IS 



'■1 







y_ 





SLOPE 



y 



o 





a 



0.100 



/ 





LO 



• 



iSt J 



/ 



o . 









oMo ^y 











■ 





s 



- 



0.5 















o 





* 







/ . 









0.5 



1-0 



1.3 



C«IC 



Figure 57. Comparison of laboratory mean currents with integrated currents 

 obtained from strong current theory (after Liu and Dalrjrmple, 

 1978). 



current data over a wide range of variables. Stated in another way, the 

 field data and radiation stress theory both demonstrate that the midsurf 

 velocity is insensitive to changes in independent variables. It is a poor 

 choice to compare theory and experiment for this reason. Consequently, 

 Kraus and Sasaki (1979) used the maximum longshore current velocity, Vm 

 in their analysis, which will be discussed later. 



Theoretical results of the large-angle strong current theory of Liu 

 and Dalrymple (1978) were compared with the data set of Putnam, Munk, and 

 Traylor (1949) which had large breaking angles. An average longshore cur- 

 rent velocity was computed from the theoretical profile by intergrating 

 over the surf zone and dividing by the surf zone area. Their results 

 (Fig. 57) show computed values consistently larger than measured for all 

 three bed slopes tested when a friction factor, f^^ =0.01 was employed. 

 The friction factor could be adjusted to improve the agreement. Liu and 

 Dalrymple (1978) concluded that their large-angle strong current model 

 is primarily valid for steep beaches and large incidence angles as commonly 

 occur in laboratory studies. Comparisons with other data sources were 

 not made. Similar comparisons of integrated mean theoretical values and 

 observations can be found in Thornton (1969, p. 88), Sasaki (1977, p. 147), 

 and Sonu (1975, p. 63). 



171 



