Study finds Banks Channel stormwater intervention effective at reducing polluted runoff

Crews install permeable pavement at the Hanover Seaside Club to reduce stormwater runoff and pollutants in 2017. (Port City Daily photo/Courtesy N.C. Coastal Federation)

WRIGHTSVILLE BEACH — A collaborative research effort shows the efficacy of a stormwater intervention project installed in Wrightsville Beach. 

In a study published in January in the Journal of Environmental Management, UNCW Center for Marine Sciences researchers Amy Grogan and Dr. Michael Mallin charted how an infiltration chamber and pervious surface area reduced both runoff and the pollutants that come in it from reaching Banks Channel. 

The study showed between a 47% and 87% decrease in pollutant loads with the barriers in place; before the intervention, runoff would flow directly from a parking lot grate at the Hanover Seaside Club, cross underneath Waynick Boulevard, and dump out onto Banks Channel, just feet from where people are known to park beach chairs. 

Funded by the N.C. Clean Water Management Trust Fund, the N.C. Coastal Federation and local firm Coastal Stormwater Services, Inc. replaced a portion of the club’s parking lot in November 2017 with permeable pavement. The group also disconnected nearby drains from other Banks Channel outfall pipes and rerouted them into a new, engineered infiltration system constructed underneath the parking lot, according to the Coastal Federation. 

“Instead of it going straight out into the sound, you direct it first into one of these chambers,” Mallin explained. 

Rather than flowing unencumbered into Banks Channel, runoff instead soaks into the ground and through the infiltration system before reaching the waterway. 

“It’s simply cutting down on the volume — you can cut it down by an order of magnitude,” Mallin explained. “So right there is a huge achievement by just reducing all the stormwater running in. 

First reported by Coastal Review, the study found the buffering impact of the infiltration and pervious interventions were sharply evident when comparing rain events before and after the installation. 

Designed to handle a 1.5-inch rainstorm, the system also proved effective at reducing the concentration of pollutants, particularly fecal coliform and nitrogen. Mallin was especially pleased with the reduction in nitrogen observed — “that’s the nutrient that’s most stimulatory to algal blooms,” he explained. 

Wrightsville Beach Town Manager Tim Owens said he was unfamiliar with the study but added the town has “a restrictive stormwater ordinance that requires trapping and treating the stormwater.”

Densely developed urbanized watersheds like Wrightsville Beach are most often subject to a “first flush” phenomenon; when rain hits, initial runoff collects a high concentration of pollutants. 

The stormwater intervention absorbs and treats the first flush before it reaches the channel, the study concludes. 

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