N.C. beaches yield their secrets a grain at a time Military program mapsthe shifting sands of Atlantic coast

October 26, 1997|By NEW YORK TIMES NEWS SERVICE

DUCK, N.C. -- Just before a recent, rosy dawn on the Outer Banks of North Carolina, a few men crossed the dunes and walked across the beach toward the waterline, where a strange contraption squatted in the sand. A metal-pipe framework 7 feet square at the base and about as high, it was hung with dozens of sensors, meters and other devices.

After a few minutes of tinkering and tightening, the men pronounced the metal frame and its instruments fit. Then a still stranger contraption pulled up, a wheeled 35-foot-high tripod with a tiny cab and grumbling engine at its apex. The men attached a chain from the frame, called a sled, to this Coastal Research Amphibious Buggy and it rolled slowly into the surf, towing the sled behind it.

The CRAB shuddered with each wave as it moved into deeper water.

"How far do you want to go out with this?" its driver inquired by walkie-talkie.

The men on the beach pondered their reply as the sled slowly vanished beneath the water.

'Where the action is'

"When the back wheels start coming off the bar," Dr. Timothy P. Stanton, one of the men on the beach, answered. The CRAB tilted crazily as first its front wheel and then its two back wheels passed over the sandbar. "That's good," Stanton declared. "In the trough. That's where you get the maximum amount of sand going longshore and cross-shore. That's where the action is."

The sled had almost disappeared in the depression along the bar. The sun was up. Another day's work was about to begin.

Stanton, an oceanographer at the Naval Postgraduate School in Monterey, Calif., is a principal investigator in the largest coastal research project ever undertaken. Its aim is to understand what drives the movement of sand along the shore, and how beaches change shape in response to changing wind, wave and water conditions.

If all goes well, the work will help engineers repair eroded beaches, assist the Navy in planning amphibious landings and offer geologists a clearer view of the forces that shape the landscape.

Work began in June when researchers and their truckloads of instruments began arriving at the Army Corps of Engineers Field Research Facility here on an undeveloped stretch of beach. For weeks, with scientists and engineers based at the center, they worked to install instruments in the surf and prepare others for deployment on sleds, boats and jet skis. Scientists set up mini-laboratories in trailers parked in the dunes.

They joined engineers and technicians from the Army who work here full time. Theirs is a day-by-day, yearlong effort to gather data on weather, water and beach topography using the CRAB, a 140-foot tower and a 1,840-foot pier at the research center. Every few years, dozens of scientists gather to pool these data with their own expertise in intense large-scale research efforts. This year, more than 100 scientists are working on dozens of projects.

Sandy Duck

While efforts in the past have focused on waves or currents, this year's experiment, named Sandy Duck, is intended to answer vexing theoretical and practical questions about how sand moves in the surf zone, the region whose features largely determine whether and how a beach will erode.

Like others working here, Stanton is studying ripples, the parallel ridges of sand that form in the oscillations of the surf zone. He is interested in the boundary layer at the bottom, where sand becomes suspended in water and begins to move. "It turns out ripples have a first-order effect on the mechanics of the bottom layer," he said. The more the bottom is rippled, the more sand moves around in the water, he said.

He designed one of the instruments on his sled, a Bistatic Coherent Doppler Velocimeter, which measures water velocity and sediment concentrations centimeter by centimeter from the bottom up to one meter. (Full-timers at the pier "tell me I need work on my acronyms," Stanton confessed.) Other instruments on the sled measure water levels, wave frequency and even bubbles in the water.

Using the CRAB, Stanton and his colleagues can position the sled in deep water more than 200 yards from shore and then slowly pull it in, collecting data all the way.

Though theories of how sand moves have been tested in experimental wave tanks, it is impossible to duplicate natural conditions in the laboratory. In addition, at Sandy Duck Stanton and his colleagues can draw on the results from dozens of other scientists gathering other kinds of data nearby. For example, researchers led by Daniel M. Hanes of the University of Florida and Chris Vincent of the University of East Anglia, in England, have observed sand moving off ripples in puffs as waves pass by overhead. Another researcher, Professor Peter Howd of the University of South Florida, is taking core samples of ripple fields to measure how much sand is deposited in ripple fields over time.

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