Inverse Tsunami Flow Modeling Including Nonequilibrium Sediment Transport, With Application to Deposits From the 2011 Tohoku‐Oki Tsunami

Abstract

Tsunami deposits provide important clues to understand ancient tsunami events. Several inverse models have been proposed to estimate the magnitude of paleotsunamis from their deposits. However, existing models consider neither nonuniform transport of suspended sediment nor turbulent mixing, which are essential factors governing sedimentation from suspension in tsunami flows. Here we propose a new inverse model of tsunami deposit emplacement, considering both transport of nonuniform suspended load and entrainment of basal sediments. This inversion model requires the spatial distribution of deposit thickness and the pattern of grain size distributions of the tsunami deposit along a 1‐D shoreline‐normal transect as input data. It produces as output run‐up flow velocity, inundation depth, and concentration of suspended sediment. To solve for advection of nonuniform suspended load, a transformed coordinate system is adopted, which increases computational efficiency. Tests of model inversions using artificial data successfully allow reconstruction of the original input values, suggesting the effectiveness of our optimization method. We apply our new inversion model to the 2011 Tohoku‐Oki tsunami deposit on Sendai Plain, Japan. The thickness and grain size distribution of the tsunami deposit was measured along a 4 km long transect normal to the coastline. The result of our inversion fits well with the observations from aerial videos and field surveys. We conclude that this method is suitable for the analysis of ancient tsunami deposits and that it has the advantage of requiring relatively little information about the condition of the emplacing paleotsunami for reconstruction.