United States
Environmental Protection
Agency
Watershed Management Unit
Water Division, Region V
Chicago, IL
December 1990
oEPA
Stormwater Control Benefits
of Managed Floodplains and
Wetlands
Introduction
loodplain management and wetland
protection both have objectives that
include the preservation of a
valuable natural resource.
Floodplain management is aimed at
minimizing modifications to streams
reducing flooding hazards, and
protecting the water quality of streams. Wetland
protection shields the hearty and diverse wetland
biological community from destruction through
development and also protects downstre?:
waterbodies from stormwater through flc
attenuation and pollutant removal.
Floodplains are lands that are subject to severe
periodic inundation. This inundation is usually a
result of high water levels, which are caused by
rain events. Wetlands are characterized by
hydrophytic vegetation, hydric soils, and wetland
hydrology. Hydrophytic vegetation refers to plants
that grow in water, soil, or on a substrate that is at
least periodically deficient in oxygen as a result of
excessive water content. Hydric soils are those
soils that develoo anaerobic conditions during the
growing season as a result of saturation, flooding,
or ponding. Wetland ..ydrology is defined as
permanent or periodic inundation, or soil saturatic
to the surface, at least seasonally. There are a
number of factors that can contribute to the
creation and sustenance of a wetland, such as
periodic inundation from rain events, proximity to
the water table, and depressions in the
topography. Wetlands often develop within
floodplains.
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Pollutant and Sediment Removal
he water quality benefits of
protecting floodplains and wetlands
are numerous. Managed floodplains
can provide a buffer between
streams and developments. This
buffer, in the form of vegetated land
cover such as erosion resistant
grasses, can effectively remove
pollutants by the filtering action of grasses,
deposition in low velocity areas, and infiltration into
the soil. Wetlands provide trapping of participate
pollutants through settling, and trapping and
subsequent biological removal of soluble
pollutants, such as nitrate and ortho-phosphorus.
The retention of stormwater afforded by the
storage capacity of wetlands allows for rapid and
fairly extensive trapping of sediments and trace
metals, especially those trace metals that have a
close affinity with suspended sediments, such as
lead. Soluble forms of nutrients are trapped and
ultimately can provide sustenance for plants,
algae, and bacteria that can grow in wetlands.
Managed floodplains and protected wetlands also
provide water quality benefits by reducing potential
land use areas that can generate nonpoint source
pollutant loads. A possible negative ramification of
managed floodplains and wetlands is that they
enhance infiltration, which can result in transport
and interaction of pollutants with ground water.
Erosion Control
anaged floodplains and wetlands
can also provide erosion control for
receiving streams. Vegetated land
cover associated with managed
floodplains can reduce runoff
velocity, which can attenuate peak
runoff discharge, ultimately lowering
peak stream flow and velocity. The pervious land
cover can also allow infiltration of stormwater
runoff into the soil, reducing runoff volume.
Wetlands can attenuate flood flows through
storage capacity, reduction in runoff velocity (flow
paths through wetlands are generally wide and
shallow, with vegetation providing added flow
resistance), and to a lesser degree, by infiltration.
Managed floodplains and wetlands bordering
streams can also provide stream bank stability
through root structure of vegetation that binds to
soil.
Regulatory Authority
loodplain management often takes
the form of provisions to county
zoning ordinances. These provide
the county with consistent criteria
and methods of evaluation for
reducing flooding hazards and
minimizing impacts on streams.
Wetland protection often takes the same
approach as floodplain management, depending
upon national legislation and local support through
provisions to development ordinances and State
codes. Wetland protection does not often have the
same safety and insurance ramifications as
floodplain management, but the environmental
consequences are real and important; wetlands
represent an unparalleled natural resource of
floral, fauna, and insect communities. Wetlands
and floodplains can also provide stormwater relief
to receiving streams.
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Federal, State, and local legislation is currently
aimed at no loss of wetlands. In some jurisdictions,
codes indicate no net loss of wetlands. In other
words, trade-offs can be made between existing
and created wetlands. Criteria for evaluating the
loss of natural existing wetlands and the
acceptability of created wetlands must be carefully
formulated. The ability to "create" wetlands is still a
matter of debate. Other jurisdictions do not allow
any development of wetlands unless practical
alternatives are not available for a necessary
project. In any case, any modification to a wetland
should require a detailed environmental impact
study along with public meetings. Public
perception is extremely important in any project
that involves environmental alteration. By including
the public in the evaluation and decision-making
process, State and local officials can form an
alliance with the public, instead of a perceived
alliance with developers that the public may feel
runs counter to their interests.
References
Schueler, Thomas R., 1987. Controlling Urban Runoff: A
Practical Manual for Planning and Designing Urban
BMPs. Washington Metropolitan Council of
Governments, Washington, D.C.
U.S. Army Corps of Engineers, U.S. Environmental
Protection Agency, U.S. Fish and Wildlife Service,
and U.S.D.A. Soil Conservation Service, 1989.
Federal Manual for Identifying and Delineating
Jurisdictional Wetlands. U.S. Government Printing
Office, Washington, D.C.
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