SECT. 3] 



BEACH AND NEARSHORE PROCESSES 



511 



In the present context the term "friction coefficient" will be used exclusively 

 in the former, solid sense to denote the ratio of shear stress to normal stress, 

 as applied to the solid phase alone. 



The term "concentration", N, applied to one substance dispersed within 

 another is generally understood in physics to denote the ratio of the space 

 occupied by one substance to the whole space (at a particular place if the 

 dispersion is not uniform). It will here be used only in this sense. Hydraulic 

 engineers currently use the same term to mean the ratio by weight of the time 

 rates of sediment -to -water transport in a stream. This is a different quantity 

 and may have a very different value. 



(a) 



Fig. 2. (a) Continuous contact friction ; (b) intermittent contact friction by diffusion of 

 grain momentum. 



Taking an aloof view of the phenomenon as a whole, it is at once apparent 

 that aU its aspects without exception share several common characteristics. 



(i) The sediment is heavier than the fluid. Its grains are, therefore, puUed 

 towards the bed by a gravity component normal to the bed siu'face. If a mass 

 m of sediment of density ps overlies unit area of any plane parallel to the bed 

 surface which is inclined at an angle j8 to the horizontal, equihbrium demands a 

 normal stress to be exerted across the plane, of value [{ps —p)lps]gm cos ^ 

 whether the sediment is moving or stationary. Here p is the density of the 

 pervading fluid. 



(ii) There must also exist in all cases a tangential stress [{ps -p)jps\gm sin /3 

 due to the tangential component of gravity. 



(iii) In all cases the movement of sediment over the bed involves the shearing 

 of the pervading, inter-granular fluid and a shearing of the sediment grains 

 over one another and over the bed surface. 



(iv) The shearing of any layer of grains settled on the bed and in static 

 contact with one another requires, as O. Reynolds showed, some degree of 

 dispersion or dilatation of the grains from one another. 



(v) In aU cases this dispersion must necessarily be upwards normally to the 

 bed surface and therefore against the normal component of gravity. 



