What causes freak waves?

2008-05-01 00:00

On Sunday, April 13, an unexpectedly large wave capsized a shark diving boat at Gansbaai, on the west coast of South Africa, killing three tourists. National Sea Rescue Institute (NSRI) spokesperson Craig Lambinon said that there was no way to anticipate the freak wave that capsized the boat, causing the three drownings.

“There is no official term for something like this, it’s just something that happens on the ocean,” he said. “Every now and again you have a wave that picks up higher than normal for an unknown reason”.

Bearing out what Lambinon says, in February 1995, the liner Queen Elizabeth II encountered a 29-metre high rogue wave during a storm in the North Atlantic.

Captain Ronald Warwick described it as “a great wall of water… it looked as if we were going into the White Cliffs of Dover.” At 5 am on March 2, 2001, a 30-metre wave bore down on the tourist ship Caledonian Star, in the South Atlantic. It smashed over the ship, flooded the bridge and destroyed navigation and communication equipment. The First Officer, Göran Persson, reported: “Out of nowhere … a wave of twice as high as average. The ship went down like free fall.”

What could be the cause of these freak waves, which appear unexpectedly out of a train of waves to capsize, damage or sink boats and ships?

Waves are generated by wind blowing over the sea. They are characterised by their height H (from trough to crest) and their length L (distance between crests), which is inversely related to their frequency (how many successive crests pass a point in a given time). Their steepness is determined by H/L. The velocity of the wind, its duration and its “fetch” (the distance that it blows over the sea) determine the height and frequency of waves in the deep sea: generally, the larger these factors, the larger and steeper the wave.

The energy of a wave is proportional to its height and inversely proportional to its length, i.e., large, short waves are more energetic.

At their origin, waves tend to be steep. Moving from their source they lose energy to friction and flatten out. Nevertheless, they may travel thousands of kilometres. Most waves reaching our coasts are generated by winds in the roaring forties. The highest are thus found in the south, with the average height declining northwards. Because of the distance they have travelled to our east coasts, here they tend to be long and low — typical summer waves. In winter, though, passing cold fronts can generate big, steep, high-frequency waves.

The sea off our east coast is notorious for freak waves, which have damaged ships and may be the cause of unexplained ship losses. Analysis of conditions when freak waves were reported showed that they occur when a southwesterly wind, accompanying a cold front, blows up the coast. When the resulting waves encounter the Agulhas current, flowing in the opposite direction, they gain energy from the current. Their wavelength shortens and their height increases, leading to very high seas.

Well and good, this explains high seas, but what causes freak waves? Scientific evidence, from monitors on buoys in the Great Lakes, on oil rigs and on the European Space Agency’s satellites, suggests that freak waves are not uncommon and are, in fact, a normal occurrence in wave trains —hardly “freak” in other words.

Al Osborne is a Texas-born wave mathematician, who divides his time between the University of Torino in Italy, and the Office of Naval Research in Arlington, Virginia, United States. He has devised equations to describe open ocean wave patterns, based on the Schrödinger Equation, a complex way of expressing the probability of an event occurring. His model suggests that in certain conditions, waves can acquire energy from their neighbours, growing in size while shrinking adjacent waves. Such waves are called “breathers” — since they “breathe” themselves up at the expense of their neighbours. His equations agree closely with the observed behaviour of ocean waves. See www.brucestutz.com/ articles/rogue1.htm.

It seems that breather waves can occur in any wave regime, but are more damaging when they emerge from storm seas. For many years, ships have been built to withstand waves of up to 15 metres, because linear mathematical modelling predicted that waves higher than this should only occur once in 10 000 years. Problematic freak or rogue waves — those which damage ships — emerge when the average height of the wave train approaches the 15-metre design limit of ships. Any breather wave emerging from such a wave train will be bigger than 15 metres and will thus be capable of damaging ships.

Off our east coast a southwester wind, opposing the Agulhas current, raises the height of the entire wave train and sets the base from which damaging, rogue “breather” waves can — and do — emerge. An implication is that ships should be engineered to withstand waves much greater than 15 metres, and probably will be in future. In the meantime, they are advised to avoid the Agulhas current when a southwester is blowing.

Bearing in mind that a freak wave can emerge from any wave train, shore fishermen should be aware that an unexpectedly large wave could appear at any time and it seems to be just such a wave that capsized the shark boat at Gansbaai.

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