Bioretention: Sizing and modeling

Before beginning the sizing calculations most of the following parameters must be known or estimated. The exceptions are the depth (d) and Permeable area (P), as only one of these is required to find the other. Note that some of these parameters are limited:

• The maximum total depth will be limited by construction practices i.e. not usually > 2 m.
• The maximum total depth may be limited by the conditions underground e.g. the groundwater or underlying geology/infrastructure.
• The minimum total depth may be limited by the need to support vegetation i.e. not < 0.6 m.

• The area P for a green roofs, absorbent landscapes and permeable paving may be very similar or equal to the catchment area, so that the I/P ratio is close to 1.
• Infiltration trenches, chambers and bioretention cells have a maximum recommended I/P ratio of 20.

Inputs

Symbol	Units	Parameter
D	hrs	Duration of design storm (for MOECC volume based caclulations set to 1)
i	mm/hr	Intensity of design storm (for MOECC volume based calculations use whole storm depth (link to map))
q	mm/hr	Infiltration coefficient, calculated from measured infiltration rate and applied safety factor
n	-	Porosity, as measured (or default to 0.35)
I	m2	Impermeable area i.e. catchment
d	m	depth of Infiltration facility or BMP
P	m2	Permeable area i.e. area of the facility or BMP

To calculate the required depth, where the area of the facility is constrained: $d={\frac {D\left(\left[I/P\right]i-q\right)}{n}}$

• The applicable infiltration coefficient (q) is calculated by applying a safety factor to the measured infiltration rate.
• Determine the impervious catchment area (I), and outline the location of the bioretention facility. To begin with, budget for at least 10% of the total catchment area to be required for the BMP. The area of the facility on the ground will be 'P' (pervious).
• Unless the biomedia fill material has been tested and certified otherwise, a porosity value (n) of 0.35 may be assumed (i.e. void ratio 35%).
• Bioretention facilities are usually between 0.5 m and 1.25 m deep, owing to construction considerations. Determine the maximum available depth of the facility (d_{f max}) from the site constraints. The Infiltration page lists these.

Identify the duration (D) and intensity (i) of design storm(s). Calculate the required facility area (P).

$d_{f}={\frac {D\left(\left[I/P\right]i-q\right)}{n}}$

$\left[I/P\right]={\frac {Cn+qD}{in}}$

Download .xlsx calculation tool

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

The ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.

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)

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

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

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

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