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Constructed Wetland

General

Constructed wetlands are artificial wetland systems that are designed to collect and temporarily store runoff. These devices reduce stormwater flow velocity and trap sediment and other associated pollutants. Stormwater enters the wetland and is temporarily retained, allowing pollutants to settle out. Pollutants, such as metals, nutrients, sediment, and organic substances, are removed by the settling of particulates, biological uptake, consumption, and decomposition.

Constructed wetlands can be used in residential, commercial, and industrial areas if the contributing watershed is large enough to maintain water depth in the basin. Their implementation also depends on several site factors, including soil types, depth to bedrock, depth to groundwater, and available land area. Due to the permanent ponding of water, constructed wetlands can remove large amounts of pollutants and are more effective in removing nutrients than most other management practices. The large volume of storage helps to reduce peak discharges from storm events, which, in turn, reduces down-stream flooding and streambank erosion. However, constructed wetlands are designed primarily to treat stormwater and should not be considered restored or mitigated wetlands.

Accumulated sediment can reduce the capacity, life span, and effectiveness of constructed wetland systems. As a result, they are best used in conjunction with other management practices.

Advantages

  • Wildlife habitat is created when the ponds are properly designed and maintained, including habitat for mosquito predators
  • Can be aesthetically pleasing if designed properly, which can increase adjacent property values
  • Capable of removing both solid and soluble pollutants
  • Reduces peak flows

Disadvantages

  • Requires a large land area
  • Generally requires a large contributing watershed
  • Pond discharges usually consist of warm water, so their use may be limited in areas with temperature sensitive fisheries
  • Improperly maintained wetlands may result in nuisance odors, algae blooms, and rotting debris
  • Cost

Design

Constructed wetlands may be designed with irregular and curvilinear shorelines in a variety of configurations depending upon the individual needs of the site. Regardless of design, these devices should be capable of controlling the runoff from the 1-year, 24-hour storm event.

Forebays/Inlets

Forebays, which are separated from the rest of the wetland by a gabion or berm, receive runoff and prevent concentrated flow from entering the device, allowing sediment to settle out before it reaches the wetland. They also simplify maintenance by concentrating sediments and extending the holding capacity and life of the structure. Forebays should have an area equal to 10-25% of the wetland’s surface area, with a length to width ratio of at least 2:1 to provide proper flow and a depth of 2-4 feet. Forebays should be located opposite of the outlet structure to increase detention time and should provide access for the removal of accumulated sediment.

Shape

Constructed wetlands should be designed so that the previously held stormwater is replaced by the newer stormwater, a process referred to as plug flow. Plug flow allows the water to remain in the wetland long enough to facilitate the settling of sediment and the adsorption of pollutants and sediment. To encourage plug flow, the wetland should be designed with a length to width ratio of at least 3:1. These shapes encourage proper mixing of the water column and increase retention time. In addition, these shapes lessen the amount of sediment stirring caused by wind, allowing pollutants to remain settled in the sediment. If these shapes are not feasible, structures that lengthen the flow path, such as gabions and baffles, should be installed.

Soils

Soils should have infiltration rates low enough so that base flow or stormwater runoff can maintain a permanent pool of water in the wetland. Sites with highly permeable soils or those that are close to the water table should incorporate an impermeable liner into the design to facilitate ponding and to prevent stored water from mixing with groundwater.

After the basin has been excavated and graded, at least 4 inches of soil should be placed in the basin. This layer, which should also be applied on top of any liner, provides a substrate necessary for plant growth.

Depth

The depth requirements for each constructed wetland are dependent upon the individual characteristics of each site. However, they should be constructed with varying depths to encourage biological diversity and to improve pollutant removal. Shallow depths encourage vegetative growth and are more effective for pollutant removal than deeper areas and should have a surface area that it equal to or larger than deeper areas. Deeper areas provide storage capacity and facilitate sedimentation while providing habitat for submerged and floating species of vegetation. Aquatic benches should be incorporated into the design of deep pools. These benches improve the diversity of the pool and the safety of the practice and should be 3-10 feet wide with 10:1 slopes.

Vegetation

Vegetation may be planted from seed or by applying a layer of wetland soil to the depression. When seeding is necessary, plant selection will depend upon individual site characteristics of the site. However, care should be taken to avoid the use of invasive and exotic species. Selected species should be capable of thriving in hydric conditions.

In many applications, vegetation can be established by applying a layer of wetland soils to the basin. Wetland soils generally contain a large number of seeds that will propagate under proper hydrologic conditions.

Outlets

Constructed wetlands should have an outlet structure that is designed to handle the peak flow of the structure and should discharge to a stable outlet. Outlet structures should incorporate a multi-stage riser and should be resistant to clogging and may incorporate trash racks, skimmers, or other devices. Outlets should be designed with stability in mind and should be able to endure frost heave and settling.

Constructed wetlands should also incorporate an emergency spillway and a dewatering outlet into the design of the structure. Emergency spillways are designed to safely pass flows that exceed the design capacity of the basin. These structures prevent large flows from overwhelming the capacity of the structure without causing damage to the basin or downstream structures and should discharge to a stable outlet. A dewatering outlet is a gate controlled drain that is capable of draining the wetland within 24 hours. They are used on a limited basis for maintenance purposes.

Construction

  • Constructed wetlands should be kept flooded after excavation and final grading until vegetation is planted
  • Outlets should allow access to maintenance personnel while restricting access to the general public

Maintenance

  • Accumulated sediment should be removed from forebays as needed
  • Inspect the outlet and emergency spillway after all large storm events for erosion and displacement- all repairs should be made immediately
  • Trash and other debris should be removed as necessary
  • Gabions, berms, and other hydraulic control structures should be inspected after large storm events for damage- all repairs should be made immediately
  • Constructed wetlands should be inspected during and after ice break-up for damage and blockage
  • Plants should be cut and removed in the fall to prevent the release of stored nutrients and other pollutants

Method Used to Determine Practice Efficiency

Constructed wetlands reduce the flow velocity of runoff, allowing suspended particles to settle out. In addition, these devices remove nutrients, heavy metals, and other pollutants from stormwater by utilizing several biological processes. The efficiency for this practice is dependent upon the size of the wetland, the size of the drainage area, and other site characteristics and the proper design, installation, and maintenance of the constructed wetland. As a result, the efficiency for this practice must be calculated using factors unique to each site.