18. In the second experiment, Kaihatu and Briggs (in preparation) 

 examined the system response (primarily diffraction) in the lee of a (modeled) 

 semi- infinite breakwater subject to various incident wave fields. Different 

 frequency and direction distributions were used, similar to the mound study. 

 Peak wave energy direction was maintained along the normal to the breakwater 

 axis. A reference wave gage was placed near the point of the breakwater. 

 Wave gages to monitor system response were arrayed along straight transects 

 behind and at an angle to the breakwater, with transects originating at the 

 breakwater point. As with the mound study, results varied with incident wave 

 conditions. The dominant variable characterizing system response was direc- 

 tional spread of the incident waves. In contrast to the mound study, it was 

 found that wave heights in the lee of the breakwater increased as spread 

 increased. As much as twice the reference wave height (again, four times the 

 energy) was observed with widespread (about 60-deg) incident waves as compared 

 to unidirectional waves. This occurrence may have been, in part, because of 

 the direct propagation of energy into the lee of the breakwater by waves from 

 high incident directions on one tail of the main directional distribution. 

 Nonetheless, it illustrates the point that peak direction alone is not a 

 sufficient characterization of wave directionality. 



19. Collectively, the two experiments indicated that coastal engi- 

 neering system response can vary dramatically for fixed values of the three 

 conventional wave-field descriptors (characteristic wave height, peak period, 

 and peak direction) . At least one measure of the angular spread of energy is 

 required. The experiments suggest strongly that a parameter characterizing 

 directional spread is at least as important as the three conventional 

 parameters . 



Common Directional Wave Gages 



20. One can imagine any number of other possibilities for the distribu- 

 tion of wave energy in frequency and direction. For engineering application 

 in real projects, it is necessary to know the energy distribution in real seas 

 rather than in a set of imagined, idealized seas. The present status of 

 guidance in this regard is that while there is a physical wave basin as well 

 as several mathematical theories which can use virtually any sea state de- 

 scription as input for modeling, there is very little detailed knowledge of 



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