Infragravity wave dynamics on Seisho Coast during Typhoon Lan in 2017

The extreme typhoon Lan caused devastating damage to the Seisho Coast of Japan in 2017. This study presents nearshore wave measurements conducted at different locations along the shore and numerical hindcasts to elucidate the extreme wave field during the typhoon focusing on infragravity wave dynamics, which was comparable in magnitude to incident short waves in shallow water. During the peak of the typhoon, there was a significant wave height of ~8 m, and the infragravity wave height exceeded 2 m at a 14 m depth. The measurement data suggest that infragravity waves developed as bound waves from incident wave groups and partially dissipated during the typhoon peak in the surf and swash zones through their energy transfer to high-frequency waves. Numerical wave hindcasting models successfully reproduced the measured extreme wave field and indicated that the wave refraction over Oiso Spur caused an incident wave concentration in the severely damaged area where high waves collapsed a seawall during the typhoon. Moreover, infragravity waves also exhibited significant alongshore variation, which was due to the different bottom slopes and incident short-wave properties. This study identifies several key features that may have caused the localization of significant damage along the Seisho Coast.