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Rogue Waves E-mail
(3 - user rating)
Written by Brian Grover   
Friday, 16 February 2007 09:31
Backpackers following the intertidal shelf should be aware of something known as "rogue waves". Unless seismic in origin, waves are usually created by the transferance of energy from wind to water. Three factors, wind speed, duration and fetch or the distance which a wind can blow, unimpeded, contribute to wave size. The energy moves wave-like through the water, displacing that water but not in fact transporting it. In other words the energy moves but the water doesn't.

Long wave-trains, waves marching in succession, can travel over great distances across the water. At any given time the waves from several wave-trains from disparate origins may come crashing on any particular beach. Rogue waves occur when the crests of individual waves from different wave-trains momentarily coincide. In short rogue waves occur when two or more waves displace the same water at the same time. When two wave crests come together they create a bigger than average wave. When a crest and trough coincide the result is a smaller than average wave.

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Image Rogue waves occur when the crests of individual waves from different wave-trains momentarily coincide. Hokusai's famous Tsunami, pictured here, is actually not a rogue wave at all.

Statistically the coincidence of two waves doubling up is thought to occur as frequently as one in 23 normal-sized waves. Certainly often enough to warrant caution whenever mucking about at the very edge of rocky shelves. When walking along the edge of metre high surf the margin of safety will rapidly disappear whenever a two metre wave hits the beach. Backpacker and knapsack may very well disappear as well if three crests overlap. Fortunately triple-sized waves are predicted to occur only once out of 1, 175 while four crests meeting simultaneously is as rare as one in three hundred thousand.

A further dynamic, not fully understood yet, occurs when coinciding waves are further pumped up by ocean currents. Though not a frequent occurrence off the B.C. coast, waves on steroids have been measured as high as a 10-story building from crest to trough. Coinciding troughs are known to create momentary liquid black holes from which an unlucky freighter or tanker would be unlikely to reappear. Every year a handful of cargo ships vanish from the oceans of the earth. At least a few of these are thought to have fallen prey to rogue waves.

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