O
408 OCEAN WAVES PAUL D. KOMAR Oregon State University Wind-generated ocean waves are generally the most signi ficant physical force involved in the erosion of coasts and experienced by wave-swept plants and animals that live along the shore. The heights of the waves generated by storms over the ocean commonly reach to meters, with the highest reliably measured wave having achieved meters (measured in in the South Pacific to the east of the Philippines). The energy carried by the waves as they cross the expanse of the ocean is eventually delivered to the coast, where they break on beaches or crash against cliffs. There the waves expend their energy as they wash across the intertidal plants and animals and, over the long term, etch out the natural weaknesses in the rocks to locally form tidepools. CHARACTERISTICS OF OCEAN WAVES Waves are generated on the ocean whenever the wind blows across the water’s surface, the process involving the transfer of energy from the wind to the waves. The greater the speed of the wind, the greater the heights and energies of the waves that can be formed. The highest waves are generated by major storms, since they generally have the strongest winds and can persist for several days during which energy is transferred to the waves. Therefore the heights of the waves depend on the duration of the storm as well as on the speed of its winds, and also on the storm’s fetch: the area or length over which the energy is transferred from the winds to the waves. Techniques have been developed by researchers to predict the heights of waves generated by storms, depending on the wind speeds and on the storm’s fetch and duration. The characteristics of relatively simple ocean waves are depicted in Fig. , with the waves consisting of a series of crests and troughs. The wave height, H, is the vertical distance from the trough to its crest, while the wavelength, L, is the horizontal distance between successive crests. The motion of waves is periodic, repeating through a relatively fixed interval of time: the wave period, T. From the geometry of the waves it is apparent that they will move a horizontal distance L during the period T, so their speed, or wave celerity, is calculated as C = L/T (Eq. ) As the waves pass it can be observed that a floating object (such as a cork) rises and falls as the crests and troughs alternately affect its movement, with some to-and-fro horizontal movement as well; the paths of the floating objects are thereby circular to elliptical. This FIGURE 1 Regular ocean waves, characterized by the wave height, length, and wave period, the time interval of the passage of successive wave crests. O pattern of movement within the wave extends downward with depth beneath the water’s surface (see Fig. ), but with the diameters of the circular movements progressively decreasing and flattening, so that at the sea- floor the water movement is a simple horizontal orbital motion. The water itself makes no net advance in the direction of wave movement, unless there is also a net ocean current in that direction. The waves thereby transfer energy across the ocean’s surface, in the form of the wave itself and its internal orbital water movements , but with a negligible net drift of the water. Having derived their energy from the storm’s winds, the waves are capable of carrying that energy for hundreds to thousands of kilometers across the expanse of the sea, ultimately delivering it to the coasts that form the ocean’s boundaries. The energy of the waves, E, is directly related to their heights: E = 1 8 gH2 (Eq. ) where is the density of the water and g is the acceleration of gravity. It is seen from this relationship that if the wave height is doubled, the energy is increased by a factor of . Important is the rate of transfer of this energy across the ocean to the coasts, which is calculated by multiplying E by the wave celerity C to obtain the wave power, P = ECn (Eq. ) The factor n has been included because waves travel across the ocean in groups, those formed by a particular storm. It turns out that although individual waves travel at their celerity, C, in deep water the group as a whole travels at half that speed (that is, n = ⁄).The observation is that an individual wave progressively moves to...
OR
