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People who return to the same beach year after year have most likely noticed that sometimes the beach is big and sandy, whereas other times it seems to have all but disappeared. For years scientists have understood how pounding waves during violent storms suck sand offshore. How it reappears on beaches once the stormy seas have passed, however, remains a mystery, and without that information, accurately predicting beach evolution has been impossible. Now researchers are one step closer to foretelling sand's journeys. A model that successfully forecasts how sandbars that protect beaches from harsh wave action migrate away from shore and back again over several weeks is described today in the journal Science.
Nearly a decade ago, Steve Elgar of the Woods Hole Oceanographic Institution (WHOI) set up a series of floating pressure gauges, sonar devices and water velocity meters on a beach in Duck, N.C., in an attempt to determine what changes in wave action bring sand back onshore. For two months he measured the changing currents and sand distribution from a point on the shoreline out to a water depth of five meters. Based on those measurements, Elgar and Fernanda Hoefel, a graduate student at the Massachusetts Institute of Technology and WHOI, have spent the past few years developing a mathematical model that accounts for changes in wave velocity and predicts sandbar migration. Their model is the first to accurately anticipate observed changes in onshore and offshore sandbar movements, and it did so successfully over 45 days.
The broad, sandy beach at Duck is representative of most East Coast beaches because it has one sandbar, and the waves that pummel the coastline there are the same as those that affect sand transport from Maine to Florida, Hoefel explains. But because it's difficult to set up measuring devices in the surf zone, as they did at Duck, the pair is now working on a way to measure wave properties using a single offshore buoy. If the new technique can accurately collect data, the model will be easier to apply elsewhere to predict when a strong undertow might endanger swimmers or when it might be appropriate to begin beach replenishment. Their goal, Hoefel says, is to one day be able to say: "Tell me what the waves are like offshore and I will tell you what the beach will look like in a few months."