section of energy spreading. However, this method has been applied in other 

 investigations of i 

 shown in Figure 7 • 



] 2 

 investigations of wave-current interaction and some of the computed results are 



Effect of Water Depth 



Waves in finite water depths affect wave measuring platforms such as the 

 ENDECO Wave-Track buoy. Also known as an orbital following buoy, it derives wave 

 directional measurements from tilts induced by the wave orbits. Since wave orbits 

 are affected by water depth, it becomes necessary to account for the depth effect 

 when converting the tilt data to slope form. 



In deep water, the depth effect is neutralized when the Fourier coefficients 

 become normalized, as per equations (6) and (T). Meanwhile, the dispersion rela- 

 tion of equation (5) indicates that (2nf) /gk is equal to tanh(kd) and approaches 

 unity. In a finite water depth the normalized Fourier coefficients also 

 neutralized the depth effect of hyperbolic functions in (32). However, the 

 relation of (2irf)/gk no longer approaches unity and the correction becomes 

 necessary. The correction will not change the values of mean direction, but will 

 reduce the rms values of spreading angle. This result is consistent with the 

 general observation that the waves become narrow banded as they approach the shore. 



BIMODAL AND MULTIMODAL WAVE SPECTRA 



The ocean is a dynamic system. As the wind blows along the ocean surface, 

 waves are generated at different locations and propagate in various directions. 

 The combination of two or more wind wave systems generate bimodal or multimodal 

 wave spectra. Bimodal sea systems are enhanced by the passage of a storm or a 

 rotating wind 2 9»30 ( see pi gure 8). The appearance of bimodal wave spectra have 

 been estimated to occur in about 20 to 30 percent of the measured field data. J 

 The multimodal wave spectra also represent an important surface wave mechanism. 



The superposition of two wave systems, which originate from two different 



31 

 storms, is a common technique applied to estimate the bimodal wave spectrum. J 



Based on field data, several models have been developed to fit the bimodal wave 

 spectra and to investigate the wave mechanisms. '^ The mechanisms are complex 

 and must take into account the wind-wave and wave-wave interactions of two- 

 dimensional waves. The results have not been conclusive. 



20 



