650 INMAN AND BAGNOLD [CHAP. 21 



maximum in force applied to the beach bed by breaking waves, ECnjUo, and 

 the maximum in longshore current velocity, ui, would result in an appreciable 

 decrease in the longshore transport rate of sand as postulated by the relation- 

 ship of equation (22). Also, in this regard, one would like to know what deter- 

 mines the spacing between rip currents, and why the longshore current is 

 contained within the surf zone. Here again, the model appears to be in better 

 agreement with the prototype in the case of very coarse sand beaches. Very 

 coarse sand beaches, although less common in nature, appear to have poorly 

 developed nearshore circulation systems when the waves break directly on 

 the beach face. Fiu'ther and more detailed observations of natural circulation 

 are necessary before a rigorous testing of hypotheses is possible. Establishing a 

 water budget for nearshore circulation along the various t3rpes of natural 

 beaches which can then be tested and duplicated in the laboratory appears to 

 be an essential step towards an understanding of the complex mechanism of 

 the longshore transportation of sand. 



In the present state of our knowledge, the most promising approach to the 

 prediction of littoral sediment movement would seem to be from a broad 

 consideration of how the incoming wave energy is transformed and dissipated. 

 But progress here is limited by lack of sufficiently comprehensive data. This 

 can best be remedied by laboratory experiment under controlled conditions. 

 The data are required both as clues to, and as checks upon, general influences 

 drawn from the basic principles of nature. 



One of the most urgent needs is an experimental estimate of the proportion 

 of wave energy which is dissipated in such ways that it is not available to 

 transport sediment in the process of its dissipation. This unavailable energy 

 falls under two heads: (a) that lost from the nearshore water system by 

 radiation, as reflection, and as microseismic waves and noise; and (b) that 

 converted into heat other than in the neighbourhood of the sediment, as in the 

 decay of turbulence generated from above by the breaking wave rather than 

 from below by boundary drag. Intuitively one might expect (b) to constitute 

 the largest item in the energy budget. Thus a clearer understanding of the 

 process by which a wave breaks would seem to be of prime importance. 



Symbols 



Symbol Dimensions Description 



a, a', c Coefficients of proportionality, a' = ps'lps is the 



correction for pore-space 

 Number of grain layers in motion 

 Phase velocity of surface wave 

 Drag coefficient, flpu^ or TJpw^ 

 Drag coefficient, tt'/w* = ^IV^d 

 Grain diameter, roughness of bed boundary 

 Subscript denoting deep-water-wave properties 

 Mean energy per unit surface area of a wave, IpgHa^ 



