Bioretention: Sizing and modeling

From LID SWM Planning and Design Guide
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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 (df 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).

Download .xlsx calculation tool