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A January
B July
1 5
Significant wave height
6 m
Worldwide distribution of altimeter wave heights measured by the TOPEX/POSEIDON satellite A) January 1995, B) July 1995 (Jet Propulsion
Laboratory, 1995a, b).
Fig. 8.4
involves the meeting of several waves of different
height, period, and direction that exist in the spectrum
of waves generated by winds over the ocean. For
example, the energy per meter of wave crest in a 4 m
wave with a period of 10 seconds in deep water is
Sometimes waves will increase in height when they
meet an opposing current. The region along Africa's
south-eastern coast is notorious for high waves due
to current interaction. Here, waves generated in the
roaring forties travel against the Agulhas current moving
south, trapped between the submerged Agulhas Plateau
and the continental shelf. Waves steepen in this region
as a result. One confluence of wave crests steepened
by this current swamped and sank a supertanker, the
Gigantic , in the late 1970s. There are other regions of
the world - including the east coast of New South Wales
where storm waves often move into the East Australian
Current - that are as dangerous.
Algebraic summation of interacting waves does not
explain the incidence of rogue waves currently being
10 5 joules (Equation 8.4). The energy of a 3 m
wave of the same period is 17.6
10 5 joules. If the two
waves intersect then the summation of energy is
10 5 joules. Using Equation 8.4, the resulting wave
height is 5 m. This wave height is unique in time and
space. If a ship happens to be at the point of confluence
at that time, then it is under threat of capsizing. Such
waves have always plagued shipping, but today they also
pose a potentially serious hazard to oil platforms, with
potential ecological damage and investment loss.
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