Numerical Modelling of Tsunami Generated by Submarine Landslides on Irregular Bathymetry

We present a two-layer, non-hydrostatic model in horizontal Cartesian coordinates, consisting of a depth-integrated model for landslide motion coupled to an upper layer consisting of the non-hydrostatic, sigma-coordinate wave model NHWAVE (Ma et al., 2012) which solves either the Euler equations or Navier-Stokes equations with turbulence models. NHWAVE has previously been shown to be an accurate predictor of highly dispersive tsunamis generated by submarine landslides. The landslide model utilizes a rheology based either on a viscous Newtonian fluid or a saturated granular flow with stresses based on Coulomb friction to predict the motion of submarine landslides over arbitrary, irregular bathymetry. The depth-integrated lower-layer includes contributions from non-hydrostatic pressure exerted by the upper layer as well as the effect of vertical acceleration of the slide. A quadratic vertical profile of non-hydrostatic pressure is imposed in landslide to improve the model dispersion properties, and a new algorithm for granular rheology closure is introduced, where the internal state of stress is evaluated in local, slope-oriented coordinates and then transferred to the horizontal Cartesian coordinates of the upper layer model. The model is validated against analytical solutions and laboratory observations, to illustrate the importance of non-hydrostatic pressure in cases with steep slopes when describing slide motion and final deposit. We further investigate the tsunami generated by the collapse of the Anak Krakatau volcano in the Sunda Straits of Indonesia on Dec. 22, 2018, and compare results to field observations.