In sxjmmary, it is concluded that all irregular wave theories for MWL 

 change in the nearshore area suffer from a lack of concrete experimental 

 confirmation of their validity. This situation is unfortunate since the 

 theories of Battjes and Goda show much promise in this regard. If success- 

 ful comparisons exist in the published literature, they have escaped the 

 review in this study. 



Several possible reasons have been offered to explain why the experi- 

 mental measurements of wave setup disagree with the theory. Air entrain- 

 ment by breaking waves is one possibility since the mixture has a lower 

 density. Another is the neglect of the instantaneous bed shear stress 

 resulting from the asymetrical wave orbital motion which can produce a 

 nonzero average stress (Bijker and Visser 1978).^^ 



II. LONGSHORE CURRENTS 



Of primary interest is a comparison of laboratory and field measure- 

 ments with radiation stress theory for uniform longshore currents. In 

 most cases the accuracy of both laboratory and field data is relatively 

 low due to observational difficulties in measurement of breaker heights 

 and wave angles. Also, nonuniform boundary conditions, alongshore varia- 

 tions in breaker height, winds, etc. can contribute to the uncertainties 

 in the measured values. Current variation with water depth is another 

 factor to be considered regarding the available data base. 



1 . Mean Currents . 



Before 1967, all theories were for a mean longshore current (Galvin, 

 1967). Horikawa (1978a) provides a more recent summary which includes 

 Japanese research efforts. What was meant by mean velocity in the theory 

 and what was measured is not always clear. For example, Komar (1976b, 

 p. 184) used Vi for the average longshore current in a review of all 

 theories and also the current at midsurf (p. 190) when referring to data 

 in Figure 56. The data by Harrison et al. (1968) in Figure 56 were said 

 to plot higher than the others because the data were measured just shore- 

 ward of the breakers as a maximum longshore current (Komar, 1976b). 



In the theory (Ch. 3) , the following time-averaged longshore currents 

 are defined: 



v, = velocity at breaker line 

 b 



V = maximum current velocity 



m 



Vj = the midsurf velocity 



^'^BIJKER and VISSER, op. ait. 



168 



