Damage to Seisho Bypass due to Storm Waves During Typhoon 0709

Abstract

Some 1.1 km of the Seisho Bypass along the Seisho coast was severely damaged on September 7, 2007, due to Typhoon 0709 storm waves, closing the road to traffic for emergency repairs. Damage to the highway, caused primarily through wave impact and secondarily by long-term shoreline recession due to decreased fluvial sand supply from the Sakawa River and sand loss into a submarine canyon, was investigated through field observations. A factor contributing further to shoreline recession was the obstruction of eastward longshore sand transport by the Ninomiya fishing port breakwater. Storm waves hit the coastline counterclockwise to normal to the shoreline, causing offshore and westward longshore sand transport with part of the sand transported by the longshore sand transport flowing into the submarine canyon, causing a net sand loss.