Difference between revisions of "Bioretention: Sizing"

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This article is specific to [[bioretention]], vegetated systems that infiltrate water to the native soil. <br>
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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.]]
If you are designing a planted system which does not infiltrate water, see advice on [[Planters: Sizing]].
 
[[File:Sizing Bioretention.jpg|thumb|The vertical storage zones in a bioretention cell include: ponding, mulch, filter media, choker course, pipe diameter reservoir and the storage reservoir.]]  
 
{{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 and the depth of the storage reservoir beneath the optional underdrain pipe. The order of operations in calculating these dimensions depends on whether an underdrain is desired. 
 
  
 +
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"
 
|-
 
|-
Line 10: Line 13:
 
! Recommended depth (with underdrain pipe)
 
! Recommended depth (with underdrain pipe)
 
! Recommended depth (no underdrain pipe)  
 
! Recommended depth (no underdrain pipe)  
! Typical void ratio (''V<sub>R</sub>'')
+
! Typical porosity (''n'')
 
|-
 
|-
| Ponding (''d<sub>p</sub>'')
+
| Surface ponding, maximum (''d<sub>p, max</sub>'')
| 300 mm
+
|  
 +
* 125 to 400 mm (filter media Blend A or B)
 +
* Consult local design standards where available
 
| See below
 
| See below
 
| 1
 
| 1
 
|-
 
|-
| [[Mulch]]
+
| [[Mulch]] (''d<sub>m</sub>'')
 
| 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]]
 
|-
 
|-
| [[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 rainwater/roof runoff)
+
* 300 mm to support turf grass and accept only roof runoff
* 600 mm to support flowering [[perennials]] and decorative [[grasses]]
+
* 600 mm to support shrubs, flowering [[perennials]] and deeply rooting decorative [[grasses]] and accept both roof and pavement runoff
* 1000 mm to support [[trees]]
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* 1000 mm to support [[trees]] and accept both roof and pavement runoff
 
|  
 
|  
*[[Bioretention media storage| 0.4]] for sandy mix
+
* 0.4 for sandier Blend A (drainage rate priority)
* 0.35 for a more loamy mix.
+
* 0.35 for more loamy Blend B (water quality treatment priority)
 
|-
 
|-
| [[choker layer|Choker course]]
+
| [[choker layer|Choker layer]]
| colspan="2" |100 mm
+
| 100 mm
| 0.4 typical
+
| Not applicable
 +
| 0.4
 
|-
 
|-
| Pipe diameter reservoir
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| [[Pipes|Perforated pipe]]
| Is equal to underdrain pipe diameter
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| 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>s</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>
 +
* Catchment area (i.e. contributing drainage area), A<sub>c</sub> (m<sup>2</sup>)
 +
* Catchment runoff coefficient, C
 +
* Catchment impervious area, A<sub>i</sub> (m<sup>2)
 +
* Design storm intensity, i (mm/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)
 +
* 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
 +
* Design infiltration rate of the underlying native soil, f' (mm/h), median field measured value, f divided by a safety factor, z
 +
* Safety factor, z (dimensionless value between 2 and 3) chosen by designer based on consideration of risk factors
 +
* 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
 +
* 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)
 +
* 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>
 +
{{Clickable button|[[Media:Infiltration Sizing 20220617 locked (1).xlsx|Download the infiltration practice sizing tool]]}}
  
 +
==Decide if an underdrain will be included==
 +
* Step 1: Based on the median measured infiltration rate of the native soil at the proposed location and bottom elevation of the practice, or estimated infiltration rate from median grain-size distribution results, decide if an underdrain will be included in the design. See [[Design infiltration rate]] for guidance on field tests, methods and number of test locations and tests (i.e. measurements) to be performed.<br>
 +
If the median field measured or estimated infiltration rate of the underlying native soil (f) is less than 15 mm/h, include an underdrain.
  
==Calculate the maximum overall depth==
+
==Select a surface ponding depth to begin sizing with==
*Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above.
+
* Step 2: Determine a maximum surface ponding depth (''d<sub>p, max</sub>'')
*Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth. *Note that this component does not apply if a downstream riser is being used to control an extended saturation zone.
+
For practices without underdrains:
*Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (''d<sub>s, max</sub>'', mm):
+
<math>d_{p, max}=f'\times24</math>
<math>d_{s, max}=f'\times t</math>
+
{{Plainlist|1=Where:
 +
*''f''' = Design infiltration rate (mm/h), and
 +
*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 of the in-situ (native) soil at the base of the practice.<br>
 +
<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>
 +
* Step 3: Select a design surface ponding depth, d<sub>p</sub>' (m) to begin sizing with:<br>
 +
For practices with soft (i.e. landscaped) edges and bowl-shaped ponding areas calculate the mean surface ponding depth:
 +
<math>d_{p}'=d_{p, max}\times 0.5</math>
 
{{Plainlist|1=Where:
 
{{Plainlist|1=Where:
*''f''' = Design infiltration rate (mm/hr), and
+
*d<sub>p, max</sub> = maximum surface ponding depth (m)}}<br>
*''t'' = [[Drainage time]] (hrs). Check local regulations for drainage time requirements.}}
+
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>
  
===Additional step for system without underdrain===
+
==Determine the infiltration water storage depth of the practice==
* Step 4: Determine maximum permissible ponding depth (''d<sub>p, max</sub>''):
+
 
<math>d_{p, max}=f'\times48</math>
+
* 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:
 +
<math>d_{i}=d_{p}'- (d_{m}\times (1-n_{m})) + (d_{f}\times n_{f})</math>
 
{{Plainlist|1=Where:
 
{{Plainlist|1=Where:
*''f''' = Design infiltration rate (mm/hr), and
+
*d<sub>p</sub>' = Design surface ponding depth (m)
*48 = Drainage time of the ponding (hrs)
+
*d<sub>m</sub> = Depth of mulch (m)
*Note that conceptually the drainage of the ponded area is limited by ex-filtration at the base of the practice.}}
+
*n<sub>m</sub> = Porosity of mulch
* Step 5: Sum total depth of bioretention, and compare to available space above water table and bedrock. Adjust if necessary.
+
*d<sub>f</sub> = Depth of filter media (m)
 +
*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:
 +
<math>d_{i}= f'\times t</math>
 +
{{Plainlist|1=Where:
 +
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
 +
*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 45 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.
  
==Calculate the remaining dimensions==
+
==Determine the required surface area of the practice==
* Step 6: Multiply the depth of each separate component by the void ratio and then sum the total to find the 1 dimensional storage (in mm).
+
* 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 7: Calculate the required total storage (S<sub>T</sub>, m<sup>3</sup>):
+
For practices where flow is delivered to a surface ponding area:
<math>S_{T}=RVC_T\times A_c\times C\times 10</math>
+
<math>A_{p}=i\times D\times A_{i}/[d_{p}' + (K_{f} \times D)]</math>
 +
{{Plainlist|1=Where:
 +
*i = Design storm intensity (m/h)
 +
*D = Design storm duration (h)
 +
*A<sub>i</sub> = Catchment impervious area (m<sup>2</sup>)
 +
*d<sub>p</sub>' = Design surface ponding depth (m)
 +
*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>
 +
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>
 
{{Plainlist|1=Where:
 
{{Plainlist|1=Where:
*''RVC<sub>T</sub>'' is the Runoff volume control target (mm),
+
*d<sub>i</sub> = Infiltration water storage depth (m), see above for equations for with and without underdrain designs
*''A<sub>c</sub>'' is the catchment area (Ha),  
+
*f' = Design infiltration rate of the underlying native soil (m/h)
*''C'' is the runoff coefficient of the catchment area, and
+
*D = Design storm duration (h)}}<br>
* 10 is the units correction between m<sup>3</sup> and mm.Ha.}}
+
* Step 6: Compare required surface area of the practice to available space.<br>
* Step 8. Divide required storage (m<sup>3</sup>) by the 1 dimensional storage (in m) to find the required footprint area (''A<sub>p</sub>'') for the bioretention in m<sup>2</sup>.
+
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 9. Calculate the peak [[flow through perforated pipe|flow rate through the perforated pipe]],  
+
* Step 7:  Calculate catchment impervious area to practice permeable (footprint) area ratio, R, also referred to as I/P ratio:
* Step 10. Calculate the peak [[flow through media|flow rate through the filter media]],
+
<math>R=A_{i}/A_{p}</math><br>
* Step 11. Determine if downstream [[flow control]] is required to achieve hydrologic objectives.
+
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.
  
==Additional calculations==
+
==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>
 +
{{Plainlist|1= Where:
 +
*V<sub>i</sub> = Infiltration volume target for the design storm event (m<sup>3</sup>), based on average annual water budgets for pre- and post-development scenarios, and the estimated infiltration volume deficit (pre-development minus post-development infiltration).
 +
*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)
 +
*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 and adjust the ponding depth down to limit surface storage to what is required to fully capture the design storm runoff volume.
 +
<br>
 +
*Step 9: Calculate the required storage reservoir depth, d<sub>r</sub> (m):
 +
<math>d_{r}=d_{i}'/n_{r}'</math>
 +
{{Plainlist|1= Where:
 +
*d<sub>i</sub>' = Design infiltration water storage depth (m)
 +
*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<sub>r</sub>').
  
===Calculating infiltration practice drainage in 1 dimension===
+
==Calculate the total depth of the practice, d<sub>T</sub>==
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).  
+
* 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 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 two 45 degree couplings - ease for maintenance) 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 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>
 +
<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>
 +
For more on sizing bioretention practices on constrained sites see [[Bioretention: Sizing and modeling]].
  
{{Clickable button|[[Media:Darcy drainage.xlsx|Download drainage time calculator(.xlsx)]]}}
+
==Calculate peak flow rates==
 +
* Step 14. Calculate the peak [[flow through perforated pipe|flow rate through the perforated pipe]],
 +
* Step 15. Calculate the peak [[flow through media|flow rate through the filter media]],
 +
* Step 16. Determine if downstream [[flow control]] is required to achieve hydrologic objectives.
  
===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 one dimensional (1D) drainage===
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br> P = 12 m (left), P = 20 m (right)]]
+
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.  
In some situations, it may be desirable to reduce the size of the bioretention required, by accounting for rapid drainage.
 
Typically, this is only worth exploring over sandy soils with rapid infiltration.  
 
  
Note that narrow, linear bioretention features (or [[bioswales]]) drain faster than round or blocky footprint geometries.
+
{{Clickable button|[[Media:Darcy drainage.xlsx|Download the Darcy drainage time calculator tool]]}}
  
*Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
+
===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>===
*To estimate the time (''t'') to fully drain the facility:
+
[[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br> x = 12 m (left), x = 20 m (right)]]
:<math>t=\frac{V_{R}A_{p}}{f'P}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math>
+
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.
 +
 
 +
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'')
 +
*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]''
 
{{Plainlist|1=Where:
 
{{Plainlist|1=Where:
*''V<sub>R</sub>'' is the void ratio of the 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),  
*''P'' 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 ponding zone (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>
When you wish to model the extent of groundwater mounding beneath an infiltration facility. 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>
----
+
 
 +
==References==

Latest revision as of 18:59, 21 May 2024

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.

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.



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.

Component Recommended depth (with underdrain pipe) Recommended depth (no underdrain pipe) Typical porosity (n)
Surface ponding, maximum (dp, max)
  • 125 to 400 mm (filter media Blend A or B)
  • Consult local design standards where available
See below 1
Mulch (dm) 75 ± 25 mm
  • 0.7 for shredded wood
  • 0.4 for stone
Filter media (df)
  • 300 mm to support turf grass and accept only roof runoff
  • 600 mm to support shrubs, flowering perennials and deeply rooting decorative grasses and accept both roof and pavement runoff
  • 1000 mm to support trees and accept both roof and pavement runoff
  • 0.4 for sandier Blend A (drainage rate priority)
  • 0.35 for more loamy Blend B (water quality treatment priority)
Choker layer 100 mm Not applicable 0.4
Perforated pipe 200 mm diameter pipe Not applicable 0.4 for clear stone aggregate in which the pipe is embedded
Internal water storage reservoir (dr) See below See below 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;

  • Catchment area (i.e. contributing drainage area), Ac (m2)
  • Catchment runoff coefficient, C
  • Catchment impervious area, Ai (m2)
  • Design storm intensity, i (mm/h)
  • Design storm duration, D (h)
  • Infiltration volume target for the design storm event, Vi (m3), 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 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
  • Design infiltration rate of the underlying native soil, f' (mm/h), median field measured value, f divided by a safety factor, z
  • Safety factor, z (dimensionless value between 2 and 3) chosen by designer based on consideration of risk factors
  • 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
  • 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)
  • 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.
Download the infiltration practice sizing tool

Decide if an underdrain will be included[edit]

  • Step 1: Based on the median measured infiltration rate of the native soil at the proposed location and bottom elevation of the practice, or estimated infiltration rate from median grain-size distribution results, decide if an underdrain will be included in the design. See Design infiltration rate for guidance on field tests, methods and number of test locations and tests (i.e. measurements) to be performed.

If the median field measured or estimated infiltration rate of the underlying native soil (f) is less than 15 mm/h, include an underdrain.

Select a surface ponding depth to begin sizing with[edit]

  • Step 2: Determine a maximum surface ponding depth (dp, max)

For practices without underdrains:

Where:

  • f' = Design infiltration rate (mm/h), and
  • 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 of the in-situ (native) soil at the base of the practice.

For practices with underdrains see recommended range for maximum surface ponding depth in the table above, and consult local design standards where available.

  • Step 3: Select a design surface ponding depth, dp' (m) to begin sizing with:

For practices with soft (i.e. landscaped) edges and bowl-shaped ponding areas calculate the mean surface ponding depth:

Where:

  • dp, max = maximum surface ponding depth (m)


For practices with hard edges and vertical-walled ponding areas (e.g., stormwater planter, stormwater tree trench) use the maximum ponding depth:

Determine the infiltration water storage depth of the practice[edit]

  • Step 4: Calculate the infiltration water storage depth of the practice, di which is the depth of water stored by the practice that can drain by infiltration alone.

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:

Where:

  • dp' = Design surface ponding depth (m)
  • dm = Depth of mulch (m)
  • nm = Porosity of mulch
  • df = Depth of filter media (m)
  • nf = Porosity of filter media


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, dr, 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:

Where:

  • f' = Design infiltration rate of underlying native soil (m/h)
  • 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


For practices with an underdrain where the perforated pipe is installed on the bottom and connected to a riser (e.g., standpipe and two 45 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.

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.

Determine the required surface area of the practice[edit]

  • Step 5: Calculate the surface area of the practice (Ap) needed to capture the volume of runoff produced from the catchment by the design storm event.

For practices where flow is delivered to a surface ponding area:

Where:

  • i = Design storm intensity (m/h)
  • D = Design storm duration (h)
  • Ai = Catchment impervious area (m2)
  • dp' = Design surface ponding depth (m)
  • Kf = minimum acceptable saturated hydraulic conductivity of the filter media (m/h), see Filter media for guidance



For practices where flow is delivered directly to a subsurface (underground) water storage reservoir:

Where:

  • di = 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)
  • D = Design storm duration (h)


  • Step 6: Compare required surface area of the practice to available space.

To decrease Ap, 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:


You can use the infiltration practice sizing tool noted above to adjust dp, di, Ap or Ai 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 internal water storage reservoir[edit]

  • Step 8: Calculate the surface area of the storage reservoir, Ar (m2) needed to capture the infiltration volume target for the design storm event:

Where:

  • Vi = Infiltration volume target for the design storm event (m3), based on average annual water budgets for pre- and post-development scenarios, and the estimated infiltration volume deficit (pre-development minus post-development infiltration).
  • di = 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)
  • D= Duration of design storm (h)


For practices where inflow is directed to a surface ponding area, if Ar is greater than Ap, use the value for Ar 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.

  • Step 9: Calculate the required storage reservoir depth, dr (m):

Where:

  • di' = Design infiltration water storage depth (m)
  • nr' = Effective porosity of the storage reservoir fill material(s).


To minimize the total depth of the practice, dT, 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 (nr').

Calculate the total depth of the practice, dT[edit]

  • Step 10: Determine what the planting needs are and assign an appropriate depth of filter media (df), using the table above.
  • 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 two 45 degree couplings - ease for maintenance) 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 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.

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.

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.

For more on sizing bioretention practices on constrained sites see Bioretention: Sizing and modeling.

Calculate peak flow rates[edit]

Calculating infiltration practice drainage time assuming one dimensional (1D) drainage[edit]

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.

Download the Darcy drainage time calculator tool

Calculating infiltration practice drainage time assuming three dimensional (3D) drainage[1][edit]

Two practice areas of 9 m2.
x = 12 m (left), x = 20 m (right)

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.

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 (Ap) by the perimeter (x)
  • To estimate the time (t) to fully drain the facility

t = [nr' * Ap/f'*x] * ln[(dr + (Ap/x)/Ap/x]

Where:

  • nr' is the effective porosity of the storage reservoir fill material(s),
  • Ap is the area of the practice (m2),
  • f' is the design infiltration rate of the native soil (mm/h),
  • x is the perimeter of the practice (m),
  • ln is the natural logarithm and,
  • dr 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[edit]

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).
Download the Hantush groundwater mounding tool
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and hosted by the USGS.

References[edit]

  1. 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.