(3) surf zone energy dissipation models, 



(4) different mixing models inside the surf zone and beyond 

 breakers, and 



(5) by use of two-dimensional numerical models to compute 

 solutions. 



In general, the experimental data from both field and laboratory 

 measurements substantiate the qualitative aspects and trends of the 

 radiation stress theory. However, sufficient data under widely varying 

 conditions are lacking to quantitatively verify all aspects of the theory, 

 to select the best submodels, and to determine the closure coefficients. 

 In some cases (e.g., rip current formulations and spacing) mechanisms 

 are still being studied as part of the theory so that controlled labora- 

 tory investigations still play a major part in the research effort. 

 Extensive new field data collection efforts (NSTS, ARSLOE) will signi- 

 ficantly enhance efforts to verify the radiation stress theory and estab- 

 lish the needed coefficients in the near future. 



This background provides a beginning in understanding the current 

 state-of-the-art, and the radiation stress approach will continue to be 

 researched. Further refinements will be made by use of nonlinear wave 

 theories in both the radiation stress and wave field computations, use of 

 better wave breaking and surf zone simulation models that include breaker 

 type and beach profile features, determination of rational procedures 

 to select closure coefficients, and incorporation of wave-current 

 interactions in all aspects of the computation. Accurate two-dimensional 

 numerical models will be constructed which include all the physically 

 important terms and minimize the numerical inaccuracies. However, signi- 

 ficant improvements in knowledge will always be limited by the time- 

 averaged nature of the radiation stress approach. 



In effect, a plateau of advancement has been reached in which a 

 somewhat rational framework is now available for time-averaged coastal 

 hydrodynamic theory. This approach will benefit from further research 

 but only moderate improvements in fundamental, physical knowledge are 

 anticipated because the time-averaging omits many local, time-varying 

 details. Waves in the nearshore and surf zones are highly nonlinear. 

 Solitons form and harmonics appear in nature with energy levels on the 

 same order as the dominant period. Unsteady currents appear due to many 

 forces including wave breaking rollers, eddies, and turbulence. The 

 longshore current is by definition a time-averaged current yet a univer- 

 sally accepted averaging time is not known. Efforts to verify the radia- 

 tion stress theory by new field measurements will be hampered by the need 

 to average the time-series obtained over some time interval yet to be 

 determined. 



The Boussinesq theory offers the possibility to eventually raise 

 the fundamental knowledge of coastal hydrodynamics to a higher level. 

 No time-averaging is involved. Nonlinear wave propagation and resulting 



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