Wave Patterns Point To Coastal Erosion "Hot Spots"

FAYETTEVILLE, Ark. Ð A University of Arkansas professor and his colleague have developed a model that shows why certain parts of a North Carolina barrier island erode faster than others. The model may help scientists pinpoint the causes of other problem areas along the rapidly-eroding shoreline on the Eastern Seaboard. The erosion problem is particularly severe in North Carolina, where hurricanes pound State Highway 12 along the barrier islands annually.

Steven K. Boss, assistant professor of geosciences, and Bill Hoffman of the North Carolina Geological Survey will present their findings Friday, March 24, at the annual meeting of the southeastern section of the Geological Society of America meeting in Charleston, S.C.

Boss and Hoffman focused on Pea Island, which has a rapidly-eroding shoreline on the North Carolina Outer Banks. The island has several "hot spots" with severe erosion, interspersed with areas that have lower erosion rates. They suspected some aspect of the offshore wave action might be causing the hot spots.

Boss and his colleagues have surveyed the ocean floor from Oregon Inlet to Cape Hatteras and westward from Cape Hatteras to Ocracoke Inlet. The researchers used sound waves bounced off the ocean floor to produce "images" of the seafloor and the layers beneath it. They also used side-scan sonar, which produces images that look almost like an aerial photograph.

"This gives us a ÔfishÕs eye viewÕ of the bottom," Boss said.

From these images, the researchers created three-dimensional images of the different layers and produced a topographic map of the seafloor.

The map shows that Pea Island has a narrow shore face that drops abruptly into a 20-25-meter deep trough, Boss said. A group of shoals lie about three miles offshore.

Using Geographic Information Systems and spreadsheet software, the researchers developed a wave refraction model by creating a series of points offshore to represent a wave front and entered them into the computer-generated map. They then calculated the speed of the waves, known as celerity, at one-minute intervals as the waves approached shore. As the waves move shoreward, the wave front bends, or refracts, as it crosses the shallow shoals. This bending of the wave fronts focuses wave energy on particular spots along the coast Ð resulting in development of erosion "hot spots." The model developed by the Arkansas and North Carolina scientists accurately predicts the location of the erosion "hot spots" on Pea Island.

"It turns out to be a simple physics problem," Boss said. "Where the offshore shoal sits determines where the greatest erosion occurs on Pea Island."

This may lead to clues to coastal erosion in other areas along the Eastern Seaboard, Boss said.