SECT. 3] BEACH AND NEARSHORE PROCESSES 549 



deposition will occur near B and cause the form of the bulge to migrate up- 

 stream, decreasing the angle of inclination, as shown in Fig. 12 (subsequent 

 shoreline). An excess in deposition over erosion between C and D will favour 

 the formation of a spit. Once formed, the spit will further decrease the transport 

 in the vicinity of D by protecting it from wave action. This in tiu-n will result 

 in a downstream propagation of the disturbance by erosion. Certain waves and 

 angles of approach will favour the formation of a spit, while others will merely 

 destroy it. In general, if the bulge is large, waves approaching from a single 

 direction as postulated in this example would be expected to form a spit. It 

 seems quite probable that this process has been operative in the Gulf of Cali- 

 fornia, in regions where waves are predominantly from one direction and large 

 bulges of loose sand are rapidly deposited by flash floods along otherwise 

 straight coastlines. 



12. Discussion 



A sound understanding of natural littoral processes awaits further experi- 

 mentation and development of our knowledge, especially in the following 

 areas : the mechanics of wave motion in and near the surf zone, the budget of 

 water flowing in the nearshore circulation system, and the modes and mech- 

 anics of sand transport along the beach. In addition, it is necessary to gain a 

 better understanding of the extent to which models are caj)able of reproducing 

 nature. 



The behaviour of breaking waves is not in agreement with existing wave 

 theory, and it is not clear that waves in small models always follow the be- 

 haviour of their natural prototype. The distinctive diff"erences both in the mode 

 of breaking and in the after-effects of the wave once it has broken have not 

 been satisfactorily reproduced in models. The persistence of the bore of white 

 water once the wave has broken, common to natural beaches of fine sand 

 (gentle slopes), appears to be absent from models ; this may in part result from 

 the attenuation of smaller waves by surface tension. Also, it appears doubtful 

 that the effect of sand ripples on the rate of transport of sand is equivalent 

 from model to prototype ; ripples in the model frequently attain the relative 

 size of what would be termed a bar in nature. Nevertheless, model profiles 

 reproduce the main characteristics of natural coarse-sand beaches to a degree 

 which is surprising, in view of the great discrepancy in the sediment scale ratio 

 in terms of wave height to grain size. 



While the two-dimensional model experiments appear in many respects to 

 approach prototype conditions, it is apparent that there is yet a wide dis- 

 crepancy between what has been made to occur in the three-dimensional model 

 and the more complex littoral processes found along natural beaches. The 

 currents in the model surf zone appear to lack many of the features, such as rip 

 currents, that are common to nearshore circulation systems in nature. Special 

 attention should be given to amplitude modulations of incident wave height in 

 models so that the fluctuation in longshore current velocity characteristic of 

 the prototype is produced. For example, a difference in phase between the 



