Understanding Rip Currents

克里斯,水上运动,冲浪,身体寄宿,工具包e surfing, and so on. They've all become increasingly popular in recent years, but the number of people who are getting into trouble in the water has correspondingly also gone up and part of the problem are what are called rip currents. These are unexpected and fast moving currents that can drag you out to sea even if you're a very good swimmer. In fact, I had a little brush with one of these when I was in Australia recently. Now though, a group of surfers and scientists, I suppose you could call them surfer scientists, are trying to understand the processes of these rip currents so that we can make beaches safer places to be. Planet Earth podcast presenter Sue Nelson has donned her bikini and quite possibly a spotty handkerchief with knots tied in the corner, and she's been to Perranporth in North Cornwall to meet Dickon Berrimen from the RNLI, that's the Royal National Lifeboat Institute, along with Tim Scott, and former European surf champion, yes, he really is a surf champion, but he's also Professor Paul Russell from the University of Plymouth...

Paul - It's a 3-year project and we're looking at the factors that cause rip currents to vary. The factors are the incoming waves which drive the whole system, measuring the rip currents themselves using the drifters, and the shape of the beach - the sandbanks, the rip channels in the beach and how those change. The crucial thing about the UK is that we have these large tidal ranges so the water is moving on and off the sandbanks quickly and the waves are changing quickly, and therefore, rip currents can turn on and off very quickly, and this is what causes a major hazard.

苏-Now you mentioned the drifters, part of the equipment that you're using to measure rip currents. Tim Scott, you were involved in designing this equipment. Take us through the equipment that you're using. Probably best to start as it's already been mentioned with the drifter. What's a drifter?

Tim -The drifters themselves stand about a meter tall. The base of them is made up of a cylinder which is about the size of a 2-litre drinks bottle and above that is a mast which extends up to about waist high. The drifters themselves float, they're neutrally buoyant, so when we put them in the surf zone they float with the mast sticking upright and standing up. They have a damping plate on the bottom which stops them from surfing on the waves, and they very effectively mimic what would happen to a person if they were trapped in the surf and they were moving around on the rip currents.

苏-How many would you use?

Tim - Well we use between 15 and 25. Each drifter itself has got a GPS unit that tracks its location and it takes a position every second. We use a couple of interesting survey techniques where we can use a GPS base station to make these measurements much more accurate. These enable us to get very accurate velocities and positions within the surf zone using these bits of equipment.

苏-Paul, we've got a sort of taste of some of the parameters that are being measured there. What else are these drifters, these GPS tracking devices, actually looking for in the water?

保罗-流浪者either get taken out to sea on a rip current or they get washed up back in by the wave, so we're continually reseeding them. So what we're looking for is the changes in the system. So not just measuring it once but continually changing it as the tide changes, as the waves change, and as the seabed topography, the sandbars change. And we've just done a 6-week experiment here which encapsulated quite a range of conditions and we're coming back in October when the beach will be different and the waves will be different, and the rip currents will be different, and we'll be repeating all the measurements then.

苏-Now both Tim, yourself and Paul are keen surfers. Have you noticed just purely with your surfing? I mean, because there are people here, body boarding, taking surf boards out obviously to enjoy the waves. Have you ever noticed yourself that there are certain times of the year or the day when the rip current seem to be stronger than usual?

Tim - Yeah. Well as a surfer, you spend a huge amount of time in the sea and you see rip currents all the time and you know, you're constantly in rip currents and moving around, and many surfers might not know exactly the physics behind what's happening but they're certainly observing them all the time, and they have a really good intuitive knowledge of how these rip currents work. The fact is they do change all the time and from seconds, from groups of waves, all the way through to seasons and annually, you get different kind of rips on different kinds of beaches.

苏-Dicken, is this what makes them so dangerous as a lifeguard then as surfers like Tim have said that they're there all the time and they're really quick?

Dicken - Yeah, potentially. I mean, we got to remember that it's not the rip current that kills someone. It's the inability to cope and potentially, the inability to see it in the first place. So, this science backs up what we think we know already and that's also interesting to come out. It gives us a sort of confidence in the safety message that we give. But also the lifeguards will be generally risk assessing all the time, dynamically. As the tides dropping out here at Perranporth, it will get worse as the tide drops, and certainly towards low tide is the most dangerous time that we know on this beach and that's built up from experience. So, the guys will now be adjusting potentially where they put their swimming area. They'll be adjusting the advice they give to swimmers, and they may possibly even go in the water themselves to prevent those accidents happening.

Chris - Dicken Berrimen from the RNLI with Tim Scott, and Paul Russell from the University of Plymouth. They were talking with Sue Nelson.