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Freak wave probability higher than thought

Terrifying ocean Devastating freak waves the size of a 10-storey building can be more common than previously thought, say researchers.

The findings, by civil engineer Dr Alessandro Toffoli, of Swinburne University of Technology, and colleagues, have been accepted for publication in the journal Physical Review Letters.

"They call them freak because they are not well understood," says Toffoli.

Freak waves are steep waves that can appear to come out of nowhere. They are hundreds of metres long and can be two to three times higher than the surrounding waves at the time.

They are rarely seen and were once considered a sailor's myth.

"Ten, 20 years ago, mariners would say a ship was sunk by a rogue wave and no one would believe it," says Toffoli.

But since then evidence for their existence has been building.

In 1995, a 26.5-metre high freak wave was recorded passing an oil platform in the North Sea, says Toffoli. The surrounding waves at that time had an average height of 10 metres.

Three years later a freak wave was involved in the 1998 Sydney to Hobart yacht race tragedy.

In 2001, European Space Agency satellites captured more than 10 individual giant waves around the globe more than 25 metres high during three weeks of data collection.

Probability estimates

Little is known about what causes freak waves, but a number of theories and models have been developed to predict the probability of them occurring.

One theory suggests that 1 in 10,000 waves in a typical stormy sea would be a 'rogue' wave - as the researchers prefer to call them.

But this "linear" theory does not take into consideration the fact that waves can become unstable, says Toffoli. One wave can steal energy from surrounding waves and grow at their expense.

A more sophisticated theory does take this 'instability mechanism' into account and estimates 1 in every 1000 waves could be a rogue one.

Toffoli and colleagues have taken this theory one step further by including the fact that the probability of rogue waves increases when waves travelling in one direction meet a current travelling in the opposite direction.

Toffoli says this can happen at sea or near shore where there are strong tidal currents.

The greater the speed of the current and the waves, the larger the rogue wave will be.

Toffoli says the latest model can be used to predict the probability of rogue waves under different conditions.

"It will provide more accurate predictions in the areas where currents are located," says Toffoli.

When waves hit a current travelling in the opposite direction, the likelihood of rogue waves increases to 1 in every 300, says Toffoli.

This figure assumes the waves are travelling in the same direction as each other but if the waves approaching the current are coming from different directions, the probability will be lower than this, he says.

"As we're predicting rogue waves, that has safety implications for marine operations, but it can also help design practices, to properly account for the wave-load on structures," says Toffoli.

He says the research could also improve navigational software, which could suggest alternative routes for ships based on the likelihood of a rogue wave occurring.

Serious stuff

Another expert in ocean wave modelling and physics says rogue waves are not only terrifying, but can split the hulls of ships, endangering lives and releasing dangerous cargo.

"It's serious stuff, especially for the insurance industry," says Dr Michael Banner, an Emeritus Professor at the University of New South Wales.

Banner says it has long been known that currents - such as the Agulhas Current off the coast of Africa - affect the chances of rogue waves developing.

But he says this latest model is the most detailed to incorporate the role of currents in generating rogue waves.

Organisations, such as the European Centre for Medium Range Weather Forecasting, are moving towards the creation of likelihood maps of rogue waves that would be updated as the wind fields and the currents change, says Banner.

But he says it's challenging work because global models rely on average wave heights and don't have the resolution to predict individual extreme waves.

Also, says Banner, we still don't understand all the scenarios in which energy can be focused to create freak waves.

"The capacity to make accurate forecasts is not there yet," he says.

"This clearly belongs to a class of problems that are non-linear and non-linearity brings in a huge number of opportunities for variance ... it's pretty hard to nail them all."