PART II. LITTORAL PROCESSES 



D. L. InmanI and R. A. Bagnold 



(For list of symbols and definitions, see p. 550) 



8. Introduction 



The application of the general principles of sedimentation to conditions close 

 to a shore as propounded in Part I of this Chapter is complicated by the 

 essential presence of an upward bed slope, and by the fact that this slope, 

 extending to the water surface, introduces a barrier to the mass transport of 

 water associated with surface waves and winds. Further complications are 

 caused by tidal variations of water depth relative to any given point on the 

 littoral slope. Not only does the ui)ward httoral slope distort the oncoming 

 waves until, becoming unstable, they begin to dissipate their energy by break- 

 ing, and thus cease to behave consistently with existing wave theory, but also 

 it introduces special currents having components both longshore and offshore. 



Littoral processes have been watched at work and their effects frequently 

 described. The physical mechanics of the processes have been speculated upon, 

 but no claim to a reliable understanding of a natural process can be accepted 

 until it can be explained in terms of basic natural principles, and all of its 

 effects can be predicted to the extent of successful reproduction at wiU under 

 controlled conditions in laboratory models. It is our intention here to outline 

 the degrees to which the natural processes controlling the form assumed by the 

 littoral sea bed have been successfully reproduced in models, and to offer some 

 tentative physical explanations. 



9. Littoral Profile : Two-Dimensional Case 



A. Definitions 



Although the precise demarcations are blurred in nature by tidal variations 

 of levels, it is convenient to divide the littoral boundary into three marine 

 zones : 



(1) The offshore zone, extending from the bed contour at which the depth 

 admits disturbance of bed sediment by wave motion inwards to the contour at 

 which the waves begin to break. 



(2) The surf zone, in which the waves dissipate their energy by surface 

 turbulence. 



(3) The swash zone, comprising the beach proper, in which the residual wave 

 motion consists of successive surges up and down the beach face. 



The great majority of model experiments designed to gain an understanding 

 of the natural processes, rather than to further engineering projects, have been 

 restricted, on grounds of economy, to the ideal two-dimensional case. Waves 

 of specified height and w^avelength (or period), generated mechanically, are 



1 The support of the Office of Naval Research over many years has made this work 

 possible. 



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