ABSTRACT
Figure 3-1: (a) Sinusoidal ocean wave form; (b) Wind generating sea and swell (Holthuijsen, 2007). As the depth into the ocean increases, the radius of the circular motion decreases. By a depth equal to half the wavelength λ, the orbital movement has decayed nearly to zero. The speed of the surface wave is well approximated by:
where c is phase speed, λ is wavelength, d is water depth and g is acceleration due to gravity at the Earth's surface. In fact, the real ocean waves do not, of course, have a sinusoidal shape and rarely are found with a single wavelength or wave period. The ocean surface is quite chaotic and made up of
many component waves of different periods and directions. The major driving force for ocean waves results from the wind forces on the ocean surface. Wind speed and wave activity are closely related. Besides the wind speed, there is the duration of storm and fetch. Fetch is the distance the wind blows over the water to generate waves. The wave speed is usually variable. Such variation produces waves of various sizes. The relationship between wind and waves is formalized in Beaufort scale. The waves still under the action of the winds that created them are called sea. Whereas, the waves that have moved out of the generating area are known as swell. After, waves travelled a distance from the generating area, they have lost some energy (due to air resistance, internal friction, etc.) leading to a decrease in energy density. Thus, waves become lower in height. Seas usually have shorter periods and lengths and their surface appears more disturbed than for swells. Swells, being no longer under influence of wind, appear more orderly with well defined long crests and relatively long periods than seas (Figure 3-1b). Ocean waves can be classified in at least into four classes based on the water depth, method of generation, wave period, and relationship to generating force) as described below.
