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

sink for pollutants and generally have high water
holding capacities. They will also facilitate plant
growth and propagation and may hinder invasion of
undesirable species. Care must be taken to ensure
that soils used are free of invasive or nuisance plant
seeds.
• To enhance habitat value, visual aesthetics, water
temperature, and pond health, a 25-foot buffer
should be provided, measured outward from the
maximum water surface elevation. The buffer
should be planted with trees, shrubs, and native
ground covers. Except in maintenance access
areas, turf grass should not be used. Existing trees
within the buffer should be preserved. If soils in the
buffer will become compacted during construction,
soil restoration should take place to aid buffer
vegetation.
Additional design considerations for constructed
wetlands
• Underlying soils must be identified and tested.
Generally, hydrologic soil groups “C” and “D”
are suitable without modification, “A” and “B”
soils may require a clay or synthetic liner. Soil
permeability must be tested in the proposed
constructed wetland location to ensure that
excessive infiltration will not cause it to dry out.
Field results for permeability should be used in the
water balance calculations to confirm suitability. If
necessary, constructed wetlands should have highly
compacted subsoil or an impermeable liner to
minimize infiltration.
• Organic soils should be used for constructed
wetlands. Organic soils can serve as a sink for
pollutants and generally have high water holding
capacities. They will also facilitate plant growth
and propagation and may hinder invasion of
undesirable species. Care must be taken to ensure
that soils used are free of invasive or nuisance plant
seed.
• About half of the emergent vegetation zone should
be high marsh (up to six inches deep) and half
should be low marsh (six to 18 inches deep).
Varying depths throughout the constructed wetland
can improve plant diversity and health (Table 7.9).
• The open water zone (approx. 35 to 40 percent of
the total surface area) should be between 18 inches
and six feet deep. Allowing a limited five-foot deep
area can prevent short-circuiting by encouraging
mixing, enhance aeration of water, prevent
Wet Pond with Buffer
Source: Township of West Bloomfield
resuspension, minimize thermal impacts, and limit
mosquito growth. Alternating areas of emergent
vegetation zone (up to 18 inches deep) and open
water zone– can also minimize short-circuiting and
hinder mosquito propagation.
Additional design considerations for underground
detention
• Underground systems that provide storage within
the void space of a stone layer should be wrapped
(bottom, top, and sides) in nonwoven geotextile
filter fabric to prevent migration of the subsoils into
the voids.
• Control of sediment is critical. Rigorous erosion
and sediment control measures are required to
prevent sediment deposition within the underground
system. Nonwoven geotextile may be folded over
the edge of the system until the site is stabilized.
To minimize maintenance and prevent siltation
of the system, pretreatment devices are strongly
recommended.
• Aggregate, if used for storage, should be clean,
durable and contain a high percentage of void space
(typically 40 percent).
• Perforated pipes, if used to distribute runoff to/
from the system, should connect structures (such as
cleanouts and inlet boxes).
• Cleanouts or inlets should be installed at a few
locations within the system at appropriate intervals
to allow access to the piping network and/or storage
media and complete removal of accumulated
sediment.
LID Manual for Michigan – Chapter 7 Page 180