Naval Postgraduate School GPS-Drifter Research Makes Waves

By Barbara Honegger, Naval Postgraduate School Public Affairs

May 4, 2010 - MONTEREY, Calif. (NNS) -- In surf zones near the Naval Postgraduate School (NPS)and around the world, a NPS oceanography assistant professor is quietly revolutionizing the understanding of rip currents, turning them from a swimmer's worst nightmare into a friend.

Instead of expensive GPS sensors that relay position data in real time, Jamie MacMahan and his students mount low-cost GPS devices with memory cards inside swarms of drifters whose brightly-colored florescent poles bob in the waves like giant pick-up sticks. The dozens of position-tracking sensors are then retrieved and their saved location and velocity data post-processed to reveal detailed surf zone flow patterns across various wave and tidal conditions.

This more comprehensive data produced paradigm-shifting discovery. The conventional view of rip currents -- that they're fast, narrow flows moving away from shore and that everything in the current goes out to sea -- is dangerously incomplete. Rather, they're a system of giant whirlpools whose circular eddies act like conveyer belts to carry a swimmer who treads water safely back to shore; and only 10 to 20 percent of the currents exit the surf zone.

"I've jumped in a lot of rip currents all over the world, and I've always ended up back on shore," said MacMahan. "Many experienced surfers know this, and even talk about 'riding the rip.'"

According to MacMahan, who won the Naval Postgraduate School Menneken Faculty Research Award in 2009, the whole idea is to keep the drifters cheap, so you can field 40 or 50 instead of just four or five, and then do an aggressive analysis afterwards.

"This new view matters because rip currents are the third deadliest natural hazard in the U.S., after heat waves and floods, and kill more people in Florida than lightning, hurricanes and tornados combined," said MacMahan. "Each year in the U.S., 18,000 swimmers and divers have to be rescued from these currents, representing 80 percent of all surf zone rescues, and 100 of them drown. We're working with lifeguards on how to integrate the new view into their training and public awareness campaigns. We presented our research results and the new, fuller paradigm they represent at the first International Rip Current Symposium held at Florida International University in February."

The new rip current paradigm is also important for littoral, amphibious and diver operations, as it overturns the conventional view that rip currents transport most suspended material out of the surf zone to the inner shelf and don't have a significant alongshore impact. The more complete view -- that 80 to 90 percent of these currents remain within the surf zone -- means that the vast majority of floating sediments, bubbles, plankton and nutrients simply re-circulate in the zone.

The mechanisms behind rip currents are also important for tracking and predicting the trapping of pollutants from rain runoffs and harmful algae within surf zones.

As for the future, surfers know "The Seventh Wave" is the biggest. With the new rip current paradigm, it looks like MacMahan has caught his "Big One" and that his NPS research will be making waves for a long time to come.