Taking the surprise out of sneaker waves

by Mary Caperton Morton
Tuesday, May 1, 2018

Most beaches in the Pacific Northwest and Northern California have signs warning beachgoers of sneaker waves. Credit: Daveynin, CC BY 2.0.

Since 2005, more than two dozen confirmed fatalities in California and Oregon have been caused by so-called sneaker waves, which surge far ashore with little warning, sometimes catching beachgoers by surprise. Most beaches in the Pacific Northwest and California have posted signs warning visitors of the hazard, but few scientific studies have been done on sneaker waves and, currently, there is no consensus on their definition or origin. A new incident database is helping clarify where and when sneaker waves are most likely to occur, an important step toward creating better warning systems.

“Here in Oregon, all the beaches have signs warning of sneaker waves, but when you search for scientific studies, the only information you typically find are newspaper stories of tragic events,” says Gabriel García Medina, a coastal engineer at Oregon State University (OSU), who is developing the database and who presented related research at the Ocean Sciences meeting in Portland, Ore., in February. Sneaker waves tend to make the news only when somebody is swept out to sea. “That means reports are very biased toward events where somebody is hurt or killed and somebody else was there to witness the event,” he says.

Unexpected wave incidents also occur in Southern California and on the East and Gulf coasts. But the warmer water and calmer wave and rip current conditions that prevail along those coasts, relative to the U.S. West Coast, mean those incidents are less likely to end in tragedy. “From the middle of California all the way up into Canada, getting swept into the water can be life-threatening,” says Troy Nicolini, a meteorologist with the National Weather Service based in Eureka, Calif., who was not involved in the research presented at the Ocean Sciences meeting but has collaborated with García Medina and his colleagues on related research.

“There are many reasons why people get in trouble in the coastal zone,” Nicolini says, which can make discerning sneaker events difficult. “It can be hard to separate a rough wave event or a cold-water drowning from a sneaker wave event” without eyewitness reports, he says.

García Medina and his colleagues began their study by defining a sneaker wave as “a wave event when somebody thought to be in a safe zone was caught off guard.” They then characterized beaches and regions where sneaker waves are more likely to occur by performing threshold analyses for various beach settings to determine the likelihood of anomalous waves reaching beachgoers. They also sought to identify the physical controls on large wave run-ups, such as beach topography and the existence of protective structures such as jetties. Sneaker waves also occur on beaches with no witnesses, of course, but without cameras or monitoring equipment, they’re very difficult to identify.

“I’m very excited to see the OSU team bring this amount of rigor to the study of sneaker waves,” Nicolini says. “It has always been a struggle to connect the dots with these kinds of events. We have hypotheses about why they occur when and where they do, but very little data with which to test those hypotheses.”

The team identified 27 cases in Northern California and Oregon in which witnesses confirmed that victims standing on beaches were struck by surprisingly large waves. The researchers also determined that sneaker wave events occurred over a wide range of wave heights —emerging not only from stormy seas but sometimes also from calm seas — but that most were associated with long-period swells arriving from distant storms. For the majority of the fatalities, simulated wave run-ups on the particular beaches where accidents occurred correctly predicted that the water level could reach victims' locations, suggesting that an accurate warning system could be created for individual beaches.

“A warning system would have to operate on a local basis, starting at beaches that have had multiple incidences of sneaker waves,” García Medina says. “We’d be looking to predict the likelihood of a wave overtopping a beach, berm or jetty, given certain weather and wave patterns, which could be monitored using buoys or cameras. That might be much more effective than just issuing blanket warnings for entire coastlines.”

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