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

Stormwater Functions and
Calculations
Infiltration practices can provide excellent benefits for
managing volume and water quality protection. While
some BMPs are better than others in managing peak
rates, all infiltration BMPs provide some peak rate
benefit by removing direct connections from impervious
surfaces and increasing time of travel. Table 7.12
provides a summary of the stormwater functions by
BMP type.
Calculations for Infiltration BMPs
Infiltration area
The minimum infiltration area should be based on the
following (according to the loading ratio):
Minimum Surface Infiltration Area = [Contributing
impervious area] / 5*
*May be increased depending on soil infiltration
capacity (e.g., where soils are Type A or rapidly
draining). For carbonate, geologic areas may be
decreased to three.
This actual infiltration area (Table 7.13) should be
greater than the minimum infiltration area.
Table 7.13
Definition of Infiltration Area for Infiltration BMPs
BMP Infiltration Area Definition
Protecting Groundwater Quality
The protection of groundwater quality is of utmost importance
in any Michigan watershed. The potential to
contaminate groundwater by infiltrating stormwater in
properly designed and constructed BMPs with proper
pretreatment is low.
Numerous studies have shown that stormwater infiltration
BMPs have a minor risk of contaminating either
groundwater or soil. The U.S. Environmental Protection
Agency summarized in “Potential Groundwater
Contamination from Intentional and Non-intentional
Stormwater Infiltration” (Pitt et al., 1994) the potential
of pollutants to contaminate groundwater as
either low, low/moderate, moderate, or high. Of the
25 physical pollutants listed, one has a “high” potential
(chloride), and two have “moderate” potential
(fluoranthene and pyrene) for polluting groundwater
through the use of shallow infiltration systems with
some sediment pretreatment.
While chloride can be found in significant quantities
due to winter salting, relatively high concentrations
are generally safe for both humans and aquatic biota).
Pentachlorophenol, cadmium, zinc, chromium, lead,
and all the pesticides listed are classified as having a
“low” contamination potential. Even nitrate which is
soluble and mobile is only given a “low/moderate”
potential.
Infiltration Berms Total Infiltration Area (Ponding Area) = Length of Berm x Average Width of ponding behind berm.
Infiltration Basin The Infiltration Area is the bottom area of the basin. This is the area to be considered when evaluating the
Loading Ratio to the Infiltration basin.
Infiltration Trench The Infiltration Area* is the bottom area of the trench. This is the area to be considered when evaluating the
Loading Rate to the Infiltration basin.
[Length of Trench] x [Width of Trench] = Infiltration Area (Bottom Area)
* Some credit can be taken for the side area that is frequently inundated as appropriate.
Dry Well A dry well may consider both bottom and side (lateral) infiltration according to design.
Subsurface Infiltration The Infiltration Area is the bottom area of the bed. Some credit can be taken for the side area that is
frequently inundated as appropriate.
LID Manual for Michigan – Chapter 7 Page 210