Breaking of the waves implies that when the sea waves approach a coast, they are forced to break. The point at which the breaking of waves occurs is determined to a great extent by the height of the waves.
It is true that this phenomenon occurs in water which is about 4/3 of the height of the wave at the break point. Thus, one meter high wave at its break point will break in water which is 1.3 meters deep.
In fact, the cause of breaking of waves is not mainly the result of friction between the floor of sea over which it is passing and the wave. However, it cannot be denied that the wave may lose a part of its energy in this way if the slope happens to be very gentle, but the wave will not break as a result of this friction.
It is the change in the orbital velocity and size of the orbit which is the main causative factor for the process of breaking. As the water depth decreases to half the distance between the wave crests, the orbital motion of the water is retarded by friction with the bottom.
The speed at which the wave travels becomes slower; there is a decrease in the wavelength; and the height of the wave is characterized by increase. Ultimately the increase in the wave height reaches a point when the wave becomes unstable and the wave may collapse producing surf, or topples forward producing a curly breaker.
If water at some distance from the shore is shallow, the waves will certainly break far away from the shore. On the contrary, if there is deep water right at the coast, the waves are forced to break against the land.
When the breaking of waves occurs near the shore, a wave of translation may develop. This wave has a singular peculiarity of having only a crest, and no trough. In the wave of translation the water particles move in the direction of progress, and the water particles are, therefore, displaced forward as the wave advances.
This is possible when a mass of water is rapidly added to the calm water, and may be produced as the crest of a breaking wave topples down on the water surface in front of it. In the open sea this type of wave is unimportant, but on a shallow coast it may be prominent.
Another important factor that contributes to the breaking of a wave is the fact that with the increase in the size of the orbit the amount of water in the wave is decreased by the decrease in length of the wave.
The result is that there is not enough water to complete the orbit. Finally the crest is practically unsupported in the front, and then, because of the forward movement of the particles at the top of the wave, the crest topples forward.
It must be remembered that as the waves enter shallow water their progress is retarded so that they are closely packed, the orbital motion of the particles becomes elliptical, the major axis of the ellipses being horizontal.
This results in hollowing out of the crest and the ultimate breaking of waves. Landwards of the breaker the water alternately advances and retreats up and down the beach as the swash, and backwash. The backwash takes place under the action of gravity.