It is not necessary to know the value of f in order to solve for q in the 



above procedure. Once q is known, the local volumetric sediment transport 



rates q and q for the cell may be determined by multiplying q by 

 ^x y ''• 



u^/v^ and v^/v^ , respectively. 

 Transport beyond the surf zone 



65. Beyond the surf zone, waves are not breaking. Currents (tidal, 

 littoral, and rip) still transport sediment, but the sediment load is much 

 smaller than the load in the surf zone. Waves still assist in providing power 

 to support sand in a dispersed state. However, there is little turbulent en- 

 ergy dissipation, and frictional energy dissipated on the bottom represents 

 most of the energy dissipation. Bed load is the primary mode of sediment 

 transport beyond the surf zone according to Thornton (1972). 



66. Since beyond the surf zone it is the tractive forces of currents 

 (including wave orbital velocity currents) that produce sediment movement, a 

 sediment transport by currents approach is taken. Again, since the complete 

 physics of the problem is not completely understood, a semiempirical approach 

 must be taken. In this model, the approach of Ackers and White (1973) is 

 followed after appropriate modification for the influence of waves. 



67. Ackers and White (1973) studied sediment transport due to currents. 

 They used the results of 925 individual sediment transport experiments to 

 establish various empirical coefficients. The approach considers both 

 suspended load and bed load. It is assumed that the rate of suspended load 

 transport is dependent upon the total shear on the bed. Therefore, the shear 

 velocity v^ is the important velocity for suspended load transport. Bed 

 load transport, however, is assumed to depend upon the actual shear stress on 

 individual sediment grains. Ackers and White (1973) assume that this stress 

 is comparable with the shear stress that would occur on a plane granular sur- 

 face bed with the same mean stream velocity. Thus the mean velocity of flow 



V is the important velocity for bedload transport. 



68. Considering only currents (not waves). Ackers and White (1973) 

 derived sediment transport rate in a dimensionless form. For convenience in 

 practical application, this may be written as 



q = j^-^^ D (^^ ^ -^ (F - A)""! 

 (1 - p) V^*/ ""l 



(66) 



39 



