in which the nonlinear characteristics become stronger. Several ocean environments 

 are linked to this condition. Common phenomena related to strong nonlinear 

 interactions in the field are breaking waves, waves encountering a strong current, 

 or waves entering a shoaling zone in a shallow water zone. 



Breaking waves are caused by strong wind, by wave-current interaction, and by 

 interaction with the bottom topography in the nearshore zone. When waves break, 

 the particle velocity at the peak increases and overtakes the propagating wave. 

 The surface elevation forms a discontinuous profile and generates a strong tur- 

 bulent vortex under the water. Since breaking is such a nonlinear event, there 

 is no clear way to describe this process. The effect of buoy response to the 

 breaking has not been taken into account in the analysis. If the breaking is 

 caused by a strong and steady wind while the wind and waves follow the same direc- 

 tion, the Wave-Track buoy's accuracy of the measurement of wave heights may suffer 

 but not the wave approach angles . 



In general, wave characteristics show weak nonlinearities in open water. The 

 assumption of a linear model applied to the computation of wave direction generates 

 small percentages of error . As the waves approach shallow water , or strong 

 currents, the characteristics of nonlinearity increases. The effects of water 

 depth, currents and other wave-wave interaction systems will be discussed in the 

 following paragraphs. 



Current Effects 



When the buoy is deployed in a free drift mode, it may drift with the current 

 or wind, provided either exist. When this occurs, the buoy is measuring waves in a 

 moving coordinate system. If it is desired to know the wave environment in a 

 fixed coordinate system, a spectral transformation must be performed. In trans- 

 forming the ocean wave energy from a moving, or encountered, spectrum to a fixed, 

 or ordinary, wave spectrum, the principle of conservation of wave energy must be 

 applied. That is to say, 



(f>(o) e ,9)da) e = <J>( o>, 9)du) (37) 



Given (— )d«o = d(o e (38) 



3o) 



IT 



