Scientists investigate erosion, sand movement on North Carolina's coast

NC State graduate students Periann Russell and David Bernstein take global positioning system measurements near Cape Fear. Full size image available through contact

It's not without reason that the sandy coast of North Carolina is known as the Graveyard of the Atlantic. Shallow, shifting shoals and violent currents have wrecked ships along the Outer Banks for hundreds of years.

Those same dangerous conditions are the reason scientists know relatively little about how the Atlantic Ocean shifts and deposits sand along the shores of North Carolina's barrier islands. A geologist at North Carolina State University, however, is working to unlock the secrets of sand transport and beach erosion from Corolla to Cape Fear.

Dr. Tom Drake, assistant professor of geology at NC State, is investigating two phenomena crucial to scientists' understanding of how the ocean affects sandy barrier islands like the Outer Banks. He is examining the dynamics of erosion "hotspots" on those islands, and he is studying how ocean currents transport sand to massive and treacherous shoals off Cape Hatteras, Cape Lookout and Cape Fear.

That research could help predict where erosion is likely to threaten homes and businesses, and could improve the success of projects that pump sand onto eroding beaches. Real estate development has boomed along North Carolina's coast in recent years, and beach-related tourism is an important part of the state's economy.

Drake and his colleagues, Dr. Jesse McNinch at the Virginia Institute of Marine Sciences and current graduate student David Bernstein, will present their findings Friday, April 6, at the Geological Society of America's Southeast Section annual meeting in Raleigh.

Among their significant discoveries is that "hotspots" � areas of the beach that "erode like crazy" during hurricanes and nor'easters, but recover quickly afterward � are caused by layers of mud and gravel just below the sea bed and near the shore.

"There's something under the sand causing these hotspots," Drake said. "It's a fact that there are definitely geological structures there under the sand. But we still don't understand how waves move huge amounts of sand during a storm and then quickly replace it." He speculates that the mud and gravel substrates inhibit the development and migration of submerged near-shore sand bars, which help protect the beach from the ocean's pounding waves.

The research results were not easy to come by: The shallow and energetic zone along the beach isn't well understood by scientists because it's too dangerous for most research boats. Drake and McNinch, however, used a U.S. Army Corps of Engineers amphibious vessel called the LARC (Lighter Amphibious Resupply Cargo) to survey a hotspot near Duck, N.C., at the Corps' field research facility there.

Specifically, they used both bathymetric and chirp sonars to map the constantly changing surf zone bottom, and to determine what lies beneath it. "We found that the bottom of the surf zone is not just a uniform layer of sand," Drake said. "The thickness of the sand layer is highly variable, with a non-erodible substrate just underneath it."

Such findings are important because erosion hotspots pose problems for beach nourishment projects � efforts to rebuild and widen beaches by transporting sand from offshore.

In a separate research project, Drake and Bernstein are studying the constantly changing shapes of North Carolina's Cape Lookout and Cape Fear as a way to better understand how "longshore currents" move sand along the coastline.

Those currents are the reason the shoals off Cape Lookout, Cape Hatteras and Cape Fear have claimed so many ships � and so many lives. "The longshore current is very much like a river, a conveyor of sand that dumps it out in the shoals beyond the capes," Drake said. "Those shoals seem like the ultimate resting place for all that sand. As far as we can tell, it's a one-way trip."

Studying how currents, waves and tides shape the shoals themselves is far too hazardous, even on a small craft like the LARC. Instead, the researchers measure how the beaches at Cape Lookout and Cape Fear shift under differing weather and wave conditions.

"One thing that surprised us is the incredible variability of the capes, changing with the tides literally hundreds of meters," Drake said. That, he explains, means that the tides and currents and waves move a massive amount of sand each and every day.

Where that sand goes depends on the size and direction of ocean waves. During winter nor'easters, sand is pushed south, creating a south-southwest-pointing sand spit that can extend 50 meters past the base length of the Cape Lookout point. During other times of the year, when the current shifts directly south, the point may be lengthened by more than 150 meters to the southeast.

Drake and his colleagues are studying the shape of Cape Fear on Bald Head Island to monitor how the ocean redistributes sand after an upcoming beach nourishment project.

Drake's near-shore geology research is supported by the U.S. Army Research Office and the National Research Council. The research on North Carolina's cape points is supported by the Army Research Office, the Bald Head Island Conservancy and the National Park Service.

NC State News Services
Campus Box 7504
Raleigh, NC 27695-7504
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Contacts: Dr. Tom Drake, 919-515-7838; [email protected] or Kevin Potter [email protected].

Note to editors: For photos of NC State researchers taking Global Positioning System measurements near Cape Fear, contact NC State News Services at 919-515-3470 or http://[email protected].