Changing the Tide on Rip Currents

Rip currents can turn a carefree day into a tragedy. At least three people drowned off the North Carolina coast last summer after getting dragged offshore, and lifeguards and bystanders had to pull hundreds of others to safety from the powerful currents. Dr. Jie Yu, an assistant professor in the Department of Civil, Construction and Environmental Engineering, is trying to gain a better understanding of the mechanisms involved in the formation of rip currents so coastal communities can better forecast them and make beaches safer.

An expert in fluid mechanics, Yu is studying the interaction between waves, currents, and the movement of sand. A rip current can begin to form when waves break near shore, she says. "Once a wave breaks, its energy and momentum dramatically change and are transferred to form surf-zone currents," Yu says. Under certain conditions, these currents move in a circular pattern—water moves onshore in some places and flows back out to sea in others. The beach topography affects this circular flow since wave crests tend to follow the depth contours in the shallows near shore. Undulations in the beach can make the wave non-uniform when it rolls in perpendicular to the shoreline, Yu says, forcing the water to move sideways and converge into offshore flows—rip currents—at the low spots in the beach topography.

As the circular flows develop, they interact with the waves. "This can make the sideways and circular motions speed up," Yu says. When two of these circulation patterns are in close proximity, the outward flow confined between them becomes very strong. "Is the lack of uniformity on the beach causing the rip current, or does the circular motion create the beach undulations?" she says. "It's a chicken-and-egg proposition."

To solve the riddle, Yu is developing mathematical models that include data like wave height, the angle at which waves hit the beach, and the location of nearby sandbars. In a five-year project funded by the National Science Foundation, she will combine models to reflect the interaction of waves and currents, nonlinear dynamics, and sediment movement along the shore. "Rip currents threaten the public safety and affect the form and structure of beaches," Yu says. "If we have more knowledge about them, I hope we can make them less of a hazard."

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Dr. Jie Yu uses mathematical models to better understand the interaction between waves, currents and topography that result in rip currents.

In 2010, the U.S. Lifesaving Association reported that of 10,449 rescues performed nationally last year, 6,498 were due to rip currents.