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Last Updated: Wednesday, 13 August, 2003, 14:09 GMT 15:09 UK
Tracking Australia's lethal 'flash rips'
A scientist in Australia is trying to uncover the mystery of the strange undercurrents beneath the waves - known as flash rips - that can instantly transform a good surfing beach into a lethal one.

Bondi beach, AP
Johnson hopes his research will make Australia's beaches safer places
All rips are dangerous currents that run out to sea, and can drag hapless surfers out with them. A flash rip is caused by a large build-up of surf in a short period of time, and usually appears without warning.

But oceanographer David Johnson is trying to work out where and why they would be likely to occur in order to make the country's beaches safer.

"I've been looking at what we call transient rip currents - they're also known as flash rips in the United States," Mr Johnson told BBC World Service's Science In Action programme.

"These are rips that can really form anywhere at any time. Rip currents can reach up to one metre per second.

"An Olympic swimmer may be able to swim against that, but your average everyday beach user has no hope of swimming against that."

'Baywatch with science'

Mr Johnson has spent much of his career studying inshore currents and has made the flash rip his latest challenge.

But though his workplace may seem idyllic - he does, after all, spend all day working on the beach - he said that it was not an easy place to do science.

"Even though nobody ever believes me, it's actually surprisingly hard work to deploy instruments in the surf zone because you're getting bashed around all the time by waves," Mr Johnson explained.

Surf. EyeWire
You don't get to see them coming, or anything like that - they basically just appear
David Johnson

"Even though it's an enjoyable day, it's actually quite physically demanding.

"You have to be very careful yourself, in terms of not getting drowned or squashed by your instruments when you go and put them in the water."

The basis of these instruments - called "drifters" - is a piece of grey sewage pipe. Inside is a Global Positioning System (GPS) receiver, pinpointing the location of the pipe every second.

That information is later downloaded, so a map can be drawn of where the device has been and help create an image of where the currents are.

Mr Johnson agreed his method was akin to "Baywatch with science".

"Because these are so small and light you literally pick them up, run in and throw them in the water," he said. "Once it's been out and done its thing, we swim out and pull them back in."

Practical use

From the paths that the GPS pipes take, the speed of the rip currents can be worked out, as well as their dispersion - the extent to which the currents mix with each other.

Surfer Glyndyn Ringrose, AP
Good surf makes for good surfing - but can also be deadly
"I think the main thing is to improve our overall understanding of how the near-shore zone works," Mr Johnson said.

"What hasn't received a lot of attention in surf zone research is what we call transient-type phenomena - stuff that really is very variable in time and space."

Mr Johnson added that he was hopeful his work could soon be put to practical use by beach inspectors and lifeguards.

"In terms of practical implementation and public safety, it might be nice for the lifeguards that work on the beach from day to day to know that what they're seeing is actually being studied from a scientific point of view."

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