28 



Physiography 



33' 



3^ 



120 



119' 



Figure 28. Rate and direction of littoral movement of sand along beaches, compiled from various sources, chiefly 

 Handin (1951) and Johnson (1956). Rate is expressed in thousands of cubic yards of sand per year trapped behind 

 breakwaters and contributed by streams. Bars divide the shore into five segments along which the beaches increase 

 in width toward the southeast until ended by a submarine canyon which begins close to shore and ends in a submar- 

 ine fan (cross-hatched areas). Small circles show the positions of beach sand samples collected November 23, 1956. 



29) sometimes occur, and these are accom- 

 panied by variations in beach width of as 

 much as 100 feet. The chief cycle is an an- 

 nual one, which for most beaches of southern 

 California results in a lowering of the level 

 of the top of the beach during winter and a 

 building up of it during summer. Sand car- 

 ried seaward during the winter is deposited 

 seaward of the breaker zone, so that that 

 area is shallower in winter than in summer 

 (Fig. 30). This cyclical change of profile is 

 caused chiefly by the greater size of waves 

 occurring during winter as compared with 



those of the summer, although seasonal dif- 

 ferences in the direction of wave approach 

 also plays a part, particularly for short 

 beaches. 



As shown by Grant (1943), waves of me- 

 dium height approaching shore are able to 

 hft sand of medium grain size as the crest 

 approaches, transport it landward with the 

 crest, and set it down as the crest passes on 

 so that it is not carried seaward during the 

 backwash of the trough. Large grains may 

 not be moved at all whereas small ones may 

 be kept in suspension so that they are moved 



