Sediment transport due to the 2011 Tohoku-oki tsunami at Sendai: Results from numerical modeling

Numerical modeling of sediment transport on the Sendai Plain in northeast Japan, which was caused by the 2011 Tohoku-oki tsunami, was performed to investigate why onshore tsunami deposits included small amounts of marine materials and to explain the gap between the maximum inland extent of the recognizable sand layer and the limit of inundation. The simulation results generally corresponded with the thicknesses of the deposits observed in the field by previous studies. The simulation revealed that the tsunami caused a significant amount of erosion on the beach and in the coastal forest; however, erosion was significantly limited on the offshore seafloor. In addition, the bore of the highest tsunami wave offshore limited the amount of suspended sediment due to a steep bathymetric profile of the shoreface and a moderate initial drop in water level prior to the arrival of the bore. This resulted in limited suspension and shoreward advection of the sediments and a minor contribution of the seafloor sediments to the onshore tsunami deposit. Visualization of the onshore sediment transport indicated that the sediment transport was significantly affected by artificial topographic features. The flow speed and height varied across the topographic highs, and sediments were trapped on the seaward side of these highs. In coastal areas, the sediment supply was limited due to the presence of engineered dikes along the coast. In inland areas, embankments of paved roadways caused increased deposition on the seaward sides of the embankments and less deposition on the leeward sides of the embankments. Although some aspects of the relevant physics were not fully implemented in the model, in which uncertainties exist, the results may explain the processes and consequences of sedimentation from a tsunami event.