Wave measurements are examined from three relatively deepwater field sites in Lake Michigan, the Pacific Ocean, and the Gulf of Mexico. Approximately 1 hour of data representing high waves, single-peaked spectra, and nearly constant significant heights and peak spectral periods was selected for analysis. The data represent actively growing waves at two sites and swell at the third site. Analysis is done in both the frequency and the time domain. The fast Fourier transform (FFT) spectral analysis procedure is shown to possess limitations in resolution of frequency and phase. Phases are shown to be subject to erratic variations. Shortcomings of the FFT procedure are circumvented by using a multiple regression screening (MRS) technique to identify frequency, amplitude, and phase for major constituents in the frequency domain. The time domain analysis is designed to extract wave grouping information directly from the time series. A wave group is conceptualized as a small area of sea surface containing relatively high energy. Groups are identified as sections of the time series in which the local variance is high relative to the variance of the complete record. Local variance is computed over a time approximately equal to twice the peak spectral period. Fluctuations in local variance provide information on both the intensity and time scale of wave grouping. A new dimensionless parameter indicative of wave grouping is defined as the ratio of standard deviation of local variance fluctuations to variance of the time series. The autocorrelation between individual wave heights, between periods, and between amplitudes is also considered. The autocorrelation between successive heights ranged from about 0.2 to 0.5. Analyses of the data are used to test the following six hypotheses about the nature of ocean waves: (A.) Spectral components are sometimes discrete and are not smeared over a broad continuous spectrum. (B.) Spectral components are sometimes related in a deterministic, nonrandom way. (C.) The detailed spectral shape may be partially explained by the theory of Benjamin and Feir (1967). (D.) Waves in deep water tend to be organized so that high waves occur in groups. (E.) The modulation period of wave groups is sometimes related to the period and steepness of the waves. (F.) The extent of grouping in each time series and the modulation period are related to certain features of the spectrum. Evidence supporting the hypotheses leads to the conclusion that some commonly held conceptions of ocean waves, including the notion of a random wave field represented by a continuous random-phase spectrum, are open to serious question.
Nonrandom Behavior in Field Wave Spectra and Its Effect on Grouping of High Waves
Technical Report ; TR 82-2
1982
19018681
Report
Elektronische Ressource
Englisch
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