You’ve likely seen gravel wetlands in your travels, but you probably assumed it was just a pond. You’ll often find these manmade structures near parking lots and in industrial areas, or near schools.
Overall, wetlands have a positive impact on water quality, and these systems work hard to manage pollution. Whether the wetland is natural or constructed, it has a purpose. That purpose is why we are here; we’re going to tell you about one type of wetland that we’ve installed to manage stormwater runoff.
What is a Gravel Wetland?
You might hear the term subsurface gravel wetland, subsurface flow wetland, or gravel wetland – these terms are used interchangeably.
What’s important is what it is. A gravel wetland is a constructed wetland where the water sits below the surface. There are several layers, from top to bottom:
- Wetland plants and grass
- Soil/Sand – a sandy loam is perfect for this layer
- Native Soil
This type of wetland remains wet under the surface, never fully draining and never drying out. It must maintain some level of moisture, particularly within the gravel layer, to sustain the plant life that filters the water during a rain event. Each layer provides a different level of filtration.
The forebay captures large coarse particles, like sticks and branches, sediment, or trash. The main wetland and gravel layers capture and filter smaller particles, removing pollutants and harmful nutrients such as:
- Solids, roadway pollutants
The liner has two jobs. It makes sure the water level below the surface, in the gravel layer, is maintained. Its second job is to protect the native soil and groundwater from the pollutants. If the gravel layer isn’t fully saturated, then it cannot correctly drain and manage pollution.
Subsurface Gravel Wetlands are Carefully Constructed
You’ll want to work with an engineer familiar with Best Management Practices (BMPs) for stormwater runoff, and one who also knows how to construct a gravel wetland. Without proper installation and combining the use of appropriate BMPs, your efforts and money will literally go down the drain.
Above, we talked about the five layers of a gravel wetland, and now we’ll outline the construction process
- Remove vegetation in the area of construction
- Grade and excavate the bottom of the wetland (forebay/sedimentation basin)
- Build the berm (depending upon the area, it may not be necessary)
- Install the riser(s) and pipe(s)
- Add crushed stone
- Add concrete sand
- Install the fabric liner
- Combine a mixture of fill dirt made up of compost and topsoil and apply over the fabric liner
- Plant native wetland plants and grass
- Let it do the work!
Benefits and Outcomes
Anytime we can apply a method to reduce stormwater runoff and nutrient loading into the lake, we’re doing a good thing. A gravel wetland provides stormwater management and can decrease peak stormwater flows by as much as 77% to 85% in some cases.
Reducing peak stormwater flows also helps with flood control, reduces soil erosion, and, ultimately, improves overall water quality.
Wildlife typically found in wetlands will often make a gravel wetland it’s home. Which just feels good knowing wildlife can return to the area they once called home.
We’re already seeing results. The State of Vermont released the 2019 Clean Water Initiative Performance Report at the beginning of 2020. It told us that our joint efforts kept 16.4 Metric Tons of Phosphorous out of the lake. This is excellent news, and we expect this to continue.
Whatever we can do to keep the momentum for a healthy lake, we will do. A gravel wetland is one of those options.