Sand, sea and surf--a U.S. Geological Survey report takes a holistic approach to studying coastlines to determine how best to save the beaches
Carina Storrs, Scientific American 30 Oct 09;
Myrtle Beach's popular oceanfront is retreating at a rate of up to 30 centimeters per year. But visitors who flock to that part of South Carolina's Atlantic coast continue to enjoy its wide, sandy stretches, because the state refills them every seven years or so with sediment dredged from the sea bottom.
Deciding whether to re-sand an area of beach is one impetus behind a study by researchers from the U.S. Geological Survey (USGS) and the South Carolina Sea Grant Consortium, who are presenting its findings this week at the International Geological Program Annual Conference in Myrtle Beach. Called the South Carolina Coastal Erosion Study (SCCES), the multiyear project examined the 100-kilometer-long stretch of the northern South Carolina coast known as Long Bay, which includes Myrtle Beach. By gathering information for the entire bay area, the scientists can make better predictions about erosion rates and the best ways to restore beaches. Moreover, the study's wide-scale approach could be applied to improve erosion projections in other areas.
"It's a holistic system as opposed to how we might have done things 20 years ago," says Paul Gayes, director of the Center for Marine and Wetland Studies at Coastal Carolina University in Conway, S.C. Gayes is one of the collaborators involved in the study, which began in 1999.
Departing from the decades-old approach, the SCCES surveyed a broad swath of land and sea. "What's typically done is you go out and you might have a footprint that's one percent of [our 100-kilometer area]. We take the big picture approach," says Walter Barnhardt, a USGS marine geologist who worked with the study. As Barnhardt explains, erosion is caused by "energy in the ocean that's working against the architecture of the beach"—energy that typically is generated by storms and rising seas. Its erosive effects are not just determined by the conditions in the 1 percent footprint, but also by the ocean's depth, current and geology all along a segment—in this case, Long Bay.
To get a broad picture of Long Bay, the SCCES scientists used sonar to measure water depth and explore the geology along the ocean floor. They also placed a series of three-meter-tall tripods along the seabed from which they hung various sensors to measure wave height, current and sediment flow.
The measurements revealed a number of variations in the ocean conditions along Long Bay. For one, different sections along the beach have deeper waters that generate larger waves, which pound at the sand strongly and accelerate erosion rates. Variations in ocean depth could help explain why different parts of the beach are eroding at different rates.
The team also found that currents and ocean geology are causing sand on the sea bottom to move in a southeasterly direction, starving the waters to the north of sediment. This movement has created deposits in the southeastern section of the bay that could serve as sources of sand for dredging to replenish the beaches, Barnhardt says.
USGS scientists have already launched similar bay-wide surveys of coastlines in North Carolina and Massachusetts. As with SCCES, the same types of sonar and sensor measurements will be taken in these locations to measure water depth and flow as well as seafloor geology. Although these measurements will span several years, they will provide information about coastlines in a shorter time frame than those taken in topographical studies, which require decades of data to reveal patterns, Barnhardt says.
With the predictive information about erosion rates at hand, Long Bay's state and local governments can find the most promising dredging locations before making an expensive investment. The decision to replenish Myrtle Beach every seven or eight years may be an easy one, considering that the area brings the state billions of dollars in tourist spending. But to the south, where history and the recent geologic studies predict faster erosion, the beaches might require dredging and refilling every couple of years.
The mapping data will also aid planning for new construction projects. In Massachusetts, for example, engineers are using this information to design foundations for offshore wind turbines. "If you want to build something on land, you would want to know the geology. We're just doing it underwater," Barnhardt says.
