Author(s): Masayuki Sato; Norio Tanaka
Linked Author(s): Norio Tanaka
Keywords: River embankment; Erosion; Scouring; Tsunami; Overtopping
Abstract: The tsunami caused by the Great East Japan Earthquake on 11 March 2011 propagated inland and rivers, and caused catastrophic damage to people and buildings in the Tohoku and Kanto regions of Japan. Although tsunami propagation into rivers were already reported in the previous tsunamis, this tsunami was the first case that large amount of tsunami flow propagated into rivers, overtopped from river embankments, and broke or washed out houses along rivers. Damage situation of houses and river embankments was investigated in April 2011 along the Abukumagawa River where severe erosion by overtopping of river embankments was occurred. For the quantitative estimation between the damaged situation and hydraulic parameters, the tsunami was simulated by non-linear long wave equations and fault motion model. This simulation validated in comparison with the observed data and simulated one as to maximum water level near the Abukumagawa River. The field investigation showed that the river morphology, meandering and bridge girder, had large influences on the local distribution of maximum water level along the river. In the right-hand side, the river-side wall of houses near the embankment were mostly broken, large scoured region around houses were generated, and some houses near the embankment were completely washed out by the overtopping flow. Even at the far side area from river mouth, the scoured volume was large notwithstanding the simulated maximum kinetic energy was low, because the overtopping duration had been lengthened by the local condition of the river, i. e. meandering and damming by the bridge girder. Moreover, the tsunami in river run faster than inland, and overtopped from river embankment to hinterland where the tsunami still not came or came but the water depth was small. In that case, the duration is lengthened. The damage of embankment and houses along the embankment was not very large in the left-hand side of the river where the river embankment was almost parallel to the sea embankment because of the meandering of the river, and the simulation shows that the tsunami inundated from sea side was easily accumulated at the site before the tsunami current overtopped from the river. It is assumed that the difference of the damages on the embankment and hinterland between each side of the river could be caused not only by river morphology but by the time difference of overflow from river embankment and tsunami inundation from seaside.