Occurrence probability of freak waves in nonlinear wave field

Freak wave is an extreme phenomenon of waves and may cause damage to seaman or people who live or act at coastal area or in the sea. It is essential to predict its probability by stochastic approach since the mechanism of freak wave occurrence is still not fully understood. Nonlinear assumption is used to describe the ocean waves in the field in this study. Derivation of occurrence probability proposed by Mori and Janssen [1] is reported. Long-term and continuous wave data measured at Northern Taiwan waters are used to verify the theoretical result. The result shows the occurrence probability of freak wave in nonlinear wave field is higher than Gaussian Sea and proportional to the kurtosis of sea surface elevation. The comparison shows good agreement between theoretical derivation with field data when kurtosis is small. In addition, the relationship between kurtosis and BFI is established. BFI is derived from spectrum that is the output of numerical wave modeling. This means the prediction of probability of freak wave occurrence is possible estimated from wave model.