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

Precast concrete vault
Source: American Concrete Industries
Applications
Detention systems can be used in a wide variety of applications
when the necessary space is available. Their use
is limited in ultra urban areas and some redevelopment
projects simply due to a lack of available space (in
these cases underground and/or special detention may
be used). The following applications can readily use
detention systems:
• Residential development,
• Industrial development,
• Commercial development, and
• Urban areas.
Design Considerations
Storage volume, depth, and duration
• Detention basins should be designed to mitigate
runoff peak rates for the one-year through 100-year
rainfall events.
• An emergency outlet or spillway capable of
conveying the spillway design flood (SDF) must be
included in the design. The SDF is usually equal to
the 100-year design flood.
• Detention basins should be designed to treat the
runoff volume produced by the water quality
design storm unless additional upstream BMPs are
provided.
• Detention time is defined as the time from when
the maximum storage volume is reached until only
10 percent of that volume remains in the basin. In
order to achieve a 60 percent total suspended solids
removal rate, a 24-hour detention time is required
within an extended detention basin.
• The lowest elevation within an extended dry
detention basin should be at least two feet above
the seasonal high water table. If high water table
conditions are anticipated, then the design of a wet
pond, constructed wetland, or bioretention facility
should be considered.
• The maximum water depth of the basin should not
exceed 10 feet.
• Inflow and discharge hydrographs should be
calculated for each selected design storm.
Hydrographs should be based on the 24-hour
rainfall event. Specifically, the NRCS 24-hour Type
II rainfall distribution should be utilized to generate
hydrographs.
• Basins should have one or more sediment forebays
or equivalent upstream pretreatment to trap coarse
sediment, prevent short circuiting and facilitate
maintenance (i.e., sediment removal). The forebay
should consist of a separate cell, formed by a
structural barrier. The forebay will require periodic
sediment removal.
• Distances of flow paths from inflow points to
outlets should be maximized.
Detention basin location
• Basins should be located down gradient of
disturbed or developed areas on the site. The basin
should collect as much site runoff as possible,
especially from the site’s impervious surfaces
(roads, parking, buildings, etc.), and where other
BMPs are not proposed.
• Basins should not be constructed on steep slopes,
nor should slopes be significantly altered or
modified to reduce the steepness of the existing
slope, for the purpose of installing a basin.
• Basins should not worsen the runoff potential of the
existing site by removing trees for the purpose of
installing a basin.
• Basins should not be constructed within 10 feet of
the property line or within 50 feet of a private well
or septic system.
• Detention basins should not be constructed in
areas with high quality and/or well draining soils,
which are adequate for installing BMPs capable
of achieving stormwater infiltration and, hence,
volume reduction.
LID Manual for Michigan – Chapter 7 Page 176