On average five people are killed in rip currents each year. Of approximately 700 rescue missions, surf lifeguards attribute around 85 to these fast offshore flows of water. University of Canterbury Coastal Geomorphologist Dr Seb Pitman is developing a way of mapping these rip tides on Muriwai beach and joined Kathryn Ryan to discuss how his GPS ‘drifters’ could predict rips and make sea swimming safer.

Photo: RNZ / Maxine Jacobs

Listen ( duration 10 ′ : 09 ″ )

Listen Download: Ogg | MP3

The GPS drifters are designed to mimic a human body treading water on the beach. Pitman is currently in a two to three-week campaign to drop these into surf zones and see at which point currents turn on and off. The GPS pings every second giving them information about what conditions, in terms of wind, waves, and tides are making the current speeds more hazardous and quicker. They’re also looking at the conditions that create rips which exit far offshore, compared to ones which return back to shore.

“Really crucially it’s the flow pattern: does the rip current take someone far offshore, or are we in a circulation pattern where people are retained within the first 50 -100 meters off the shore – which is obviously a lot better if they’re looking to escape or get back to the beach,” Pitman says.

Pitman says once they’ve got the base information on the currents, they can begin to investigate more thoroughly what the best escape strategy might be.

Rips are driven by waves which, as they start to come onto the beach, interact with sandbanks and shallow water – where they break. In between the sandbanks are deeper channels where the breaking water on the beach re-routes back to the sea. It’s these channels, which are narrow and constrained, which are hazardous to people.

“We can narrow it down quite nicely into a hazard window. Tide is really one thing, the lower the tide level, the more the waves are interacting with the sea bed morphology, and we can pick that out really clearly. There are also controls on rips by wave height or wave periods, but that’s a little bit harder to get a range of conditions.”

Photo: Supplied

In addition to the GPS drifters, Pitman and his team have access to eight or nine years-worth of lifeguard incident reports. Every time they’re involved in a rescue, lifeguards fill out detailed incident reports which log a wide range of data including time, place, and whether they think an accident could be attributed to a rip current.

“We can pull all of these logs where, if the lifeguard feels this was a rip current incident, we can look at what the wind, wave, and tide conditions were doing at the time. So, beyond this two or three-week data collection we’ve got going on now, we’ve got eight or nine years leading up to this. With that data, we’re able to really accurately pick out when that rip current might turn on or off.”

Pitman says one of the big problems with a beach like Muriwai is that the visitors are transient and often don’t know the risks associated with swimming there.

“They perhaps don’t get that information, or don’t have that knowledge. Locals generally aren’t the people that this will be targeted at so much. It’s more for people who are going to a new beach and perhaps don’t understand the hazards. It’s to give them that real-time indication of what the beach might be like on their chosen day and time.”

A long-term plan for the study is to incorporate real-time data about rips, currents, and strong offshore winds into the Safeswim website, which currently focuses on water quality with only generic warnings about rips.

Pitman says if you happen to be caught in a rip this summer, there are three pieces of advice to follow: relax, raise and ride.

“Stay calm and take stock of what’s going on. It’s a misconception that rip currents pull you under the water, it’s only going to move you laterally. Raise your hand and signal a lifeguard or someone on the beach that you’re in trouble. And finally, ride the rip. Rather than fight against it, just stay in it and see what happens.”

This advice is a departure from what many Kiwis were told growing up, which was to stay calm and swim sideways. Pitman says this is because new data has shown rip tides often move parallel to the shore.

“If you were to chose the wrong direction when you swim sideways, you’re still fighting against that current. If you were just to ride it, wait until the rip current weakens, and then swim toward the breaking waves, you’ll be able to get back in far easier.

“What we really don’t want people doing is panicking and fighting against the rip."