534 



IN MAN AND BAGNOLD 



[chap. 21 



from the beach and depositing it offshore, with the formation of a pronounced 

 bar at the point at which the waves started to break. 



This reproduces quahtatively what is seen to happen in nature, but quantita- 

 tively it lias so far not been found ])ossible to reproduce the same profiles on 

 differing model scales. One probable reason is that the smaller the model scale, 

 the more do surface-tension effects modify the mechanics of energy dissipation 

 within the upper water layers when the waves have broken. The dynamics of 

 a breaking wave are not yet understood. For this reason, perhaps, only little 

 experimental attention has been paid to the marked difference in the manner 

 of breaking, as between long and steep waves breaking on flat and steep 

 beaches. Also, little attempt has been made to reproduce the features of natural 

 fine sand beaches in the laboratory. Here again, the very gentle slopes (Fig. 9) 



MAXIMUM SEASONAL CHANGE IN SAND LEVEL 



250 



125° 



62 



10 ^- 

 oi- 



<UJ 



"J 



3000 



2000 



1000 



DISTANCE 



FEET 



Fig. 9. Profile of a natural fine sand beach at La Jolla, California, siiowing the median 

 sand size, the maxinium seasonal change in the level of the sand associated with 

 winter and summer waves, and the thickness of the modern littoral sand over the 

 wave-cut terrace. Note the very gentle slopes. (After Inman, 1953.) 



and the small size of sand together appear to give rise to a type of transverse 

 sweejiing motion which cannot be reproduced in narrow two-dimensional 

 models. It seems likely that this sweeping motion, the cause of which is not 

 understood, may give rise to the formation of beach cusps. 



For reasons already suggested little attention has been paid to the littoral 

 profile offshore of the plunge line. In many regions the sea bed remains 

 sufficiently shallow to be affected by wave action for great distances offshore. 

 Formation of ripples and active movement of sand on the sea bed has been 



