BASIC PRINCIPLES OF OCEAN WAVESOcean waves are advancing crests and troughs of water propagated by the force of the wind. When winds start to blow, the frictional effect of the wind on the water creates ripples that form more or less regular arcs of long radii. As the wind continues to blow, the ripples increase in height and become waves. A wave is visible evidence of energy moving in an undulating motion through a medium, such as water. As the energy moves through the water, there is little mass motion of the water in the direction of travel of the wave. This can best be illustrated by tying one end of a rope to a pole or other stationary object. When the free end of the rope is whipped up and down, a series of waves moves along the rope toward the stationary end. There is no mass motion of the rope toward the stationary end, only the energy traveling through the medium, in this case the rope. A sine wave is a true rhythmic progression. The curve along the centerline can be inverted and superimposed upon the curve below the centerline. The amplitude of the crest is equal to the amplitude of the trough, and the height is twice that of the amplitude. Sine waves are a theoretical concept seldom observed in reality. They are used primarily in theoretical groundwork so that other properties of sine waves may be applied to other types of waves such as ocean waves. Principles of other types of waves are modified according to the extent of deviation of their properties from those of sine waves. Waves that have been created by the local wind are known as sea waves. These waves are still under the influence of the Sea surface forecasting local wind and are still in the generating area. They are composed of an infinite number of sine waves superimposed on each other, and for this reason they have a large spectrum, or range of frequencies. Sea waves are very irregular in appearance. This irregularity applies to almost all their properties. The reason for this is twofold: first, the wind in the generating area (fetch) is irregular both in direction and speed; second, the many different frequencies of waves generated have different speeds. Figure 6-1 is a typical illustration of sea waves. The waves found in this aerial photograph are irregular in direction, wave length, and speed. As the waves leave the generating area (fetch) and no longer come under the influence of the generating winds they become swell waves. Because swell waves are no longer receiving energy from the wind, their spectrum of frequencies is smaller than that of sea waves. Swell waves are also smoother and more regular in appearance than sea waves. Figure 6-2 illustrates typical swell wave conditions. Figure 6-1.-Aerial photo of sea waves. |
