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Line 1: − + − The exceptions are the depth (''d'') and Permeable area (''P''), as only one of these is required to find the other. + − + − + − + + + + + + + + + + + + + + + + + + − ==Size a bioretention cell for constrained depth== − Determine if there is a constraint to the depth (''d<sub>T</sub>'') of the bioretention practice: − {{:Depth to GW}} − |'''or''' if the land area is limited, determine the I/P ratio::<math>R=\frac{A_{c}}{A_{p}}</math> − + − + − + − |-+ − !colspan="2" style="text-align: center;"|Then+ − |-+ − |To calculate the require facility area or footprint where the depth is constrained:+ − − |To calculate the required depth, where the area of the facility is constrained: − − {{#widget:WolframAlpha|id=88d135fd5507a36a9770deaa8106c975}} − |} − + − + − *''q''' = Infiltration coefficient in mm/hr (accounting for SCF)+ − *''SCF'' = Safety correction factor+ − *''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>+ − + − *''d'' = Depth of infiltration practice in m.}} − The following equations assume that infiltration occurs primarily through the base of the facility. − They may be easily applied for any shape and size of infiltration facility, in which the reservoir storage is mostly in an aggregate. − − − <strong>[[Media:Infiltration Sizing.xlsx|Download .xlsx calculation tool]]</strong> + + − + − + − + − + − + − + − + − + − +
no edit summary
Before beginning the sizing calculations most of the following parameters must be known or estimated.
This article describes recommended design approaches when available space for the practice is constrained.<br>
<br>
Note that some of these parameters are limited:
Before beginning the sizing calculations certain parameters must be known or estimated. See [[Bioretention: Sizing]] for parameter descriptions and conceptual diagram illustrating key components of bioretention practices. Note that some of these parameters are limited:
#The ''maximum'' total depth will be limited by construction practices i.e. not usually > 2 m.
#The ''maximum'' total depth will be limited by construction practices i.e. not usually > 2 m.
#The ''maximum'' total depth may be limited by the [[Infiltration| conditions underground]] e.g. the groundwater or underlying geology/infrastructure.
#The ''maximum'' total depth may be limited by the [[Infiltration| conditions underground]] e.g. the groundwater or underlying geology/infrastructure.
#The minimum total depth will be limited by the need to support vegetation i.e. not < 0.6 m.
#The minimum total depth will be limited by the need to support vegetation (e.g not less than 0.6 m to support deep rooting perennials and shrubs).
#[[Bioretention]] has a maximum recommended I/P ratio of 20.
#[[Bioretention]] has a maximum recommended catchment impervious area to practice permeable (footprint) area ratio, R (or I/P ratio) of 20.
==Size a bioretention cell receiving flows directly to the storage reservoir for a constrained depth==
If there is a constraint to the depth (''d<sub>T</sub>'') of the practice, calculate the required storage reservoir footprint area (''A<sub>r</sub>''), as:
<math>A_{r}=\frac{i\times D\times A_i}{(d_{r}\times n')+(f'\times D)}</math>
{{Plainlist|1=Where:
*''A<sub>r</sub>'' = Area of the infiltration practice storage reservoir (m<sup>2</sup>)
*''A<sub>i</sub>'' = Catchment impervious area (m<sup>2</sup>)
*''D'' = Duration of design storm (h)
*''i'' = Intensity of design storm (mm/h)
*''f''' = [[design infiltration rate]] (m/h)
*''n''' = Effective porosity of the fill material in the storage reservoir of the practice
*''d<sub>r</sub>'' = Storage reservoir depth, based on depth available between the elevation of the invert of the underdrain perforated pipe and one (1) metre above the seasonally high water table or top of bedrock (m) or other value determined to be suitable through groundwater mounding analysis.}}<br>
If R is greater than 20, consider decreasing catchment impervious area (A<sub>i</sub>) by draining less area to the practice.
==Size a bioretention cell where drainage area and practice area are fixed==
If the land area is limited, determine the I/P ratio, which is the ratio of catchment impervious area (A<sub>i</sub>) to practice pervious footprint area (A<sub>p</sub>):
:<math>R=\frac{A_{i}}{A_{p}}</math>
{{Plainlist|1= Where:
{{Plainlist|1= Where:
*''R'' = Ratio of catchment area (''A<sub>c</sub>'') to BMP footprint area (A<sub>p</sub>) syn. I/P ratio.
*''R'' = Ratio of catchment impervious area to practice pervious footprint area, also referred to as I/P ratio
*''A<sub>p</sub>'' = Area of the infiltration practice in m<sup>2</sup>
*''A<sub>p</sub>'' = Practice pervious footprint area in m<sup>2</sup>
*''A<sub>c</sub>'' = Catchment area in m<sup>2</sup>}}
*''A<sub>i</sub>'' = Catchment impervious area in m<sup>2</sup>}}
Then calculate the required storage reservoir depth (''d<sub>r</sub>''), as:
<math>d_{r}=\frac{D \left[ (R\times i)-f'\right]}{n'}</math>
<math>A_{p}=\frac{IiD}{V_{R}d+q'D}</math>
<math>d=\frac{D\left[Ri-q'\right]}{V_{R}}</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''D'' = Duration of design storm in hrs
*''D'' = Duration of design storm (h)
*''i'' = Intensity of design storm in mm/hr
*''i'' = Intensity of design storm (m/h)
*''f''' = Design infiltration rate (m/h)
*''n''' = Effective porosity of the storage reservoir fill material}}
These equations assume that infiltration occurs primarily through the base of the facility.<br>
*''R'' = Ratio of catchment area (''A<sub>c</sub>'') to BMP footprint area (A<sub>p</sub>) syn. I/P ratio.
<br>
This spreadsheet tool has been set up to perform all of the infiltration practice sizing calculations shown above.<br>
*''A<sub>c</sub>'' = Catchment area in m<sup>2</sup>
{{Clickable button|[[Media:Infiltration Sizing 20220617 locked (1).xlsx|Download the infiltration practice sizing tool]]}}
This spreadsheet tool has been set up to perform either of the above calculations.<br>
==Calculate drawdown time==
==Calculate drawdown time==
[[file:Hydraulic radius.png|thumb|Two footprint areas of 9 m<sup>2</sup>.<br>
[[file:Hydraulic radius.png|thumb|Two footprint areas of 9 m<sup>2</sup>.<br>
Perimeter = 12 m (left) Perimeter = 20 m (right)]]
Perimeter = 12 m (left) Perimeter = 20 m (right)]]
{{Clickable button|[[Media:Darcy drainage_20200528_locked.xlsx|Download the Darcy drainage time calculator tool]]}}
In some situations, it may be possible 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.
In some situations, it may be possible 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 drain faster than round or blocky footprint geometries.
Note that narrow, linear bioretention features drain faster than round or blocky footprint geometries.
*Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
*Begin the drainage time calculation by dividing the storage reservoir area of the practice (''A<sub>r</sub>'') by the perimeter (''x'').
*To estimate the time (''t'') to fully drain the facility:
*Use the following equation to estimate the time (''t'') to fully drain the facility:
:<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>
:<math>t=\frac{nA_{r}}{f'x}ln\left [ \frac{\left (d_{r}+ \frac{A_{r}}{x} \right )}{\left(\frac{A_{r}}{x}\right)}\right]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''V<sub>R</sub>'' is the void ratio of the media,
*''n'' is the porosity of the storage reservoir fill material
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>),
*''A<sub>r</sub>'' is the storage reservoir footprint area (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), and
*''d<sub>T</sub>'' is the total depth of the practice, including the ponding zone (m).}}
*''d<sub>r</sub>'' is the depth of the storage reservoir (m).}}
This 3 dimensional equation makes use of the hydraulic radius (''A<sub>p</sub>''/''P''), where ''P'' is the perimeter (m) of the facility. <br>
This 3 dimensional equation makes use of the hydraulic radius (''A<sub>r</sub>''/''x''), where ''x'' is the perimeter (m) of the facility. <br>
Maximizing the perimeter of the facility directs designers towards longer, linear shapes such as [[bioswales]].
Maximizing the perimeter of the facility directs designers towards longer, linear shapes such as [[bioswales]].
[[category: modeling]]
[[category: modeling]]
[[category: infiltration]]
[[category: infiltration]]