OCEAN WAVES 



Fig. 8. British National Institute of Oceanography developed this 

 floating buoy that carries an accelerometer and two gyroscopes. 

 Instruments give data on pitch, roll, and vertical displacement of 

 buoy, from which directional wave spectra can be derived. 



To all things there comes a time 



As wind waves and swell finally approach a coast and the depth 

 of water decreases, the water particles in orbital motion far beneath 

 the waves begin to "feel bottom." Orbits closer to the bottom of the 

 wave structure gradually flatten into ellipses, and the forward-back- 



Water depth contours (ft) 

 » Rays for 7-sec period waves 

 ""■"""^Rays for 14-sec period waves 



Fig. 9. Differences in water depth along a coast cause waves to 

 refract, as shown here by rays showing direction that waves travel 

 Lower frequency waves such as swell refract more than the waves 

 of higher frequency, complicating the picture of how energy is dis- 

 tributed along coast. 



ward motion of particles becomes greater than the wave's height at 

 the surface. As these changes occur along a gently shoaling bottom, 

 both the group and the phase velocity of the waves decrease. Waves 

 in shallower water are more retarded than the waves just behind 

 them that are still in somewhat deeper water; as a consequence 

 wavelengths also decrease, but wave periods remain the same. 

 Differences in depth along the coast account for differences in the 

 amount of retardation experienced by a wave along its length. This 

 in turn causes the waves to refract as they approach the shore — waves 

 of lower frequency, such as swell, refracting more than waves of 

 higher frequency — as shown in Fig. 9. 



Like the frequency or period, the all-important amount of energy 

 being transported shoreward by the waves at all stages of their prog- 

 ress also remains approximately constant. To conserve energy as 

 their velocities decrease, the waves must grow higher and steeper — 

 until turbulence in the surf and the final plunge up the beach dissipate 

 the energy completely. This is the energy that ooastal engineers must 

 contend with. 



Just how much of the wave energy in deep water is ultimately 

 destroyed by breaking at a beach, how much by whitecaps and turbu- 

 lence in deep water, and how much by friction against the bottom in 

 shallow water is not known. Barber and Tucker of NIO estimate, 

 however, that a run-of-the-mill ocean swell — perhaps 2 m high in deep 

 water — contains 5 X lO^ergs of energy per cm^of sea surface. If the 

 period of this swell is 10 sec, its group velocity in deep water is about 

 7.8 m/sec, which means that the swell is transmitting energy at the 

 rate of 3.9 X 10' ergs/sec for each cm of length along the crest. When 

 this swell reaches a coast this energy is nearly all spent in turbulence 

 in the surf or breaker zone. It amounts to approximately 40 kw along 

 every meter of shoreline. And how many meters are there along the 

 shorelines of the world? 



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