Swell Waves
On leaving a fetch, waves lose their energy source,
and change their character. The height of the waves
decrease, while the period increases. The height, period, and direction of these
waves also become much more regular in comparison
to wind waves.
The wave-dissipation
process, or wave decay, is brought about by
(1) internal friction within the waves,
(2) resistance met as waves overtake the wind,
(3) restraint caused by crosswinds,
(4) action of ocean
currents in the path of waves,
and
(5) effects of seaweed (alghe marine), ice,
shoals (secche o fondali bassi), islands, or continents
in the path of waves.
Even with all these factors working to bring about
wave dissipation, swell waves dissipate very
gradually. As an example of such gradual dissipation, oceanographers
at the University of California at San Diego tracked waves that developed in
storms near Antarctica, crossed the equator and eventually
reached the shores of Alaska. That’s almost the
entire length of the Pacific Ocean, or looked at in another way, halfway around
the world.
These waves
come about when wind waves (WW) are superimposed on swell waves (sw). The
interaction of wind waves and swell waves produces larger
waves. However, observers do not report combined
sea heights; they simply report the wind and swell. The resultant combined
wave height (Cwh) is computed using the formula
or determined using
combined sea-height tables (see table 1-1-3). Compute the
combined-wave height using 8-foot wind waves and
15-foot swells. The combined height of these two waves works out to 17 feet,
as follows:
Now, use the combined sea-height table,
using the same wind and swell wave heights, and you should
come up with the same answer. If your answer is something other than 17 feet,
you have misread the table.
Combined sea-height charts (analyses and prognoses)
are most often produced at the oceanography centers
and transmitted via radio facsimile. The importance of such charts to mariners
is that it lets them know the highest seas or highest forecast seas in a
particular operating area or along a particular
route.
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