Wetland Mitigation at DuPage County Complex
To Furnish Stormwater Management and Ecological Benefits
The new DuPage County Courthouse and government complex will be a study in contrasts when it's
completed. On the one hand, it will be a state-of-the-art
judicial facility designed to serve the needs of one of the
nation's fastest growing counties. And yet, bordering
the distinguished building will be a re-created wetland,
engineered to correspond as closely as possible with the
original character of the land.
This feat of intelligent engineering will be made
possible through the collaboration of -Wight & Company of Dowsers Grove, Illinois; the planners of DuPage County, Illinois; and the Kane-DuPage Soil and
Water Conservation District.
Wetland History and Legislation
The preservation of natural wetlands has gained
considerable attention in the last two decades. Wetlands have disappeared at an alarming rate since the
pilgrims came to America, but between 1947 and 1967
alone, the nation lost an estimated 21 to 36 percent of its
wetland area. Under the 1972 Clean Water Act, the
Army Corps of Engineers and the Environmental Protection Agency are charged with protecting this dwindling natural resource with benefits that far exceed
wildlife conservation alone.
Even the U .S. Supreme Court has been involved in
the conservation effort, giving the Army Corps of Engineers broad power to curb development on millions
of acres of marshy land in every state. Since 1975, the
Corps has claimed that the Clean Water Act requires
federal permission to develop any wetlands adjacent to
navigable waters — even wetlands entirely within a
single state.
While the Corps approves the development of
about 500, 000 acres annually, it has halted some
projects and limited the size of others. In 1984, a U.S.
appeals court said that the Corps had overreached its
authority in barring the development of a wetland, but
a unanimous Supreme Court upheld the sweeping
Corps regulations.
DuPage County is located in northeastern Illinois.
Most of Illinois' remaining wetland territory is in this section of the state. A large portion of this wetland area
has been lost to development — a cause for alarm
because of the substantial stormwater detention and
water quality benefits that this type of ecosystem
offers,
Wetland Characteristics and Benefits
The wetlands themselves are characterized by soil
saturated with water and plants adapted to life in this
type of habitat. Few ecosystems can compare to wetlands in the variety of wildlife — especially endangered
species — that they support. By slowing water runoff
and temporarily storing excess water, wetlands also
help to protect communities from flooding. In addition,
they are chemical and biological oxidation basins that
cleanse both surface and groundwater systems.
Wetland as a Stormwater Management Tool
At the DuPage County government center, the new
courthouse will be the fourth large scale building on the
120-acre site. Wight & Company, in collaboration with
HOK, St. Louis, Missouri, designed the new court-
house, now under construction, as well as an addition to
an existing jail planned for groundbreaking in 1992. The
construction of these two structures, in tandem with
two new parking structures, has reduced on-site water
storage, fostering the need for a complex stormwater
management system.
Two retention ponds flanking the new7 courthouse
are an important component of the plan: an existing
pond designed for an additional two feet of storage and
a new detention facility with wetland characteristics.
The larger pond will hold 30.3 acre feet of water,
while the smaller detention basin will have a capacity of
29.4 acre feet. According to Wight & Company vice
president and project director, Ottavio Finaldi, a complex pumping system will bring water to the detention
basins from all areas of the site.
"The stormwater pumping system will be located
underground, running from the north side of the complex to the south," Finaldi explained. "An underdrain
system will be installed around the courthouse. Parking
deck grades have been designed to ensure positive
drainage away from the structure."
February 1991 / Illinois Municipal Review / Page 15
"In the early design stages, observation wells were
established at various points around the site to monitor
water depths," Finaldi continued. "This data was integrated with historic rainfall records to establish the
necessary water storage area as well as pumping rates."
The Kane-DuPage Soil and Water Conservation
District offered several recommendations to planning
engineers. The Kane-DuPage authority pointed out that
stormwater retention facilities should be designed to
retain and slowly release all additional storrmwater run-
off at a maximum release rate of .10 cubic feet per
second per acre.
Soil erosion and sedimentation, the authority
pointed out, are another concern since excessive erosion on construction sites is a contributing source of
water pollution. Eroded soil sediments are capable of
choking storm sewers, reducing the capacity of detention/retention areas, increasing the risk of flooding,
degrading water quality and destroying aquatic ecosystems.
Wight & Company has made provisions for controlling costly erosion including keeping disturbed areas to
a minimum, maintaining vegetative ground cover and
reducing runoff velocities.
DuPage County Government Center Wetland Design
The pervasive need for additional water storage at
the county site furnished planners with an ideal opportunity for incorporating wetland characteristics into the
expansion, an activity typically known as "wetland mitigation." Since wetland shelves will be constructed at
levels below7 the normal pond elevation, no loss of
stornnwater storage capacity will result.
A 1988 soil analysis of the government complex site
revealed prior wetland characteristics. Wetland features are established on the basis of three primary characteristics: presence of a hydric or wet soil, a predominance of hydrophytic or water-tolerant vegetation, and
evidence of seasonal or periodic flooding.
Wetland mitigation is gaining popularity as a tool
for stormwater detention/retention, preferable from
an ecological standpoint. Wetland mitigation has also
been found to be superior for preventing soil erosion.
Performing a Wetland Restoration
To create the new7 wetland, the perimeter of the
retention pond will be shelved in steps between one
tenth of a foot and two feet below water level. Different varieties of wetland plants require varying water
levels to flourish. The shelving system also permits fish
to swim in and amongst the plants and eat mosquito
larvae.
"An important consideration in the particular design
of the wetland shelves is the fact that wetland plants
need water to survive, but they can also be drowned,"
explained Joe Chaplin, Engineering Technician, Department of Environmental Concerns for the DuPage
County Stormwater Division. "Therefore, we have to
be careful about the change in water level in the pond.
For example, if the water level rises for a brief period
because of a storm, they could probably survive. But
over longer periods, they would die."
According to Randy Stowe, a natural resource planning consultant to the county, another critical factor in
wetland mitigation is the soil quality. "You can usually
have some degree of success in re-creating the plant
community, and from an engineering standpoint, it is
possible to restore water quality, but the wetland soils
have a complex profile which involves many layers."
"One alternative the county is considering," Stowe
said, "is to obtain topsoil stripped from an existing
wetland site. The mitigation process, in this case, would
be to carry out rough clay grading and then top dress
the wetland shelves with at least one foot of this organic
wetland topsoil. Often, the numerous seeds, rhizomes
and tubers found in this soil will bring back much of the
wetland on its own."
A period of three to five years will be required to
establish the wetland plant community. In its first year,
virtually no top growth will be apparent, since most of
the early wetland plant activity is in its roots. Not unlike
a prairie ecosystem, wetlands are managed with controlled burning which kills off the more invasive yet
shallow-rooted cattails and canary reeds which tend to
take over a wetland area to the detriment of the other
plant varieties.
Engineers have chosen approximately 10 hardy
plant species to begin the wetland mitigation. A buffer
area of native prairie plants will help to protect the
tenuous plantings. Native tree material is also planned
for the wetland perimeter, chosen for both aesthetic
and stabilizing benefits.
When the new courthouse is completed, the pond
waters will lap at the south side of the building where a
generous terrace will capture a view of both the wetland wildlife and a magnificent fountain to be set in the
center of the pond.
The combination of complex stormwater management techniques and wetland mitigation benefits at the
DuPage government center will make it a model facility from the standpoint of both stormwater and environmental planning. DuPage County planners anticipate
that the project will generate considerable interest as a
prototype for other county governments as well as municipalities and commercial land developers. •
Page 16 / Illinois Municipal Review / February 1991