Temporal sea surface elevation data picking up giant freak waves with wave heights exceeding 2H1/3 and 10m are analyzed to investigate their statistical characteristics. The freak waves occur in sufficiently developed a seasonal wind wave field with and unidirectional spectra having steep single peak and obeying the wind wave low of f-4. The crest and trough amplitude distributions of the observed sea waves including the freak waves are different from the Rayleigh distribution, although the wave height distribution fairly agree with the Rayleigh distribution. The run length distribution of the observed observed waves are also examined and found to be well predicted by the Kimura theory.
A freak wave is defined as the maximum wave with the larger wave height than twice of the significant wave height. The freak wave is sometimes featured by a single and steep crest giving severe damage to offshore structures and ships. Although there is no doubt on the occurrence of freak waves from many reports on their observations and the damages of offshore platforms at deck level, a sound information on them is very few. Moreover, the mechanisms and detailed statistical properties of the freak waves are still unknown(Dean 1990). The state of the art on freak waves was summarized at NATO Advanced Research Workshop(Tørum and Gudmestad, 1989), where it was explained by Dean (1990) that both of nonlinearity and directionality are primary possible causes of freak waves. Stansberg (1990) carried out a laboratory study on freak waves and then demonstrated that freak waves can be generated through nonlinear wave-wave interactions in a two-dimensional wave flume. Further, Yasuda et al. (1992) numerically showed that the typical freak wave having a single and steep crest is generated by the third order nonlinear interaction.
