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Federal Highway Administration > Publications > Public Roads > Vol. 61· No. 3 > Wetland Mitigation: An Early Effort

Nov/Dec 1997
Vol. 61· No. 3

Wetland Mitigation: An Early Effort

by Cheryl M. Nash and Morgan Cotten

Workers replanting vegetation.

Workers are replanting vegetation at the Greene Valley mitigation site.

In 1986, the Illinois State Toll Highway Authority began construction of the 28-kilometer North-South Tollway (Interstate 355) in the western suburbs of Chicago. Construction of this roadway filled slightly more than 30 hectares of wetlands, and Section 404 of the Clean Water Act required compensatory replacement or mitigation. Personnel from the U.S. Army Corps of Engineers, the Morton Arboretum, the Forest Preserve District of DuPage County, the U.S. Environmental Protection Agency, the Illinois State Toll Highway Authority, and Envirodyne Engineers Inc. (now Consoer Townsend Envirodyne Engineers Inc. [CTE]) formed a Wetland Working Group (WWG) to develop the wetland design criteria needed for this mitigation effort.

This effort resulted in the creation or enhancement of about 51 hectares of wetlands along the East Branch of the DuPage River, and for this project, CTE received the 1995 Honor Award for Environmental Engineering Excellence from the Consulting Engineers Council of Illinois. This project is now a model for similar wetland mitigation in the Midwest.

At the time, researchers knew little about what factors were necessary for successful wetland creation in the Midwest. As a result, CTE designed different wetland types to increase the probability of success and to monitor which factors resulted in successful wetland mitigation. WWG reviewed and approved these wetland types. The project was designed to study wetland restoration and design and to serve as a long-term monitoring project for water quality mitigation of the DuPage River. Another goal of the project was the recreation of a historical ecological community, including plants and animals typically found in Illinois wetlands.

The five chosen mitigation sites -- East Branch DuPage River, Route 53, Roosevelt Road, Hidden Lakes, and Greene Valley -- are adjacent or parallel to the East Branch of the DuPage River. The locations were hydrologically connected to the river and were dependent upon its water levels. CTE developed five types of vegetative habitats for each of the wetland locations and incorporated both passive and active water entry and control into the design. Passive wetlands were to be isostatic with the water levels of the DuPage River. Active wetlands were to have control structures that could be used to control water levels within the wetland.

CTE, with guidance from WWG, began the wetland design work by first studying the hydrology of the chosen sites. They conducted hydraulic modeling of the river channel to determine the average annual water levels, and then used this data to determine all of the wetland vegetation zones. These studies determined the necessary water levels and elevations to produce the different vegetative zones.

In addition to annual water levels, CTE considered hydroperiods, the water level fluctuations over time, to be a critical design factor. Although much data was available for large flood flows in DuPage County, data for smaller, more normal flood flows, which are crucial to wetland hydroperiods, was not available. CTE was able to determine the hydroperiods for the mitigation sites by using three methods of analysis: computer modeling, other stage data examination and evaluation, and mitigation field surveys.

CTE used both hydrologic and hydraulic analyses in the computer modeling to predict hydroperiods. They used a hydrologic model (the U.S. Army Corps of Engineers' HEC-1 package) to predict normal flows for the East Branch DuPage River watershed. They also used hydraulic modeling (the U.S. Army Corps of Engineers' HEC-2 model) to predict normal water surface elevations along the river, including the effects of obstructions such as bridges, piers, and culverts. The HEC-2 model required the input of reliable flood stage data, which they obtained from the U.S. Geologic Survey's gage station operated at Lisle, Ill. The water surface profiles generated by the HEC-2 modeling enabled WWG to establish a relationship between different flood elevations in the East Branch of the DuPage River and the resulting water elevations at the five wetland sites.

After CTE completed the computer modeling, they field-checked the generated information for accuracy. They compared flood elevation data generated by the computer models to the Lisle gauge station's water elevation data. The stage data was also used to check the frequency of storm occurrences as predicted by the computer modeling. In addition, CTE conducted stream vegeta- tion field surveys, focusing on the elevation of streambank vegetation. They surveyed reed canarygrass (Phalaris arundinacea), which is not known to grow in standing water, along the streambanks to determine normal stream water elevations. Finally they obtained soil borings to determine the groundwater elevations at each wetland site.

Each analysis produced a set of flow elevations from which CTE could choose design water levels. A comparison of the computer modeling data and the gauge station data revealed that, in general, these two data sources were in accordance with each other. Comparison of modeling/gauge station information and the surveyed reed canarygrass elevations revealed, however, that the canarygrass elevations were variable. Reed canarygrass elevations varied from the model projected elevations by anywhere from one foot below the projected one-year storm elevation up to the projected two-year storm elevation.

This variability resulted in WWG choosing the lower water elevation as a design criteria to ensure that an acceptable amount of annual flooding occurred to support healthy plant growth and to provide periodic flushing.

When the hydrologic studies on the five wetland sites were completed, CTE concentrated on the design of the wetland habitats. WWG's goal was to create both high-quality wildlife habitat and high-quality plant communities. They designed the wetlands to provide a good growth of submerged, emergent, and mudflat vegetation; to create a scattered pattern of plant communities; to provide good interspersion of water areas with emergent vegetation to promote the habitat necessary for aquatic insect production and suitable nesting for birds; to create native wet prairie and sedge meadow to provide a vegetative fuel matrix that could be easily burned; and to provide scattered, open water areas that would be wet year round. Based on these characteristics, WWG developed the detailed designs so that the newly constructed wetlands would have the proper hydro-period, contours, elevations, and soils. They replicated natural wetlands to establish the desired wetland plant communities.

WWG studied four types of wetland habitats to determine their physical and biological characteristics: wet mesic prairie, wet prairie/sedge meadow, erect emergent habitat, and open water habitat.

Studies revealed that the wet mesic prairie is characterized by grasses and herbaceous plants adapted to temporary inundation during the spring; inundation is generally of a short duration with water depths of up to nearly a half meter. Wet mesic prairie habitat is essentially flat.

The wet prairie/ sedge meadow habitat is also flat, but contains sedges and herbaceous plants that adapt to seasonal flooding during both the spring and fall. Inundation is once again generally of a short duration, but the soils remain saturated through most of the year.

The erect emergent habitat is semi-permanently flooded with 150 to 450 millimeters (mm) of standing water throughout the growing season. Slopes in this hahitat type range from 2 percent to 5 percent. Sedges, arrowheads, and bulrushes favor this habitat.

The open water habitat is permanently flooded with about a half meter to 2 meters of water. Slopes range from 10 percent to 20 percent. The habitat may hold random pockets of deeper water, which provide a suitable winter habitat for fish. Pond lilies and pond weeds characterize this habitat.

CTE considered the wet prairie/ sedge meadow to be the most difficult wetland habitat to develop. When WWG designed these wetlands, the lack of seed source for many of these rarer native plants, combined with inexperience in creating such a habitat, made the creation of a high-quality sedge meadow or wet prairie difficult. To improve a diverse and native plant community, WWG removed the top 600 mm of soil from the higher quality wetlands being affected and then stockpiled this soil to use during the construction of the new wetlands. WWG hoped that the soil would contain a high-quality seed bank for native wetland plant species that would become established at the mitigation sites, enhance diversity, and preserve the area's native character, which otherwise would have been lost.

Applied Ecological Services Inc. began planting wetlands in spring 1988 through a variety of means. They conducted plantings one to three times at each wetland, planting thousands of rhizomes, bulbs, and tubers. In late fall of that year, Applied Ecological Services augmented the plantings by hand seeding. In addition to seeding, they introduced wetland substrates and rescued plants from wetlands and prairies that were being destroyed in local communities. They rescued and delivered a total of 85 loads of wetland plants to the mitigation wetlands between January 1990 and April 1990.

Applied Ecological Services used a combination of planting techniques at the mitigation sites. They conducted the initial plantings using modified tractors and delivered the seed using a TRAUX drill. They also hand planted each wetland. On some sites they used an all-terrain vehicle mounted cyclone seeder and drag unit. Those involved in the planting pushed a random mix of individual plants, bulbs, tubers, and rhizomes into the substrates and planted them in a pattern in the design locations of each wetland. They heeled rescued materials into the respective zones using shovels and hand labor, and they planted buffer trees and shrubs with a tree planter and hand labor. In 1988, 1989, and 1990, CTE planted up to 50,000 sedge and Spartina plugs (50 mm by 50 mm) cut from the peat sod of a wetland in an agricultural farm in the mitigation wetlands.

Route 53 mitigation site.Planting of the mitigation wetlands proceeded with minor divergence from the original design over a period of four years. During that time, a variety of management strategies, including prescribed burns, were also undertaken to promote the mitigation effort. Members of WWG made frequent site visits to assess the progress of the project throughout this period.

In late 1991, the Illinois State Toll Highway Authority commissioned a formal study of the status of the mitigation efforts. This study confirmed what WWG had surmised through earlier site visits: the mitigation wetlands were not performing as well as WWG had initially hoped.

Vegetative diversity was not as extensive and vegetative cover was not as inclusive as originally planned. The problems were traced to the following conditions:

  • Initial planting began in the spring of 1988, and in the summer of 1988, the area experienced an extreme drought. Most of the plantings at several sites did not survive the drought. Plantings at other sites were considerably stressed. In addition to the drought conditions, the hydrology of several of the wetland sites did not perform as expected. At the Greene Valley site, debris collected at the inlet structure and vandalism of the stop log structure created rapid water level fluctuations.The Roosevelt Road site also experienced rapid river fluctuations; while at the Hidden Valley site, flow through caused scouring and sediment deposition. Beavers controlled the water level at the Route 53 mitigation site. Problems such as these caused the water elevation of the different vegetation zones to change from the original design elevations, resulting in the plants' failure to thrive.
  • Besides water quantity issues, there were water quality problems. Salt spray from adjacent roadways and nutrient loadings from point and non-point sources severely restricted the types of plants that could become established in the wetlands. Electrical conductivity tests and sodium analyses of the soils at the mitigation sites revealed excessively high salt contents in more than half of the soil samples. The dominance of salt-tolerant species in the areas that tested high for salts corroborated the soluble salts data. These halophytic species had invaded the wetlands and were excluding other, more desirable wetland species. Testing for nitrates and nitrogen also revealed excessive levels. Urban runoff and nearby sewage treatment facilities contributed to the high nutrient loads. These conditions favored the establishment of weeds such as reed canarygrass and cattails.
  • Finally, predation by herbivores was another unexpected problem. Although wetland plantings can experience extensive damage from Canada geese and muskrats, researchers had not anticipated this factor during early mitigation efforts. Those working on the project tried several different herbivore control methods. Eventually, they designed goose exclosure cages to eliminate this problem. These exclosures protected plantings during sensitive growing stages, and they also allowed seed dispersal to surrounding areas once the plants reached the stage of seed and rhizome production. It was the design of this solution that set the standard for the type of herbivore protection that is being used in mitigation wetlands today.

Because of these problems and the failure of the wetland vegetation to flourish, researchers redesigned the Greene Valley site. The failure of this site was linked to extreme water fluctuations, poor water quality, and steep slopes of the wetland banks. CTE designed a preconcept plan that created an additional 4.4 hectares at this site, using the excavated soils of the new wetland to correct water levels and shoreline steepness of the established wetland. This resulted in the creation of 8.8 hectares of wetlands at the Greene Valley site. Christopher B. Burke Engineering Ltd. designed the final engineering plans and oversaw construction of the wetland in 1995. This wetland complex is currently performing as expected and is anticipated to eventually be the highest quality wetland of the five sites.

In summary, this wetland mitigation effort encountered numerous problems in the early years of development. Given these problems, however, some of the sites performed better than expected. Additional plantings, once actual water levels were determined, increased species richness and vegetative abundance in those sites that experienced less success originally. Ultimately, however, those working on the project had to change the performance requirements to more closely agree with what can actually be achieved by such sites.

Most of the problems encountered in this mitigation effort were directly related to regional issues and the urban setting in which these mitigation wetlands were located. These factors were beyond the control of the design team. Salt spray and high nutrient loads associated with urban settings will always restrict the success of wetland restoration programs. Factors like weather and hydrologic alterations from unexpected events will always be factors for which a team cannot design. The effects were simply realized as the experiment began and were dealt with as it matured. WWG considers this wetland mitigation effort to be a success. All of the wetlands are performing according to the revised standards. The U.S. Army Corps of Engineers confirmed that these wetlands fulfill the permit requirements and approved the mitigation; the Corps no longer requires annual monitoring. This project provided a tremendous learning experience. The wetland mitigation project began and continues to serve as an important living experiment on wetland creation in an urban setting.

These created sites will serve the future as a barometer of urban ecological system conditions and will accurately reflect the prevailing land-use behavior and associated water quality concerns within the watershed.


Cheryl M. Nash is an environmental scientist employed by Consoer Townsend Envirodyne Engineers Inc. in Chicago, Ill. She possesses a master's degree in environmental biology from Governors State University and has been working in the field of wetland science for five years. Consoer Townsend Environdyne Engi- neers Inc. is the general consultant for the Illinois State Toll Highway Authority.

Morgan Cotten is a senior project coordinator for the Illinois State Toll Highway Authority. Before joining the Toll Highway Authority four years ago, he worked for 15 years as a design engineer, engineering project manager, and construction project supervisor. He has a bachelor's degree in forestry from Southern Illinois University, Carbondale, and a civil engineering degree from Midwest College of Engineering. He has also earned a master's degree in civil engineering from Illinois Institute of Technology. Cotten is a licenced professional engineer in the states of Illinois and Wisconsin.

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