Low Impact Development Manual for Michigan - OSEH - University ...

Stormwater Functions and
Calculations
When designing a bioretention area, it is recommended
to follow a two-step process:
1. Initial sizing of the bioretention area based on the
principles of Darcy’s Law.
2. Verify that the loading ratio and the necessary
volume reductions are being met.
Initial sizing of the bioretention area
Bioretention areas can be sized based on the principles
of Darcy’s Law, as follows:
With an underdrain:
Af = V x df / [k x (hf + df) x tf] Without an underdrain:
Af = V x df / [i x (hf + df) x tf] Where:
Af = surface area of filter bed (ft2 )
V = required storage volume (ft3 )
df = filter bed depth (ft)
k = coefficient of permeability of filter media (ft/day)
i = infiltration rate of underlying soils (ft/day)
hf = average height of water above filter bed (ft)
tf = design filter bed drain time (days)
A “quick check” for sizing the bioretention area is to
ignore the infiltration rate and calculate the storage
volume capacity of the bioretention area as follows:
A inf = (Area of bioretention area at ponding depth +
Bottom area of bioretention area) divided by two =
Infiltration area (average area)
The size of the infiltration area is determined by the
volume of water necessary to remove as determined by
LID criteria, depth of the ponded area (not to exceed 18
inches), infiltration rate of the soil, loading ratio, and, if
applicable, any subsurface storage in the amended soil
or gravel.
This volume can be considered removed if the bioretention
is not underdrained. If the bioretention cell is
underdrained, consider the bioretention cell as a detention
device with the volume calculated above discharged
to a surface water over time t f .
Verification of meeting volume reduction requirements
The bioretention facility should be sized to accommodate
the desired volume reductions (see Chapter 9 for
Volume Control Criteria). This can be based on water
quality volume (e.g., first inch of runoff from the site)
or based on size storm event (e.g., no net increase based
on presettlement conditions of the two-year, 24-hour
event).
The volume of a bioretention area can have three components:
surface storage volume, soil storage volume, and
infiltration bed volume. These three components should
be calculated separately and added together. The goal is
that this total volume is larger than the required volume
reduction that is often included in local ordinances.
If the total volume is less than the required volume,
another adjustment may be needed to the bioretention
area (e.g., increased filter bed depth).
Total volume calculation:
1. Surface storage volume (ft3 ) = Average bed area
(ft2 ) x Maximum design water depth (ft)
2. Soil storage volume (ft3 ) = Infiltration area (ft2 ) x
Depth of amended soil (ft) x Void ratio of amended
soil.
3. Subsurface storage/Infiltration bed volume (ft3 ) =
Infiltration area (ft2 ) x Depth of underdrain material
(ft) x Void ratio of storage material
Total bioretention volume = Surface storage volume +
Soil storage volume (if applicable) + Infiltration bed
volume (if applicable).
Peak rate mitigation
Chapter 9 provides information on peak rate mitigation
methodology and addresses links between volume
reduction and peak rate control. Underdrained bioretention
acts as a detention practice with a discharge rate
roughly equal to the infiltration rate of the soil x the
average bed area.
Water Quality Improvement
The reported water quality benefits of bioretention can
be expected to remove a high amount of total suspended
solids (typically 70-90 percent), a medium amount of
total phosphorus (approximately 60 percent), and a
medium amount of total nitrogen (often 40-50 percent).
In areas with high sediment loading, pretreatment of
runoff can significantly reduce the amount of bioretention
maintenance required (See Chapter 9 for water
quality calculation procedures).
LID Manual for Michigan – Chapter 7 Page 143