Extreme wave runup at the Seisho Coast during Typhoons Faxai and Hagibis in 2019

Two successive extreme typhoons, Faxai and Hagibis, hit Eastern Japan and left devastating damage along its long coastline due to high waves. During the two typhoons, the characteristics of extreme waves at Seisho Coast, the most affected coast, were investigated. The investigations were based on wave observation data, post-event surveys, and numerical simulations. The observation data confirmed that extreme wave fields were generated on the coast during both typhoons and that Hagibis produced higher waves than Faxai. The maximum spectral significant wave height of incident short waves exceeded 8 m at the depth of 13 m and that of infragravity waves reached 2 m in shallower water during Hagibis. The post-event surveys revealed that the typhoons produced different variations of runup heights over nonuniform bathymetry along the coast. Wave hindcast results suggested that the overall variability of runup heights was primarily attributed to alongshore variations of the beach slope rather than offshore wave conditions. Further numerical investigations of nearshore waves using SWASH were conducted to reveal the mechanisms of local extreme runup, which could not be explained solely by crossshore wave processes. The model results suggested the significant involvement of two-dimensional wave processes in the observed variations. Further detailed analyses revealed that local high runup was caused by concentrations of incident short waves during Faxai. Conversely, infragravity waves trapped on the coastal bathymetry contributed to extreme runup during Hagibis. This study highlights the importance of considering nonlinear wave processes over a broad coastal area for predicting local wave impacts under extreme wave conditions.