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Multiscale coupled three-dimensional model analysis of the tsunami flow characteristics around the Kamaishi Bay offshore breakwater and comparisons to a shallow water model

ABSTRACT A two-way coupled two-dimensional (2DH) shallow water to three-dimensional (3D) Navier–Stokes equation model, named 2-way Coupled Long Wave to Navier–Stokes 3D (2CLOWNS-3D), is applied to the 2011 Tohoku-oki Earthquake Tsunami in Kamaishi Bay, Japan. 2CLOWNS-3D simulates the entire evolution of the tsunami from its source to inundation at the coast, in which the 3D model component is used to model the flow through the submerged opening of a large-scale offshore tsunami breakwater. 2CLOWNS-3D is compared with a 2DH model simulation in order to identify where it becomes beneficial. It is found that flow rates through the submerged breakwater opening, and thus the resulting inundation heights, are similar between 2DH and 2CLOWNS-3D model simulations when an appropriate momentum dispersion coefficient is applied to the former. However, significant differences between the model simulations are identified in relation to; hydrodynamic forces acting on the submerged caissons, velocities in the large-scale jet structure emanating from the breakwater, and bed shear stresses in the vicinity of the breakwater. Furthermore, the characteristics of the large-scale jet structure simulated by the 3D model are reasonable when employing the realizable k-ε turbulence closure. Our results demonstrate the practical merit of 2CLOWNS-3D for simulating complex geophysical scale flows.