Bioretention: Sizing

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This article is specific to bioretention, vegetated systems that infiltrate water to the native soilThe natural ground material characteristic of or existing by virtue of geographic origin..
If you are designing a planted system which does not infiltrate water, see advice on Planters: Sizing.

The vertical storage zones in a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell include: ponding, mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales., filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles., choker course, pipe diameter reservoir and the storage reservoir.

Many of the dimensions in a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. 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 underdrainA perforated pipe used to assist the draining of soils. pipe. The order of operations in calculating these dimensions depends on whether an underdrainA perforated pipe used to assist the draining of soils. is desired.

Component Recommended depth (with underdrainA perforated pipe used to assist the draining of soils. pipe) Recommended depth (no underdrainA perforated pipe used to assist the draining of soils. pipe) Typical void ratio (VR)
Ponding (dp) 300 mm See below 1
Mulch 75 ± 25 mm
  • 0.7 for wood based
  • 0.4 for stone
filter media (dm)
  • 300 mm to support turf grass (and accept only rainwater/roof runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.)
  • 600 mm to support flowering perennials and decorative grasses
  • 1000 mm to support trees
  • 0.4 for sandy mix
  • 0.35 for a more loamy mix.
Choker course 100 mm 0.4 typical
Pipe diameter reservoir Is equal to underdrainA perforated pipe used to assist the draining of soils. pipe diameter Not applicable 0.4
Storage reservoir (ds) See below See below 0.4

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 underdrainA perforated pipe used to assist the draining of soils. 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.
  • Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (ds, max, mm)\[d_{s, max}=\frac{f'\times t}{0.4}\]

Where:

  • f' = Design infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (mm/hr), and
  • t = Drainage time (hrs). Check local regulations for drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). requirements.
  • 0.4 = Void ratio of clear stone

Additional step for system without underdrainA perforated pipe used to assist the draining of soils.

  • Step 4: Determine maximum permissible ponding depth (dp, max)\[d_{p, max}=\frac{f'\times48}{0.4}\]

Where:

  • f' = Design infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (mm/hr), and
  • 48 = Drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). of the ponding (hrs)
  • 0.4 = Void ratio of clear stone *Note that conceptually the drainage of the ponded area is limited by ex-filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. at the base of the practice.
  • Step 5: Sum total depth of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation., and compare to available space above water tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body.Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone. 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 (ST, m3)\[S_{T}=RVC_T\times A_c\times C\times 0.1\]

Where:

  • RVCT is the RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. volume control target (mm),
  • Ac is the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. area (Ha),
  • C is the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. coefficient of the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. area, and
  • 0.1 is the units correction between m3 and mm.Ha.
  • Step 8. Divide required storage (m3) by the 1 dimensional storage (in m) to find the required footprint area (Ap) for the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. in m2.
  • Step 9. Calculate the peak flow rate through the perforated pipe,
  • Step 10. Calculate the peak flow rate through the filter media,
  • Step 11. Determine if downstream flow control is required to achieve hydrologic objectives.

Additional calculations

Calculating infiltration practice drainage in 1 dimension

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 timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). for the purposes of groundwater mounding (where a shorter drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). causes a greater impact).

Download drainage time calculator(.xlsx)

Drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). (3D)[1]

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

In some situations, it may be desirable to reduce the size of the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. required, by accounting for rapid drainageNatural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).. Typically, this is only worth exploring over sandy soils with rapid infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..

Note that narrow, linear bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. features (or bioswales) drain faster than round or blocky footprint geometries.

  • Begin the drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). calculation by dividing the area of the practice (Ap) by the perimeter (P).
  • To estimate the time (t) to fully drain the facility:

\[t=\frac{V_{R}A_{p}}{f'P}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]\]

Where:

  • VR is the void ratio of the media,
  • Ap is the area of the practice (m2),
  • f' is the design infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (mm/hr),
  • P is the perimeter of the practice (m), and
  • dT is the total depth of the practice, including the ponding zone (m).

Groundwater mounding

When you wish to model the extent of groundwater mounding beneath an infiltration facility. This tool uses Hantush's derivation (1967).
Download groundwater mounding calculator(.xlsm)
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and hosted by the USGS.


  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.