Closure coefficients required in these models can be obtained by 

 fitting the theory for longshore current profile to available data. 

 Some results are given in Table 9. They must all be used with caution 

 since they only apply to the theory and limited range of data involved 

 in their determination. No state-of-the-art values will be given here 

 for this reason. The wide range of time-averaged bottom friction 

 coefficients in the literature is partly due to the many assumptions 

 in the time-averaged bed shear-stress models employed. Use of the maxi- 

 mum longshore current and its location to fit the data is better than 

 use of mean values which are relatively insensitive to bed slope and 

 bottom roughness. Both friction and mixing coefficients are calculated 

 from the data by this method. An alternate method would be to specify 

 the bed roughness, calculate the friction coefficients from fundamental 

 theory, and use the longshore current profile data to estimate mixing 

 coefficients. These closure coefficients would be based on the overall 

 longshore current profile and can be obtained from field or laboratory 

 experiments. Local closure coefficients in the surf zone can also be 

 obtained directly from equation (42), local velocity measurements in 

 the surf zone and independent measurements of S^y. This was done by 

 Huntley (1976) and Thornton (1980) who both neglected the lateral mixing 

 stress gradient in the analyses. It may be possible to independently 

 obtain estimates of v^ from the velocity time histories using auto- and 

 cross- correlation techniques. Then the full equation (42) could be uti- 

 lized to calculate friction coefficients. Only detailed and extensive 

 two-component field data are appropriate for this purpose. 



The empirical wave breaking ratio, Yb ^^^ surf zone energy dissi- 

 pation model complete the list of required coefficients to theoretically 

 estimate current profiles. As stated by Battjes (1978a), use of a con- 

 stant Y ratio imparts an excessive sensitivity of the currents to bottom 

 profile variations. Comments above for wave setup and setdown are equally 

 appropriate for longshore current. It is anticipated that some fundamental 

 new concepts in wave breaking criteria will result from the present numeri- 

 cal modeling simulations of the wave breaking process. 



d. Nearshore Circulation Systems . All available models are based 

 on vertically integrated or depth-averaged flows. This implies that 

 the dominant motions are horizontal and relatively uniform with only 

 weak secondary currents in the vertical taking place. Little data are 

 available to substantiate (or refute) this assumption. 



The two-dimensional equations of motion written in conservation form 

 (eqs. 107, 108, and 109) are preferred over the Eulerian form since the 

 discharges per unit width include wave-induced mass transport. Many 

 terms were discarded in the early analytic theory. For general application, 

 all must be included which means numerical solution methods are now neces- 

 sary. Additional interaction stresses resulting from mean flow, oscilla- 

 tory motionj and turbulence interactions are present but for lack of suffi- 

 cient detailed data, they will continue to be lumped with boundary shear 

 and lateral shear terms. 



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