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As a general guideline, the state of Maryland suggests that a bioretention area occupy between 5 and 7 percent of the drainage area. The following 3-step process can be used to determine the specific footprint required. View the Rain Garden Design Factsheet (2001) for detailed guidance. Other references in the Urban Waterways Series include: Bioretention Performance, Design, and Construction and Maintenance (2006) and Coldwater Stream Design Guidance for Stormwater Wetlands, Wet Ponds, and Bioretention (2007). Another design reference is the NCDENR BMP Manual Chapter on Bioretention.
Determine runoff depth using the following equation from the Natural Resources Conservation Service:
- Runoff Depth in Inches = (P-0.2*S)^2 / (P+0.8*S)
- P = Precipitation (typically use 1 inch)
- S = 1,000 / CN - 10
- CN = Curve Number (see table below)
Calculate runoff volume to be treated
- Runoff Volume (ft³) = Watershed Area (ft²) * Runoff Depth (ft)
- Remember to convert runoff depth from inches to feet (1 foot = 12 inches)
Calculate required surface area
- Bioretention Surface Area (ft²) = Runoff Volume (ft³) / Avg. Depth of Water (ft)
- Average depth of water is typically 9 inches, but depends upon the height of the overflow structure
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