Bioretention: Sizing

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.
If the space on the ground is the tighter constraint to design, try this article on sizing bioretention for space.

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Step 1, total volume

The three storage areas 'bowl', 'filter media' and 'reservoir' are sized together.

The bioretention facility will comprise three vertical zones:

1. a 'bowl' permitting water to pond occasionally (maximum depth, d_p = 0.5 m)
2. a layer of filter media to support plant growth (minimum depth, d_m = 0.6 m) , and
3. a reservoir of stone to store and facilitate infiltration of additional water (depth, d_R, includes 0.1 m choking layer).

Firstly, all three zones will be sized together within the limited depth to estimate the footprint area of the whole bioretention facility. To do this the mean void ratio (V_R) will estimated as ~ 0.4.

To calculate the required practice area (A_p) or footprint where the depth is constrained:

$A_{p}={\frac {V}{V_{R}d}}$

Where:

• V = Required volume of storage in m³
• V_R = Void ratio (porosity), as measured (or default to 0.4)
• A_p = Area of the infiltration practice in m²
• d_T = Total depth of infiltration practice in m (= d_b + d_m + d_R)

Step 1 alternative, accounts for infiltration during a design storm event

This may be desirable where the practice is located on particularly freely draining soils and/or where the design storm is of longer duration. To calculate the required practice area (A_p) or footprint where the depth is constrained:

$A_{p}={\frac {A_{c}iD}{V_{R}d+f'D}}$

Where:

• D = Duration of design storm in hrs
• i = Intensity of design storm in mm/hr
• f' = Design infiltration rate in mm/hr
• V_R = Void ratio (porosity), as measured (or default to 0.35 for all aggregates)
• A_p = Area of the infiltration practice in m²
• A_c = Area of the catchment in m²
• d_T = Total depth of infiltration practice in m (= d_b + d_m + d_R)

Step 2, geometry and perimeter

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

Fit the bioretention facility into the site plan. It is necessary to have an idea of the perimeter of the facility.
Narrow, linear cells drain faster than round or blocky footprint geometries.
Divide the area of the practice (A_p) by the perimeter (P).

Step 3, depth of bowl,d_b

The duration of surface ponding is usually determined by the emptying of the facility below.

The 'bowl' area is simply the depressed elevation of the surface compared to the surrounding areas; a space to permit temporary ponding.
The depth of the bowl would not typically exceed 0.5 m due to safety concerns.
Some jurisdictions or particular sites may demand a lower maximum depth of ponding. It is worth bearing in mind that in most cases this ponded water will only occur once or twice per year.
The ponded water within the bowl should drain within 48 hrs. This addresses public concerns about mosquitoes and reduces the probability of losing vegetation due to saturated growing conditions.
The drainage of the bowl is not usually constrained by the surface infiltration (filter media should drain > 25 mm/hr). The infiltration of water from the bowl is determined by the drainage of the filter media and reservoir into the soils.
To estimate the time (t) to fully drain the facility:
$t={\frac {V_{R}A_{p}}{f'P}}ln\left[{\frac {\left(d+{\frac {A_{p}}{P}}\right)}{\left({\frac {A_{p}}{P}}\right)}}\right]$

Final step

Check the mounding of groundwater beneath the facility to ensure no incompatibility with nearby sensitive receptors.
Download groundwater mounding calculator(.xlsx)
Note that some adaptations have been made to this tool, from the original hosted by the USGS.

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

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.

A body of water smaller than a lake, often artificially formed.

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

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