Janet Aird 2015-10-12 15:12:22
Many people think of wetlands as smelly, mosquito-ridden nuisances, but healthy wetlands provide habitat for birds, fish, amphibians, bats, and other insects that live on mosquitoes. Wetlands are home to more than one-third of the United States’ threatened and endangered species, according to EPA. They buffer storm surges in coastal areas. They protect the edges of oceans, lakes, rivers, bays, and the smallest streams. Their vegetation holds soil in place, absorbs the energy of waves, and breaks up the flow of stream or river currents. They also trap snowmelt and stormwater runoff. Sediment in the water, and the nutrients and pollutants attached to the sediment, settle in the bottom, and cleaner water flows out. In fact, wetlands are so valuable that states are restoring them and mitigating for their loss by constructing new ones. “Constructed wetlands can be a lot of things and take a lot of different forms,” says David Whitney, a civil engineer and owner of EcoSolutions LLC in Westford, VT. “What they all have in common is that they all incorporate plants and they are all saturated with water for extended periods of time.” Many constructed wetlands are used as stormwater BMPs. They also might mitigate for land reclamation or the loss of protected habitats to development. While natural wetlands have a wide diversity of plants and animals that have evolved over long periods of time, wetlands constructed as stormwater BMPs have less diversity because it is simply not possible for designers to completely recreate natural wetlands with native plants, stone, and other materials. The plants are critical because they have adapted to the wetlands and filter stormwater well: “the workhorses of the plant world,” Whitney calls them. “A constructed wetland made for stormwater treatment isn’t necessarily habitat restoration. It’s intended to be a repository for sediment, where in between storm events the contaminants associated with that sediment can be biologically broken down. We’re putting it right in line with a pollutant load. We want plants that will clarify the water.” When the sediment settles, so do the pollutants attached to it. They may include industrial and municipal waste, oil and grease, decaying plant and animal matter, and pesticides and fertilizer. Organic matter attached to the sediment is broken down by bacteria that consume the oxygen in the water, which results in an unhealthy aquatic environment, he says. However, well-chosen plants move oxygen from the air to the plants’ roots. The roots benefit from the oxygen, and bacteria, especially next to and in the roots, break down the pollutants into forms that are useful to the rest of the plant. “If you can create this well-balanced environment, it’s very robust,” says Whitney. “Plants and animals adapt very well. They can survive under a very wide range of conditions.” The first consideration in planning a well-functioning wetland is the site. Some questions to ask, notes Whitney, include: How much space is there? Is the wetland a retrofit or large tract of land? What is the watershed like? Does it have a high pollutant load or is runoff coming off a metal factory roof, which has hardly any pollution? Another question: Is there much of a slope? Wetlands on slopes require berms on the downhill side to hold water back and grading of the slope out to meet the existing soil. This can result in large footprints, and the steeper the existing slope, the larger the footprint. “However, wetlands typically have a smaller footprint that traditional stormwater ponds, and therefore they are preferable on steep-slope terrain such as ski resorts,” says Whitney. The second consideration is the design. A well-designed wetland requires little maintenance. Stormwater flows through a forebay before it reaches the larger pool of the wetland. The forebay usually is a small pool roughly 10% of the volume of the wetland, although in tight spaces it may consist of one or more inlet structures. The water should be conveyed to the forebay in a way that minimizes the potential for erosion. The forebay helps settle out coarse sediment particles, which reduces the frequency of dredging needed in the larger pool. Because much of the dredging takes place in the forebay, it should have access for maintenance. The large pool usually has two depths, typically about 18 inches and 3 feet, to allow more diverse vegetation to grow. A large amount of land may be needed to store the required volume of water. Where land is limited, there may be additional storage above or below ground. The constructed wetland should have a drain to draw down the water level during maintenance. There are two general designs for the large pool. In a shallow, or free-water, constructed wetland, the pool is essentially a marsh. In a gravel-based or subsurface flow wetland, runoff flows through a filter of gravel or sand and wetland plants. The water level is just below the surface of the media. The flow can be directed horizontally across the surface, or vertically from top to bottom or from bottom to top. “You don’t see the wetland,” says Whitney of the subsurface flow variety. “It’s within the stone media. These wetlands do a great job of trapping sediment.” There may or may not be a small pool at the outlet of the wetland, but either way, it should be designed to prevent scour. Pipes should be larger than 3 inches in diameter and be either reverse-slope pipes or weir outlets with a trash rack to prevent clogging. In areas with large storm events, an emergency spillway conveys excess stormwater directly downstream. Proper vegetation beautifies the wetland as well as removing pollutants. Designers should use native wetland plants whenever possible, in the form of dormant rhizomes, live potted plants, and bare rootstock. Plant lists are available for specific regions through wetland nurseries, university extension services, and conservation districts. The plantings may be supplemented with soil from a natural wetland (often referred to as “wetland mulch”), which contains seeds and other organisms from the wetland. The mulch can help enhance plant diversity, but it also may contain seeds from invasive plants. Constructed wetlands need regular maintenance. According to the Maryland Department of the Environment’s stormwater design manual of 2000, maintenance includes: • Mowing slopes and inspecting them for damage • Inspecting the inlet and outlet structures for damage, repairing undercut or eroded areas, and removing sediment • Inspecting the main pool and removing sediment • Harvesting wetland plants that have been “choked” by sediment. If vegetation covers less than 50% of the surface area of the wetland after the second growing season, it should be supplemented with additional plants. • Inspecting the wetland for invasive vegetation and removing it where possible The following are two examples of constructed wetlands. The first, the US 36 Habitat Mitigation Project, was to mitigate for an especially sensitive habitat in Colorado. The second, the Farrell Park Wetland in South Burlington in Vermont, was a water-quality-improvement project to treat stormwater runoff that previously had flowed untreated into an adjacent stream. US 36 Habitat Mitigation Project Several wetland drainages, including a protected and critical willow habitat in the riparian zone of South Boulder Creek, are home to two federally protected species: the tiny Preble’s meadow jumping mouse and the beautiful but elusive Ute ladies’ tresses orchid, says Patrick Hickey, project manager and wetland and wildlife specialist with the Colorado Department of Transportation (CDOT). This type of habitat is generally threatened by development throughout Boulder and the Front Range of Colorado. In 2004, a project to widen US Highway 36, which connects Boulder and Denver, threatened one of these wetlands. CDOT began consultations with the US Fish and Wildlife Service (USFWS) to minimize impacts and mitigate for the unavoidable ones. CDOT, the Federal Highway Administration (FHA), USFWS, and the city of Boulder collaborated on the project, which took place on a parcel known as the Granite Property, a degraded 24-acre parcel of land just about a mile downstream of the protected wetland. The Granite Property had suffered from extensive grazing and dumping and had an abundance of noxious weeds. Weeds are designated as noxious when they are injurious to natural habitats, ecosystems, humans, livestock, or agricultural and horticultural crops. “The project was a good marriage of interests,” says Hickey. “The city wanted to protect the site from being developed. We met our environmental obligations by purchasing and restoring the parcel for mitigation, which will be managed as a nature preserve by the city.” The project took place in the spring of 2014. The new habitat includes a considerably larger area than was originally affected by the highway widening, because the USFWS requires a larger amount of land to mitigate for the land that is lost. The FHA provided most of the funding. CDOT created, restored, or enhanced 15.9 acres of wetlands and restored 8.4 acres of upland buffer habitat, for a total of 24.3 acres. The main goal of the project was to recreate a matrix of habitat types on the Granite Property to suit the two threatened species. This included willow habitat, a relatively dense combination of grasses, forbs (wildflowers), and shrubs. The secondary goal was to transplant some of the orchids, along with large segments of the wetland sod associated with them, says Hickey. There was no requirement to relocate the Preble’s meadow jumping mouse, which uses several of the drainages across some 3 acres within the highway project limits, including one that was directly adjacent to the constructed wetland. The year before the project began, the threatened area was mowed so the mouse would hibernate elsewhere during the winter. “We hoped if we created a better habitat on the Granite site, then the adjacent mouse population would be able to expand its local range. The assumption was, ‘If you build it, they will come,’” he says. “We intend to monitor the site to determine if that is true.” Jacobs Engineering, based in Pasadena, CA, designed the wetland and CDOT provided support. Both Jacobs and CDOT provided construction oversight. Rocky Mountain Excavation of Castle Rock, CO, was the prime contractor and did all the major earthwork. Generally, the water table in these wetland areas ranges from zero to 24 inches below the surface during the growing season. Part of the Granite Property was already wetland. Rocky Mountain excavated another half of the land to create a larger one. “The lower surface elevation was necessary to connect the wetland habitat to the supporting water table,” explains Hickey. On the driest land, Rocky Mountain excavated some 6 inches, primarily to remove the weed-infested soil. Crews excavated other areas to a depth of 3 feet to allow more diverse wetland vegetation to grow. No groundwater was exposed. Western States Reclamation, which has offices in Frederick and Loma, CO, and in Kayenta, AZ, handled all the vegetative and erosion control components of the project except for the mowing. Crews used heavy equipment to drive pilot holes into the cobbly gravel soil for the coyote willow (Salix exigua) stakes and planted more than 9,700 stakes in specific areas of some 6.4 acres of the wetland. They also planted 6,000 riparian shrubs, including chokecherry (Prunus virginiana), golden current (Ribes aureum), Wood’s rose (Rosa woodsii), and Western snowberry (Symporicarpos occidentalis) in more than 13.7 acres. They overseeded with native grasses such as blue grama (Chondosum gracilis), side oats (Bouteloua curitipendula), and sand dropseed (Sporobolus cryptandrus). In addition, they put in native pollinator attractors such as prairie coneflower (Ratibida columnifera), sidebells penstemon (Penstemon secundiflorus), and white sagebrush (Artemisia ludoviciana). In the protected wetland, the orchids were growing in an area of about a tenth of an acre. Transplanting them posed an interesting challenge. “The orchids don’t come up every year,” says Hickey. “We surveyed the site for several years to understand where they were occurring.” Western States Reclamation dug up five orchid plants by hand and excavated large sections of the soil associated with them. This ensured that their root systems would be undamaged. It also ensured that seeds, mycorrhizae, bacteria, and other elements necessary to the orchids’ health and reproduction would be transferred to the constructed wetland. Within the constructed wetland, Rocky Mountain excavated an existing 40-foot by 800-foot section of a swale to approximately 18–24 inches deep. Western States Reclamation crews placed the orchid sod in the swale, which raised the soil level up to that of the surrounding land. “We overseeded the wetland areas to increase species diversity,” says Hickey. “The local native grasses and forbs weren’t really present anymore.” The same upland wetland species were used throughout, including Canada wildrye (Elymus Canadensis), western wheatgrass (Pascopyrum smithii), alkali sacaton (Sporobolus airoides), Dudley’s rush (Juncus dudleyi), purple verbena (Verbena hastate), large leaf avens (Guem macrophyllum), and golden banner (Thermopsis montana). The city of Boulder’s Open Space and Mountain Parks Program will maintain the site, he says. “The city is very active in open-space preservation. This parcel will be closed to the public and maintained as a nature preserve.” Western States Reclamation is maintaining the wetland, mainly watering, weeding, performing noxious weed control, and other vegetative maintenance, with the assistance of LT Environmental in Arvada, CO, which is responsible for the weed monitoring and reporting. “We’ve been pleasantly surprised by the results of the project,” says Hickey. “We had a very wet year with some big flood cycles, which is helpful for the plant communities we’re trying to establish.” Approximately four months after construction, the willow habitat had expanded. Approximately 90% of the planted willows were doing well. The native herbaceous understory species also had expanded. Some 95% of the planted shrubs survived, and the native seed was germinating well. “I give a lot of credit to Western States Reclamation,” says Hickey. “I think their execution of the project was very good, which is reflected in the early seeding and planting success. That’s a function of their maintenance effort and favorable climatic conditions. They really exceeded our expectations.” Farrell Park Wetland The Farrell Park Gravel Wetland is a gem. At only 10 feet by 30 feet by 3.5 feet deep, and brimming with native plants, the wetland is sized to treat a 0.9-inch rain event, or 1,769 cubic feet, in 24 hours. In 2008, the South Burlington, VT, stormwater utility constructed the wetland to treat stormwater runoff that previously had flowed untreated into the adjacent Potash Brook. “This was a general water-quality-improvement project,” says Tom DiPietro, project manager for the stormwater utility, which owns, operates, and maintains the wetland. “We have a number of stormwater-impaired streams that are unable to support aquatic biota—fish and macroinvertebrates.” The project was accomplished with funds from the stormwater utility and the Winooski Natural Resource Conservation District, which provided funding from an EPA grant. “Because we have a stormwater utility, we’re always looking for project partners,” he says. “We work with various watershed groups within the area, mostly the conservation district. It isn’t just the right thing to do, it’s also a requirement of our MS4 permit.” The wetland is in a small sliver of land in the 23-acre recreational park between the parking lot and Potash Brook. Most of the approximately 1.5 acres of the drainage area is permeable. Half is the 3/4-acre gravel parking lot. A portion is part of an impermeable bike path. The soils are a mix of loamy sands, clayey loams, and fill associated with parking lots. The runoff has a high pollutant load of predominantly sediment from the parking lot: small particulates, heavy metals, and organic compounds as well as cigarette butts and other debris. Dave Whitney of EcoSolutions LLC designed the wetland as an Advanced Wetland Stormwater Filter (AWSF) system, a subsurface flow gravel wetland. Captured sediment can be flushed from the system through an innovative valve and pipe setup, also designed by Whitney. Stormwater flows down a grassy hillside to the parking lot and portions of the bike path to a pretreatment system. This system consists of three swales covered with erosion control mats and planted with native vegetation, as well as three underground concrete settling tanks. “The tanks simulate a pond forebay,” says DiPietro. “We didn’t have room for a traditional pond. We squeezed the tanks in between the parking lot and the stream. They remove a lot of sediment.” Runoff flows through the first swale into an inlet that is flush with surface to the first tank, through the second swale to the second tank, and through the third swale to the last tank. From there it flows through a 3.5-inch pipe into the wetland. The stormwater utility excavated the wetland, stabilized the sides with plywood, and lined the bottom and the sides with a 45-mil nonwoven EPDM pond liner. Crews placed a thin layer of 2- to 4-inch gravel on the liner. Once the stone was in place, the plywood was removed. In the gravel, they placed an adjustable standpipe system that maintains the water level at just below the surface of the pea stone; a series of manifolds, which determine the flow of the water; and the flushing system developed by Whitney. The flushing system is used primarily in upflow gravel wetlands. Pipes and a submersible pump in the bottom of wetland circulate the water. It’s also possible to hook up an air compressor to circulate the air in the bottom. “The idea is to stir up the sediment that’s collected on the bottom,” says DiPietro. “Then we can pump it out into a silt bag, capture the sediment, let the water out, and haul out only the sediment.” Crews then layered approximately 2.5 feet of a gravel media with a 35% void space for water storage. They used a total of 12 inches of 2- to 4-inch gravel on the bottom, 24 inches of 0.75-inch gravel in the middle, and 6 inches of decorative pea stone on top. They planted native iris, cardinal flower, and other plants in the gravel. Stormwater flows from the third settling tank into the bottom of the wetland. The manifolds spread it out horizontally until it reaches the surface at the outlet, where the clarified water drains out a 6-inch pipe to Potash Brook. During large storm events, water fills up the gravel wetland and backs up into the third underground tank. This tank contains an overflow pipe that discharges water to Potash Brook after it flows through the upstream tanks and swales. This prevents too much water from flowing into the wetland and damaging it. Maintenance has been simple, says DiPietro. “We don’t have to go in with an excavator and clean the sediment out. We use our vacuum truck to remove sediment from the tanks every year or two, and the utility has flushed the sediment from the wetland twice.” “The project fit the limited space we had very well and the funds made it very affordable,” he says. “We’re very happy with the whole project.” EC Janet Aird is a writer specializing in agricultural and landscaping topics.
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