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It has previously been thought that rogue waves are rare but recent research carried out by the European Space Agency indicates that they are much more frequent than originally thought. They are now no longer called ‘rogue’ but ‘abnormal’ waves in recognition of that fact.

The Agency examined satellite photos of the oceans and found frequent examples of abnormal waves – some of them up to 30m in height. This height confirms the theoretical maximum calculated heights of abnormal waves.

Fortunately, however, the majority of abnormal waves are short lived and, after half a minute or so, gravity overcomes the wave energy and the wave collapses. The other fortunate fact is that the majority of such waves do not move and the only forward energy they have is the normal cyclical movement of a wave.

However, should any ship be unlucky enough to be in the path of an abnormal wave during its short life, the weight of water falling on the ship when the wave breaks is enough to damage it severely, if not sink it entirely. The other danger is that for every abnormal wave, there is an abnormal trough – of exceptional depth – preceding it.

Linked to strong currents

Researchers are still trying to pinpoint the conditions that might trigger abnormal waves but it is clear that they are more common where there are powerful currents, such as the Agulhas off South Africa, the Kuroshio off Japan and the Gulf Stream off the eastern United States. It is well known, for instance, that abnormal waves and troughs can form in the Agulhas current when the wind suddenly switches from being a steady north-easterly to a strong southerly blowing against the prevailing current.

Another mechanism by which the waves can form is where wave trains traveling in the same direction but at different speeds pass through one another. When they synchronize, they combine to form abnormal waves.

It is not yet possible to forecast the possibility of abnormal waves though this could happen in the future. However, South African authorities do issue predictions in respect of the Agulhas current.

Ensuring forward buoyancy

In the meantime, we seafarers should continue to take precautions such as ensuring that there is as much buoyancy as possible in the fore part of the ship. Good seamanship practice should be followed and all weather tight openings at the fore part of the ship should be closed and secured whenever the ship is expected to encounter heavy weather.

 

 
Anatoly Orel
Anatoly Orel on Jan 11, 2013
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