UF Researchers Developing More Accurate Method To Predict Rip Currents

May 21, 2003

GAINESVILLE, Fla. — As people flock to the beach for Memorial Day weekend, the rolling waves and blue seas could hide a deadly threat: rip currents.

But a better tool for predicting the currents – which cause more deaths on average each year than hurricanes, tornadoes, storms and lightning combined – could be available soon.

By correlating records of rip-current rescues with wave conditions, University of Florida researchers have come up with an index that shows promise to more accurately predict the currents than the one now used by the National Weather Service. The index appears to be particularly good at predicting days when there will be lots of rip currents – information that, if available early each day, could prompt beach managers to add lifeguards or even temporarily close beaches.

“Lifeguards could use a heads-up to the effect that this is not just going to be a rip-current day, this is going to be a major rip-current day,” said Bob Thieke, an assistant professor of civil and coastal engineering who heads the ongoing project.

Rip currents occur when water pushed between a sandbar and the beach rushes seaward through a channel in the bar. Drownings happen when swimmers, finding themselves pulled rapidly out to sea, fight the current and become exhausted. Swimmers who don’t struggle may escape by swimming parallel to shore out of the rip, but many people panic and drown, researchers say.

Rip currents, which cause about 150 deaths nationally each year, killed an average of 19 people annually in Florida between 1989 and 1999, according to a study by the National Weather Service’s East Central Florida Rip Current Program.

However, most people caught in rip currents are rescued – and nowhere more than in Volusia County on Florida’s east coast, where the UF researchers gathered their data.

With 3,000 rescues of all kinds annually, Volusia County averages more rescues each year than all of Florida’s other counties combined, according to the weather service study. That’s partly because of the natural occurrence of offshore sand bars along Volusia’s coast but mostly because of the huge popularity of Daytona Beach as a swimming destination, experts say.

The UF study examined Volusia County lifeguards’ records of rip-current rescues for 1996. The researchers chose that year because that’s when a separate Army Corps of Engineers project gathered extensive data on wave conditions in the area.

Lifeguards recorded 686 rip-current rescues in 1996, Thieke said. A comparison of these rescues to the wave conditions documented in the Army Corps records revealed something not documented before: The rescues occurred most often when the waves were hitting the beach square on instead of at an angle. Because local storms or windy conditions tend to cause waves to arrive at an angle, the researchers discovered that rip-current rescues often increased as storms moved away from the coast and wave heights dropped – just as many swimmers would expect the danger to be falling.

The researchers drew on their discovery to craft the new index for predicting rip currents. The index used by the National Weather relies on variables including wave height, wind, the presence of rip currents previously and wave period, or the length of time between waves. The UF researchers added wave direction to the mix, then compared the accuracy of both indexes in predicting the 1996 rescues.

The result: The UF formula more accurately predicted the date of the rescues, especially the days when lifeguards performed a large number of rescues. For example, there were eight days when lifeguards rescued at least 20 people. The UF model predicted all eight, while the National Weather Service model predicted only five. Overall for the year, the UF model was 31 percent more accurate at predicting days with rip-current rescues than the National Weather Service model, and it predicted 23 percent less false alarms.

Thieke cautioned that reported rescues are not the most accurate indicator of rip currents. For example, on a cloudy day when few people are swimming, there probably are few rescues, but there may very well be rip currents. Cold weather also discourages swimming and reduces reported rescues.

To get around that problem, he and a team of researchers recently completed a three-week research project at Ormond Beach on Florida’s east coast that used sensors instead of rescues to gauge the presence of rip currents. That project, which will combine data about wave conditions with the more accurate data about the presence of rip currents, is expected to make the new model even more accurate.

Thieke credits former UF student Jason Engle with much of the research so far. Engle, who graduated from UF in May with a master’s in civil and coastal engineering, did the research for his thesis. Other participants include UF civil and coastal engineering Professor Dan Hanes and graduate research Professor Robert Dean. The project has been funded by Florida Sea Grant, which has contributed about $250,000 so far.

Joe Wooden, deputy beach chief of Volusia County beaches, said Volusia’s experienced lifeguards are good at locating rip currents and determining whether a day is likely to present rip-current problems. He said a more accurate index would be useful provided it was reliable.

“Anytime you have research or additional resources or tools to rely upon to protect a bather or public safety, of course it’s a good tool, but it’s got to be accurate and something that’s consistent, and when you’re dealing with Mother Nature, that can be difficult,” he said.