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 layer, embedded pipe diameter 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 porosity (n)
Surface ponding (dp)	380 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 runoff) 600 mm to support shrubs, flowering perennials and deeply rooting decorative grasses 1000 mm to support trees		0.4 for sandier Blend A (drainage rate priority) 0.35 for more loamy Blend B (water quality treatment priority)
Choker layer	100 mm		0.4
Perforated pipe and surrounding aggregate	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 perforated 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 permissible storage reservoir depth beneath the underdrain perforated pipe (d_{s, max}, mm):

$d_{s,max}=f'\times t\times 1/n$

Where:

• f' = Design infiltration rate (mm/hr),
• t = Drainage time (hrs). Check local regulations for drainage time requirements; and
• n = Porosity of the reservoir aggregate

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, which increases hydraulic head and infiltration rate at the base of the practice. See Low permeability soils for more information.

Additional step for system without underdrain

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

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

Where:

• f' = Design infiltration rate (mm/hr), and
• 48 = Maximum permissible drainage time for ponded water (hrs)
• Note that in designs without underdrains, conceptually the drainage of ponded water is limited by exfiltration at the base of the practice.

• Step 5: Sum total depth of bioretention components, and compare to available depth between the surface grade and the seasonally high water table or top of bedrock elevations.
• Step 6: Adjust component depths to maintain a separation of 1.0 metre between base of the practice and 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.

Calculate the remaining dimensions

• Step 7: Multiply the depth of each separate water-retaining component layer in the profile of the practice by its corresponding porosity and then sum the total to find the 1 dimensional storage (in mm).
• Step 8: Calculate the required total storage (S_T, m³):

$S_{T}=RVC_{T}\times A_{i}\times 10$

Where:

• RVC_T is the runoff volume control target (mm),
• A_i is the impervious area within the catchment (Ha), and
• 10 is the units correction between m³ and mm.Ha.

• Step 9. 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 10. Calculate the peak flow rate through the perforated pipe,
• Step 11. Calculate the peak flow rate through the filter media,
• Step 12. 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 time for the purposes of groundwater mounding (where a shorter drainage time causes a greater impact).

Download drainage time calculator(.xlsx)

Drainage time (3D)

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

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 textured native subsoils with rapid infiltration.

Note that narrow, linear bioretention features (or bioswales) 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 (x).
• To estimate the time (t) to fully drain the facility:

$t={\frac {nA_{p}}{f'x}}ln\left[{\frac {\left(d_{T}+{\frac {A_{p}}{x}}\right)}{\left({\frac {A_{p}}{x}}\right)}}\right]$

Where:

• n is the porosity of the filter media,
• A_p is the area of the practice (m²),
• f' is the design infiltration rate (mm/hr),
• x is the perimeter of the practice (m), and
• d_T is the total depth of the practice, including the surface ponding depth (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.

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

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

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 downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

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.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

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

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

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.