Coasts 



27 



area toward the south by Uttoral drift is sug- 

 gested by data from a compilation of rates 

 of accretion of sand behind breakwaters and 

 groins (Handin, 1951; Johnson, 1956). The 

 rate of transportation from the west toward 

 the middle of the embayment is approxi- 

 mately three times that of transportation 

 southward away from the middle (300,000 

 versus 100,000 cubic yards per year) as shown 

 by Figure 28. These figures for rate of lit- 

 toral transport are minimum ones because 

 much sand probably bypasses the obstacles, 

 particularly when the areas of trapping are 

 nearly filled. 



A large amount of sand may escape the 

 beaches by leakage to deep water, counter- 

 balancing the ultimate tendency of beaches 

 to prograde the shoreline of southern Cali- 

 fornia into a smooth broad curve. 



Four submarine canyons (the Hueneme- 

 Mugu-Dume group, Redondo, Newport, and 

 Scripps-La Jolla) extend so close to shore 

 that large quantities of sand moving along 



the beaches can be intercepted. Sliding of 

 sandy sediment is known to have occurred 

 at the heads of Mugu, Redondo, Newport, 

 La Jolla, and Scripps Canyons, and large 

 fans are present in deep water at the mouths 

 of Hueneme, Mugu, Dume, Redondo, and 

 probably Newport and La Jolla Canyons 

 (Table 4, p. 47). Interruption of the littoral 

 drift is also indicated by the scarcity of 

 beaches in areas immediately east or south of 

 the canyons — Santa Monica Mountains, 

 Palos Verdes Hills, Laguna Beach, and La 

 Jolla, respectively. Thus the submarine 

 canyons appear to divide the region into five 

 subequal and more or less independent beach 

 units. 



Sand moves seaward and landward across 

 the beaches in areas that are free of canyons. 

 The movements are periodic and produce 

 cyclical changes in the profiles of beaches 

 that have been measured for many years 

 (ShepardandLaFond, 1940; Shepard, \950b). 

 Vertical variations of more than 10 feet (Fig. 



Figure 27. Watershed tributary to continental borderland. 



