540 INMAN AND BAGNOLD [CHAP. 21 



the surf zone or on the beach foreshore it appears that about 100 times that 

 amount of irradiated material would be necessary to trace successfully the 

 movement of sand ])articles for any distance along the beach. 



11. Littoral Processes: Three-Dimensional Case 



The transition from two-dimensional models to natural littoral processes 

 involves the introduction of mass transport, momentum and like components 

 in directions other than normal to the shore. It also involves the introduction 

 of the effects of tides and of random or ill-definable variations in the character 

 of waves, winds, currents and sediments. The complexity of these factors and 

 the difficulties involved in their observation have resulted in inadequate 

 descriptions of the motions of water and sediment in the field. No serious 

 attempt has yet been made in the laboratory to sort out these motions according 

 to their causes. Before proceeding to a working model for the littoral transport 

 of sand, it is instructive to consider field and laboratory studies of currents in 

 the surf zone as well as observations of sand transport rates. 



A . Currents in the Surf Zone 



Waves in deep water cause a volume transport, or "discharge", of water in 

 the direction of their travel. This discharge increases as the waves pass into 

 shallowing water near the shore. The beach, however, presents an almost 

 impermeable obstacle which must reduce to zero the entire component of the 

 discharge normal to the shore. In effect the rise in water level referred to 

 previously constitutes a normal outward force which superimposes an outward 

 discharge; this just balances the inward discharge by the waves. That such a 

 state of equilibrium must exist on the average over a sufficient length of shore 

 is indisputable, but it is doubtful how far the equilibrium is stable at any 

 given i)oint, i.e. to what extent the water transported shoreward at one point 

 may return seaward elsewhere. 



If, on the other hand, the direction of wave travel is not normal to the shore, 

 the accompanying discharge has in any case a longshore component over and 

 above that due to any local disequilibrium in the force components normal to 

 the shore, and a definite longshore current must result. For some, as yet un- 

 explained, reason this longshore current is found to be confined largely within 

 the surf zone. 



The waves discharge water into each unit length of the surf zone. So, assuming 

 the resulting longshore current to be so confined, the velocity of the current 

 would, were it not for the existence of outward-fiowing rip currents at discrete 

 places, go on increasing indefinitely with distance along the shore. On this view 

 the existence of outward rip currents is inevitable. 



The net onshore transport of water by wave action in the breaker zone, the 

 lateral transport inside the breaker zone by longshore currents, the seaward 

 return of the flow through the surf zone by rip currents, and the longshore 

 movement in the expanding head of the rip current all constitute a nearshore 



