Directional Effects on Freak Wave Prediction

The last decade freak waves have become an important topic in engineering and science. The freak waves are sometimes featured by a single and steep crest causing severe damage to structures and ships. The freak wave studies started in the late 80’s and the high-order nonlinear effects on the freak waves were discussed in the early 90’s (i.e.Yasuda et al., 1992). Owing to the many research efforts, the occurrence of freak waves, their mechanism and detailed dynamic properties are now becoming clear. Janssen (2003) showed that the deviations from the Gaussian pdf of the surface elevation are related to the presence of resonant and non-resonant four-wave interactions. The kurtosis, fourth-order moments of the surface elevation, is the measure for extreme events. Mori and Janssen (2006) investigated explicit formulation between the maximum wave height distribution and kurtosis change through the four-wave interactions. This give us the possibility of the freak wave prediction in the open ocean. However, there is no validation of the theory at present. The purpose of this study is to verify the Mori and Janssen (2006) theory to compare with observed data and modification of the theory for making an operational prediction system of freak wave. First, the present theory is verified by the observed data. Second, the directional effects on freak wave occurrence is discussed by the numerical simulations. Finally, the modification of freak wave prediction theory including the directional effects is discussed.