3.0 



(cm) 



2.0 



o TJ measured 



X % calculated from Sxx measured 



• T) calculated from Hprns measured 



Figure 50. Comparison of measured and calculated mean water level varia- 

 tions using laser-Doppler velocity meter (after Stive, 1980). 



questionable. The use of linear wave theory for S^^j^ and constant y across 

 the surf zone is a major assumption being addressed. Wave setdown based 

 on the same theory is definitely incorrect at the breaker line. These 

 results are important in that investigators have used this theory to correct 

 for setup effects in the longshore current theory. 



Experiments to verify the theory for oblique wave approach (eqs. 39 

 and 4) are not known. They reduce to the normal incidence forms and hence 

 are based on the same questionable assumptions. 



2. Nonlinear and Irregular Waves , 



James (1974a) only included the two comparisons of his nonlinear theory 

 ,and experiments shown in Figure 51. No real conclusions can be drawn from 

 this limited comparison. Predicted setup is sometimes greater and sometimes 

 less than measured by Galvin and Eagleson (1965), and of the correct order 

 of magnitude. Setdown is much less near the breaker line and more in agree- 

 ment with observations by Bowen, Inman, and Simmons (1968) than linear 

 theory. James' model is said to be limited to spilling br eake rs on gently 

 sloping beaches and for the case of large P values (P = T//gd^) where non- 

 linear effects are more pronounced. 



The cnoidal theories for wave setdown (Svendsen and Hansen, 1976) were 

 compared with experimental results on a 1:35 slope using a regular wave 

 generator that eliminates free second harmonics. Example results are 

 reproduced in Figure 52 where the dark line is experimental data. Linear 

 wave theory (eq. 30) is shown to greatly overestimate the magnitude of set- 

 down (labeled eq. 4 in figure). The curve (eq. 9 in figure) does a better 



162 



