Longshore currents typically have mean values of 0.3 meter (1 foot) per 

 second or less. Figure 4-23 shows a histogram of 5,591 longshore current 



(O 



c 

 o 



o 



> 



w 



O 



e 



2400 



2000 



1600 



1200 



800 



400 - 



0^ 



-1.50 



Total of 5591 Observations 

 March -December 1968 



NORTH 



J. 



-4 



-2 







(ft/s) 



_J 



SOUTH 



-1.00 



-0.50 







(m/s) 



0.50 



1.00 



1.50 



Longshore Current Velocity 



Figure 4-23. Distribution of longshore current velocities (data taken 

 from CERC California LEO Study (Szuwalski, 1970)). 



velocities measured at 36 sites in California during 1968. Despite frequent 

 reports of exceptional longshore current speeds, most data agree with Figure 

 4-23 in showing that speeds above 0.9 meter (3 feet) per second are unusual. 

 A compilation of 352 longshore current observations, most of which appear to 

 be biased toward conditions producing high speed, showed that the maximum 

 observed speed was 1.7 meters (5.5 feet) per second and that the highest 

 observations were reported to have been wind-aided (Calvin and Nelson, 

 1967). Although longshore currents generally have low speeds, they are impor- 

 tant in littoral processes because they flow along the shore for extended 

 periods of time, transporting sediment set in motion by the breaking waves. 



The most important variable in determining the the longshore current 

 velocity is the angle between the wave crest and the shoreline. However, the 

 volume rate of flow of the current and the longshore transport rate depend 

 mostly on breaker height. The outer edge of the surf zone is determined by 

 the breaker position. Since waves break in water depths approximately 

 proportional to wave height, the width of the surf zone on a beach increases 



4-53 



