Freak Waves
From: SA Sailing Directions Vol 1 page 43:
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3.9 ABNORMAL WAVES. As described in section 3.8.9, the Agulhas Current
flowing off and parallel to the East coast of South Africa is about 60
miles wide and attains rates of up to 5 knots on occasions. This
current is normally kept outside the continental shelf by the fact
that it extends downwards to a depth of more than 200m. It attains its
greatest rate along its western edge. Between Durban and Port St.
Johns the average width of the continental shelf is 5 miles, and it is
in this area between the shore and the western edge of the Agulhas
Current that a counter current is sometimes generated when a strong
wind from the SW is associated with an atmospheric depression moving
to the ENE. This current, moving in a NE direction, is composed of a
gradient current caused by the level of the sea being raised in the
low pressure area and the surface drift caused by wind friction.
In the NW quadrant of the depression, strong to gale force SW winds blowing contrary to the direction of the Agulhas Current cause very steep waves, especially in the western part of the current immediately to seaward of the shelf edge. These waves are approximately 5 - 1O m high, have a frequency of about 10 seconds and a length of 60 - 80 m. At the same time there may be wave trains emanating from storm centres further south, whose lengths are much longer and whose frequency is about 16 seconds. These also travel in a general NE direction against the current. lt is thought that a combination of the waves in these different wave trains, together with some aberration in the Agulhas Current caused by the influence of the counter current, has on several occasions caused an exceptionally large wave to form, the notorious Freak Wave of the Natal Coast. For some reason, as yet not fully understood, a very deep trough precedes the crest of the wave, with the result that a ship steaming against the sea suddenly and without any warning, plunges into it, and before the bows can lift to the oncoming wall of water, which may be as much as 20 m high, the forepart buries itself in this mountainous wave with disastrous results. The lifetime of such a freak wave is very short, and it will extend over a distance of not much more than 11 cables, so that the chances of a ship encountering it are small. Nevertheless, mariners are warned to treat that section of the coast between Richards Bay and East London with caution when steaming SW into a rough sea when the barometer is low and there is a strong SW wind blowing, on a course lying within 20 miles to seaward of the 100 m bathymetric contour. Between 1964 and 1973 six ships reported having encountered freak waves in this area. One of these, the tanker World Glory, broke her back and sank on 13th June 1968. |
World Glory
(click for enlargement)
Esso Nederlander
(Photographed from Bridge)