(a) Bed shear-stress formulations, 



(b) extensions to irregular waves, and 



(c) use of two-dimensional numerical models. 



Extensive use of empirical surf zone models remains a weak link in the 

 theory, expecially in determining, when, where, and why waves break. Use 

 of the method also requires prior specifications of wave heights throughout 

 the region of interest. This aspect poses difficulties in its own right 

 due to wave shoaling, refraction, diffraction, reflection, transmission, 

 and wave-current interaction computations required. Additional research 

 is needed; e.g. , the modified bed shear-stress and lateral mixing stress 

 models used with an accurate two-dimensional numerical formulation to study 

 both regular and irregular wave conditions. 



Time-averaging masks the physical processes that occur at scales within 

 each dominant wave period. The Boussinesq approach unlocks this information 

 and allows finite-amplitude waves in the nearshore zone to propagate and 

 interact with the surroundings in a fundamental way. Solutions to the gov- 

 erning equations are only possible with the aid of large high-speed computers 

 A new research tool is thus emerging to perhaps raise the level of theoreti- 

 cal understanding above that obtainable from radiation stress theory. 



In either case, only depth- integrated flow characteristics are consid- 

 ered. No truly three-dimensional model of coastal hydrodynamics has been 

 attempted. 



Which theory is correct? All the physical observations (Ch. 2) have 

 purposely been separated from the theory (Ch. 3) and left for a comparison 

 of each in Chapter 4. It will become apparent that the number, extent, and 

 detail of the theory far exceed the number of good data sets available for 

 comparison and verification. 



156 



