Bioretention: Sizing

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.

The vertical storage zones in a bioretention cell include: ponding, mulch, filter media, choker course, pipe diameter reservoir and the storage reservoir.

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.

Component	Recommended depth (with underdrain pipe)	Recommended depth (no underdrain pipe)	Typical void ratio (VR)
Ponding (dp)	300 mm	See below	1
Mulch	75 ± 25 mm		0.7 for wood based 0.4 for aggregate
filter media (dm)	300 mm to support turf grass (and accept only rainwater/roof runoff) 600 mm to support flowering perennials and decorative grasses 1000 mm to support trees		0.3
Choker course	100 mm		0.4 typical
Pipe diameter reservoir	Is equal to underdrain 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 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.
• Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (d_{s, max}, mm):

$d_{s,max}={\frac {f'\times t}{0.4}}$

Where:

• f' = Design infiltration rate (mm/hr), and
• t = Drainage time (hrs), e.g. 96 hours, check local regulations for drainage time requirements.
• 0.4 = Void ratio of clear stone

Additional step for system without underdrain

• Step 4: Determine maximum permissible ponding depth (d_{p, max}):

$d_{p,max}={\frac {f'\times 48}{0.4}}$

Where:

• f' = Design infiltration rate (mm/hr), and
• 48 = Drainage time of the ponding (hrs)
• 0.4 = Void ratio of clear stone *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 (S_T, m³):

$S_{T}=RVC_{T}\times A_{c}\times C\times 0.1$

Where:

• RVC_T is the Runoff volume control target (mm),
• A_c is the catchment area (Ha),
• C is the runoff coefficient of the catchment 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 (A_p) for the bioretention 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

Drainage time (3D)

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

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.

• Begin the drainage time calculation by dividing the area of the practice (A_p) 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:

• V_R is the void ratio of the media,
• A_p is the area of the practice (m²),
• f' is the design infiltration rate (mm/hr),
• P is the perimeter of the practice (m), and
• d_T 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.

The natural ground material characteristic of or existing by virtue of geographic origin.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

The 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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A perforated pipe used to assist the draining of soils.

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

The portion of water precipitated onto a catchment area, which then 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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)

The period between the maximum water level and the minimum level (dry weather or antecedent level).

The technique of removing pollutants from runoff as it infiltrates through the soil.

The 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.

The portion of water precipitated onto a catchment area, which then 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.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.