control stormwater runoff
Despite the current economic slump, bulldozers and buzz saws can be heard throughout the Triangle as development continues. And as pavement replaces soil, stormwater runoff poses a greater hazard to water quality.
North Carolina Cooperative Extension Service specialists recently completed a series of bioretention areas at Carpenter Village, a residential development in Cary. Bioretention areas, also known as rain gardens, use vegetation to slow the flow of stormwater runoff, directing water through the ground instead of over it. The 800-square-foot bioretention site treats water runoff from three acres.
The purpose of the rain garden is to hold and treat stormwater runoff before it rushes into drainage pipes. Using natural processes to capture and filter stormwater, rain gardens use layers of soil, sand and mulch, along with plants and trees, such as red maples, wax myrtles and pines, that absorb pollutants. The strategy will be studied to determine its effectiveness in preventing water pollution and the alternation of stream channels.
“The site is basically an outdoor classroom,” said Dr. Nancy White, director of Extension programs in North Carolina State University’s College of Design. “We are trying to reroute and filter water to minimize the effect of land-disturbing activities.”
Bioretention areas are emerging as a water-management tool communities can use to offset environmental consequences of development, according to Bill Hunt, Extension stormwater specialist. “Runoff will pond near the top of the bioretention area. As the runoff infiltrates the soil, sediment and nutrients are removed. The vegetation takes up some of the nutrients for growth,” he said, adding that microbes in the soil also break down dissolved pollutants.
“This research will provide baseline data linking development practices directly to water quality,” said Mike Hunter of WW Partners, developer of the Carpenter Village. “Hopefully this project will contribute to understanding how communities can better manage stormwater and erosion to protect our waters.”
Bioretention also may help communities deal with flooding, though it is most effective for smaller storm events, with rainfall of less than two inches, Hunt said. Rain that once idled down ravines and gullies in wooded forests now travels rapidly over impervious surfaces like streets and parking lots, the products of growth and urbanization.
“By increasing the retention time of the water, we are hoping to reduce both the peak and total volume of the stormwater flowing from such sites by spreading it out over a longer period,” White said.
The Wake County site will be used for both research and demonstration purposes. Educational tours involving local leaders, water quality professionals and even homeowners are planned.
As for research, strategically located monitoring equipment records data before, during and after storms. Researchers will monitor the site for three years.
“We have put in monitoring equipment that allows us, after a period of calibration, to sample the water based on the volume of flow,” White said. “It takes about a year to develop a chart that shows flow rate.”
Data from this site, combined with rain garden studies in Chapel Hill, Wilson and Greensboro, she said, should double the national database of information on bioretention.
—Andy Fisher
