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

surface into the pond to a maximum depth of 18
inches, also at slopes no greater than 10 percent.
• Slopes in and around wet ponds should be 4:1 to
5:1 (horizontal:vertical) or flatter whenever possible
(10:1 max. for safety/aquatic benches). Wet ponds
should have an average depth of three to six feet
and a maximum depth of eight feet. This should be
shallow enough to minimize thermal stratification
and short-circuiting and deep enough to prevent
sediment resuspension, reduce algal blooms, and
maintain aerobic conditions.
Additional design considerations for constructed
wetlands
• Constructed wetlands should be designed so
that the 10-year water surface elevation does not
exceed the normal water surface elevation by more
than three feet. Slopes in and around constructed
wetlands should be 4:1 to 5:1 (horizontal:vertical)
whenever possible.
• All areas that are deeper than four feet should have
two safety benches, each four to six feet wide. One
should be situated about one to 1.5 feet above the
normal water elevation and the other two to 2.5 feet
below the water surface.
Pocket wet pond
Source: Maryland Stormwater Manual, 2000
Embankments
• Vegetated embankments less than or equal to
three feet in height are recommended. However,
embankments must be less than 15 feet in height
and should have side slopes no steeper than 3:1
(horizontal to vertical).
• The basin should have a minimum freeboard of one
foot above the SDF elevation to the top of the berm.
• Woody vegetation is generally discouraged in
the embankment area because of the risk of
compromising the integrity of the embankment.
• Embankments should incorporate measures such as
buried chain link fencing to prevent or discourage
damage from tunneling wildlife (e.g., muskrat).
Inlet structures
Erosion protection measures should be used to stabilize
inflow structures and channels.
Outlet design
• The low-flow orifice should typically be no smaller
than 2.5 inches in diameter. However, the orifice
diameter may be reduced to one inch if adequate
protection from clogging is provided.
• The hydraulic design of all outlet structures
must consider any significant tailwater effects of
downstream waterways.
• The primary and low flow outlets should be
protected from clogging by an external trash rack or
other mechanism.
• Online facilities should have an emergency
spillway that can safely pass the 100-year storm
with one foot of freeboard. All outflows should be
conveyed downstream in a safe and stable manner.
Additional design considerations for dry detention
• When designed to meet discharge criteria for
a range of storms, basins should incorporate a
multistage outlet structure. Three elements are
typically included in this design:
° A low-flow outlet that controls the extended
detention and functions to slowly release the
water quality or channel protection design storm.
° A primary outlet that functions to attenuate the
peak of larger design storms.
° An emergency overflow outlet/spillway. The
emergency spillway should be at the top of the
berm.
• The primary outlet structure should incorporate
weirs, orifices, pipes, or a combination of these to
control runoff peak rates for multiple design storms.
Water quality storage should be provided below the
invert of the primary outlet. When routing basins,
the low-flow outlet should be included in the depthdischarge
relationship.
• Energy dissipaters should be placed at the end of
the primary outlet to prevent erosion. If the basin
discharges to a channel with dry weather flow, care
should be taken to minimize tree clearing along the
downstream channel, and to reestablish a forested
LID Manual for Michigan – Chapter 7 Page 178