Bioretention cells do a beautiful job of removing pollutants from stormwater runoff
ANDERSON – Over the course of two sun-drenched mornings, Clemson University’s Carolina Clear and its collaborators turned an unattractive stormwater detention basin into a state-of-the-art filtration system that is as pretty as it is environmentally friendly.
The rain garden, which recently was built at Green Pond Landing and Event Center in Anderson, technically is known as a bioretention cell. But by any name it’s a carefully crafted and multilayered assembly of plants, soil, compost and stone that is designed to filter, trap and remove stormwater runoff and its accompanying contaminants. This is good news for nearby Hartwell Lake, an enormous manmade reservoir that provides drinking water to more than 200,000 South Carolinians.
“Stormwater rushes off hard, sloping surfaces and gathers pollution in its wake,” said Rachel Davis, water and natural resources agent for Carolina Clear, a statewide service of Clemson Cooperative Extension that educates communities about solutions for stormwater pollution. “This bioretention cell will slow the water down and absorb most of the pollutants before they can flow into the lake.”
Carolina Clear and its regional consortium, the Anderson and Pickens Counties Stormwater Partners, chose the Green Pond Landing site because of its high visibility. Every project of this kind benefits the environment in a variety of ways, so the more the merrier when it comes to attracting attention and building interest.
“This is an Anderson County park with an events center that hosts lots of fishing tournaments and other events,” Davis said. “We’re going to put a sign out so that people can come by and become educated about what this is all about. And Green Pond was also a good location because it had this big, empty detention pond that was a perfect fit for what we wanted to accomplish.”
Though it took only two mornings to construct the bioretention cell, the overall project was almost a year in the making. Chuck Jarman, water resources engineer for Extension, began preparations months ago, surveying the site, conceiving the design and drawing up the plans.
“Part of what I do is to get demonstration projects going that can teach people how to design bioretention cells from an engineering and technical standpoint,” said Jarman, who is based at the Baruch Institute in Georgetown but who routinely coordinates projects through the state.
“So a site like this serves a dual purpose,” he said. “For one, it enhances the environment. We’ll be getting 98 percent sediment removal out of this system, as well as the elimination of large percentages of nutrients existing in stormwater runoff, such as nitrogen, phosphorus and potassium. But this is also a great teaching tool. We’re trying to help people understand that undertakings such as these should be embraced by public and private landowners.”
Bioretention is defined as the process by which contaminants and sedimentation are removed from stormwater runoff. The cells soak up and store water that might otherwise contribute to flooding. They also provide habitat for a variety of animals and insects, beautify the landscape and naturally remove toxins. Stormwater that collects in the depressed garden bed is absorbed by the plants and filtered through the soil. Depending on size and location, the design of each cell presents its own unique challenges. But all of them are built with methods that are similar to the ones used at Green Pond Landing:
- First, an overflow outlet was installed and connected to an existing perforated drainage pipe that goes into the storm drainage system and outfalls into the lake.
- Next, a thick bed of gravel was smoothed out at the base of the depression and covered with a geotextile fabric. This allows water to flow through while keeping the soil from clogging the stone layer and drainage pipe.
- Then, an engineered soil media consisting of a mixture of sand and compost was placed on top of the fabric.
- Finally, a combination of native and non-native plant species was planted in the sand/compost mixture, which was topped off with a layer of hardwood mulch.
“The surface of the cell has to be level so that the water will spread out and filter down evenly,” Jarman said, “and the layers of sand and compost are rototilled to create a good, uniform mix. This helps to ensure that the plants will thrive and the filtering system will work as effectively as possible.”
Sarah White, associate professor and nursery Extension specialist at Clemson University, selected a variety of plants that are “rain-garden friendly,” meaning they are tolerant to both drought and flooding. White’s choices included muhly grass (native), Virginia sweetspire (native), glossy abelia (introduced) and butterfly bush (introduced.)
“Muhly grass helps to manage erosion on the sides of the basin, and it also reduces maintenance because it only needs to be cut once a year,” said White, who chose smaller plants – in two-inch containers – because these were more likely than larger ones to survive the establishment phase of growth.
“Virginia sweetspire blooms white in the summer and with colorful foliage in the fall, and it forms dense mounds of foliage that attract wildlife. Glossy abelia and butterfly bush are pretty and hardy. Once established, these plants will mature in two to three years, and by then they’ll be really beautiful.”
Clemson’s Cherry Crossing Research Facility supplied compost produced from food waste and landscaping residuals collected on campus.
“Unscreened bulky compost is used in stormwater control applications for its water holding capacity, ability to support hardy vegetation and improved infiltration capacity,” said David Thornton, organics and biofuels coordinator for Clemson University Facilities.
Anderson County stormwater manager Jon Batson, another key player from the outset, provided his own expertise and also a crew of tireless professionals.
“We want this to be a demonstration project for the public that shows how stormwater management techniques can benefit the environment and enhance water quality,” Batson said. “We want to always be evolving. So this is one of the newer, more-innovative projects to improve water quality, and we hope that others will see it and put similar systems in place in commercial and residential developments.”
Carolina Clear is a comprehensive approach developed by Clemson University to inform and educate communities about water quality, water quantity and the cumulative effects of stormwater. Carolina Clear addresses the special significance of South Carolina’s water resources and the role they play in the state’s economy, environmental health and overall quality of life.