OSU News Service

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.

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.

These strong currents of water rushing out to sea may appear as dark choppy water. Debris and foam floating out to sea is sometimes a sign of a rip current.

Rip currents annually kill an estimated 100 persons each year in the United States alone, and many more throughout the world.

"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."

Seen from above, a rip current is easier to identify - a sandy plume of lighter-colored water heading out toward the ocean carrying sand, debris and organic material, Holman said.

Unfortunately, waders and swimmers do not have the advantage of that view.

Anyone caught in a rip current should swim parallel to the shore until out of it, then swim at an angle back to shore. People who can't swim should tread water and wave or call for help.

Often 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. This can create an offshore river channel with a powerful current, moving at speeds of 5 feet per second or more - forceful enough that even strong swimmers have trouble bucking the current.

Holman has collected a series of time-exposure images that allow researchers to delve into the mechanics of the ocean that result in rip currents.

"Rip currents are notoriously hard to predict," said Tuba Ozkan-Haller, an assistant professor of oceanography at OSU. "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 off the California coast as part of a project funded by the Office of Naval Research. They 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."

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 who survived after being caught in a rip current at the Oregon coast. Ozkan-Haller has a friend whose father was killed when caught in a rip current in South Africa.

"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."

"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."

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 who survived after being caught in a rip current at the Oregon coast. Ozkan-Haller has a friend whose father was killed when caught in a rip current in South Africa.