Spectra 



Short wave spectra were required to compute N r It is also helpful to examine 

 spectra for specific events of interest to identify any presence of multiple wave 

 systems or to confirm that H s and T p adequately characterize the sea state. 



Spectra are especially useful in relation to long waves. Average long wave 

 spectra were computed by month for each gage to reveal general structure in the 

 long wave response of the harbor. A maximum energy density value (over all 

 records during the month) at each spectral frequency was also identified. The 

 results for January 1994 are typical (Figure 20). Average spectra are surprisingly 

 similar from month to month. Maxima are typically an order of magnitude 

 higher than the average, but they define a shape very similar to the average 

 spectra. The maxima clearly show more statistical variability than the averages, 

 as would be expected. 



Long wave spectra were examined in more detail to better understand harbor 

 response during high energy long wave events. Spectra for one such event are 

 shown in Figure 21. Averages of the event spectra are also shown. Individual 

 spectral values fluctuate over a very wide range. Averages follow the charac- 

 teristic shape of monthly averages, suggesting that each area of the harbor tends 

 toward a signature response curve which varies in energy level according to 

 incident wave variations but not in shape. The energy level across all frequen- 

 cies of the extreme event average spectra is considerably higher than that of the 

 monthly average spectra (Figure 20). Mean water level variations during the 

 event had no clear impact on energy level of the spectral peaks at Pier 2, but they 

 did appear to cause very small shifts in the frequency at which the peaks 

 occurred. 



To explore whether high energy long wave events consistently excite the 

 main resonant peaks of the signature response curve, the time-history of energy 

 level at specific resonant frequencies was plotted. Figure 22 shows results for a 

 dominant resonant frequency at Pier 2. Two adjacent frequencies are shown 

 because varying conditions, such as tidal water level, caused the peak frequency 

 to vary over this very small range. By comparing with Figure 16, it is clear that 

 high values of H slong are accompanied by high energy in this resonant peak. 

 Other resonant peaks show similar correspondence, indicating that when H sIong is 

 high, all of the characteristic resonant frequencies have high energy levels. 



Correlations for predicting incident infragravity wave energy 



A special study was conducted to relate incident infragravity (long) wave 

 energy to offshore wave conditions, for which long-term information is available. 

 The purpose of the correlation study was to determine the ability to predict 

 infragravity energy levels incident to the harbor from deepwater, wind wave 

 parameters (Merrifield and Okihiro 1996). Correlations and linear regressions 

 were calculated between observed infragravity energy (converted to an H slong ) in 

 the frequency range 0.002-0.040 Hz (500 to 25 sec) at the array just outside of 

 the harbor and reduced parameters (H^ Tp, and 6 'J measured at the offshore 



24 



Chapter 2 Field Wave Measurements 



