Minimum width = 15 to 25 feet
Minimum length = 30 to 50 feet
Length to width ratio of 2:1 for widths > 15 feet
Maximum ponding depth = 6 inches
Maximum planting soil depth = 4 feet
Drainage area = 0.25 to 1.0 acres
Maximum entry velocity = 3 feet/second
In order to maintain a suitable micro-environment and to help simulate conditions which exist within an existing forest community, bioretention facilities must have a minimum area coverage. The sizing criteria presented in the previous slide ensures the necessary treatment area and volume to accommodate the WQv, but additional criteria, such as those presented in this slide, are necessary to assure the survival and success of the planted material.
The minimum width allows for random spacing of trees and shrubs, it also permits planting densities which help create a micro-environment where stresses from urban stormwater pollutants are minimized. The 2:1 length to width ratio maintains a longer flowpath for the settlement of particulates and maximizes the edge-to-interior ratio. The maximum ponding depth of 6" provides surface storage for stormwater runoff (approximately 40% of WQv) but is not so deep as to adversely affect plant health. The 6" depth also will dissipate within a reasonable time (less than 3 days) which maintains flexibility in species selection, and minimizes the likelihood that the bioretention area will become a breeding ground for mosquitoes. The four foot planting soil bed depth is sized to provide adequate storage for the WQv, suitable capacity for root system growth and adequate moisture in the soil during dryer periods.