Abnormal waves : Caution

A well-found ship properly handled is designed to withstand the longest and highest waves she is likely to encounter as long as they retain their original shapes. But when waves are distorted by meeting shoal water, a strong opposing tidal stream or current, or another wave system, abnormal steep-fronted waves must be expected. Abnormal waves may occur anywhere in the world where appropriate conditions arise.



In places where waves are normally large, abnormal waves may be massive and capable of wreaking severe structural damage on the largest of ships, or even causing them to founder.

Abnormal waves at sea
Fig:abnormal-waves


Where conditions are considered to exist which may combine to produce abnormal waves liable to endanger ocean-going craft, a warning is given in Admiralty Sailing Directions and in Ocean Passages for the World.

Description

Reports of such occurrences, and indeed all wave measurements, are very few, and in many parts of the world are non-existent.



Off the coast of SE Africa, however, some research has been made into abnormal waves. To show how these waves are believed to occur in this particular case, the relevant article from Africa Pilot Volume III, is quoted below in full.

“Under certain weather conditions abnormal waves of exceptional height occasionally occur off the SE coast of South Africa, causing severe damage to ships unfortunate enough to encounter them. In 1968 ss World Glory (28300 grt) encountered such a wave and was broken in two, subsequently sinking with loss of life.

These abnormal waves, which may attain a height of 20m or more, instead of having the normal sinusoidal wave-form have a very steep-fronted leading edge preceded by a very deep trough, the wave moving NE at an appreciable speed. These waves are known to occur between the latitudes 29°S and 33°30'S, mainly just to seaward of the continental shelf where the Agulhas Current runs most strongly; a ship has, however, reported sustaining damage from such a wave 30miles to seaward of the continental shelf.

No encounters with abnormal waves have been reported inside the 200m depth contour. When heavy seas have been experienced outside the 200m depth contour, much calmer seas have been found closer inshore in depths of 100m.

Abnormal waves are apparently caused by a combination of sea and swell waves moving NE against the Agulhas Current, combined with the passage of a cold front. Swell waves generated from storms in high latitudes are almost always present off the SE coast of South Africa, generally moving in a NE direction. These are sometimes augmented by other swell waves from a depression in the vicinity of Prince Edward Islands (47°S, 38°E) and by sea waves generated from a local depression also moving in a general NE direction. Thus there may be three and sometimes more wave trains, each with a widely differing wave-length, all moving in the same general direction.



Very occasionally the crests of these different wave trains will coincide causing a wave of exceptional height to build up and last for a short time. The extent of this exceptional height will be only a few cables both along the direction the waves are travelling and along the crest of the wave. In the open sea this wave will be sinusoidal in form and a well found ship, properly handled, should ride safely over it.



When the cold front of a depression moves along the SE coast of South Africa it is preceded by a strong NE wind. If this blows for a sufficient length of time it will increase the velocity of the Agulhas Current to as much as 5kn. On the passage of the front the wind changes direction abruptly and within 4hours may be blowing strongly from SW.



Under these conditions sea waves will rapidly build up, moving NE against the much stronger than usual Agulhas Current. If this occurs when there is already a heavy NE-going swell running, the occasional wave of exceptional height, which will build up just to seaward of the edge of the continental shelf, will no longer be sinusoidal but extremely steep-fronted and preceded by a very deep trough.



A ship steering SW and meeting such a trough will find her bows still dropping into the trough with increasing momentum when she encounters the steep-fronted face of the oncoming wave, which she heads straight into, the wave eventually breaking over the fore part of the ship with devastating force. Because of the shape of the wave, a ship heading NE is much less likely to sustain serious damage.”





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