along the foreshore in a saw-toothed or zigzag path due to upwash and 

 backwash of obliquely approaching waves." Material is also moved in sus- 

 pension in the surf zone by longshore currents and the energy associated 

 with breaking waves. A third mechanism is bedload transport which is 

 sediment moved close to the sediment-water interface by sliding, rolling, 

 and bed form migration. The longshore current measurements taken during 

 the study period (according to U.S. Army, Corps of Engineers, Coastal 

 Engineering Research Center, 1966) would be considered a combination of 

 beach face velocity and wave- induced currents in the surf zone. 



Computations were carried out for theoretical conditions on a mature 

 beach profile in which all of the parameters used in the Eagleson formula 

 for longshore drift were held constant except for slope angle of the 

 beach. Beach- face gradients taken from beach profile data were inserted 

 to approximate velocity conditions as the tide rises (2 to 3 slope of 

 the low tide terrace) and begins to flood the beach face (7 to 10 slope) 

 As the tide rises, assuming constancy of other variables (i.e., breaker 

 height, breaker depth, and breaker angle), the current velocity on the 

 high tide beach face was greater than twice that of the low tide terrace. 

 This points out that the longer that waves can act on the high tide beach 

 face, as during spring tides or storm surges, the greater the longshore 

 transport of sediment and the greater the net erosion. 



There is a direct relationship between breaker angle and longshore 

 drift velocities for breaker angles less than 45°. However, as breaker 

 angles become greater than 45°,. a decrease in longshore current velocities 

 occurs (Vollbrecht, 1966). 



The strength of longshore currents necessary to produce littoral 

 transport can be qualitatively estimated from Figure 11. The range of 



6 



J 1 



-3 



—I 



! Unit* if) 



Figure 11. Mean velocities required to erode sand 

 (U.S. Army, Corps of Engineers, Coastal 

 Engineering Research Center, 1966) . 



30 



