Ripped and torn
Rip currents on Perranporth beach
7 November 2011 by Sue Nelson
Sue Nelson met oceanographer and former European surfing champion Paul Russell and his colleague Tim Scott from the University of Plymouth, and lifeguard Dickon Berriman from the RNLI, on blustery Perranporth beach in Cornwall. She asked them about a new research project that will keep people safer in the sea.
What is a rip current?
Dickon: Put simply a rip current is a body of water trying to find its own level. Right now there's maybe five-foot of surf breaking - that's lots of water forcing its way onto the beach. That water's got to get back out again, so it'll look for the path of least resistance. Here at Perranporth it'll be a break in the sandbanks, but it could also be a pier or jetty, or some rocks. The water will follow the edge of those man-made structures or physical hazards and flow back out to sea.
What problems do rip currents cause for you as a lifeguard?
Dickon: They account for at least two thirds of our rescues, so a lot. We're always advising people to avoid them and swim between our red and yellow flags. The UK's large tidal ranges mean the water's moving on and off the sandbanks quickly and rip currents can turn on and off very quickly too, so they're a major hazard.
Rip current sensors
Tell me about your project.
Paul: We'll spend three years looking at what causes rip currents to vary - so we'll measure the incoming waves which drive the whole system, the rip currents themselves and the shape of the beach.
How will you measure the currents - isn't it dangerous?
Tim: We'll use these 'drifters' which effectively mimic the movements of someone trapped in the surf zone. They're about a metre tall, with a floatation cylinder and a mast, and they float upright. A damping plate on the bottom stops them surfing on the waves. Each drifter has a GPS which takes a position every second and we can accurately plot their speed and position in the surf zone.
Paul: We've also got fixed instruments measuring the current at one point, while the drifters will follow the current just like a surfer. In big waves like this a surfer will get washed along the shore a bit and then taken out in a rip current. Then you'll be riding waves - that's the water being dragged back in over the bar. Then you're washed along shore again. This is exactly the sort of circulation that our drifters follow when they're out in the waves.
What are these devices actually looking for in the water?
Paul: We're looking for the changes in the system, so we're not just measuring it once, but continually - as the tide changes, as the waves change and as the sandbars change. We've just done a six-week experiment here and we're coming back in October when the beach, the waves and the rip currents will be different. As keen surfers, do you notice when the rip currents are stronger?
Tim: Surfers see rip currents all the time - we're constantly moving around in them. Many surfers might not know the physics behind what's happening, but they have a good intuitive knowledge of how rip currents work. And the fact is, they do change all the time - with the seasons, and even over seconds with different groups of waves. You get different kinds of rips on different beaches.
Have you ever been tempted to get in with the drifters and see if they work?
Paul: Absolutely - in fact we've spent the last six weeks doing just that. We've been drifting with GPSs on our heads to make sure humans and drifters track similar directions at similar speeds - which they do. That means whole groups of us being swept out in rip currents and then swimming back in. Plus drifters continually get washed in and we're continually taking them back out. It's incredibly hard work!
How are you going to use this research?
Dickon: It will help in several areas, firstly with risk assessment - helping us work out how to use the lifeguards and equipment we have, and what level of training we need on every beach in the UK. The research is providing base data - a scale of relative danger for each beach - that will combine with the observations to determine what we need. It will also help us improve the safety advice we give to schools and on the beach, as well as the training we give our lifeguards. These young men and women have potentially grown up in the surf, but this will back up their knowledge and teach them how to watch the surf for rip currents. So it's really, really useful. And, of course, it'll be relevant wherever there are rip currents around the world.
Finally, what should people do if they get caught in a rip current?
Dickon: The important thing is to conserve energy and not to panic - the rip current will carry you quickly away from the beach but it won't pull you under the water. So swim parallel to the beach until you're outside the current, and then swim diagonally towards the shore. If the rip is really strong and you can't swim out of it, float or tread water until it loses strength. Never try to swim against the flow of water to get back to the beach - you'll never out-pace the current and you'll quickly get worn out; that's when people drown.
Professor Paul Russell. firstname.lastname@example.org
This Q&A is adapted from the Planet Earth Podcast 12 July 2011.