Coasts 



29 



both landward and seaward, with the latter 

 movement predominating, owing to the ef- 

 fect of the seaward slope of the bottom. It 

 is evident that waves of medium size tend to 

 build up the beach with contributions of 

 medium sand if sand of that size is available. 

 With larger waves the medium sand behaves 

 as did the fine sand for the medium waves, 

 so that the large waves erode the upper part 

 of the beach, leaving only coarse sediment 

 there. Rip tides or rip currents (Shepard, 

 Emery, and LaFond, 1941) also shift sand 

 seaward, particularly during winter when the 

 waves that produce them are large. So much 

 sand may be carried away that the beach 

 becomes incised by channels several feet be- 

 low the general sand level (Fig. 31). As the 



size and direction of waves change, the chan- 

 nels shift in position and size (Shepard and 

 Inman, 1950, \95lb). 



Another cycle of beach level, biweekly, 

 has been related to the lunar tidal cycle by 

 LaFond (\939b). Maximum erosion of the 

 upper part of sand beaches occurs during 

 spring tides when waves can effectively reach 

 that part of the beach. During the interven- 

 ing time of neap tides only the thin edge of 

 waves can reach the top of the beach, and 

 these redeposit there the sand which had 

 been shifted to deeper water during spring 

 tides. Variations of beach height are usually 

 only a few inches, but these are known to 

 and are taken advantage of by the grunion, 

 a fish that flops ashore at spring tides and 



Figure 29. Change of beach 

 north of La Jolla. In the top 

 photograph taken in August 

 1955 a broad sand berm has 

 been built and the sand is 

 piled high against the sea wall; 

 in the bottom photograph 

 taken in February 1942 the 

 sand had been completely 

 stripped from the beach and 

 the waves had exposed the 

 underlying terrace material. 

 Courtesy F. P. Shepard. 



'^^^^ 



