In 1993, the devastating floods that ravaged regions bordering the upper Mississippi River washed millions of tons of topsoil off the land. At first, this monster flood was dubbed a “once-in-a-century” phenomenon. Though it might be called “unusual,” and a flood of that size can be expected to cause extensive damage and considerable erosion, scientists insist that the volume of stormwater carried downstream during this event was in large part aggravated by the loss of water-absorbing wetlands along the upper Mississippi watershed. Had more wetlands been intact along the river, the scope of the devastation would have been mitigated. Without wetlands, such devastating floods will likely be more frequent occurrences.
Once viewed as wasteland, good only for draining and filling, wetlands are now increasingly being appreciated. In the United States, more than 50% of wetlands have been destroyed, with agriculture accounting for 87% of the losses. Today wetland destruction is largely controlled or prohibited by governments–local, state, and federal. In fact, the trend today is toward restoring and, in some cases, constructing wetlands for the many benefits they provide: wastewater and stormwater purification, both point- and nonpoint-source pollution control in urban and agricultural areas, wildlife habitat, sediment filtration, and erosion reduction.
Wetlands, rather unpoetically called “the kidneys of the land,” control erosion through their water-storage capabilities. Wetlands regulate water levels, absorbing and reducing floodwater. Absorption of floodwater by upstream wetlands reduces the flow of floodwater downstream.
Restored or constructed wetlands must be designed, monitored, and managed to maintain the desired water level; ensure the survival of wetland plants, which trap sediments and help neutralize pollutants; and ensure that surface water does not back up onto adjacent land. Well-constructed wetlands that, along with natural and restored wetlands, make up 5-10% of a watershed can provide a 50% reduction in peak flood period compared with watersheds having no or insufficient wetlands.
A constructed wetland is a designed, manmade structure consisting of saturated substrates, submergent and emergent plants, wildlife, and water levels that simulate those of natural wetlands. In agricultural areas, wetlands constructed to reduce downstream flooding must be used in conjunction with ecologically sound agricultural practices, such as no-till cultivation, to achieve maximum benefit.
Nationwide, wetlands are being constructed for their water-quality value. Wetlands are excellent filters of pollutants in wastewater, stormwater, and runoff. Research has shown that properly designed and maintained wetlands effectively remove 90-100% of agricultural nonpoint-source pollutants, including suspended solids, phosphorus, nitrogen, and other pollutants. The ability of many aquatic plants and microorganisms to uptake and transform certain pollutants and to utilize nutrients can reduce pollutant loadings. Since numerous pollutants wash off land during flood events, construction of a wetland to limit erosion will necessarily also reduce the amount of pollutants, from both point and nonpoint sources, carried in floodwater. Thus, wetlands constructed for erosion control fulfill a dual purpose.
Constructed wetlands, especially, are used for water-quality improvement, including treatment of effluent from wastewater treatment plants or even, in some cases, raw effluent. In fact, many of the same biological and physical processes that occur in wetland are used in conventional wastewater treatment systems. However, the sensitive ecosystems of naturally occurring wetlands are easily altered, and they are less suited for wastewater treatment than are constructed, managed wetlands.
There is no one “right” kind of constructed wetland. Conditions–of hydrology, soil, climate, and so on–are extremely varied from place to place, and these conditions dictate the type of wetland to be constructed, as does the objective for which the wetland is being built. Over the past decade, several different approaches have been used to construct wetlands to stabilize streambanks and reduce the impact of floods. Though construction of a topographically flat wetland that supports plants that trap sediment and hold soil in place (soil bioengineering) is considered the base model for built wetlands, research has yet to determine what specific kind of wetland is best for downstream flood mitigation. Certain design characteristics, however, have proven effective in wetland constructed for flood attenuation.
Placement. Construction of several small freshwater wetlands on the upper reaches of a watershed is more effective in flood attenuation downstream than construction of a single, large wetland in the watershed’s lower reaches. Numerous upper-reach wetlands retain more water and lower the volume and the velocity of floodwaters. The construction of many smaller wetlands upstream is also the best approach to flood and erosion control on land with a high topographical gradient. The multiple-upstream-wetlands approach is also generally considered preferable for improving water quality. Upstream wetlands are better situated to trap and neutralize nonpoint-source pollutants from agricultural lands, which are often sited along the upper reaches of a watershed.
If a downstream wetland is required or planned, one large wetland is generally best. In fact, the farther downstream the wetland is located, the larger it needs to be to ameliorate floods effectively. Large, lower-reach wetlands–which in the past occurred naturally at the mouths of large rivers such as the Ohio, Mississippi, and Missouri–are ideal as wildlife habitat and for above-normal flood events. Large wetlands are also more hydrologically stable, as small upstream wetlands might even dry out during some periods and thus be harder to maintain.
Size. The larger the watershed, the larger or more numerous the constructed wetlands must be to control floodwaters and erosion. Downstream flow rates in medium-size streams during peak and flood flows were found to decrease between 3.7% and 1.4% for every 1% increase in wetland area in the watershed. Another study showed that a watershed containing 4-5% wetlands realized a 50% reduction in peak flood period, compared with watersheds of the same size that lacked wetlands.
Thus, it is clear that the size of a constructed wetland depends on the size of the watershed. Yet a watershed containing only 10% wetlands offers effective flood and erosion control, as well as water-quality benefits such as removal and retention of sediment and nutrients.
Buffers. A buffer is a swath of vegetation sited between the wetland and the adjacent land. As a transition zone between a stream and the land, wetlands must alleviate the stress each ecosystem has on the other by “responding” to extremes in each. Wetlands themselves are also vulnerable to stress from both ecosystem types. Buffer zones are areas of native vegetation planted to limit stress on the wetland and the adjacent ecosystems. Construction of a buffer zone for erosion control should include a permanent stand of woody plants, including mature trees, in a strip between 30 and 200 ft. wide immediately adjacent to the stream, followed by a 30-ft.-wide grass buffer strip. The few studies that have been done show that this configuration effectively removes suspended solids and other pollutants in runoff and significantly increases the flood-retention capacity of the wetland.
The type of vegetation used in constructed wetlands varies depending on the region and the site. In all cases, native vegetation should be used. The types of native vegetation planted as a buffer, however, also vary according to the function they must perform. For example, wetlands constructed in agricultural areas require native plants that best neutralize agricultural chemicals; wetlands in more urban areas need plants that improve water quality compromised by stormwater and combined sewer overflows.
Soil Bioengineering. Once the site and size have been determined, plant selection and installation is the single most important aspect of an effective constructed wetland. Soil bioengineering refers to the selection and installation of living plants in a constructed or restored wetland. These plants comprise the main structural component of the wetland and are the most important factor in controlling land instability and reducing erosion and sedimentation. In all soil bioengineering, native plants suited to the climate and location of the wetland are used. Such plants, including shrubs and trees, may be collected from around the site, if they are available. Likewise, seeds or live plants may be purchased for planting.
Setbacks. Setbacks are restrictions on construction of standing structures along streambanks. Most states set standards for setbacks, especially for “highly erodible” areas (where erosion is greater than 1 ft./yr.). Before wetland construction, it is vital that the legal setback requirements for the area are known. On occasion, setback standards may be modified to permit construction of an effective wetland for erosion control. Once the wetland is constructed, minimum setback requirements must be maintained. Establishing prudent setback standards in the vicinity of a wetland is one of the best and most immediate ways of realizing reductions in nonpoint-source pollution.
Examples of Constructed Wetlands
Wisconsin. Wetland construction need not be a long and arduous process. Duffy’s Marsh, in Marquette County, WI, is a 1,732-ac. wetland consisting of 1,000 ac. of open water and 732 ac. of grassy wetland upland. To create the wetland on the former farm field, a network of ditches that previously had been used to drain the cropland was plugged. Embankments were constructed from native soil, and a single rock spillway was built as a water outlet. The entire project took one month. The restored wetland can hold 55 million ft.3 of water, which has translated into significant reductions in flood, runoff, and erosion problems downstream.
Indiana. In the Muscatatuck River Basin in Indiana, a landowner built a 49-ac. wetland on his farm. The plan called for the construction of a short, 2-ft.-tall, 600-ft.-long dike containing a water-control structure to permit management of water levels. The water in half of the wetland has a depth ranging from 6 to 12 in. or, in some areas, to 2 ft., underlain by saturated soil. The other half of the wetland, about 24 ac., is a woodland buffer zone. The construction/restoration project was completed in three days.
Washington. Severe flood events led to the construction of meanders along the Asotin Creek in Washington State in 1997. Erosion from repeated flooding on one site, called the J-bar-S, led to floodwaters that were up to 10 times the normal height, and when floodwaters collapsed huge quantities of rock and gravel on the right side of the south fork’s stream corridor, the entire channel was redirected more than 100 ft. This led to the erosion of thousands of tons of topsoil on adjacent property. An Asotin County Conservation District team devised a plan to construct a meander and to restore the natural flood-absorbing capabilities of the floodplain. Woody debris was used to build numerous pools as salmon habitat, but the pools also were designed to follow and enhance the scour pattern unique to the Asotin Creek in this location. The construction project took two weeks and cost about $17,000.
New Mexico. Reduction of flood peaks was the primary goal of wetland restoration/construction along Bluewater Creek in New Mexico. Here, too, natural meanders were re-created, wetlands were constructed, and large buffer zones were installed using native plants. The reestablished meander successfully lengthened the stream channel, reduced slope erosion, and decreased the gradient along a critical site. Limitations on cattle grazing and the closure and reconstruction of roads crossing valuable wet meadows were also part of the plan and contributed to its success.
North Dakota. Construction and restoration of prairie potholes, through the Conservation Reserve Program, has significantly reduced soil erosion from highly erodible farmland in North Dakota. Sedimentation rates have been cut 50% by the existing restored or constructed wetlands. When the program is extended to cover wetland restoration in all of the state’s highly erodible land districts, experts say that between 9 and 18 million tons of topsoil will be saved yearly.
Mississippi. A 56-ac., three-tier system of constructed wetlands was built in West Jackson County to treat mainly suburban wastewater, which normally flows at a rate of 1.6 million gal./day (mgd), but which increases to 2.6 mgd during flood events. The wetlands were constructed to be able to handle and treat 2.6 mgd and to eliminate treatment bypass during floods. The project involved construction of three slightly sloping “cells,” which contain and are bordered by native plant buffers and receive influent from a wastewater pretreatment plant via pipes. The effluent flows through the cells for up to 12 days, after which it is pure enough to be sprayed on croplands bordering the wetland. Naturally occurring bacteria and fungi in the wetland sediments at the bottom of the three constructed cells act on the wastewater to purify it. Plant stems, too, take in and neutralize organic compounds and nutrients from the wastewater that would otherwise run into and pollute (nutrify) adjacent streams. Though originally planted with cattails and bulrushes, since construction the site has been colonized by at least 43 other native wetland plants, which contribute to the wastewater purification process. The reductions in wastewater contaminants realized through this system, largely the result of the cleansing action of wetland plants and other organisms, as well as aeration, are summarized in Table 1. The entire wastewater treatment system is not only extremely effective, it requires little maintenance and has very low operational and energy costs.
| Table 1. Water-Quality Improvements Achieved With a Constructed Wetland (West Jackson County, MS) |
||||||
| Month/Year |
BOD (mg/l) |
Total Suspended Solids (mg/l) |
Nitrogen (mg/l) |
|||
|
IN |
OUT |
IN |
OUT |
IN |
OUT (as NH3) |
|
| October 1991 |
27 |
4 |
35 |
5 |
14 |
3 |
| November 1991 |
46 |
3 |
36 |
4 |
13 |
4 |
| December 1991 |
39 |
4 |
29 |
7 |
7 |
1.3 |
| February 1992 |
19 |
5 |
12 |
4 |
14 |
1.6 |
| April 1992 |
28 |
4 |
18 |
4 |
12 |
1.2 |
| May 1992 |
24 |
4.5 |
31 |
6 |
7 |
0.05 |
| BOD= biochemical oxygen demand | ||||||
| NH3= ammonia | ||||||
| Source: Adapted from EPA, www.epa.gov/owow/wetlands/construc/wetjack/21.perfor.html | ||||||
There are many approaches to and methods for constructing wetlands. As mentioned before, no one method is right for all sites and conditions. Therefore, clear objectives must be set out before a wetland is planned or constructed. Once the objectives are clear, research can begin to determine the hydrological and soil conditions on the site, the type and size of the wetland needed, and the best and most cost-effective method of construction. In addition, the types of native vegetation–perhaps the most crucial component of any wetland–to be planted to best serve the objective, whether it’s erosion control or wastewater treatment, should also be determined. Local or other government expertise should be solicited to determine these factors. Finally, once the wetland has been built, it is imperative, of course, that its integrity and continued usefulness be monitored and that it be maintained to fulfill its function.
The flow chart gives an overview of the processes and steps involved in wetland construction.
Constructed-wetland costs vary considerably, depending on the site, the size of the wetland, and the objective to be accomplished. For example, beaches and coastal wetlands have recently been constructed and restored around Galveston Bay in Texas. Coastal erosion in the area was destroying the beaches and marshes around Galveston Island, destroying habitat for wildlife and lowering property values. And as the wetlands disappeared, wastewater pollutants built up in coastal waters. About 1,500 ac. of tidal wetlands and marshes will be built or restored. Total cost: about $1 million.
Wetlands constructed to eliminate streambank erosion in Allegheny County in North Carolina, totaling 56 sites covering 456 m, cost a little more than $18,000–or about $40/m of restored streambank. These costs include construction, bank stabilization, livestock crossings, fencing, and labor.
The Galveston wetland was funded primarily by the US Fish and Wildlife Service and the Texas Parks and Wildlife Department, as well as with a grant from the National Coastal Wetlands Conservation Grant Program. The North Carolina construction was paid for jointly by the state Wildlife Resources Commission and the US National Park Service.
EPA reports some admittedly out-of-date costs for different aspects of wetland construction in a handbook published at its nonpoint source (NPS) Web site: www.epa.gov/owow/NPS/MMGI/Chapter6/ch6-4.html. Among the costs listed:
- Bottomland buffer vegetation (1 ac./50 ft. wide): $40-$60/ac., or about $0.08/lin. ft. of streambank (1991)
- Riparian area restoration, including extensive site work (grading, riprap, sediment traps, fencing, and planting): $5.94/ft. or $2,527/ac. (1988)
- Vegetative erosion control, fringe marsh construction (Chesapeake Bay), including plantings, grading, seeding: $20.48/lin. ft. (1988)
Wetland construction, restoration, and preservation have become a priority in the United States in recent decades, during which time more than 100,000 ac. of wetlands have been saved, restored, or built. Every state and numerous federal agencies have programs specifically designed to provide monies to localities wishing to construct or restore wetlands for any reason–erosion control, flood control, pollution reduction, wildlife habitat, or simply fishing and recreation. Funding is often available outright through grants or may be offered via very low-interest loans.
The first place to begin to investigate funding sources for constructed wetlands is state environmental or wildlife agencies. Most of these state agencies have money available for worthwhile wetland projects. All states also participate in the Clean Water State Revolving Funds Program, which offers $3 billion in very low-interest loans for wetland revival. The fund is administered through EPA, and the loans’ attraction is that they require no up-front payments.
EPA also runs the Five-Star Restoration Program and Action Plan for the restoration of river corridors and wetlands. The program brings together interested parties and funding from a variety of sources, which range from local and county governments to nonprofit organizations and commercial interests. Through the program, EPA provides grants averaging about $10,000 for wetland construction and stream restoration, with other monies coming from other partners in the program. The program also offers technical assistance, peer information exchanges, and challenge grants. See the Resources sidebar.
The following are some of the US government and other agency wetland restoration/construction programs:
- Bureau of Land Management: Public Rewards from Public Lands Program
- Department of Agriculture: Wetlands Reserve Program, Restoration Cost-Share Agreements
- National Oceanic and Atmospheric Administration/National Marine Fisheries Service: Community-Based Restoration Program
- North American Wetlands Conservation Council: Grants Program
- Restore America’s Estuaries: Funding for Habitat Restoration Projects
- National Coastal Wetlands Conservation Grant Program
- National Resources Conservation Service
- Wetlands Reserve Program (US Fish and Wildlife Service)
- Partners for Fish and Wildlife (US Fish and Wildlife Service);
- Forest Stewardship Incentive Program (US Forest Service).
