energy appears to be absorbed by the primary peak. The structure of 

 the bimodal, high-frequency tail remains, and its energy increases. 



e. As storm waves reach their peak wave energy, the directionally broad, 

 energetic, low-frequency spectral peak and directionally bimodal, high- 

 frequency tail of the spectra remain. If driving winds persist, this 

 structure can continue relatively unchanged for some time. In Event N, 

 for example, wind speed remains nearly constant at 16 to 18 m/sec, and 

 wind direction persists at 60 deg to 80 deg from shore-normal for more 

 than 2 days (see page A47). The sequence of observations on pages 

 A48 to A50, showing S(f,6) from 1300 on 7 Mar 89 to 1900 on 9 Mar 

 89, illustrate a structure that changes very little, suggesting that some 

 equilibrium condition has been attained. 



f. In the waning stages of a storm (the final plots on page A50 for spectra 

 of Event N), winds usually decrease in speed and change direction. 

 Active local wave generation abates, and the high-frequency part of the 

 spectrum decays. Low-frequency waves that were generated offshore 

 continue to arrive, still with a broad directional spread. 



g. In the end stage, conditions generally return to the prestorm spectrum 

 of low-frequency, low-energy swell, having broad directional spread 

 and being centered near the shore-normal direction. 



The above description was derived specifically from Event N, but the other 

 24 storm events have the same features to a greater or lesser degree. Even 

 the two dissimilar storms, Events E and R mentioned above, have some of the 

 same features. The ubiquitousness of these spectral patterns indicates a high 

 degree of consistency in storm wave evolution at the FRF. The unexpected 

 features of these spectra, low-frequency side lobe of energy in the secondary 

 growth stage, as well as a directionally broad, low-frequency peak coupled 

 with a bimodal high-frequency tail in the most energetic stages, require some 

 explanation so that guidance appropriate to engineering projects can be de- 

 rived. 



There are a number of lines of approach for explaining these observations. 

 Several of them require work that is beyond the scope of this report, but the 

 nature of the necessary investigation can be mentioned here. A primitive 

 question is whether the observed S(f,6) are real or some artifact of the inter- 

 action of the true wave field and the IMLE algorithm as used with the FRF 

 linear array of gauges. If the wave energy is all propagating onshore, there is 

 no known reason to suspect the spectral estimation process. Tests using syn- 

 thetic data indicate the system is well capable of resolving the broad direction- 

 al spreads and 20-deg-separated bimodal distributions observed here. If there 

 is wave energy reflected from nearshore, the directional estimation system 

 used in this report cannot resolve it, and the reflected energy will appear to be 

 coming from offshore. This question can be addressed with a more recently 

 installed 360-deg array, but that analysis has not been completed at this writ- 

 ing. Nonetheless, it is unlikely that reflections will account for the present 

 observations because the spectral pattern recurs at many tidal stages and over 



18 



Chapter 4 Discussion 



