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 cross-shore 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.

连续两次极端台风“法茜”和“海贝思”袭击了日本东部，并由于巨浪而在其漫长的海岸线上造成了毁灭性的破坏。在两次台风期间，对受影响最严重的西正海岸的极端海浪特征进行了调查。调查基于波浪观测数据、事后调查和数值模拟。观测数据证实，在两次台风期间，海岸都产生了极端波场，海贝思产生的波浪高于法茜。Hagibis 期间，入射短波的最大光谱有效波高在 13 m 深度超过 8 m，次重力波在浅水区达到 2 m。事后调查显示，台风在沿海的非均匀水深测量中产生了不同的爬升高度变化。波浪后测结果表明，爬升高度的整体变化主要归因于海滩坡度的沿岸变化，而不是离岸波浪条件。使用 SWASH 对近岸波进行了进一步的数值研究，以揭示局部极端爬高的机制，这不能仅用跨岸波过程来解释。模型结果表明二维波浪过程在观测到的变化中具有重要意义。进一步的详细分析表明，局部高升是由法茜期间入射短波的集中引起的。相反，被困在沿海水深测量中的次重力波导致了海贝思期间的极端爬升。