Buried Treasure: System Cleans Runoff
Yellow signs dot North Carolina's beaches, warning people about stormwater discharge from nearby outfall pipes. Heavy rains wash contaminants off coastal roads and parking lots, and the pipes flush the bacteria and other debris onto a beach or directly into the ocean. State health officials sometimes issue beach advisories or alerts when tests show high bacteria levels near outfall pipes, so Kure Beach and state transportation officials asked Dr. Mike Burchell, assistant professor and extension specialist in NC State's Department of Biological and Agricultural Engineering, to find a way to keep their local beaches open rain or shine.
Burchell's team decided to use the most obvious feature of the beach town—sand—to filter the stormwater. "Coastal property is so expensive and scarce that it would be difficult to locate a treatment system downtown or in a neighborhood," Burchell says. So, he designed a system that runs under the dunes and allows the stormwater to seep into the groundwater, where it can flow laterally toward the ocean. Figuring sediment and debris would clog the small pipes in a septic drain field-type of system, he instead used large, open-bottom chambers that are commercially available to create three 100- to 200-foot-long underground infiltration systems situated parallel to the shoreline. Stormwater is diverted before it reaches outfall pipes, filling the chambers before gradually filtering through the underlying sand.
During heavy storms, some overflow still makes it to the outfall pipe, but Burchell says tests have shown that the dune infiltration system captures 80 to 95 percent of the stormwater. "The degree of difficulty in managing stormwater increases as we get closer to the center of town," he says, noting one of the systems is located near the Kure Beach pier, where more parking lots mean more runoff. "We still capture most of the bacteria that washes off early in the storm." Monitoring wells in the dunes also show no appreciable increase in bacteria levels in the groundwater after the runoff drains through the sand. "We want to make sure that diverting stormwater into the groundwater does not have a negative impact," Burchell says.
The dunes atop the filtration system have been replanted with sea oats and grasses, so beachgoers don't even know the underground chambers are there. Burchell says the system could be used on other North Carolina beaches, but it's more efficient on higher and wider beaches, where runoff can filter through more sand before reaching the groundwater. "There is no silver bullet for handling stormwater runoff," Burchell says. "But this system presents a low-cost, low-tech option for many communities."
Agencies providing funding for research cited in this story include:
- NC Department of Transportation
- Water Resources Research Institute of the University of North Carolina