A rip-roaring tale of danger

THE danger signs are out at Fitzroy Beach. A teenage surfer pauses to read the rip warning, scans the sea for the fast-moving channel and launches straight in. For this young man, a rip means a free ride out to where the waves are breaking. For a swimmer, it can lead out to sea and into peril.

These hazardous conditions can develop swiftly around Taranaki's exposed coast, but there are scientists throughout New Zealand who are slowly, methodically, getting to grips with rips.

One is Waikato University masters student Shari Gallop, who has been studying rip current dynamics during storms. She has been using video images of Tairua Beach on the Coromandel to locate rips and study the data.

"It's mostly computer work I spent eight months just programming."

Part of her work has been to look at how rips relate to wave conditions.

"It's very complicated."

Gallop says the rip currents are highly dependent on wave sizes, duration of storms, changing beach shape and previous weather events.

Gallop hopes her computer programme, the first of its type in the world, will be used to help lifeguards predict rip conditions.

In the meantime, Niwa principal scientist Terry Hume and his team have been working on an internet information system called Coastal Xplorer, which is available free at http://wrenz.niwa.co.nz/webmodel/coastal.

This website looks at beaches around New Zealand and classifies them.

Let's look at Taranaki. Click on beach type and you will discover that most of the region's coast is covered in little brown flags indicating low tide terrace. This is a moderately steep beach face joined to an attached bar or terrace exposed at low tide. The bar extends alongshore, is flat and featureless or cut every tens of metres by small rips.

But there are three beaches Fitzroy, Oakura and Opunake (Back Beach has yet to be classified) that bear red flags, which stand for transverse bar and rip. The definition section says that this is a beach where bars transverse perpendicular to and attached to the beach, separated by distinct rip troughs at 150-300m spacing, with breakers 1 to 1.5 metres high in a surf zone that is 50 to 150 metres wide.

In other words, take care.

The plan is to give each beach a hazard rating, but that information is not yet available. Hume says there's a misconception that tides cause the currents on beaches.

"It's waves. These are wave-generated currents. Rip currents are water from broken waves escaping back out to sea that's what carries people out."

New Plymouth oceanographer and surfer Peter McComb says we need to think of waves as energy.

"When waves approach the shore, that energy has to go somewhere," says McComb, of MetOcean Solutions. When a wave breaks, most of the energy is dissipated on the shoreline. However, some of the energy gets reflected back out to sea, and some gets converted to a current flow particularly if the waves are consistently larger on one part of the beach than another.

"The current flows tend to take the path of least resistance."

That path can be a channel scoured into the seabed, which is the underlying beach morphology, or shape, of a rip.

At Fitzroy Beach, near where McComb lives and works, danger comes in "packets of energy".

His observations show that if big waves come in sets, then there's a higher chance rips will form.

Also, the conditions can lull swimmers into a false sense of safety. They might see that for much of the time, the sea is either flat or has moderate-sized waves that look harmless. So they venture in. Then a large set comes through, hitting a beach already shaped by channels and bars, and the rips get roaring.

McComb says this is why people shouldn't rush into the water.

"When you go to the beach, you need to look at it for 10 or 15 minutes before you send your kids out there. Are there groups of large waves arriving every five minutes or so?"

If they are, then look out for rips.

The clues are clear: A channel of churning, choppy water, an area where the water colour is notably different (often brown), a line of foam, seaweed or debris moving steadily out to sea, or a break in an incoming wave pattern. McComb would like to see a traffic light system for all New Zealand beaches.

"You could look up on a website and see, Oh, it's green today, or it's amber or it's red. It's entirely possible."

Green would be for safe conditions, amber would mean take precautions and red would be flat out danger. Even when people take precautions, they can still drift into rips and find themselves sucked out to sea within seconds.

They panic, try to swim towards shore, fight the invisible current and become exhausted. That's when they are in danger of drowning.

For years, surf lifesavers have advised people not to fight the current, but to swim parallel to the beach to get out of the rip and then head for shore. However, a story in a New Scientist magazine by journalist Hugh Powell shows a different take. Research by the Naval Postgraduate School in Monterey, California shows that rips might actually carry people back to shore, or at least point them in the right direction.

Using GPS-equipped drifters and even themselves the researchers watched what happened. The journalist was on hand to see the scientists release eight drifters into a rip. After bobbing close to the shore for a few minutes, they each slipped into the out-flowing current.

"Thirty seconds later, they are 100 metres from shore. According to old models, they should just keep on going," the story says.

"Shortly before the surf zone gives way to open water, however, the drifters turn sharply to the left and then double back on themselves." Study team leader Jamie MacMahan demonstrated, allowing the same rip to sweep him out to sea.

"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 four minutes," Powell writes.

While it's too early to change the standard advice, science may be showing that going with the flow may save your life. But when in doubt, put your hand up for help. Even better, swim between the red and yellow flags.

Freaky facts

1. Rips can flow faster than Michael Phelps at full speed. While a typical flow is about half a metre per second, rips can flow as fast as 2.5m per second. Looking at Phelps' 200m freestyle world record time of 1min43.86sec, that equates to just under 2m per second. The rip would win.

2. Surf Lifesaving New Zealand statistics for 2008 show that people don't just get into trouble on rugged beaches with big surf. "Wave size of less than 1.5m was the case in 82% of rescues this year," it reports. In fact, 75% of those rescued were at beaches where the conditions were glassy or there was a slight chop. Only 10% of those rescued were in rough surf.

3. Young men are the most likely to drown in New Zealand waters. Water Safety New Zealand statistics from 2003-2007 show that 83% of all beach drownings were male. The age breakdown shows that 22% of those who drowned were aged 15 to 24. The next at-risk group are 65 or older, which accounted for 21% of drownings.

4. An undertow is not a rip. It is simply the backwash of a wave along the sandy bottom. It can trip up small children and send them seaward, but the next wave will wash them back towards the beach unless there is a rip. Undertows don't pull you underwater, either.

5. To understand the impact of waves, you need to square the height, says New Plymouth oceanographer Peter McComb. Therefore, doubling the wave height leads to four times the wave energy. In July last year, storm waves off New Plymouth were as high as 12m and were strong enough to toss concrete blocks across the lee breakwater.

- © Fairfax NZ News