Hundreds gathered for Senior Sergeant Lyn Fleming’s vigil, whilst Kiwis are urged to look out for each other. Dame Tariana Turia will be held by iwi for one more day.
Rip currents are a factor in 85% of average beach rescues and nearly two in 10 beach drownings
Surveys suggest just half of Kiwis think they can spot one –and of those, just half can correctly
While scientists are working to understand more about what creates rip currents, surf-lifesaving experts have some basic messages for swimmers caught in them
They’re responsible for nearly 20% of drownings at New Zealand surf beaches – yet there’s much about rip currents that scientists are still trying to understand. Jamie Morton explains.
What are rips?
It might look like a calm patch amid the surf that looks like a sweet spotfor a dip; rather, it’s more likely a dangerous rip current that can swiftly drag unsuspecting swimmers out to sea.
Rips are a factor in about 85% of the average 1200 rescues that lifeguards make each year and they’ve been responsible for 17% of beach drownings over the past decade.
While they can often have tragic consequences for those caught in them, just what forms and drives rips is something scientists have been trying to unravel.
In basic terms, they’re a result of complex interactions between tides, waves, currents and the shape of shore beds – and they typically form when there’s a sudden gradient change in the height of waves.
“The most common place for this to occur is on a beach, when the waves break, and the wave height changes rapidly from where they first start to break to the shoreline,” said Professor Karin Bryan, a coastal scientist at Auckland and Waikato universities.
“The energy lost from waves gets mostly channelled into rip currents.”
Anything that changed those breaking patterns – such as sand banks and reefs - in turn causes a change in rip currents.
A rip current. Photo / Supplied
Low tide is an especially dangerous time for rips to form, as it brings more wave-breaking.
But not all rips are the same.
“We used to think that rip currents were generally directed straight offshore, but now we know that they can go in a lot of directions,” Bryan said.
“Anywhere the wave breaking patterns look complex, and the waves look like they are crossing each other, is likely to be dangerous, as there are likely channels and banks which funnel the rip currents into unexpected directions.”
A set of larger waves rolling in can quickly also make for stronger rip currents.
“This is another aspect of why they are dangerous: you can be in a place that is perfectly fine when the small group of waves is there, but then [the rip] suddenly becomes much stronger.”
They’ve been observed to reach speeds of 2.5m a second – quicker than the fastest Olympic swimmer.
What makes some beaches more dangerous than others?
Bryan cites one rule-of-thumb: the bigger the waves at a beach, the bigger the risk of rips.
That typically applies to surfer-pulling beaches within headlands, which deflect the current seaward, and those with features like large embayments, rocks or islands.
Niwa coastal scientist Dr Christo Rautenbach said these include some of New Zealand’s best-known beaches, such as Piha, Muriwai, Bethells, Raglan, Mount Maunganui and Hot Water Beach.
Coromandel's Hot Water Beach has seen some dramatic mass-rescues. Photo / Greg Bowker
Rautenbach, who’s been developing an artificial intelligence-based tool to detect rips, said the simple slope and shape of a beach can also raise the danger.
“Steeper beaches tend to have stronger rip currents because the water flows back to the sea more quickly,” he explained.
“Many beaches also have sandbars with deeper channels between them: waves break over the sandbars, and the water flows back out through the channels, creating rip currents.”
Did New Zealand’s beaches happen to be more dangerous than others overseas?
While ours might be similar in nature to those elsewhere, they often had “unique aspects” – from outcrops and headlands to steep shapes and larger grain sizes – that set them apart, he said.
“Local coastal features and wave conditions can make New Zealand’s rip currents particularly strong and unpredictable.”
Bryan points out another obvious factor.
“Our beaches are very accessible, so we probably have a greater fraction of people who are unfamiliar with surf visiting them.”
Is climate change and coastal erosion raising our rip risk?
That’s another tricky question scientists are still grappling with.
Bryan said that, while international studies indicate wave energy is broadly increasing as the planet warms, the picture isn’t so clear in New Zealand, which receives waves from four directions.
Rautenbach said climate change-driven sea level rise, too, can ultimately alter the shape and dynamics of coastal areas – and the rips within them.
One survey found just 48% of Kiwis were confident they can spot a rip - and of those, only half could so correctly. Photo / George Novak
As well, more erosion from higher seas and bigger storms can change the topography of beaches – potentially creating more rip-making channels and sandbars.
Studies showed beaches on Auckland’s West Coast were likely to see more intense wave action over time, while Hawke’s Bay was likely to face more significant exposure to waves.
But it wasn’t a matter of rip conditions getting worse everywhere.
“It’s a complex picture, with some beaches calming down and others ramping up,” Rautenbach said.
How can swimmers spot rips?
One survey found just 48% of Kiwis were confident they can spot a rip – and of those, only half could do so correctly.
“New Zealanders who think they can spot rips are mostly highly competent ocean swimmers – and those who have previously been caught in a rip,” Surf Life Saving New Zealand’s national coastal safety manager Tom Kearney said.
He listed some tell-tale signs: namely those calm patches in the surf with waves breaking on each side.
“The calm gap may look safe to swim, but a small patch of calm water in amongst surf or waves is often a rip current.”
Swimmers should also watch out for ripple patterns on the sand, small holes beneath their feet in the water, or foamy or darker areas of water – created by current picking up sand in the water as it moved out to sea.
“However, every rip current is different,” Kearney said.
“They can appear and behave differently and can become more prevalent with the changing tide.”
What should people do if they get caught in a rip?
Kearney advised people to stay calm, float, and conserve their energy.
“The rip current will not pull you under the water and is just taking you for a ride offshore,” he said.
“Try to fight the urge to swim back to shore against the current; this will use up energy that you need to stay afloat before help arrives.”
He also urged swimmers to raise their hand to signal for help as soon as possible.
Illustration / NZ Herald
“The longer a person is stuck in a rip, the more tired they will become - drowning can only take between 20 and 60 seconds if the swimmer cannot float and keep their airway above the water.”
Meanwhile, he recommended they ride the rip until it stopped, and they could swim back to shore, or until help came.
“Many rips will circulate and bring you back into shallower waters closer to the shore where you may be able to stand,” he said.
“When the current has subsided, and only if you are sure you can swim to the nearest point on the shore, should you attempt to swim to safety.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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