pattern of equally spaced rip currents separating the circulation cells 

 results from the same reasons as discussed for refraction. 



A series of controlled laboratory test results was reported by Bowen 

 and Inman (1969) and offered as confirmation of the theory. Regular, 

 normally incident waves were made on a smooth concrete beach of slope 0.075. 

 A 7,3-meter working section bounded by vertical barriers which extended 

 seaward gave well-defined boundary conditions for the experiments. A stand- 

 ing edge wave was quickly formed in the basin. It was found for both swell- 

 type waves, which did not break, and plunging-type breakers that the addi- 

 tion of these incident waves and the standing edge wave at the breaker 

 position gave a longshore variation in observed breaker height. The net 

 height was greatest where incident and edge waves were in-phase and lowest 

 where they were 180° out-of-phase. Thus, every other edge wave antinode 

 produced small breakers and rip currents appeared at these positions along 

 the coast as shown in Figure 14 (from Komar, 1976a). The key aspect of 

 these experiments and theory was that rip currents could not form unless 

 both incident and edge waves have the same period. Their efforts to con- 

 firm the theory in the field at El Moreno Beach, Gulf of California, proved 

 inconclusive. Synchronous edge waves could not be measured directly in the 

 moving cell circulation system they observed (Bowen and Inman, 1969). Guza 

 and Davis (1974) ■'^1 showed theoretically, and Guza and Inman (1975) and 

 Harris (1967) showed experimentally, that the most likely resonant edge 

 wave is the subharmonic edge wave. It does not allow a rip current to form. 

 But, Dalrjmiple (1978) points out that this model first proposed by Bowen 

 (1969a) is only for steep, reflective beach systems with no wave breaking. 

 It also requires some means such as reflection of the incident waves by a 

 structure or headlands to create the synchronous edge wave mode. 



As pointed out by Wright, et al. (1979), flat dissipative-type beaches 

 have more circulation cells than the steep reflective-type. Tait (1970) 

 and Tait and Inman (1969) showed that many of these beaches (east coast of 

 the United States) have rip spacings far greater than the fundamental edge 

 wavelength associated with the dominant, incident wavelength. Thus, there 

 must be an additional mechanism that triggers longer rip spacing on flat 

 dissipative-type beaches. 



For wide surf zones and dissipative-type beaches (e.g.. Silver Strand 

 Beach, California), Bowen and Inman (1969) suggested that some type of surf 

 beat phenomena of the incident waves could create longer period edge waves 

 to produce the greater rip spacings observed. Sasaki (1977) made an exten- 

 sive summary of the literature on rip current spacing including many field 

 experiments conducted on Japanese beaches. Using surf zone width, beach 

 slope, breaker type, and other parameters of the surf zone together with a 

 number of empirical, dimensionless plots of the data, Sasaki defined three 

 domains where he felt a hydrodynamically different mechanism produced the 

 rip currents. At one extreme were the steep, reflective beaches dominated 

 by synchronous edge waves. At the opposite extreme, for wide, flat 



GUZA, R.T, and DAVIS, R.E. , "Excitation of Edge Waves by Waves Incident 

 on a Beach," Journal of Geophysical Research, Vol. 79, No. 9, 1974, 

 pp. 1285-1291 (not in bibliography). 



49 



