beaches, and the second category mechanisms cannot. Table 1, as developed 

 by Dalrymple (1978), lists these two categories and the major subclasses 

 of mechanisms as first proposed in the literature. 



Table 1. Proposed mechanisms 

 investigators since 



Wave Interaction Models 



Incident edge wave 



Synchronous 

 Infragravity 



Intersecting wave trains 



Wave-current interaction 



Structural Interaction 



for rip current generation and primary 

 1969 (after Dalrymple, 1978). 



Primary Investigators 



Bowen (1969), Bowen and Inman (1969) 

 Sasaki (1975) 



Dalrymple (1975) 



LeBlond and Tang (1974), 

 Dalrymple and Lozano (1978) 



Bottom topography 



Coastal boundaries 



Breakwaters 

 Islands 



Barred coastlines 



Bowen (1969), Noda (1974) 



Liu and Mei (1976) 



Mei and Angelides (1977) 



Dalrymple, Dean, and Stern (1976) 



a. Structural Interaction . The subclasses listed in Table 1 reveal 

 that these structural interaction models are closely associated with the 

 planform and bathymetry features previously discussed. Structural 

 interaction probably accounts for the majority of rip currents present on 

 the world's coastlines. 



(1) Bottom Topography . Although Shepard and Inman (1950, 1951) 

 correctly stated that rip currents could be related to variations in 

 breaking wave height along the beach, it was Bowen (1969b) who explained 

 why, using radiation stress principles. The excess momentum thrust of 

 the waves normal to the beach produced a tilt or wave setup of the mean 

 water surface from the breakers to the shore. Where the breakers were 

 large the setup was large, and vice-versa. Longshore variations in 

 breaker height created MWL gradients within the surf zone that produced 

 currents flowing from positions of highest breaker height to positions 

 of lowest breakers. Here, the longshore currents converge and turn sea- 

 ward as rip currents, i.e., taking the path of least resistance and main- 

 taining a mass and momentum balance. Outside the breakers a weak return 

 flow is needed to complete a nearshore circulation pattern very similar to 

 observed cell circulation. The bottom contours offshore causing wave 



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