A significant factor affecting bioretention performance is storage volume. Bioretention cells within MSD's jurisdiction are typically sized based on the "stormwater quality volume," and sometimes the "channel protection volume."
The Water Quality Volume is defined as WQv = P * Rv * A, where
P = 0.095 feet (1.14 inches)
Rv = 0.05 + 0.009 * I, where I is the percent impervious area (whole number)
A is the drainage area (square feet)
(A minimum storage volume of 0.2 inches per acre shall be met at all sites where WQv is required.)
When sized for the stormwater quality volume, bioretention should provide a minimum storage of 75 percent of the WQv. Storage is considered as the volume provided within and above the bioretention soil. (A porosity of 30 – 35 percent voids is typically used for bioretention soil.) The ponding depth (i.e., storage above the soil) should be 6 – 18 inches when bioretention is sized for the stormwater quality volume.
When bioretention is sized to store the channel protection volume or flood detention volume, stormwater may pond up to 36 inches deep. The entire channel protection volume should be stored in the basin, and should drain through the filter bed (i.e., no orifice is needed). In some situations (typically 1-2 acre drainage areas with small amount of impervious area), flood detention can also be nested in the bioretention storage volume. Orifices, weirs, and other outlet devices are typically needed to release flood events from bioretention.
Filter Bed (Soil Bed) Area
The aerial footprint of the bioretention soil affects the time needed to fully drain the bioretention basin.
Af = WQv * df / [k * (hf + df) * tf], where
Af = Minimum surface area of filter bed (ft2)
WQv = Stormwater quality volume (ft3)
df = filter bed thickness (feet)
k = saturated hydraulic conductivity (feet/day)
hf = maximum ponding depth ÷ 2 (feet)
tf = maximum drain time (days)
Bioretention should fully drain the water quality storage volume within 2 days.
If the bioretention soil meets the material specification provided in the BMP Landscape Guide, then MSD recognizes k = 2 feet/day.
Runoff Volume Reduction
"Volume reduction" occurs when the volume of water discharging from the BMP is less than the volume of water captured. Bioretention achieves volume reduction by evaporating, transpiring, and infiltrating stormwater runoff. The MEP spreadsheets provide one method of estimating volume reduction. A more precise, site specific assessment of annual or event-specific volume reduction can be estimated using RECARGA, SWMM, and other bioretention modeling programs.