As the swells or wind waves move towards the margin of the continents over the continental shelf, they reach an area where the depth of water is less than one-half their wavelength. The shallow water interferes with the particle movement at the base of the wave, and the velocity of the waves slows down.
As one wave slows down, the following wave which is moving at the velocity unaffected by the shallow water tends to catch up with the wave that is moving at a much slower rate. Thus, the wave-length is reduced. The wave height increases at the same time. As a result, the wave crests move closer together.
The crests become narrow and pointed. On the other hand, the troughs become wide curves. Because of the increase in the wave height and the decrease in the wavelength, the steepness of the wave is increased.
As the wave steepness reaches 1/7, and when the particle velocity at the wave’s crest becomes larger than the propagation velocity of the wave, the crests break. In fact, the breaking waves with foam are known as surf.
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If the surf is composed of a swell that has traveled from the distant generating areas, it is bound to develop near the shore in shallow water. Such surf consists of parallel lines of uniform breakers.
If the surf is composed of waves produced by local storms, the surf takes the form of unstable, deep water, high energy waves with steepness of about 1/7. Such waves break some distance away from the shore and the surf will be rough and irregular.
When the depth is less than 1/20 of the wavelength, the waves in the surf zone behave as shallow water waves. Particle motion is affected by the bottom of the shelf, and the water is carried towards the shore.
It is generally observed that the short waves before reaching the surf zone disappear, and a series of waves with longer crests develop, because, according to H. Jeffreys, the short waves in shallow water become unstable sooner than the longer waves.
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Since in deep water the waves travel with greater speed than those in shallow water, the in-shore part of a long crest of the shallow water waves is held back more than the part farther away. The waves that were advancing in the open sea in a direction parallel to the shore are refracted towards the shore.
The surf waves transport considerable amount of water towards the sea beach. The water masses taken towards the beach, however, do not return with the wave, but flow back along the bottom like a current. This is called an undertow.
The undertow is often very dangerous to a swimmer who may lose his balance in heavy surf. On the other hand, if the waves reach the beach obliquely i.e. at an angle, a current along the beach may develop that feeds into rip currents. The rip currents, it may be stated, flow into the open ocean and are very dangerous to swimmers.
It is important to remember that very strong and impressive surf waves are generated where swell coming from areas of large and frequent storms reach the coast. The surf along the west coast of Africa in the Gulf of Guinea is very dangerous, and south of it, it is called Kalema which proves to be a great nuisance to the ships.
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That is why only a few harbours are found in this area. The Kalema is the breaking surf that has its origin in the wind zone of the ‘roaring forties’ in the South Atlantic Ocean at about 40° S latitude. In the North Atlantic Ocean such surf waves are called Raz de Maree along the Atlantic coast of Morocco.
These dangerous waves are produced by a swell generated by northwest storms in this part of the ocean during the stormy season lasting from December to April. Similar high surf waves develop at certain costal areas in the Indian and Pacific Oceans. They are also produced by the swell that originates in the stormy west wind zones.