How do we design wetlands to treat stormwater?
To design a wetland that will effectively treat stormwater, there are five major steps:
1. Site Selection (with a new detention basin)
The location of the floodwater detention basin will determine where the stormwater wetland will go, but requirements for the wetland should also be considered in the overall location. For example, high permeability soils (i.e., sandy and sandy loam) should be avoided and the wetland site should be easily accessible for maintenance.The outlet of the wetland should be aligned with the drainage pathway of the detention basin.
2. Sizing the Wetland
The most important factor that will determine the effectiveness of the wetland is the size of wetland relative to the contributing watershed. The larger the wetland the more effective it will be at removing pollutants. Ideally, the wetland should be capable of retaining and treating a 90th percentile storm for at least two full days before another storm arrives. A 90th percentile storm for the Upper Gulf Coast of Texas is about 2 inches of rainfall. For the most developed areas of this region, the size of a fully effective stormwater wetland should be between 5% to 10% of the contributing watershed.
If a detention basin already exists, it is unlikely that a 5-10% wetland-to-watershed ratio will be achieved. Even if that is the case, any wetland area added to the detention basin will still provide some significant pollutant removal.
3. Including the Appropriate Features
To be effective, constructed wetlands need to simulate natural wetland features as well as account for the variable stormwater flow they receive. A feature common to wetlands is a forebay. A forebay will dissipate the runoff energy and collect the large particles in the runoff, which simulates deep pools often found in natural wetlands. It should make up 10-15% of the total area and be easily accessible for periodic cleanout. The deep pools should be distributed along the main pathway of the water. Both the forebay and deep pools should be 18-30 inches deep and should account for 20-25% of the total area.
Another feature found in wetlands is the shallow water area. This constitutes the main course of the water, should be 2-4 inches deep and make up 40% of the total wetland area. The feature with the greatest capacity for treatment is the temporary inundation zone of the basin. It is designed to hold water, about 1 foot deep for at least a few days after the storm. Side slopes of the upper banks need to be no steeper than 3:1 and should be vegetated with plants that help stabilize soil and can survive under much drier conditions.
4. Outlet Structure
Typical stormwater wetlands are designed with two outlets. The first outlet is an overflow spillway (usually 1-foot weir) placed at the top of the treatment wetland elevation. This will allow all the water above the 1-foot elevation in the wetland to drain fast so that the wetland plants have a greater chance of survival. The second outlet is located below the weir and is designed with several holes to slowly drain 8-10 inches of standing water above the shallow water zone and below the overflow spillway. This outlet is designed to hold water for longer periods to allow for a longer treatment time.
5. Vegetation Selection
Unlike natural wetlands and wastewater treatment wetlands, stormwater wetlands have relatively dramatic and frequent changes in water depth. In the shallow water zone, water depth can increase from 3-15 inches and back again to 3 inches in as few as three days. Therefore, it is extremely important to include multiple species of wetland vegetation in each area and monitor their survival.
To learn more about appropriate wetland plants, visit the Texas Parks and Wildlife Department’s Plan Information Database.