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 in- 

 creases with wave height. This increase in width increases the cross 

 section of the surf zone. 



2400 



2000 



10 



c 

 o 



^ 1600 



o 



> 



w 



O 



Xi 



E 



1200 



800 



400 



T 



T 



T 



Total of 5591 Observations 

 March -December 1968 



NORTH 



SOUTH 







Figure 4-15. 



ij -3 -2 -I 1 2 3 4 5 

 Longshore Current Velocity, (feet per sec) 



Distribution of Longshore Current Velocities. Data taken 



from CERC California LEO Study (See Szuwalski 1970). 



If the surf zone cross section is approximated by a triangle, then 

 an increase in height increases the area (and thus the volume of the flow) 

 as the square of the height, which nearly offsets the increase in energy 

 flux (which increases as the 5/2 power of height). Thus, the height is 

 important in determining the width and volume rate of longshore current 

 flow in the surf zone. (Calvin, 1972.) 



Longshore current velocity varies both across the surf zone (Longuet- 

 Higgins, 1970b) and in the longshore direction (Calvin and Eagleson, 1965). 

 Where an obstacle to the flow, such as a groin, extends through the surf 

 zone, the longshore current speed downdrift of the obstacle is low, but 

 it increases with distance downdrift. Laboratory data suggest that the 

 current takes a longshore distance of about 10 surf widths to become fully 

 developed. These same experiments (Calvin and Eagleson, 1965) suggest that 

 the velocity profile varies more across the surf zone at the start of the 

 flow than it does downdrift where the flow has fully developed. The ratio 

 of longshore current speed at the breaker position to longshore current 

 speed averaged across the surf zone varied from about 0.4 where the flow 

 started to about 0.8 or 1.0 where the flow was fully developed. 



4-47 



