What makes the giant freak wave 'stable'

Jun 18, 2010 by David Meyers

The dreaded giant freak wave that can appear on the open sea out of nowhere, can now for the first time be theoretically calculated and modelled. Researchers at Umea University and the Ruhr-Universitat Bochum in Germany have developed a new statistical model for non-linear, interacting waves in computer simulations.

It explains how the water-wave system evolves, behaves and, above all, how it stabilises itself. The model is also suitable for the calculation of other “extreme occurrences” - for example on the stock market - or more complex phenomena in . Bengt Eliasson, a physics research associate at Umea University along with Bochum's physicist Prof. Padma Kant Shukla report on their findings in .

Eliasson and Shukla already managed to simulate how the giant freak wave occurs on the computer four years ago. If two or more waves meet at a certain relatively small angle, they can progressively “amplify” each other. Two non-linear interacting waves therefore act very differently to a single wave which shows normal instabilities and breaks up into several small waves, which then run diagonally to each other.

Two non-linear waves, however, cause the water to behave in a new way, for example, the emergence of downright “wave packets” with amplitudes three times higher than that of a single wave. Buoyed by strong currents and powerful - opposing - winds, the giant wave can continuously build up from there.

With their new statistical model, the scientists have now succeeded in taking another crucial step towards explaining this freak wave: it results from combined non-linear effects in the wave-to-wave interaction and the dispersion of the “wave packets” in a certain direction. This causes the energy of the water to be concentrated “in a narrow band across a confined wavelength spectrum”, and with sudden, large amplitude.

The actual instability of individual waves is “saturated” through the broadening of the wave spectrum, thus the water-wave system temporarily stabilises itself. This behaviour is typical for the localised giant wave, the researchers explain. Their calculations tally with observations from experiments in large water tanks.

“These show that long-crested water waves, i.e. groups of waves propagating in approximately the same direction, have an increased tendency to evoke extreme events,” said Eliasson and Shukla.

The fact that the giant wave is no “sailor’s yarn” has been known at least since the cruise liner Queen Elizabeth 2 encountered such a freak wave in 1995. The damage to passenger and cargo ships, but also for example to oil platforms at sea can be considerable. Eliasson’s and Shulka's is a contribution to being able to predict freak waves in certain regions - for example in the North Atlantic or the Mediterranean - and providing early warning in future. The deeper physical understanding of the giant wave and statistical calculation would have to be combined with new, improved methods of observation, the researchers say.

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More information: Bengt Eliasson and P. K. Shukla: Instability and Nonlinear Evolution of Narrow-Band Directional Ocean Waves. Physical Review Letters. Available: arxiv.org/abs/0912.0474

Provided by Umea University

4.8 /5 (15 votes)

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in7x
Jun 18, 2010
This comment has been removed by a moderator.
Jigga
1.2 / 5 (5) Jun 18, 2010
Rogue wave is behaving like sail, because of its elevation above another, smaller waves - and no complex theory or expensive research is required for such understanding.
StandingBear
not rated yet Jun 19, 2010
Maybe we can make fusion plasmas behave in this way and maximize pressure density in a localized area for maximum fusion and consequent energy extraction. Worth a shot.
jerryd
not rated yet Jun 20, 2010

I've known this for 30 yrs. But I disagree that they are stable as few last over 1 mile, most just come and go in seconds.
A_Paradox
not rated yet Jun 20, 2010
Jerryd, I believe you are right. My understanding is that 'super' waves appear because bigger waves travel faster, ocean surface waves at least. This means that relatively large waves coming from a particular region will overtake smaller waves [swells]going in the same basic direction. When the actual overtaking occurs, the two waves exhibit superposition causing their combined vertical height to suddenly appear as a single giant wave front.