Wind is the most important cause of wave generation. Most natural waves are initiated by winds. When the wind blows over the surface of the ocean, it affects wave motion through friction upon the water surface. Some of the wind’s energy is thus transmitted to the water. Waves are the result of this energy transfer.
It is interesting to note that waves thus created do not transport water horizontally from one place to another. However, when the waves break and run up on a beach, the horizontal movement of water does take place in the form of currents.
The waves travel in some definite direction, but the water does not travel with the waves. It simply moves up and down, to and fro. Finally the water returns to its original position with each wave. In a storm we see that there are waves of various sizes moving in different directions on the ocean surface.
The surface of the ocean appears to be in a confused state. It is necessary to consider the characteristics of ideal, wind- generated oscillatory waves in Stillwater the ocean, although they (such waves) may not exist alone in nature.
It may be pointed out that waves generated in deep water must be distinguished from those in shallow water. Waves in deep water are defined by their length and height, since the relation-ship between these two dimensions determine the steepness of the wave.
The following definitions of various characteristics of a progressive wave will prove helpful in the further discussion of oceanic waves.
Crest: The highest part of the wave is called the crest.
Trough: The lowest part between the wave crests is the trough.
Wave length: The distance from crest to crest, or from trough to trough is called the length of the wave.
Amplitude or height of the wave: The vertical height of the crest above the trough is the height or amplitude.
Fetch: It is a term which refers to the distance of open water across which the wind can blow without interruption.
Period: The time taken for the passing of one wavelength is the period.
Frequency: Frequency is the number of wavelengths that pass a fixed point per unit of time.
Velocity: Velocity of a wave is determined by dividing the length of a wave by the period.
Steepness: Steepness of the wave is equal to the height divided by length (H/L).
Motion of water particles in a wave:
As stated earlier, the water particles in a sea wave follow the circular orbits at the sea- surface. The length of the diameter of such a circular orbit is equal to the wavelength. Moreover, at the crest of a wave the water particles move in the direction of the energy propagation.
It may be noted, however, that the size of the circular orbits followed by water particles goes on diminishing with increasing depth below the sea surface. At a depth of one-half wavelength, the particle motion is negligible.
The water particles in the trough of a wave move in the opposite direction of the wave. Half of the orbit of the water particles in the trough is at a lower velocity than the other half of the orbit that the particles follow at the crest of the wave.
So there is a small forward movement of water particles in the direction of the propagation of the wave. This is so because the velocity of the particles decreases with increased depth below the still water line.
Remember that the above characteristics as have been discussed apply to deep-water waves only. Such type of waves travels across the ocean in which the depth of water is greater than one- half the wavelength.
It should be borne in mind that all the wind-generated waves that travel across the open ocean belong to the category of the deep-water waves. Moreover, the velocity of such waves depends on the wavelength and the period. According to Mitchell, wave-crests form an angle of 120° with surface of water.
Shallow-water waves (Long waves):
Such wind-generated waves are placed in this category as move into shallow water near the shore. Tsunami or seismic waves as well as the tidal waves are included in it. In such waves the length of the wave is much greater than the water depth. Besides, the velocity is related to the water depth rather than the wavelength.
The water particles move in an elliptical orbit instead of the circular orbit as is the case with deep-water waves. The movement of water particles approaches nearly horizontal oscillation.
The vertical component of the motion of the water particles actually decreases with increased depth. The presence of these waves can be felt even at the ocean bottom.
The type of waves that are characterized by wavelengths greater than twice the depth of water, but less than 20 times the water depth, is included in the category of transitional waves.
In these waves, their velocity depends partially on the wavelength and partially on the depth of water. It is to be remembered that the transition from the deep-water waves to the shallow-water waves begins when their periods reach 10-12 s (seconds).