If You Build It
Constructed wetlands provide an ideal solution for dealing with stormwater in increasingly urbanized environments
- By Scott D. Wallace
- Apr 01, 2006
Stormwater managers around the country are challenged by growing regulatory requirements in the face of increasingly urbanized land uses. As cities continue to grow, more and more areas are covered with roads, buildings, parking lots, and other impervious surfaces. Instead of storing and slowly releasing water, these impervious surfaces quickly shed rainfall. At the same time, contaminants on these impervious surfaces, such as salt, oils, and sediments, are picked up and carried away in the runoff. The result is both an increase in runoff volume and a decrease in water quality, contributing to the decline of urban and suburban streams throughout the United States.
This threat to our streams and rivers has lead to intense interest in stormwater best management practices (BMPs). Because stormwater managers often have very limited areas in which to install BMPs, there is a push toward systems that can provide as much multiple-use benefit as possible.
Why Constructed Wetlands?
Alterations of drainage patterns within watersheds often require the creation of new, engineered wetland systems. These constructed wetlands can be designed to achieve specific project goals. Wetlands combine the goals of water storage and release, water quality improvement, wildlife habitat, and community green space. By combining these attributes, wetlands offer many opportunities for multiple-benefit stormwater projects.
Located within depressed areas in the landscape, wetlands are natural accumulation points for stormwater runoff. After storm events, wetlands fill with stormwater runoff, which is gradually released from the wetland basin. Water stored in the wetland can be released by overland flow to surface waters, or discharged to ground water through infiltration, depending on the specifics of the project.
Managing Water in Wetlands
The key benefits provided by water storage in wetlands are volume and time. Every cubic foot of water that is temporarily stored in a wetland is a cubic foot of water subtracted from damaging peak floods. Processes that treat stormwater and reduce contaminants require time to operate. The storage and gradual release of runoff from wetlands provides the time needed for water treatment.
With knowledge of engineering hydraulics and plant hydrology, designers can create wetlands that store and release water in a manner that mimics the hydroperiod of natural wetlands. Wetland plants have developed the ability to transport oxygen from the leaves, through the plant stems, and into the root system. This oxygen transport capacity allows the plant to survive in waterlogged soils (which do not contain oxygen). However, individual plant species vary widely in this regard. Plants with a high degree of oxygen transfer can tolerate permanently flooded soils. Other plants may tolerate flooding for only a few days, or not at all. The U.S. Army Corps of Engineers has developed a classification system for this flood tolerance that ranges from Obligate (plants occur almost always (greater than 99 percent) in wetlands) to Upland (plants almost never (less than 1 percent) occur in wetlands).1 The U.S. Fish and Wildlife Service ( www.fws.gov/nwi/bha/) has classified more than 6,700 plant species according to their wetland tolerance.
In addition to this classification system, there are handbooks available to assist in the plant selection process.2 Armed with this knowledge, designers can determine the acceptable "bounce" of the wetland (how much and how long a plant community can be flooded without adverse impacts). A typical bounce target for design purposes is less than 2 feet of water level increase from the 10-year, 24-hour storm event. Outlet weirs for the stormwater wetland can then be designed to produce bounce fluctuations that are within the acceptable range of the plant community.
Wetland Treatment Processes
Wetland systems remove contaminants through a variety of physical, chemical, and biological treatment mechanisms.3 Treatment mechanisms that are particularly important for stormwater management include settling, precipitation, plant uptake, and microbial degradation.
In the process of storing water, wetlands reduce flow velocity through the dense stem networks of aquatic plants. Reduction in flow velocity allows for settling, interception, and filtration of sediment particles. Since nutrients, metals, and organic matter are absorbed into sediment particles, substantial reductions of these pollutants occur as well.
These systems act as "microbial hotels," with the microbes growing on plants, plant detritus, and sediment particles (the biofilm) performing as powerful treatment drivers for organic matter and nitrogen compounds. The wide range of oxidation/reduction (redox) potentials within the wetland environment creates conditions conducive to the precipitation of many metals, including iron, copper, and nickel.
Wetlands and Wildlife Habitat
Since wetlands combine terrestrial and aquatic habitats, they offer unique benefits for hundreds of species of wildlife. These range from breeding habitat for amphibians to "stop-over" feeding and resting areas for migratory waterfowl. The decline of many animal species can be directly linked to the ongoing loss of wetland habitat within the United States. Despite an official policy of "no net loss" of wetlands, the United States continues to lose wetlands, with corresponding declines in many species (especially migratory birds) and associated increases in flood damages.
Maintaining a healthy and diverse community of plants, insects, fish, birds and other species is a key ingredient is controlling nuisance species like mosquitoes. Well-designed wetland systems include a variety of habitat zones, ranging from open water to emergent plants to upland buffer areas. Wetland areas that are greater than 3-feet deep will generally not support emergent vegetation like cattails and bulrushes. These "deep zones" will instead be open-water areas that can support submerged aquatic plants and are attractive habitats for fish, waterfowl, and animals such as muskrats. Generally speaking, the greatest habitat diversity of a wetland system will be achieved with a 50/50 mix of open water and emergent plants.
Wetlands and People
Wetlands can provide "green islands" in an otherwise urban landscape. By combining zones of different water depth (with associated plant communities), a diverse range of habitats can be combined in a relatively small area. This creates a broad range of wildlife viewing opportunities, especially if visitor access through boardwalks or trails is incorporated into the project. This range of access can include viewing blinds for bird watching, elevated boardwalks for access into the wetland, fishing docks in deep-water areas, and associated trails through upland areas adjacent to the wetland.
Planning for visitor use requires an understanding of the type of wildlife that will use the wetland, the anticipated degree of access to the wetland, and the programming or educational goals associated with the project. For instance, wetlands that support populations of alligators or other potentially dangerous species will require boardwalks that are located at least 3 feet above the water level with a secure railing system. Educational programs may have their own specific needs, ranging from signage to a good location to gather visiting students. Fishing docks can be constructed over deep-water areas that provide good fish habitat (old pipes or brush piles can be used to increase fish habitat in these areas). The key is to identify the project goals for visitor use up-front and then design the needed infrastructure into the project to support these goals.
Wetlands have a lot to offer as a stormwater BMP. Through the storage and gradual release of water, they can reduce flood damage to downstream properties. Within the wetland ecosystem, a variety of processes naturally occur that treats runoff and reduces contaminants. Wetlands provide habitat for key wildlife species; and through the use of creative design, a diverse range of habitat areas can be combined into a small area. Finally, since wetlands are attractive to both people and wildlife, a broad range of visitor use can use the potential trails, boardwalks, and other design features.
The natural "kidneys" of our landscape, the ability of wetlands to store water and gradually release it, reducing flood damage, is well documented. Inside the wetland, complex assemblages of plants and microbes act to purify the water as it flows through the system. Wetlands offer protection from predators for many kinds of fish, amphibians, and reptiles, and they are an important link in the life chain of hundreds of species of migratory birds. Because of their abundant wildlife habitat, they offer the chance for people to "get away" and experience nature, even in urban environments.
- United States Army Corps of Engineers (1987) Wetlands Delineation Manual. Wetlands Research Program Technical Report Y-87-1 (online edition). Washington D.C., U.S. Army Corps of Engineers. ( www.usace.army.mil/inet/functions/cw/cecwo/reg/wlman87.pdf)
- Shaw D., Schmidt R. (2003) Plants for Stormwater Design -- Species Selection for the Upper Midewest. St. Paul, Minnesota, Minnesota Pollution Control Agency.
- Wallace S.D., Knight R.L. (2006) Small-Scale Constructed Wetland Treatment Systems -- Feasibility, Design Criteria, and O&M Requirements. Alexandria, Virginia, Water Environment Research Foundation.
This article originally appeared in the 04/01/2006 issue of Environmental Protection.