It is a well-known fact that the motion of a wave through water is transmitted by the circular movement of individual water particles. Surface water alone, it should be noted, does not participate in the circular motion with the passage of a wave.
Even the subsurface water also participates in the circular motion. However, with increasing water depth, the size of the circular orbits goes on decreasing.
That is why in greater depth of water the motion of water is hardly visible, despite the fact that large surface waves are present. It is interesting to learn that in very deep water the bottom of the sea or lake is not affected by the movement of waves.
With about 50% decrease of the distance between wave crest and water depth the circular motion of water is retarded as the result of friction with the bottom. In that case the wave transmission becomes slower, wave length decreases, and height of the wave increases.
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Finally, wave height reaches a point of instability, and the wave either collapses, producing surf, or topples forward, forming a curling breaker.
If the water is shallow at some distance from the shoreline, the waves will break some distance offshore. In case, the water is deep enough beside the coast, the waves are compelled to break against the land.
If the wave crest falls against the coast, its impact is sufficient to break the solid material apart. The storm waves that are created by very strong gales blowing over considerable fetch cut back coastal cliffs and headlands.
The storm waves have far more considerable effect on a coastline than ordinary waves in relatively calm weather. Generally, the waves break before hitting coastal land directly, then the water rushes forward as foaming swash.
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The swash picks up sand from the bottom and deposits the same onto a beach which consists of deposition of sand and stones upon the shore.
With the declining momentum of water, the force of gravity gains the upper hand and reverses the direction of flow which drains down the beach slope.
It is called backwash. Remember that the phenomenon of swash helps in the growth, whereas that of backwash leads to the diminution of the beach. But it all depends on the transportation capacity of these two phenomena-swash and backwash.