Keeping It Clean Downstream

Downpours often pollute North Carolina’s waterways when stormwater washes oil, gravel, pollen, and other debris off the pavement into nearby streams. One inch of rain on a one-acre parking lot can send 27,000 gallons of water gushing into storm drains. NC State researchers have developed practices for controlling such runoff and are helping craft state and local regulations to limit its impact in the future.

Dr. Bill Hunt has been a guru of stormwater management technologies since joining NC State in the mid-1990s shortly after lawmakers adopted the Neuse River rules to cut pollution that had resulted in fish kills. “The General Assembly decided that farmers shouldn’t be the only ones responsible. They wanted to make everyone limit runoff,” says Hunt, an associate professor and extension specialist in the Department of Biological and Agricultural Engineering. He has studied options from permeable pavement to bioretention areas.

Impervious surfaces like parking lots and driveways are main targets in reducing runoff. Hunt has used pavers with holes to design parking areas where precipitation drains into the pavement and not the nearest gutter. As the stormwater passes through a sand and gravel base beneath the pavement, pollutants are filtered out.

“People said it wouldn’t work, that sediment would clog the holes,” he says. “We’ve used permeable pavement on several sites across eastern North Carolina and found it works well.” After storms spawned by Hurricane Dennis dumped 5 inches of rain on the region in 2005, for example, Hunt recorded just 1¼ inches of runoff from one of his parking lots. Because of his research, the state Department of Environment and Natural Resources (DENR) now gives special permitting allowances to developers who use permeable pavement. A state law that took effect in April requires permeable pavement for all parking lots at least one acre in size.

One inch of rain on a one-acre parking lot can send 27,000 gallons of water gushing into storm drains.

Hunt has also shown that, in addition to capturing and filtering urban runoff, bioretention areas are critical to preserving fishing streams in the North Carolina mountains. Bioretention areas are depressed medians or other plots with filtering vegetation built into parking lots and along streets. The soils can hold excess water until it percolates into the ground water, is taken up by the grasses and bushes, or evaporates. The areas allow water coming off steaming asphalt parking lots to cool before flowing into nearby streams, preventing fish kills among trout and other species sensitive to changes in water temperature. State officials now recommend building bioretention areas near all new parking lots in mountain counties.

Across campus, Jay Tomlinson and Dr. Hugh Devine have incorporated some of Hunt’s findings in a development model for Brunswick County, where rapid growth threatens water quality. The DENR-funded project was designed to demonstrate the impact alternative development designs could have on preserving clean water. Devine, the director of the Center for Earth Observation in the College of Natural Resources, and research associate professor Dr. Perver Baran used satellite images and geographic information systems data on land use and watersheds to help select three test sites: one along the coast, one along the Cape Fear River, and one in the interior of the county.

Tomlinson, assistant dean for research, extension and engagement in the College of Design, led a team of students who laid out subdivisions on each tract using both conventional designs and cluster development plans that feature smaller lots. The cluster designs led to cutting the amount of impervious surface by up to 35 percent. The denser developments also left more area for green space and incorporated bioretention areas between rows of houses to channel stormwater.

Cluster developments produce less stormwater runoff and cut the levels of nitrogen, phosphorus, and sediment.

Devine and Baran then used a U.S. Geological Survey software model and a formula used by Brunswick County planners to determine the environmental impact of the cluster designs. Compared to conventional subdivisions, the developments were shown to produce less stormwater runoff, and the levels of nitrogen, phosphorus, and sediment were each cut by 13 to 40 percent. The findings show the need for a new approach to development, says Tomlinson, a former developer who designed the Long Bay Club golf course community in Myrtle Beach, South Carolina. “They now sell home sites like we’d sell carpet—by the square foot,” he says. “We should be using natural assets to create amenities in developments that encourage buyers to forgo larger lots.”

“As North Carolina grows, more communities will need to address their runoff problems.”

The DENR project is only an advisory effort, and it remains up to county officials in Brunswick and elsewhere to use the findings to guide future development. “As North Carolina grows, more communities will need to address their runoff problems,” Hunt says. “We need as many tools as possible to deal with stormwater. Continued research will allow us to predict which tools will work best in certain areas. After all, we’re all downstream.”


Dr. Bill Hunt says permeable pavement, like that used for this Centennial Campus sidewalk, reduces stormwater runoff.

A bioretention area, such as this one on Centennial Campus, is a depressed plot along a street or in a parking lot that uses vegetation to filter stormwater runoff. The soils in the bioretention area can hold excess water following a shower.

Jay Tomlinson and his College of Design students laid out mock subdivisions in Brunswick County to show that developments with smaller lots and stormwater management strategies would reduce runoff and water pollution.

Drs. Hugh Devine and Perver Baran used two different calculations to measure changes in stormwater runoff between conventional and cluster developments in Brunswick County.

A mock subdivision along the Cape Fear River in Brunswick County shows more green space and less impervious surface per home when a cluster development, right, is used than in a conventional layout, left.