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

Figure 7.21
Extended detention basin
Source: New Jersey BMP Manual
Additional design considerations for extended
detention basins (Figure 7.21)
• Extended detention basins should not be
constructed within jurisdictional waters, including
wetlands, or their regulated buffers.
• The low flow orifice should be sized and positioned
to detain the calculated water quality runoff volume
for at least 24 hours.
Basin sizing and configuration
• Basins, wet ponds, and constructed wetlands should
be shaped to maximize the hydraulic length of the
stormwater flow pathway. A minimum length-towidth
ratio of 2:1 is recommended to maximize
sedimentation. If the length-to-width ratio is lower,
the flow pathway should be maximized. A wedgeshaped
pond with the major inflows on the narrow
end can prevent short-circuiting and stagnation.
• Irregularly shaped basins are acceptable and may
even be encouraged to improve site aesthetics.
• If site conditions inhibit construction of a long,
narrow basin, baffles consisting of earthen berms or
other materials can be incorporated into the pond
design to lengthen the stormwater flow path.
• Permanent access must be provided to the forebay,
outlet, and embankment areas. It should be at
least nine feet wide, have a maximum slope of 15
percent, and be stabilized for vehicles.
Additional design considerations for wet ponds
• The area required for a wet pond is generally one to
three percent of its drainage area. Wet ponds should
be sized to treat the water quality volume and,
if necessary, to mitigate the peak rates for larger
events.
• All areas that are deeper than four feet should have
two safety benches, totaling 15 feet in width. One
should start at the normal water surface and extend
up to the pond side slopes at a maximum slope of
10 percent. The other should extend from the water
LID Manual for Michigan – Chapter 7 Page 177