NO MOMENTUM 

 MIXING MODEL 



MOMENTUM MIXING 

 MODEL 



2.0 



Figure 69. Comparison of irregular theory of Collins (1972) with 



original model of Longuet-Higgins (1970) and laboratory 

 data for regular waves (after Sonu, 1975) . 



at P = 0.4 and 0.135 is plotted along with the experimental data of Galvin 

 and Eagleson (1965). It is stated that the irregular wave model represents 

 more closely the mean trend of the experimental data in the surf zone. 

 This conclusion is debatable and the irregular wave theory clearly over- 

 estimates the laboratory data outside the breakpoint. 



As discussed in Chapter 2, field measurements of longshore currents 

 generated by irregular waves also show time variations. To illustrate 

 this fact, Meadows prepared the results shown in Figure 70 where the theory 

 is by Collins (1972) for various deepwater approach angles. The error 

 bars indicate the magnitudes of the variations in currents measured. It 

 was therefore concluded that time-averaged theories for plane beaches, 

 based upon radiation stress principles, are inappropriate for expressing 

 natural surf zone water motions that are three dimensional and unsteady 

 (see also Wood, 1976; Wood and Meadows, 1975). 



These are the only known comparisons between nonlinear and irregular 

 wave theories and experiments available in the literature. 



4. Bed Friction and Eddy Coefficients . 



Different coefficients of bed friction and eddy viscosity appear in 

 these investigations. Comparisons are complicated by the use of different 

 stress models, laboratory and field data sets, theories of longshore 

 current profile, and different methods of analysis including time-averaging. 

 For example, most researchers selected Cf (or f^^ or f) and N (or T or M) 



184 



