Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Wetlands and Shallow Marsh Systems
Wetlands and shallow marsh systems use the biological and naturally occurring chemical processes in water and plants to remove pollutants (ASCE, 1992). Oils, particulates, suspended sediment, and soluble nutrients are removed or settled out due to their residence time in the wetland system and before they enter the downstream receiving waters. Wetland and marsh systems can have additional stormwater features that help to attenuate peak storm flows. Figure 9 is an example of a shallow marsh system.
These systems can often have great habitat value. The fringe wetlands and deep water habitats provide shelter and breeding places for many species. Properly sited wetland systems can also be scenic assets along a highway corridor.
Wetland and shallow marsh systems must be carefully sited to ensure that the desired functions for the system are established and maintained. In the ultra-urban environment the feasibility of wetland establishment may be limited due to factors such as drainage area or the absence of high groundwater tables. Due to these considerations, potential sites are most likely at low-lying interchanges or medians where runoff can be directed to them, or existing open areas such as parks, which provide additional aesthetic and educational benefits.Wetland and shallow marsh systems have habitat value and can be efficient at removing pollutants. Since these systems are frequently inundated, adequate safety measures such as safety benches, fences, guardrails, and safety zones must be provided.
Properly designed wetland systems are extremely effective at removing soluble pollutants and particulates from ultra-urban stormwater runoff. Biological oxygen demand (BOD), chemical oxygen demand (COD), and metals are also significantly reduced. As the system ages and more algae and detritus are generated in the pond, the efficiency increases. When combined with extended detention, wetland BMPs may be one of the most effective systems to mitigate stormwater runoff impacts. Figure 10 illustrates the use of an extended detention pond as a pretreatment for a wetland system. Table 11 provides data from a study that monitored the pond and wetland system at the inlet and outlet to the wetland. Many of the suspended solids and some of the solubles were removed by the pretreatment in the detention facility (OWML, 1990). Average removal rates that can be expected from a stormwater wetland are 65 percent for total suspended solids (TSS), 25 percent for total phosphorus (TP), 20 percent for total nitrogen (TN), and 35 to 65 percent for metals (USEPA, 1993).
Table 11. Pollutant removal effectiveness for wetlands (%)
|Martin & Smoot (1986)
||Pretreatment by in-line detention pond.
Results are maximum removals for shallow wetland system only.
||Results are maximum removals for pond and wetland system.
Siting and Design Considerations
Hydrology is likely to be the most important limiting factor in the feasibility of a wetland or marsh system for an ultra-urban area. Such facilities may be on-line or off-line. On-line facilities allow all stormwater flows to pass through the system. Off-line facilities divert higher flows, which may have erosive velocities or which would inundate the system. There must be a sufficient drainage area to maintain base flow in the system. Water budgets should be performed to determine the ability of the pond to maintain vegetation in dry months. Adequate water will help prevent the die-off of planted vegetation, which can prevent invasive species from taking hold. The groundwater elevation is also important since it helps maintain the hydrology. A ratio of watershed area to wetlands area of at least two percent is recommended to have efficient removal capabilities (Schueler, 1992). However, smaller systems could be used in ultra-urban settings.
The wetland system should be designed to have pockets of deeper water to help trap sediments and to provide a diverse habitat. The length of the wetland system and ratio of surface area to width are important pollutant removal factors. The flow length must be long enough to provide adequate residence time to remove soluble pollutants and sufficient settling time for particulates. A length-to-width ratio of 2:1 is recommended to achieve an adequate residence time.
Proper soil conditions are necessary for wetland success. The wetland site must have existing natural soil conditions that facilitate ponding, or these conditions must be created using clay, PVC, or other types of liners. In addition, wetland pollutant removal functions are mediated in part by the supply of organic material in the site. Organic matter also affects the success of wetland plant establishment. Consequently, organic material must be incorporated into project soils if construction requirements necessitate removal of topsoil from the site.
Native plant species that are present in the area should be retained whenever possible. When planting a site is necessary, a diverse plant community of species native to the project area should be established to maximize wildlife and water quality benefits. Planting a variety of species increases the probability of establishing a vigorous plant community and reduces the chance of exotic species invasion into the site. A vegetative buffer strip included around the marsh or pond will reduce sediment inflow and provide additional pollutant filtration. Irregular shorelines, incorporation of nesting boxes, use of plants with habitat characteristics of cover or food, islands for nesting of waterfowl, and sufficient mudflat and deepwater areas will also greatly enhance wildlife habitat. For a thorough discussion of design considerations, refer to Evaluation and Management of Highway Runoff Water Quality (Young et al., 1996). Designers are generally cautioned to avoid species known to be aggressive colonizers, noxious weeds, or ones not recognized by state regulatory agencies.
Frequent maintenance and inspection, which usually involves moderate costs, are critical during the establishment of vegetation in the marsh or wetland. Invasive and undesirable plants must be culled from the planting area. The outfall structure might also have to be adjusted to maintain the proper hydrology for introduced plant species. Though sediment rates may initially be high from construction activity, it is important that sediment be removed so that the plants can become established and the pond capacity is maintained. Once established, the wetland vegetation should be periodically harvested so that the stand can regenerate and the pond is not choked off by vegetation. Systems that do not have consistent and steady base flow may become eutrophic. The outlet structure should incorporate features that protect it from blockage by debris and that allow adjustments to be made to the water surface.
Costs for ponds typically include costs for embankment, riser and spillway structures, outfall protection, vegetative stabilization, excavation, and grading. Additional costs for site preparation can include soil amendments, precision grading, plant materials and creation of occluding layers in coarse-textured soil types if wetlands systems must be created on upland sites due to project constraints. Project costs can be lowered if existing pre-construction site conditions are carefully considered and isolated areas with hydric soils contained within the footprint of the project are utilized as stormwater management facilities.
Additional maintenance costs will be incurred until the establishment of the wetland ecosystem. Invasive plants must be culled and dead plants replaced. The outlet structure may have to be adjusted, based on seasonal observations, to achieve the proper water surface in the pond.
ASCE. 1992. Design and Construction of Urban Stormwater Management Systems. The Urban Water Resources Research Council of the American Society of Civil Engineers (ASCE) and the Water Environment Federation. American Society of Civil Engineers, New York, NY. Martin, E.H., and J.L. Smoot. 1986. Constituent-Load Changes in Urban Stormwater Runoff Routed Through A Detention Pond-Wetlands System in Central Florida. U.S. Geological Survey Water Resources Investigations Report 85-4310, Tallahassee, FL.
Occoquan Watershed Monitoring Laboratory (OWML). 1990. The Evaluation of a Created Wetlands as an Urban Best Management Practice. Occoquan Watershed Monitoring Laboratory. Occoquan, VA.
Schueler, T.R. 1992. Design of Stormwater Wetland Systems: Guidelines for Creating Diverse and Effective Stormwater Wetlands in the Mid-Atlantic Region. Metropolitan Washington Council of Governments, Washington, D.C.
USEPA. 1993. Guidance Specifying Management Measures For Sources of Nonpoint Pollution in Coastal Waters. EPA-840-B-92-002. U.S. Environmental Protection Agency (USEPA), Office of Water, Washington, D.C.
Young, G.K., S. Stein, P. Cole, T. Kammer, F. Graziano, and F. Bank. 1996. Evaluation and Management of Highway Runoff Water Quality. FHWA-PD-96-032. Federal Highway Administration, Office of Environment and Planning.