Bedforms record the flow conditions of the 2011 Tohoku-Oki tsunami on the Sendai Plain, northeast Japan

Abstract

This article describes the bedforms formed by the 2011 Tohoku-Oki tsunami on the southern Sendai Plain, NE Japan. Integrated analyses of satellite images, field survey data, and X-ray radiographs revealed the natural shapes of these bedforms. The dominant bedforms are dunes and antidunes about ~ 20 cm thick (high) and relatively flat because of their meter-scale wavelengths. Their morphologies reflect mainly the tsunami run-up flow on the flat lowland and the return flow in a narrow, steep valley. Flow depth changed at sites where the level of the ground surface changed along the tsunami flow path, causing the flow velocity to fluctuate. Flow velocity decreased in depressed (deep-water) sections and increased in raised (shallow-water) sections. The types and positions of the tsunami deposit bedforms were thus strongly controlled by local ground surface roughness, such as raised roadbeds and levees. As a result, the observed thickness distribution of the tsunami deposits fluctuates. It is important to consider the possible existence of bedforms in paleotsunami surveys, especially for accurate determination of the thickness distribution of the tsunami deposits, because thickness data are essential information for reconstruction of the tsunami inundation limit as well as its flow speed. • We firstly report the bedforms of the 2011 Tohoku-Oki tsunami deposit. • Their types and positions are strongly controlled by local ground surface roughness. • Sequence of bedforms recorded the flow speed changes related to the ground roughness. • The flow speed changes are explained by the principle of conservation of mass. • This finding may enable to reconstruct the flow speeds of tsunamis from the deposits.