Waves 



119 



Figure 105. Wave record made with Vibraton wave recorder in 35 feet of water near the end of Scripps Pier, La JoUa. 

 Taken during a period of low waves, it shows characteristic envelopes developed by superposition of 12.5-3econd swell 

 and local 5-second wind waves. Courtesy of Walter H. Munk. 



trolled only by the wave length. In water 

 shallower than L/20, the equation is approxi- 

 mated by F = \/gD. Since the velocity is 

 progressively less in shallower and shallower 

 water, a wave approaching shore diagonally 

 becomes refracted, bending more or less 

 parallel to shore. A spectacular example of 

 wave refraction is exhibited at the east end of 

 San Nicolas Island where waves moving 

 southeasterly have been refracted by both 

 the north and south coasts of the island and 

 crash together at the end of a wave- and cur- 

 rent-shaped spit (Norris, 1952). Waves from 

 each side pass around the end of the spit and 

 are further refracted and diffracted so that 

 they move northwesterly, 180° from their 

 original path (Fig. 106). 



Variations in depth of water near the 

 shore produce curvatures of the wave crests 

 because of slowing of the part of the wave in 

 shallow areas as compared to the part in 

 deeper waters. The spacings of wave crests 

 as measured on aerial photographs served as 

 a wartime method of estimating water depth 

 at prospective sites for amphibious landings; 

 similarly, the relative wave heights along a 

 coast can be predicted from a knowledge of 

 the offshore bottom topography (Anony- 

 mous, 1944; Munk and Traylor, 1947; Dun- 

 ham, 1951). As a result of refraction, wave 

 energy is concentrated at seaward projecting 

 points and is diffused within bays (Fig. 107). 

 Thus, waves tend to develop a straight coast 



by erosion of points and deposition in inden- 

 tations, but straightening is opposed by vari- 

 ations in resistance to erosion of coastal 

 strata. For example, the projecting points of 

 the Palos Verdes Hills (Fig. 37) owe their 

 origin to thick sections of basalt sills which 

 are much more resistant than the Miocene 

 shales that form the cliffs within the small 

 bays. 



This refraction of waves is the first indi- 

 cation of transfer of wave energy to the 

 bottom and the beginning of appreciable 

 geological work by the waves. Since refrac- 

 tion occurs only where the water depth is 

 considerably less than half the wave length, 

 Dietz and Menard (1951) have pointed out 

 that effective wave base must be less than 

 about 10 meters off most coasts such as that 

 of southern California. Between this depth 

 and the highest shore level reached by waves, 

 nearly all wave erosion and most transpor- 

 tation and deposition take place. The turbu- 

 lence and orbital motion of the water before 

 and after the waves break causes much of 

 the erosion. The orbital motion is such 

 that a water particle on the forward slope of 

 an advancing wave crest is lifted when the 

 crest overtakes it, is carried forward with 

 the crest, drops as it is left behind, and is 

 finally carried back seaward in the trough 

 between crests to begin the cycle anew (Fig. 

 108). Before the wave breaks, the orbit is 

 more or less circular; afterward it is a very 



