Abstract
Typhoon 1721 hit the Seisho coast on October 23, 2017, and the seawall of Seisho Bypass extending along the coastline was severely destroyed by storm waves. The damaged area was approximately 600 m east of the previous damaged area of the bypass due to Typhoon 0709, and the bypass was destroyed again after ten years since 2007, similarly to the previous event in 2007. To investigate the cause of the damage, field observation was carried out on October 31 and November 13 around the damaged area. Furthermore, beach changes of the damaged area were investigated using the NMB survey data collected in 2014 and 2016.
