When planning a constructed wetland site, there are some questions to ask, says David Whitney, a civil engineer and owner of EcoSolutions LLC in Westford, VT. Questions include: How much space is there? Is the wetland a retrofit or large tract of land? What is the watershed like? Does it have a high pollutant load or is runoff coming off a metal factory roof, which has hardly any pollution?
When planning a constructed wetland site, there are some questions to ask, says David Whitney, a civil engineer and owner of EcoSolutions LLC in Westford, VT. Questions include: How much space is there? Is the wetland a retrofit or large tract of land? What is the watershed like? Does it have a high pollutant load or is runoff coming off a metal factory roof, which has hardly any pollution? [text_ad] Another question: Is there much of a slope? Wetlands on slopes require berms on the downhill side to hold water back and grading of the slope out to meet the existing soil. This can result in large footprints, and the steeper the existing slope, the larger the footprint. “However, wetlands typically have a smaller footprint that traditional stormwater ponds, and therefore they are preferable on steep-slope terrain such as ski resorts,” says Whitney. The second consideration is the design. A well-designed wetland requires little maintenance. Stormwater flows through a forebay before it reaches the larger pool of the wetland. The forebay usually is a small pool roughly 10% of the volume of the wetland, although in tight spaces it may consist of one or more inlet structures. The water should be conveyed to the forebay in a way that minimizes the potential for erosion. The forebay helps settle out coarse sediment particles, which reduces the frequency of dredging needed in the larger pool. Because much of the dredging takes place in the forebay, it should have access for maintenance. The large pool usually has two depths, typically about 18 inches and 3 feet, to allow more diverse vegetation to grow. A large amount of land may be needed to store the required volume of water. Where land is limited, there may be additional storage above or below ground. The constructed wetland should have a drain to draw down the water level during maintenance. There are two general designs for the large pool. In a shallow, or free-water, constructed wetland, the pool is essentially a marsh. In a gravel-based or subsurface flow wetland, runoff flows through a filter of gravel or sand and wetland plants. The water level is just below the surface of the media. The flow can be directed horizontally across the surface, or vertically from top to bottom or from bottom to top. “You don’t see the wetland,” says Whitney of the subsurface flow variety. “It’s within the stone media. These wetlands do a great job of trapping sediment.” There may or may not be a small pool at the outlet of the wetland, but either way, it should be designed to prevent scour. Pipes should be larger than 3 inches in diameter and be either reverse-slope pipes or weir outlets with a trash rack to prevent clogging. In areas with large storm events, an emergency spillway conveys excess stormwater directly downstream. Proper vegetation beautifies the wetland as well as removing pollutants. Designers should use native wetland plants whenever possible, in the form of dormant rhizomes, live potted plants, and bare rootstock. Plant lists are available for specific regions through wetland nurseries, university extension services, and conservation districts. The plantings may be supplemented with soil from a natural wetland (often referred to as “wetland mulch”), which contains seeds and other organisms from the wetland. The mulch can help enhance plant diversity, but it also may contain seeds from invasive plants.
Another question: Is there much of a slope? Wetlands on slopes require berms on the downhill side to hold water back and grading of the slope out to meet the existing soil. This can result in large footprints, and the steeper the existing slope, the larger the footprint.
“However, wetlands typically have a smaller footprint that traditional stormwater ponds, and therefore they are preferable on steep-slope terrain such as ski resorts,” says Whitney.
The second consideration is the design. A well-designed wetland requires little maintenance.
Stormwater flows through a forebay before it reaches the larger pool of the wetland. The forebay usually is a small pool roughly 10% of the volume of the wetland, although in tight spaces it may consist of one or more inlet structures. The water should be conveyed to the forebay in a way that minimizes the potential for erosion.
The forebay helps settle out coarse sediment particles, which reduces the frequency of dredging needed in the larger pool. Because much of the dredging takes place in the forebay, it should have access for maintenance.
The large pool usually has two depths, typically about 18 inches and 3 feet, to allow more diverse vegetation to grow. A large amount of land may be needed to store the required volume of water. Where land is limited, there may be additional storage above or below ground. The constructed wetland should have a drain to draw down the water level during maintenance.
There are two general designs for the large pool. In a shallow, or free-water, constructed wetland, the pool is essentially a marsh.
In a gravel-based or subsurface flow wetland, runoff flows through a filter of gravel or sand and wetland plants. The water level is just below the surface of the media. The flow can be directed horizontally across the surface, or vertically from top to bottom or from bottom to top.
“You don’t see the wetland,” says Whitney of the subsurface flow variety. “It’s within the stone media. These wetlands do a great job of trapping sediment.”
There may or may not be a small pool at the outlet of the wetland, but either way, it should be designed to prevent scour. Pipes should be larger than 3 inches in diameter and be either reverse-slope pipes or weir outlets with a trash rack to prevent clogging.
In areas with large storm events, an emergency spillway conveys excess stormwater directly downstream.
Proper vegetation beautifies the wetland as well as removing pollutants. Designers should use native wetland plants whenever possible, in the form of dormant rhizomes, live potted plants, and bare rootstock. Plant lists are available for specific regions through wetland nurseries, university extension services, and conservation districts.
The plantings may be supplemented with soil from a natural wetland (often referred to as “wetland mulch”), which contains seeds and other organisms from the wetland. The mulch can help enhance plant diversity, but it also may contain seeds from invasive plants.
