THE pull of the ocean lures people to beaches the world over, but its draw doesn’t stop at the edge of the sand. At many beaches, broken waves quickly regroup into hidden torrents that sweep thousands of beach-goers straight out to sea every year.
If you’ve ever spent time wading in the sea, you have probably felt one of these so-called “rip currents” tug at your ankles as it surges out through a calm spot in the breakers. That may seem harmless enough, but if you get caught in one while swimming the result can be deadly.
The advice about escaping a rip current – sometimes wrongly called an undertow – has been doled out for decades: swim parallel to the shore. Yet rips remain a real danger. In the US alone, they contribute to about 100 drownings a year and account for about 18,000 lifeguard rescues – more than 80 per cent of the total. Rips are the third most deadly natural hazard in the US after heatwaves and floods.
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People usually die in rips when, after discovering they are out of their depth, they strike for shore and exhaust themselves fighting against the invisible current. Breaking surf makes matters worse, pushing tired swimmers underwater or swamping them as they gasp for breath.
“People die in rips because they exhaust themselves fighting against the invisible current”
Despite the infamy of rip currents, oceanographers have very little first-hand experience with them. So earlier this year, a research team led by Tim Stanton and Jamie MacMahan from the Naval Postgraduate School (NPS) in Monterey, California, mounted a field campaign to get to grips with rips. The team spent more than a month hanging out on the beach, planting current sensors into the sea floor and tossing GPS-equipped drifters into the surf to map the currents. A few brave souls even swam directly into the rip, then relaxed and waited to see where they would end up. Among their initial results are the most detailed maps yet of real rip currents, and they contain some surprises.
For decades, rip currents defied researchers’ best efforts to study them in detail, tearing instruments from their moorings or preventing their installation in the first place. żěè¶ĚĘÓƵs had to settle for spotty measurements and fill in the blanks using fluid dynamics models. The result is a simplistic picture of a straight-sided river gushing out to sea for hundreds of metres, eventually petering out in a floret of small eddies called the rip head (see Diagram).
Models aside, researchers found that rip currents can drag objects nearly a kilometre from the shore at speeds of up to 2 metres per second – fast enough to overpower the strongest of swimmers. They are common on sandy beaches where waves approach at right angles to shore. Paradoxically, this can make sheltered and apparently safe beaches especially prone to rip currents because the headlands diffract arriving waves in just the right way.
Rips are also more likely where the sandy seabed has been reshaped by waves to form gullies and sandbars. As waves pile onto the beach, the gullies can funnel the water into “feeder channels” that converge into a rip current heading out to sea through a channel it has gouged in the sandbars. The currents tend to form in roughly the same places, though they shift around from day to day and week to week.
The standard view of rip currents began to change in 2001 when the NPS team ran a six-week experiment at Sand City, a beach close to their lab. They fed data from 13 fixed sensors into a fluid dynamics model operated by Ad Reniers of Delft University of Technology in the Netherlands. When they analysed the results they were puzzled to find the model predicting something other than long plumes of fast-moving water running out to sea. Instead, they saw strong, persistent eddies circulating throughout the surf zone – the area stretching from the shore to the outermost breakers.
Further investigation, however, had to await the invention of GPS-enabled drifters. The first of these was designed in 2003 by a team at Scripps Institution of Oceanography in La Jolla, California, but at $2500 a pop they were too expensive to risk losing in a rip. So MacMahan copied the idea using an off-the-shelf GPS antenna designed for cars. The $350 price tag now allows him to command a 30-strong fleet. His design – a squat plastic base that floats just under the water, topped with a 70 centimetre antenna – yields positional information to within 40 centimetres and speed to within 1 centimetre per second.
Calm winds, big waves
I joined the team on a picturesque May morning of calm winds and big waves. “We’re going to have some equipment attrition today, I’m afraid,” Stanton warns me, as stretches of surf 600 metres long break with a resounding “whomp”. I can’t make out where the rip channels are but Stanton knows. They mapped them two weeks ago.
Today is a current-mapping day. Eight scientists start hauling out drifters but a few steps into the water they are struck by a breaker, sending them skidding up the beach, hats and sunglasses flying. After regrouping, the team releases eight drifters in a clump. They bob close to shore for a few minutes then, one by one, slip into the rip current. Thirty seconds later they are 100 metres from shore. According to the old models, they should just keep on going. Shortly before the surf zone gives way to open water, however, the drifters turn sharply to the left and then double back on themselves.
Where previous experiments measured currents at a few fixed locations, MacMahan’s drifters give a far better resolution. On paper, the result is a forest of arrows clearly showing a system of large, persistent eddies around 100 metres in diameter, circulating in the surf zone and only rarely sending plumes of water out to sea.
Of course, that’s just one day’s work. Over the next few weeks, the team repeats the experiment under different tide and wave conditions. The outcome, MacMahan hopes, will be the most complete model of real rip currents ever made.
The measurements have yet to be fully analysed, but MacMahan immediately sees how swimmers might benefit from a revised mental picture. Caught in a rip that looks less like a river than a whirlpool, a swimmer who heads parallel to shore won’t necessarily get out of the current.
The initial findings also confirm that rips can pulse from sluggish to fierce in the blink of an eye. This occurs because of a phenomenon called infragravity waves: long, low-frequency waves that cause the ocean to surge in and out over a period of minutes. “We think this is what catches swimmers off guard,” MacMahan says. One minute you can be bobbing safely in calm water, the next you’re being swept out to sea. MacMahan now thinks that the best way to escape a rip may be to rely on the eddies to sweep you back into shallow water, though it is too early to change the official advice.
As if to demonstrate, MacMahan wades into the water, a GPS unit strapped to his cap. He leans back, sticks his toes out of the water like a basking sea otter and floats into the rip. After heading out nearly to the breakers, he sweeps southward, parallel to the beach. Not long afterwards, he is back in the shallows. He has been in the rip for 4 minutes.
Had MacMahan battled the current, he might have stayed in place for a minute or two before the rip exhausted him. Then he would have been in real danger, as breaking waves made it hard to catch his breath without choking. Instead, the current carried him safely back home. It won’t always work, but remember it the next time you feel the pull of the ocean. It might just save your life.
