Wetland
Hydrology arid
Flood Control
Wetlands are
transition zones
between uplands
and deeper
water, unique
ecosystems char-
acterized by their
hydrology, soils
and vegetation.
They function like
natural tubs,
storing flood
waters that over-
flow riverbanks
and surface
water that col-
lects in depres-
sional areas. In
this way, wet-
lands can help
protect adjacent
and downstream
property from
flood damage.
Wetlands: Protecting Life and
^fHil Property from Flooding
The Federal Emergency Management Agency (FEMA) states that
floods are the most common and widespread of all natural dis-
asters—except fire. Most communities in the United States
have experienced some kind offloading. FEMA encourages
the use of wetlands for stormwater detention in lieu of, or in
conjunction with, traditional structural flood control measures.
(Source: FEMA)
How Do Wetlands Help Reduce
Flooding?
The effectiveness of wetlands for flood abatement
may vary, depending on the size of the area, type
and condition of vegetation, slope, location of the
wetland in the flood path and the saturation of wet-
land soils before flooding. A one-acre wetland can
typically store about three-acre feet of water, or one
million gallons. An acre-foot is one acre of land,
about three-quarters the size of a football field, cov-
ered one foot deep in water. Three acre-feet
describes the same area of land covered by three feet
of water. Trees and other wetland vegetation help
slow the speed of flood waters. This action, com-
bined witli water storage, can actually lower flood
heights and reduce the water's destructive potential.
(Source: EPA)
The Wetlands Initiative completed an 18-month
study, "Flood Damage Reduction in the Upper
Mississippi River Basin: An Ecological Means." The
study revealed that restoring the 100-year flood zone
of the Upper Mississippi five-state watershed could
store 39 million acre-feet of floodwater, the volume
that caused the Great Flood of 1993, and save over
$16 billion in projected flood damage costs.
In Minnesota, an additional study by The Wetlands
Initiative noted that flood peaks and damage costs
would be decreased by restoring die natural hydrol-
ogy of the floodplain. The cost of replacing the
flood control function of the 5,000 acres of wet-
lands drained each year in Minnesota alone would
be $1.5 million, compared to the potentially mil-
lions of dollars lost to flooding. Preserving wetlands
in the first place and restoring some of those that
have been drained could help reduce future flood
losses. (Source: The Wetlands Initiative)
Preserving and protecting
coastal wetlands can help
reduce storm damage.
St. Stanislaus was a boy's
Catholic Boarding School
over a hundred years old in
Bay St. Louis, MS. Located
on the beach overlooking
the Gulf, it was destroyed
by Hurricane Katrina. This
picture was taken before
the building disintegrated.
-------
W h
ere
W
t I a
n
Where
Wetlands
are Helping
These studies and
others indicate
that wetlands
may play a part
in flood abate-
ment. The fol-
lowing examples
illustrate how
communities
across the coun-
try are restoring
wetlands in order
to reduce the
threat and costs
of flood damage.
Charles River, Massachusetts
Along the Charles River in Massachusetts, the
U.S. Army Corps of Engineers (the Corps) has
acted to utilize wetlands in preventing flood
damage. It was calculated that loss of all wetlands
in the Charles River watershed would have caused
an average annual flood damage cost of $ 17
million. The Corps concluded that conserving
wetlands was a natural, less expensive solution to
controlling flooding than the construction of
dikes and dams alone, and they proceeded to
acquire 8,103 acres of wetlands in the Charles
River basin for flood protection. (Source: U.S.
Army Corps of Engineers - Charles River Natural
Valley Storage Area)
Horseshoe Park, Colorado
In 1982, an earthen dam on Lawn Lake in Rocky
Mountain National Park collapsed, suddenly
releasing almost 700 acre-feet of water into the
Roaring River. A wall of water 25 to 30 feet high
moving at 9 miles per hour rushed downstream
and entered Fall River at Horseshoe Park. The
Park contained wetlands adjacent to the river, with
meadow grasses, reed and dense willow stands.
Here the flood wave spread across the broad, flat
valley and was slowed by wetland vegetation. The
height of the wall of water was reduced to about
10 feet, and the water spread out over the meadow
to a width of 1,300 feet. The flood was finally
contained by Olympus Dam on Lake Estes, but it
had claimed 4 lives and caused $31 million in
damage. If not for the wetlands and meadows at
Horseshoe Park, the damage would have been
much worse. The height and speed of flood waters
In 1982 these meadows and wetlands at Horseshoe Park in
Colorado were hit by a 25 to 30 foot wall of water. The height
and speed of the flood waters were reduced by the wetland
vegetation, and the damaging flood peak was greatly reduced.
were reduced by the wetland vegetation, and tr
damaging flood peak was greatly reduced. (Sou
Jarrett and Costa 1984)
Grand Kankakee Marsh, Indiana
In 1900, the Kankakee Marsh was one of the
largest, most ecologically diverse wetlands in th
United States. During the 20th century much
the mairsh was drained and converted to
agricultural use. Channelization of the Kankaki
River, which fed the marsh, reduced its length
from 250 to 90 miles. As a result, water qualit
was degraded and flooding increased. An
ambitious project was undertaken to address th
concerns. The project, featuring diverse partne
from all levels of government, private conservat
groups and business, was designed to restore ov
25,000 acres of wetlands. With a grant from th
Nordi America Wetlands Conservation Act anc
donations of cash and land, 3,000 acres of
wetlands have already been restored. Waterfowl
populations have increased, water quality is
improving and flooding has decreased. (Source:
National Park Service, "Floods, Floodplains anc
Folks", 1996, U.S. Fish and Wildlife Service,
Private Lands Office)
Mayview Wetland Project, Pennsylvania
The Pennsylvania Department of Transportatio
(DOT) completed a wetland restoration projec
offset impacts to 32 acres of wetlands that were
filled during the construction of Interstate 279
through Southwestern Pennsylvania and the
Southern Expressway. The site of the wetland
restoration is Mayview, a 65-acre piece of land,
flanking Chartiers Creek, a major stream. The
creek v/as subject to frequent, high velocity
flooding and constructing wetlands there is
helping control these floods. The new wetland;
provide flood storage capacity for 63 million
gallons of water and serve as an outdoor classro
for nearby schools. The Department of
Transportation is seeking funding to restore
additional acreage. (Source: National Park Serv
"Floods, Floodplains and Folks", 1996)
Prairie Wolf Slough, Illinois
The Middle Fork of the North Branch of the
Chicago River flows through an abandoned fari
field in the suburbs. The area was identified as
future location for a trail, part of the North
-------
are
Help ing
Branch of the Chicago River Open Space Plan designed
by the Friends of the Chicago River and Lake County
Stormwater Management Commission, a regional open
space advocacy organization. By restoring wetlands
hydrology, clearing non-native vegetation and planting
wetland, prairie and savanna vegetation, the functions and
values of the wetlands have been restored. Structures used
to drain die area for farming were removed, and a new water
control structure was constructed to decrease sedimentation
of the river. The result was moderation of stormwater flows
which provided the area widi flood protection, as well as
permanent open space and new environmental education
opportunities. (Source: National Park Service, "Floods,
Floodplains and Folks", 1996, Friends of the Chicago River)
Vermillion River, South Dakota
The Vermillion River has always flooded. It has a narrow
channel and flows slowly, making it "flood prone."
Thousands of years ago, this part of South Dakota was
scoured by glaciers that carved out shallow depressions
which remain today and seasonally fill with water. These
"prairie potholes" are intermittent, seasonal wetlands which
dot the landscape. They quickly thaw in spring and provide
habitat for a multitude of migratory birds and other water
fowl.
For hundreds of years, die rain and snow melt in the
watershed were held in these wetlands, and runoff across the
prairie was slowed. As South Dakota became populated,
many prairie potholes were filled to facilitate farming.
While these wetlands are small, they are numerous and can
hold a significant amount of flood water. As more wetlands
were filled, flooding increased.
The Great Flood of 1993 was devastating to the area. To
combat future flooding, structural flood controls were put
in place, but they were not sufficient. In response to this
problem, the National Park Service and the Federal
Emergency Management Agency formed a partnership with
the South Dakota Division of Emergency Management and
Turner-Lincoln-Clay Counties Water Project District.
Working together, this coalition assessed the area and
condition of the remaining network of podioles. They
developed a plan to protect the remaining wetlands and
restored some of those that had been filled.
(Source: National Park Service, "Floods, Floodplains and
Folks", 1996, The Vermillion River: Managing the
Watershed to Reduce Flooding, Federal Emergency
Management Agency)
The Special Case of Coastal
Wetlands in many locatiot
protection. Nowhere is this function more important than along
coastal areas. Coastal areas are vulnerable to hurricanes and
other powerful storms, and the fiat coastal terrain means that
land and property can be exposed to the full power of these
storms. Preserving and reconstructing coastal marshes can help
reduce storm damage. Coastal wedands serve as storm surge
protectors when hurricanes or tropical storms come ashore, in
the Gulf coast area, barrier islands, shoals, marshes, forested
wedands and odier features of the coastal landscape can provide
a significant and potentially sustainable buffer from wind wave
action and storm surge generated by tropical storms and hurri-
canes. (Source: Working Group for Post-Hurricane Planning for
the Louisiana Coast)
••.^ NO WETLANDS
TIME'
This diagram indicates that wetlands reduce peak
stormwater flows. (Source: Kusler 1983)
More Wetlands Mean Less Flooding
These examples illustrate how protecting and restoring wetlands
can reduce the destructive potential of flooding. Wedand
restoration and preservation is an important component of a
comprehensive flood protection strategy. EPA, working with
other federal agency partners, is a resource for state and local
decision-makers, providing tools and limited funding for devel-
opment of state wetland programs. Preserving wedands, along
with other flood control measures, can offer a degree of protec-
tion against flooding that is often more effective and costs less
than a system of traditional dikes and levees. If more communi-
ties protect existing wedands and increase the quantity of wet-
lands through restoration projects, we will be better protected
against the consequences of floods.
-------
EPA843-F-06-001
Office of Water
May 2006
On the Internet:
Charles River Natural Valley Storage Area www.nae.usace.arrny.mil/recreati/crn/crnhome.htm
Federal Emergency Management Agency www.fema.gov
Friends of the Chicago River www.chicagoriver.org
National Park Service www.nps.gov
The Wetlands Initiative www.wetlands-initiative.org
U.S. Army Corps of Engineers www.usace.army.mil
In Print:
Floods, Floodplains and Folks. 1996. National Park Service. Rivers, Trails and Conservation Assistance Program.
Flood Damage Reduction in the Upper Mississippi River Basin—An Ecological Alternative. 2004. Donald L. Hey, et al.
The Wetlands Initiative, Chicago, IL. Available at www.wetlands-initiative.org
Jarrett, R.D., and J.E.Costa. 1984. Hydrology, geomorphology, and dam break modeling of the July 15, 1982 Lawn
Lake Dam and Cascade Lake Dam Failures, Larimer County, Colorado: U.S. Geological Survey Professional Paper 1369.
Johnson, Rex R. 1997. The Vermillion River: Managing the Watershed to Reduce Flooding. Clay County Conservation
District, Vermillion, SD.
Additional Wetland Resources
For additional information, visit the U.S. EPA's website (www.epa.gov/owow/wetlands/), call the toll-free
Wetlands Helpline at 1-800-832-7828 or refer to the sources below.
On the Internet:
"A New Framework for Planning the Future of Coastal Louisiana
after the Hurricanes of 2005." January 26, 2006. Working Group
for Post-Hurricane Planning for the Louisiana Coast www.umces.edu/la-restore
Association of State Floodplain Managers www.floods.org
Association of State Wetland Managers www.aswm.org
"Reinventing a Flood Control Strategy." 1994. Donald L. Hey and
Nancy S. Philippi. The Wetlands Initiative, Chicago, IL www.wetlands-initiative.org
Society of Wetland Scientists www.sws.org
U.S. National Weather Service www.nws.noaa.gov
Wetlands Status and Trends http://wetlandsfws.er.usgs.gov
In Print:
Bradley, A.A., K.W. Potter, T. Price, P. J. Cooper, J. Steflfen and D. Francz. 1994. Dahl, T.E. 1990. "Wetland losses
in the United States: 1780's to 1980s." Washington, DC. U.S. Department of Interior.
"Flood analysis in DuPage County using HSPF," Proceedings of the Transportation Research Board (TRB) Annual
Meeting, Washington, DC.
Protecting Floodplain Resources, a Guidebook for Communities. June 1996. The Federal Interagency Floodplain
Management Task Force.
Shabman, L. 1994. "Responding to the 1993 Flood: The Restoration Option," Water Resources Update, University
Council on Water Resources, 95, 26-30.
U.S. National Weather Service, 1993, "Update on Midwestern floods, heat and drought in the East: Special
Climate Summary," 93/2, Climate Analysis Center.
------- |