>
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]]}}
{{Clickable button|[[Media:Infiltration Sizing 20220617 locked (1).xlsx|Download the infiltration practice sizing tool]]}}
==Decide if an underdrain will be included==
==Decide if an underdrain will be included==
* Step 2: Determine a maximum surface ponding depth (''d<sub>p, max</sub>'')
* Step 2: Determine a maximum surface ponding depth (''d<sub>p, max</sub>'')
For practices without underdrains:
For practices without underdrains:
<math>d_{p, max}=f'\times48</math>
<math>d_{p, max}=f'\times24</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''f''' = Design infiltration rate (mm/h), and
*''f''' = Design infiltration rate (mm/h), and
*48 = Maximum permissible drainage time for surface ponded water (h)}}
*24 = Maximum permissible drainage time for surface ponded water (h)}}
Note that conceptually, in designs without underdrains, when filled to their maximum storage capacity, drainage of ponded water is limited by the infiltration rate at the base of the practice.<br>
Note that conceptually, in designs without underdrains, when filled to their maximum storage capacity, drainage of ponded water is limited by the infiltration rate of the in-situ (native) soil at the base of the practice.<br>
<br>
<br>
For practices with underdrains see recommended maximum surface ponding depth for each filter media blend in the table above.<br>
For practices with underdrains see recommended range for maximum surface ponding depth in the table above, and consult local design standards where available.<br>
<br>
<br>
* Step 3: Select a design surface ponding depth, d<sub>p</sub>' (m) to begin sizing with:<br>
* Step 3: Select a design surface ponding depth, d<sub>p</sub>' (m) to begin sizing with:<br>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*d<sub>p, max</sub> = maximum surface ponding depth (m)}}<br>
*d<sub>p, max</sub> = maximum surface ponding depth (m)}}<br>
For practices with hard edges and vertical-walled ponding areas (e.g., stormwater planter, soil cell) use the maximum ponding depth:
For practices with hard edges and vertical-walled ponding areas (e.g., stormwater planter, stormwater tree trench) use the maximum ponding depth:
<math>d_{p}'=d_{p, max}</math>
<math>d_{p}'=d_{p, max}</math>
==Determine the infiltration water storage depth of the practice==
==Determine the infiltration water storage depth of the practice==
* Step 4: Calculate the infiltration water storage depth of the practice, d<sub>i</sub> which is the depth of water stored by the practice that can drain by infiltration alone.<br>
* Step 4: Calculate the infiltration water storage depth of the practice, d<sub>i</sub> which is the depth of water stored by the practice that can drain by infiltration alone.<br>
For practices without an underdrain, components contributing to infiltration water storage include the surface ponding, mulch and filter media depths (i.e. total depth of the practice). The infiltration water storage depth of the practice can be calculated as:
For practices without an underdrain, components contributing to infiltration water storage include the surface ponding, mulch and filter media depths (i.e. total depth of the practice). The infiltration water storage depth of the practice can be calculated as:
<math>d_{i}=d_{p}'+ (d_{m}\times n_{m}) + (d_{f}\times n_{f})</math>
<math>d_{i}=d_{p}'- (d_{m}\times (1-n_{m})) + (d_{f}\times n_{f})</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*d<sub>p</sub>' = Design surface ponding depth (m)
*d<sub>p</sub>' = Design surface ponding depth (m)
*n<sub>f</sub> = Porosity of filter media}}<br>
*n<sub>f</sub> = Porosity of filter media}}<br>
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:
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'\times 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 internal water storage reservoir of the practice, based on local criteria or long term average inter-event period for the location}}<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>
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 top 90 degree coupling) and reservoir bottom, and is calculated the same way as above. See [[Bioretention: Internal water storage]] page for further design guidance and information on water quality treatment benefits of internal water storage reservoirs or zones in partial infiltration bioretention designs.<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.
*K<sub>f</sub> = minimum acceptable 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 a subsurface (underground) water storage reservoir:
<math>A_{p}=i\times D\times A_{i}/[d_{i} + (f' \times D)]</math>
<math>A_{p}=i\times D\times A_{i}/[d_{i} + (f' \times D)]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*d<sub>i</sub> = Infiltration water storage depth (m), see above for equations for with and without underdrain designs
*d<sub>i</sub> = Infiltration water storage depth (m), see above for equations 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 = 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>
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.
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 internal water storage reservoir==
* Step 8: Calculate the surface area of the storage reservoir, A<sub>r</sub> (m<sup>2</sup>) needed to capture the infiltration volume target for the design storm event: <math>A_{r}=V_{i}/[d_{i}+(f'\times D)]</math>
* Step 8: Calculate the surface area of the storage reservoir, A<sub>r</sub> (m<sup>2</sup>) needed to capture the infiltration volume target for the design storm event: <math>A_{r}=V_{i}/[d_{i}+(f'\times D)]</math>
{{Plainlist|1= Where:
{{Plainlist|1= Where:
<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):
<math>d_{r}=d_{i}'\times 1/n_{r}'</math>
<math>d_{r}=d_{i}'/n_{r}'</math>
{{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)
==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 (d<sub>f</sub>), 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 150 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>
<br>
<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 raised outlet or riser to utilize the volume of the perforated pipe and surrounding clear stone aggregate material for water storage, which helps to minimize total depth of the practice. Another option is to decrease filter media depth (and adjust supported plant types), storage reservoir depth and/or catchment impervious area.<br>
<br>
<br>
For more on sizing bioretention practices on constrained sites see [[Bioretention: Sizing and modeling]].
For more on sizing bioretention practices on constrained sites see [[Bioretention: Sizing and modeling]].