Evaluating inverse models for reconstructing flow speed from sandy tsunami deposits

Abstract:

Tsunami flow speed can be interpreted from information contained in sandy tsunami deposits. Several inverse sediment transport models have been used to calculate tsunami flow speed from the vertical and horizontal grain size distributions of sandy tsunami deposits. The reconstructions are based on sediments that are interpreted to be deposited from suspension. Here we use the DELFT3D and Tunami forward numerical models of tsunami hydrodynamics to gain insight into the different phases of flow in a tsunami and the locations along an inundation path where assumptions of the inverse models are satisfied. Inverse model evaluations are constrained using field observations of tsunami flow depth, speed (limited observations), and deposit characteristics at a site near the Sendai Airport where the 2011 Tohoku-oki tsunami inundated 4.5 km inland. In addition to determining when and where flow speeds are high enough to suspend sediment, specific assumptions of suspended load capacity and particle trajectory models are tested. For the suspended load capacity model, the temporal and spatial gradients of flow are evaluated to test the degree to which the steady and uniform flow assumptions apply. For suspended particle trajectory models, the assumption that particles have a single path from where they are picked up to where they are deposited is evaluated in light of the temporal and spatial distributions of bottom shear stress. This research is a step towards combining forward and inverse models to create more robust hybrid tsunami inundation/sediment transport/geomorphic models. With increased knowledge of the applicability of inverse sediment transport models, the flow speed and size of paleotsunamis will be better understood and the ability to assess tsunami hazard from paleotsunami deposits will improve.