PERHAPS the most obvious movement of the water of the oceans is the
disturbance of the surface caused by the generation of wind waves. With
these may be included the rare, but frequently destructive, seismic
waves of tsunami, which, though inconspicuous in the open ocean, may
become very large near the coast. It is the wind, however, which is
responsible for the formation of the oscillatory waves, which move over
the surface of the water and which play such an important part in the
modification of the coastline. Looking at the sea in a storm, its surface
appears to be in a state of chaotic confusion, and it is difficult to appre-
ciate that amongst the disorder it is possible, to a certain extent at least,
to sort out the different wave trains present.

Although they rarely exist alone in nature, it is worth considering the
characteristics of ideal, wind-generated oscillatory waves in the ocean.
These can be divided broadly into two groups: those that are in deep
water and those in shallow water. The definition of deep water is that
the depth must be greater than the length of the wave; in deep water it
moves independently of the bottom, but in water shallower than this its
form and movement are affected by the bottom.

Waves in deep water can be defined by their length and height, and
particularly important from the point of view of their effect on the beach
at least, is the relationship between these two dimensions, which gives
the steepness of the wave. The wave length is defined as the distance
between two successive crests, measured perpendicular to the wave
crest. Closely associated with the wave length are the period and velocity
of the wave form. The period is the length of time it takes the wave form
to move through one wave length, and the velocity is the speed of
advance of the wave form. The three factors are related by L=C T,
where L is the length, C the velocity and T the period. The wave
velocity depends only on the wave length, the two factors increasing
together; the relationship can be expressed by C=

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