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This paper describes the topographic change and the recovery process, as well as the sediment sources, for tsunami deposits based on field surveys and analysis of digital elevation model data before and after the A.D. 2011 Tohoku-oki tsunami at the Sendai Plain, Japan. We found that the amount of sediment deposited on land was approximately four times greater than the eroded volume of sediment at the beach. Large amounts of the sediments deposited at the studied transect probably originated from liquefaction. This result suggests that the vented sediments might have been an important source of the tsunami deposits if liquefaction is generated by the strong ground motion of a near-field earthquake. In contrast, minor erosion was observed at the beach, and the beach berm was rebuilt within three months after the tsunami. Moreover, the erosional channel that had cut into the beach had been filled by sand within 13 days after the tsunami. Therefore, it is not expected that a sedimentary record of the tsunami will be preserved in the nearshore zone along the Sendai coast, although remnants of small scours on land might remain long after the tsunami. [ABSTRACT FROM AUTHOR]

Soon after the 2011 Tohoku-oki event, approximately 1300 tsunami deposit thickness data were collected spatially by the governors to cover the entire tsunami affected area (Sendai Plain) along the Sendai Bay coastline. This dataset, along with high-resolution and high-precision data of flow depth and pre-tsunami and post-tsunami elevation, enables us first to conduct a direct comparison of the sedimentation and erosion volumes as well as the tsunami hydrodynamic features (e.g., flow depth) and sediment thickness. The total balance of sedimentation and erosion volumes revealed that the volume of tsunami-deposited sediments can be explained roughly using the erosion volumes at the beach and sand dunes for sandy deposits and at rice paddy fields for muddy deposits. Both the flow depth and sediment thickness showed positive correlations with the distance from the shoreline while sediment thickness is no correlation to the elevation except in the zone closest to the shoreline where erosion is present. In addition, a statistical relation was found between the flow depth and sediment thickness. In fact, the frequency distribution of sediment concentration, defined here as the sediment thickness divided by the maximum flow depth at each survey site, fits well with the logarithmic normal distribution with geometric average of about 2%. This fit indicates that the tsunami deposits on the Sendai Plain can be explained generally if we simply assume that the saturated level of the sediment concentration in the flow is limited to about 2% on average, irrespective of the grain size. Such a simple explanation might be applicable only for the tsunami deposits on the Sendai Plain because the topography is remarkably flat and low. Therefore, the tsunami inundation process is relatively simple. Nevertheless, a possible relation between tsunami flow depth and sediment thickness suggested here would be very useful to consider the ideal sedimentary process of the tsunami deposits and to improve forward and inverse modelings. [ABSTRACT FROM AUTHOR]