Research On Various Periods Of Sea Waves

Wave characteristics (wave height, wave period, and so on) are significant parameters in ocean engineering applications, among which the significant wave height and significant wave period are extremely important. At present, calculating wave characteristics with numerical wave models becomes a prevailing method in ocean engineering community in the world. The widely used wave models, for instance SWAN and WAVEWATCH, can output the significant wave height and various spectral averaged periods other than the significant wave period. So far relationships between the significant period and the spectral peak period have been established. However, concerning the relationships between spectral averaged periods and the significant wave period, there are no such formulas which can be practically used in ocean engineering applications. In addition, the observations, both nearshore and in the open sea, show that the mixed waves, which usually are double-peaked or several-peaked in spectral form, appear more often than pure wind waves or pure swells. Thus, the investigation to the double-peaked wave periods is significant. At last, with the development of space technology, using data from satellite altimeter to retrieve wave period becomes an ordinary method. Although several methods for retrieving the wave periods from altimeter data have been proposed, none of them could give a direct or valid relationship between the retrieved period and the significant wave period.Considering those three problems mentioned above, in the present paper, the field data, the laboratory data from wind wave flume and the simulated data generated with Wen's and JONSWAP spectra have been applied to investigate the relationships between the significant wave period and the spectral averaged period, between the wave period at spectral peak and the spectral averaged period, and between the significant wave period and the wave period at spectral peak. The corresponding formulae are presented by data fitting. It is found that the relationship between the significant wave period and the spectral averaged period calculated with the negative order spectral moment is more stable, and that the significant wave period is almost identical to the spectral averaged period calculated with the spectral moment of order of minus 2, both of which demonstrates the contribution of the dominant wave to the averaged wave period. Next, with a large number of simulating mixed waves data, using the double-peaked spectrum expression presented by Guan (1996), the double-peaked spectrum is divided into low frequency and high frequency parts. The significant wave periods are correspondingly calculated. Combining these results with the theoretical consideration, the best formula for the significant period applicable to the double peaked spectrum is determined by comparison. At last, this paper presents a new algorithm of obtaining significant wave period from satellite altimeter data. The retrieved period is compared with the significant wave period obtained from buoy data. It is shown that the results are in good agreement with the periods obtained from the buoy measurements in wind wave seas than in the seas with lots of swells.