OREGON STATE UNIVERSITY

OSU Researchers Study Deadly Rip Currents That Are “More Common Than Rare”

07/06/2007

CORVALLIS, Ore. – Strong “rip currents” have been blamed for several incidents along the Oregon coast in the past week, including the disappearance of a teenager swimming near Cannon Beach. Experts say these rip currents are more common than rare, and can at times be deadly.

Yet researchers at Oregon State University, who have been studying the phenomena for years, say rip currents can be hard to see from the beach, and harder still to predict.

“Perhaps the best way to identify a rip current is to look at the long-shore current and see if it changes direction,” said Robert Holman, a professor in OSU’s College of Oceanic and Atmospheric Sciences. “If you’re looking out toward the water and you see the long-shore current coming from both the left and right, there’s probably a rip current in front of you.

“They are easier to identify from above,” he added. “You can more clearly see a sandy plume of lighter-colored water heading out toward the ocean carrying sand, as well as debris and organic material.”

Holman is nationally known for his studies of sand movement and near-shore currents. Since the early 1990s, he has collected a series of time-exposure images that allow researchers to delve into the mechanics of the ocean that result in rip currents.

Often mistakenly called “riptides,” these currents are usually caused by a gap cut into the near-shore sandbar that helps drain the water driven by waves high on the beach. But the overall effect can be the creation of an offshore river channel with a powerful current, moving at speeds of 1.5 meters per second or more – forceful enough that even strong swimmers have trouble bucking the current.

“Rip currents are notoriously hard to predict,” said Tuba Ozkan-Haller, an assistant professor of oceanography at OSU who has been involved in a project funded by the Office of Naval Research to create a predictive model for rip currents. “One day they can be there, the next day there may be local wind chop instead of an ocean swell, and they’re gone.”

Ozkan-Haller and her colleagues created a predictive model of rip currents for a section of the California coast then monitored the location for a month to see how well they did. They had some success in predicting general areas where rip currents might occur and efforts to validate those findings are under way. Having accurate underwater topography information significantly increases the researchers’ ability to predict rip currents.

“We hit on a lot of them,” Ozkan-Haller said. “And we learned from the process that the offshore terrain plays an important role. When there is a canyon beyond the surf zone, it modifies the waves as they come over and focuses them in a way that makes rip currents more likely. This non-symmetrical bathymetry is a key not only in creating rip currents, but in the direction the water gets funneled out to the ocean.”

Holman said rip currents also occur frequently next to rocky headlands, where the sand and water get drawn away from shore.

“Surfers actually use the rip currents quite a bit to get a free ride offshore,” Holman said. “But they can be dangerous, too. Trying to swim against the current is like trying to swim up a river. Your best bet is to angle away from the shore and get out of the current.”

Holman has seen children caught in a rip current at the Oregon coast and survived; Ozkan-Haller has a friend whose father was killed when caught in a rip current in South Africa. These rip currents annually kill an estimated 100 persons in the United States alone, and many more throughout the world.

“The thing to remember,” Holman said, “is that it’s a dynamic system out there. The force of the waves and the channels in the sandbars play a major role, but it can change from day to day. It may be best to assume that there is a rip current offshore.”