field is called irregular. Though recognized as important, wave direction 

 could not be determined from these gages. 



Irregular Waves Versus Monochromatic Waves 



6. The response of a structure or nearshore bathymetry to attack by 

 irregular waves can be very different from that by monochromatic waves . One 

 difference is that irregular waves can add up their amplitudes (constructive 

 interference) or cancel out their amplitudes (destructive interference) at 

 various places in space. This interference makes such phenomena as wave run 

 up, structural overtopping, and toe scour more or less intense for irregular 

 waves than for monochromatic waves. Furthermore, even though an irregular 

 wave field can be considered (to a first approximation) a linear sum of a 

 number of monochromatic wave trains, sedimentary and structural responses to 

 these waves are nonlinear. Nonlinearity means that estimates of these respon- 

 ses to irregular wave attack cannot be made by simply adding the responses 

 determined from attack by an equivalent number of separate monochromatic 

 waves . 



7. The problem of irregular wave attack is more difficult than that of 

 monochromatic waves but is not intractable. Engineering guidance for estimat- 

 ing these responses is derived from (a) gaging and monitoring existing real 

 projects, (b) performing scaled physical model tests of various projects in 

 laboratory wave basins, and (c) constructing mathematical models to represent 

 the processes involved. All of these efforts are currently under way at the 

 US Army Engineer Waterways Experiment Station (WES) Coastal Engineering 

 Research Center (CERC) in Vicksburg, MS. 



8. It is extremely useful to be able to characterize an irregular sea 

 as simply as possible. A conventional way is by referring to the wave energy 

 spectrum, which is simply a set of numbers (one for each frequency) propor- 

 tional to wave energy. The formal name is the spectrum of wave energy as a 

 function of frequency , and informally it is usually known as the frequency 

 spectrum. If the set of numbers is graphed as a function of frequency, a 

 curve through the plotted points illustrates the shape of the frequency 

 spectrum. 



