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

• Shallow wetlands are large surface area
constructed wetlands that primarily accomplish
water quality improvement through displacement of
the permanent pool.
• Extended detention shallow wetlands are similar
to shallow wetlands but use extended detention as
another mechanism for water quality and peak rate
control.
• Pocket wetlands are smaller constructed wetlands
that serve drainage areas between approximately
five and 10 acres and are constructed near the water
table.
• Pond/wetland systems are a combination of wet
ponds and constructed wetlands.
Although discussion of constructed wetlands in this
BMP focuses on surface flow as described above,
subsurface flow constructed wetlands can also be used
to treat stormwater runoff.
While typically used for wastewater treatment, subsurface
flow constructed wetlands for stormwater can offer
some advantages over surface flow wetlands, such as
improved reduction of total suspended solids and biological
oxygen demand. They also can reduce the risk of
disease vectors (especially mosquitoes) and safety risks
associated with open water. However, nitrogen removal
may be deficient (Campbell and Ogden, 1999) if most
of the incoming nitrogen is in the form of ammonia.
Subsurface flow wetlands are poor converters of ammonia
to nitrate (nitrification) but are excellent converters
of nitrate to nitrogen gas (denitrification). Perhaps
the biggest concern regarding subsurface constructed
wetlands is their relatively high cost. They can be two
to three times more expensive to construct than surface
flow constructed wetlands.
Constructed wetland at Okemos High School
Source: Tetra Tech
Underground detention
These facilities are usually intended for applications
on sites where space is limited and are not intended to
provide significant water quality treatment. Examples
include:
Underground detention beds
Underground detention beds can be constructed by
excavating a broad area and filling it with uniformly
graded aggregate. Runoff can be stored within the void
spaces of the aggregate while the aggregate bed structurally
supports overlying land uses.
• Storage design and routing methods are the same as
for surface detention basins.
• Underground detention beds may be used where
space is limited, but subsurface infiltration is not
feasible due to high water table conditions, shallow
soil mantle, or poorly draining soils.
• Underground detention beds provide minimal
water quality treatment and should be used in
combination with a pretreatment BMP.
• Except where runoff is or may become toxic and
contamination of soil or the water table below the
site is possible, underground detention beds should
not be lined with an impervious geomembrane. By
not installing a geomembrane, a minimal amount of
infiltration may still occur. If infiltration is allowed,
proper pretreatment is necessary to avoid polluting
groundwater. See the infiltration practices BMP for
more information.
Underground vaults
Underground vaults are stormwater storage facilities
usually constructed of precast reinforced concrete or
a structural high density polyethylene plastic system.
Tanks are usually constructed of large diameter metal or
plastic pipe. Concrete, metal, or plastic pipes may also
be installed with no slope as part of a network designed
for storage.
• Storage design and routing methods are the same as
for surface detention basins.
• Underground detention beds may be used where
space is limited but subsurface infiltration is not
feasible due to high water table conditions, a
shallow soil mantle, or poorly draining soils.
• Underground vaults provide minimal water quality
treatment and should be used in combination with a
pretreatment BMP.
LID Manual for Michigan – Chapter 7 Page 175