Considered a stormwater Best Management Practice (BMP) for years, constructed wetlands are far from a mainstream solution, yet proponents say they offer much more to a development than simply a place to send stormwater.

Constructed wetlands are a more beneficial alternative compared to the predominantly used wet or dry basins as a method to handle excess stormwater, according to Dr. Peter May, an environmental scientist with Biohabitats, Inc., an ecological restoration, conservation planning and regenerative design firm headquartered in Baltimore, Md.

Part of the reason they are often not the go-to solution is because the upfront cost is higher than the cost of constructed wet or dry detention basins to store stormwater.

“However, wetlands are much more efficient at pollution control and filtration of sediments, metals, nutrients and nutrient processing,” May said. “They also provide ecosystem services for wildlife and other benefits for communities that incorporate them into their development.”

Planned for low areas in the landscape, these systems are designed to treat stormwater runoff through natural means before it is allowed to move into a stream or other body of water. Wetland plants slow down the water flow, allowing some pollutants to settle out of suspension while breaking down and taking up nutrients into plant tissues. By slowing the water, these systems also reduce the potential for flooding and allow better infiltration into sub soils to recharge groundwater.

Though not perfect for every location or climate, these natural treatment systems, when incorporated into a development-wide design, can be an economical, long-term solution that combines the form of an appealing amenity with the function of a stormwater BMP.

“One important benefit of these systems is they are very low cost relative to harder, engineered systems,” May pointed out. “You don’t have the kind of piping involved, and their long-term maintenance costs are less.”

Further, May pointed out that wetlands soften the landscape of modern developments, providing people with a connection to nature and a sense of place, which can have a positive psychological effect.

“People generally appreciate natural areas and often prefer them to hardscapes,” he said. “When people feel better about where they live or work, this can also translate into better quality of life, improved productivity and even higher resale value.”

How They Work
The most-often used stormwater BMPs, detention or retention basins, serve a basic purpose, according to May.

“They just provide storage and some sediment settling capabilities. But studies have found that they don’t treat the stormwater runoff nearly as well as wetlands,” he said.

Wetlands filter and trap sediment. Natural biological processes within them involving plants, bacteria and algae further trap and convert pollutants that would otherwise be passed along to downstream natural water bodies.

Wetlands take in the runoff containing concentrations of pollutants such as heavy metals, organics and nutrients, and bind them to particles in the still or slow moving water or in organic matter. A portion of these pollutants are taken up by the wetland plants and are trapped in their tissues.

The wetland has the ability to become a sink for those pollutants.

“As plants in wetlands slowly decompose, the pollutants may be bound up in the wetland peat material,” May said.

“As the peat accumulates, layer by layer, and as long as it is kept moist and wet, the pollutants bound in the peat remain trapped, rather than being released.”

Wetlands also provide a significant flood-control measure while allowing water to infiltrate the soil and recharge groundwater systems.

Location, Location, Location
Generally speaking, stormwater wetlands can best be constructed in areas that once held them naturally, with ready access to groundwater.

“You would want to site your wetland low in the landscape so it can receive the site’s runoff. Another consideration is access to groundwater, which is important for the health and maintenance of your plant material,” said May.

Wetlands should be among the first things to be planned for a development and the last things to be constructed.

Early planning allows for the best location to be identified. Bioswales, rain gardens and other green, stormwater-control measures can be integrated throughout the development, creating more of a community-wide, surface conveyance system, which replaces more expensive, hard engineered stormwater infrastructure.

This sort of integrated, surface system slows the flow of stormwater and, coupled with shallow zones at the inlets to the wetland called a forebay, provides pre-treatment and additional soil infiltration.

Ted Brown, a water resources engineer with Biohabitats, recommends that designers and developers locate the wetlands in prominent spaces of the development, making them more of a part of the community.

“If you design them as ‘green fingers’ reaching into the development, you can create trail systems adjacent to and even through them,” Brown said. The stormwater wetland then becomes more of an amenity, a park-like setting, that creates natural areas for residents to walk through and enjoy.  

Meanwhile, as far as the construction schedule is concerned, the actual wetland, complete with proper soils, final grading and vegetation should be the last element of the development to be installed, according to May.

“You don’t want to build a wetland and fill it with plants which would only be smothered by construction runoff.

This would also reduce the interaction with the groundwater interface, which is one of the primary functions of wetland treatment.” May said. “Your final grading is also important. It is important to know where your drainage pathways are going to come into your wetland site.”

Learn by Experience
The creation of these natural, stormwater BMPs has advanced a great deal in the last 20 years as wetland restoration and treatment-wetland construction projects have become more prevalent, according to May.

“There’s a whole emerging field called ecological engineering, which takes traditional engineering and combines it with a good foundation in ecosystem processes,” he said. “From a development perspective, it is extremely beneficial to retain someone who has knowledge of these systems and how they should work.”

Brown said there are some key issues to consider.

The size of the wetland needs to be in the range of three to five-percent of the contributing drainage area.

“You want a minimum contributing-drainage area of from 10 to 20 acres and that might be dependent upon whether you are tapping into a groundwater table,” he said, to maintain water levels in the wetland.

Designers should develop what is called a “water budget” which takes into account the level of evaporation and transpiration that will occur on a monthly basis to further insure adequate water to support the vegetation.

“As you develop a planting plan, you also have to be fairly diligent the first couple of growing seasons to make sure the specified plants are thriving and not succumbing to invasive plants that may have come in,” Brown said.

Flat slopes are critical when grading the wetland.

“You don’t want to have three or four-to-one side slopes. Five to one or flatter, on your side embankments,” said Brown, who also stressed the need for a diversified design of the wetland bottom.

“You have high-marsh zones, low-marsh zones and fully inundated open water, so it is a diversity of water levels and planting zones,” he said.

Mosquitoes
The health concerns related to mosquitoes has dogged wetland creation, especially with the heightened awareness of mosquito-borne diseases, such as West Nile, encephalitis, and eastern equine encephalomyelitis.

May points out that that concern can be applied to any stormwater BMP which involves some level of open, still water.

“Mosquitos are a problem anywhere. They are in planter pots and bird baths. They will always be an issue,” he said. “But with wetlands, especially those that are well-designed, fully functional, and biologically diverse, you set the stage for more complex fauna—predator species—to combat mosquito populations.”

Those predators include everything from dragonflies to birds, like the Swift and Purple Martin. Bats are another species that prey on mosquitoes, but unfortunately, locating bat houses around a development may be disconcerting to some residents.

“For those in the right latitudes, a population of mosquito-fish can do a very good job of suppressing the mosquito population,” May said.

There are also environmentally safe and naturally occurring bacteria, such as bacillus Bt, which can be an effective measure to keep mosquito populations in check by preventing the insect larvae from emerging after they hatch in their aquatic phase.

“The bacteria doesn’t harm birds, fish or people at all,” May said.

Brown said another key to combating mosquitoes is in the design of the wetland. There needs to be good drainage to allow a constant, but slow, inflow and outflow of water. This prevents stagnation, which mosquitos prefer for breeding. Secondly, it is more conducive to supporting a broader range of predator species.

Nature’s Aesthetic
The return on investment in such designed and constructed, natural systems is far ranging and difficult to quantify in dollars and cents, according to May.

“If you look at stormwater infrastructure, a dry pond doesn’t have nearly the natural capital value that a wetland system does. A dry pond is just a hole in the ground that has turf grass on it. It has no biodiversity associated with it. It has very few ecological benefits and provides much less of a water quality benefit,” he added.

The payback for integrating natural systems into communities is improved water quality as well as some carbon sequestration by plant uptake of CO2.

“These natural systems can do work for us as a society. They provide all these natural, free ecosystems services, but they also do an incredible amount of good for the environment and help to mitigate the ecological impacts of human development,” May said.

The wetland areas also buffer the edges of manicured developments, which May contends adds a pleasant, calming psychological effect as people experience a greater connection to nature.

“If incorporated properly, these places can be really wonderful areas,” said May

Marketing the value of such sustainable systems is a challenge in some areas as there will be those property owners who prefer manicured, carefully mowed developments over the natural setting.

“That’s an education component that has to come along with the design and how developers market their development,” Brown concluded.

Existing Wetlands: Tough to translate into a stormwater solution
Trying to adapt an existing, or jurisdictional wetland to be part of a community’s stormwater control system can be a touchy issue, according to Ted Brown, water resources engineer for Biohabitats, Inc., an ecological restoration, conservation planning and regenerative design firm that has been designing wetlands for over 25 years.

“Regulators are a little bit gun shy about allowing them,” he said. “It can be done so long as the runoff from the developed area is treated fully with some suite of BMPs before it discharges to any natural wetland area. One of those BMPs could actually be a constructed wetland,” he said.

The discharge needs to be diffuse. Concentrated flows will create potential erosion and sediment problem in the natural wetland.
    “Many local governments will say that it is not consistent with the stormwater regulations for untreated stormwater to be discharged into jurisdictional wetlands,” Brown said. SLDT