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Many of the dimensions in a bioretention system are relatively constrained by the performance requirements of the individual 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.
{| class="wikitable"
|-
! Component
! Typical Void ratio
! Recommended depth (with underdrain pipe)
! Recommended depth (no underdrain pipe)
|-
| Ponding depth
| 1
| 300 mm
| See below
|-
| [[Mulch]]
|
* 0.7 for wood based
* 0.4 for aggregates
| colspan="2" |75 ± 25 mm
|-
| [[Biofiler media]]
| 0.3
| colspan="2" |
* 300 mm to support turf grass and accept only roof runoff
* 600 mm to support flowering [[perennials]] and decorative [[grasses]]
* 1000 mm to support [[trees]]
|-
| [[choker gravel|Choker course]]
| 0.4 typical
| colspan="2" |100 mm
|-
| Embedding reservoir
| 0.4
| Is equal to underdrain pipe diameter
| Not applicable
|-
| Storage reservoir
| 0.4
| See below
| See below
|}
==Calculate the maximum overall depth==
*Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above.
*Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth.
*Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (''d<sub>s</sub>''):
<math>d_{s}=f'\times38.4</math>
{{Plainlist|1=Where:
*''f''' = Design infiltration rate in mm/hr, and
*38.4 comes from multiplying desired drainage time of 96 hours by void ratio of 0.4}}
===Additional step for system without underdrain===
*Step 4: Determine maximum permissible ponding depth (''d<sub>p</sub>''):
<math>d_{p}=f'\times19.2</math>
{{Plainlist|1=Where:
*''f''' = Design infiltration rate in mm/hr, and
*19.2 comes from multiplying desired drainage time of 48 hours by void ratio of 0.4. Note that conceptually the drainage of the ponded area is limited by ex-filtration at the base of the practice.}}
* Step 5: Sum total depth of bioretention, and compare to available space above water table and bedrock. Adjust if necessary.
==Calculate the remaining dimensions==
* 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 7: Calculate the required total storage (m<sup>3</sup>):
<math>Storage=RVC_T\times A_c\times C\times 0.1</math>
{{Plainlist|1=Where:
*''RVC<sub>T</sub>'' is the Runoff volume control target (mm),
*''A<sub>c</sub>'' is the catchment area (Ha),
*''C'' is the runoff coefficient of the catchment area, and
* 0.095 is the product of a typical runoff coefficient for impermeable surfaces (0.95) and the units correction between m<sup>3</sup> and mm.Ha. (0.1)}}
* Step 8. Divide required storage (m<sup>3</sup>) by the 1 dimensional storage (in m) to find the required footprint area (A_p) for the bioretention in m<sup>2</sup>.
* Step 9. Calculate the peak flow rate (''Q<sub>p</sub>'', in L/s) through the filter media:
<math>Q_{p} = A_{p}\times K_{sat}\times 3.6 \times 10^{-3}</math>
{{Plainlist|1=Where:
*''K<sub>sat</sub>'' is the saturated hydraulic conductivity of the filter media (mm/hr), and
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>).}}
----
To size a bioretention facility using this page, the [[design infiltration rate]] (q' in mm/hr, after correction) of the native underlying soils must estimated, and the [[maximum available excavation depth]] (d in m) is the primary constraint to sizing. <br>
To size a bioretention facility using this page, the [[design infiltration rate]] (q' in mm/hr, after correction) of the native underlying soils must estimated, and the [[maximum available excavation depth]] (d in m) is the primary constraint to sizing. <br>
If the space on the ground is the tighter constraint to design, try this article on [[sizing bioretention for space]].
If the space on the ground is the tighter constraint to design, try this article on [[sizing bioretention for space]].