PART IV: EXAMPLES OF OBSERVED FREQUENCY-DIRECTION SPECTRA 



122. In this section results from three typical nearshore wind wave 

 regimes are shown. It should be remembered that sea state in any given 

 observation depends on both history of generating winds (including variability 

 in speed and direction) and spatial distribution of generating winds. The 

 latter property is important because waves at a site can result from both 

 local generation and propagation from distant generating areas. Results shown 

 here are taken out of context of the complete generation history and may have 

 detailed structure different from other cases. Nonetheless, the general 

 structure is sufficiently representative for purposes of illustration. 



Low- Wind. Low- Energy Regime 



123. Figure 5 represents the most common sea state, which is persistent 

 low-energy swell found in day-to-day observations along most coasts. Fig- 

 ure 5a shows the frequency-direction spectrum in three dimensions with the 

 integrated frequency spectrum on the right rear panel and the integrated 

 direction spectrum on the left rear panel, similar to the pattern used in 

 Figure 2. Figure 5b is a contour plot of the frequency- direction spectrum 

 normalized by its own maximum value, making the normalized spectral density 

 range between to 1 . Contours are drawn at 10-percent inteirvals . Directions 

 are as defined in Figure 3. 



124. The general features of this case are associated with a primary 

 mode (or lump) of energy with peak frequency and direction of about 0.09 Hz 

 and about -30 deg, respectively. Total energy is characterized by a represen- 

 tative wave height H^^ of about 0.5 m. There is very little energy at the 

 higher frequencies which suggests that there is little active wind generation. 

 The integrated frequency spectrum is shaped qualitatively like common models 

 of frequency spectra, having a steep low- frequency side and a high-frequency 

 side that decays more gradually. 



125. Of interest is the extent to which the energy is spread in 

 direction. In Figure 5a, the integrated direction spectrum indicates energy 

 is almost uniformly distributed from about to -40 deg with a slightly 

 enhanced peak at about -30 deg. For a given energy density, the contour plot 

 in Figure 5b indicates that the greatest spread is near the spectral peak 



47 



