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

THE BALTIMORE SUN

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.

"Our work fits very neatly with the other instruments we have at Sandy Duck," Stanton said. "They fill in all the other places. With individual lab experiments and tiny field experiments, you just can't get the same results."

The research at Sandy Duck is financed by the U.S. Geological Survey, the Army Corps of Engineers and the Office of Naval Research. For the geological survey, the experiment offers a rare opportunity to integrate observations of geological change as it occurs with the long-term evidence of the landscape. "In the last few years, we have seen coming together the process folks with the geologic folks," said William Birkemeier, the engineer who runs the research center. "Generally these groups are on different time scales. But there are some interactions going on here that are not inconsequential." For example, he said, sand that lies over ancient mud deposits moves differently than does sand lying over harder materials. One of the best ways for ZTC geologists to decipher the geological evidence of ancient beaches "is to come out here and see what is being laid down now," he said.

The missing sand

The Corps of Engineers also has a more practical interest in the nearshore. Each year, it spends tens of millions of dollars to pump sand onto eroding beaches - only to see much of it wash away quickly.

Engineers who design and build beach replenishment projects say this "missing" sand has simply moved offshore a bit, where it can function as a kind of wave-breaking buffer for the beach. By contrast, many geologists believe that the sand washes far away into inlets or into such deep water that it will never make its way back onshore.

The experiments at Duck will help determine who is right, and under what circumstances.

But the comings and goings of beach sand are crucial for more than sunbathing. More than half the financing for Sandy Duck came from the Office of Naval Research, which has been shifting its research since the end of the Cold War to what it calls "logistics over the shore," the problems of making amphibious landings and maintaining beachheads. A big factor there is the "traffic ability" of the beach, according to Dr. Thomas H. Kinder, an oceanographer who heads the Navy office's program on coastal dynamics.

Kinder is particularly interested in video and other techniques being developed at Duck that allow scientists to evaluate beach conditions and track sand movement at long distance. "We don't know what beach we want to understand in advance, and we might not be invited in to make measurements and do experiments," Kinder said. "That's a reason why you see a lot of remote sensing here."

Combining video cameras and other devices with the vast array of instruments making detailed observations allows scientists to improve the accuracy of the remote sensing systems. "There is great power in doing these techniques together," Kinder said.

The focus on sand came about, he said, because "the feeling was there had been really wonderful progress in understanding the fluid mechanics of the nearshore through field experiments and lab experiments and other techniques."

"With sediment mechanics - the forces that move individual grains, the erosion and deposition, bars and ripples - progress had not been nearly as great," he noted.

Since June, people involved in Sandy Duck have been working more or less constantly. "My family has hardly seen me," Birkemeier said.

Pub Date: 10/26/97

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