Bioretention: Sizing and modeling

Input data:

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	m22	Permeable area i.e. area of the facility or BMP

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

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.