velocity V also varies with the first four and N (or F) to give the proper 

 lateral mixing. Some examples of parameter studies have been discussed to 

 determine the relative importance of the independent variables involved. 

 Figure 30 from Kraus and Sasaki (1979) is an excellent example. More re- 

 search is needed here especially regarding the relative importance of y, C^ 

 and N (or T) that appear together in the P - parameter. 



For example, Jonsson, Skovgaard, and Jacobsen (1974) for their strong 

 current small-angle model showed that bottom roughness (i.e., C^) was more 

 important than eddy viscosity (N or F) on current magnitude. The breaker 

 height ratio y had little influence on magnitude but greatly affected the 

 location of the maximum velocity. Kraus and Sasaki (1979) concluded that 

 for dissipative beaches, P*<0.1 (see Ch. 4). It would be very instructive 

 to conduct sensitivity studies of the relative importance of y > C^ and F 

 in this range using their model. Ultimately however, more fundamental under- 

 standing of physical processes in the surf zone is needed to improve the 

 theory. 



4. Surf Zone Empiricism . 



Early in the 1970 's, it became evident to many researchers in the field 

 (e.g., Longuet-Higgins, 1972a; Battjes, 1974a) that the wave breaking and 

 surf zone empiricism required was a major weak link in the longshore current 

 theory. Long-range research began with the goal to ultimately provide more 

 fundamental descriptions of wave shoaling processes, when and where waves 

 break, flow separation at the crest, air entrainment effects, and the genera- 

 tion, transport and dissipation of turbulence across the surf zone. Numerical 

 simulations have played an important role in both wave shoaling and breaking 

 research, and only a bare outline for some of the important results is given 

 here. The complexity of turbulent fluid motions also means some empirical 

 models of surf zone hydrodynamics are still useful. 



a. Surf Similarity Parameter . Battjes (1974a, b, 1974) showed how a 

 single surf similarity parameter E. depending upon beach slope and incident 

 wave steepness (H/Lo) is useful to quantify various aspects of waves breaking 

 on a plane beach. The aspects considered were a breaking criterion, breaker 

 type, number of waves in surf zone, wave runup and setup, and beach type 

 (dissipative vs reflective) . The similarity parameter is defined as 



? = -^^^ , (96) 



(H/Lo)^ 



and was based on the work of earlier investigators as solely a wave breaking 

 criteria. Here, H is the wave height defined at the toe of the beach slope 

 and Lo is the deepwater wavelength. Table 5 modified from Battjes (1974b) 

 gives the results.^ The theory of longshore currents described above essen- 

 tially holds for 5<0.4. The criteria used to establish Table 5 were taken and 

 modified from published research efforts at that time. 



107 



