Quantitative analysis on the tsunami inundation area and erosion along the Abukumagawa river at the great east Japan earthquake

Abstract

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, broke most of the sea embankment and coastal vegetation belt and caused catastrophic damage to people and buildings in the Tohoku and Kanto regions of Japan. Field surveys were conducted to elucidate the volume of embankment erosion and their hinterlands (residential area) by tsunami propagation in river channels and overtopping of embankments. Ten rivers were selected for the field investigation. This study focuses on the situation around the Abukumagawa River where severe erosion due to overtopping of embankments was occurred. To elucidate the relationship between the scoured area and the tsunami overtopping time, numerical simulation was conducted around the river mouth area of Abukumagawa River including the hinterland. For reproducing the Japanese tsunami, non-linear long-wave equations and a fault movement model were used in this study. For calculating tsunami from large region where fault motion occurred to small region where tsunami inundation and overtopping from river embankment occurred, five different regions with different grid size were set and flux and tsunami height in boundary grid were interpolated from large size grid to small size grid by nesting method. This simulation validated in comparison with the observed data and simulated one as to maximum water level near Abukumagawa River. The simulation reproduced well the tsunami water depth and inundation area within a reasonable limit, and showed a clear positive tendency between the time while tsunami overflow occurred and the size of scoured region on and around embankments measured by the post tsunami survey in April 2011. However, the volume of embankment erosion was greatly affected by the locality along the river, i.e. meandering, sand bar at river mouth, obstruction by a bridge, even when the tsunami-overtopping time was similar. This indicates that erosion volume due to overtopping from embankments is affected not only by overtopping time but also by the locality, and the mechanism should be included in future.