Characteristics of Wave Grouping and Freak Wave Observed by Two Typhoons

Two large typhoons destroyed a concrete detached breakwater on the Suruga Coast, Shizuoka Prefecture, Japan in 2013. Although the Typhoon Man-yi, recorded a lower wave height than the other Typhoon Wipha, significant damage to the structure was observed by Typhoon Man-yi. It appeared that the significant wave height alone cannot describe the extreme wave field caused by typhoon appropriately. This study investigates typhoon wave characteristics with an objective to extract essential wave parameters influential to the stability of coastal structures. The relationships among various parameters were examined regarding the nonlinearity and the irregularity of nearshore waves, such as power spectrum, occurrence of freak waves, skewness and kurtosis, wave grouping and distribution of wave height. It is concluded that the occurrence of freak wave had strong correlation with the kurtosis μ4 and the groupiness factor GF. The nonlinearity parameter П1/3 was found to be a good parameter in the early detection of catastrophic wave field.