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Geographic website, 1 Aug 05 Ancient Channels, Seasons Shape Beach Erosion, Lab Finds John Roach for National Geographic News For William Birkemeier and his colleagues, just about every day is a day at the beach. Birkemeier is trying to figure out how coastlines keep changing. "There's a lot of mystery in the coast," he said. Just one mystery is how recently discovered channels on the ocean floor dating back to the Ice Age are affecting beach erosion. Another puzzle has to do with how beaches grow, and then shrink, according to the seasons. Birkemeier is the director of the U.S. Army Corps of Engineers' Field Research Facility (FRF) near the town of Duck on North Carolina's Outer Banks chain of barrier islands. There, Birkemeier and his colleagues make daily measurements of the relentless sculpting of the coastline by waves. On some days violent storms rip entire sand dunes out to sea. On other days the calm surf lulls the sand, grain by grain, back to the beach. According to Birkemeier, there's a natural rhythm to the process—the sand moves offshore during the stormy fall and winter months and back onshore during the calmer days of spring and summer. Nearly three decades of daily data on the churning surf have allowed Birkemeier and his colleagues to "begin asking better questions" about how and why coastlines erode, he said. One of his colleagues is Jesse McNinch, a professor at the College of William and Mary's Virginia Institute of Marine Science. McNinch points out that the FRF is one of the very few research centers of its kind. "The greatest thing is, it's a facility on an active, energetic surf zone and beach, which makes it a rare commodity in an extremely difficult place to work," McNinch said. Erosional Hot Spots McNinch has spent seven years using FRF's equipment to investigate why some areas of beach erode much more drastically than others. He had heard stories from summertime beach visitors about hurricanes obliterating the beach in front of one house, while the house next door went unscathed. "I always chalked them up to anecdotal stories with not much science to them," he said. But FRF data, combined with measurements collected by the U.S. Geological Survey, proved the stories to be true. Intrigued, McNinch is now trying to understand what drives the phenomenon. The answer will help developers understand where not to build beach houses, among other things. To probe for an answer, McNinch attached seismic gear to the facility's amphibious vehicle called a LARC (Lighter Amphibious Resupply Cargo) and mapped the seafloor in the surf zone. "That's an area that's really been a no-man's-land," he said. "We were able to map the geology underneath it for about 50 kilometers [31 miles]. "And the exciting thing for us that was amazing was, when you look at the maps [and] the collected data, you see underlying the beach in the surf zone were old Ice Age river channels," McNinch added. The channels corresponded with the areas of high beach erosion, known as erosional hot spots. The channels had eluded scientists until now, because they are filled in with sand and are invisible to the unaided eye. Now McNinch is now trying to understand why the channels cause beach erosion. Improving Models Erosional hot spots are one more piece of the coastal-dynamics puzzle. And the more pieces to the puzzle that researchers have, the better the computer models will be that Birkemeier and colleagues can supply to planners and developers. When Birkemeier started work at the Field Research Facility, he said, all the models of sand movement were one-directional—offshore, tracking sand as it left the beach. "But beaches wouldn't exist anywhere if most of the time sand wasn't moving toward the beach," he said. Waves carry sand both ways, Birkemeier pointed out: There's a forward motion and backward motion in every wave. If the sand drops to the bottom on the backward motion, the sand moves offshore and the beach erodes. But the reverse is true when the sand drops out on the forward motion. Big storm waves tend to create more backward motion, Birkemeier said. By contrast, smaller, calmer waves tend to have more forward motion. As a result, in the stormy fall and winter months, sand moves offshore, where it piles into sandbars. The sandbars then force waves to break farther out from the coast, reducing the wave action on the beach. In the spring and summer the sandbars move toward shore as waves slowly carry the sand to land, creating the beaches visitors expect. "The beach, by creating a sandbar through the erosion process, has created a level of self-defense," Birkemeier said. links Related articles on Wild shores of Singapore |
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