Bioretention Filter MediaDesign Components

Af = WQV ((df)/k((h + df)(tf)

where:

Af = Surface area of the bioretention planting bed (ft2)

WQv = Water quality treatment volume (ft3)

df = Planting soil bed depth (ft) – 4 ft recommended

k = Coefficient of permeability for planting soil bed (ft/day) - k = 0.5 ft/day: Median value of a silt loam

h = Average height of water above the bioretention bed (ft); havg = ½*hmax - h is equal to 3", assuming a maximum ponding depth of 6" above the planting soil bed

tf = Time required for the Water Quality Treatment Volume (WQV) to filter through the planting soil bed - A value of 72 hours is recommended

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Since the bioretention concept incorporates a gravel underdrain system and a porous soil filter medium and sand bed, runoff entering the shallow ponding area will slowly percolate through the soil bed in a fashion similar to other filter practices.

The equation shown in this slide is utilized to estimate the minimum surface area and then the volume capacity is checked against that required to treat the WQv.