Dynamics of submarine debris flow and tsunami

The general two-phase debris flow model proposed by Pudasaini (J. Geophys. Res. 117:F03010, 2012, doi:10.1029/2011JF002186) is employed to simulate subaerial and submarine two-phase debris flows and the mechanics of complex wave generation and interactions between the solid and the fluid phases. This includes the fluid waves or the tsunami generated by the debris impact at reservoirs, lakes, and oceans. The analysis describes the generation, amplification, and propagation of super tsunami waves and run-ups along coastlines, debris slide and deposition at the bottom floor, and debris shock waves. Accurate and advance knowledge of the arrival of tsunami waves in the coastal regions is very important for the design of early warning strategies. Here, we show that the amount of solid grain in the fluid reservoir plays a significant role in controlling the overall dynamics of the submarine debris flow and the tsunami. For very small solid particle concentrations in the reservoir, the submarine debris flow moves significantly faster than the surface tsunami wave. As the solid volume fraction in the reservoir increases, the submarine debris speed slows down. For relatively large solid volume fractions in the reservoir, the speed of the submarine debris becomes slower than the surface tsunami wave. This information can be useful for early warning strategies in the coastal regions. The fast or slow speed of the submarine wave can be attributed to several dynamical aspects of the model including the generalized drag, basal traction, pressure gradient, virtual mass force, the non-Newtonian viscous stress, and the strong phase interaction between the solid and the fluid as they enhance or diminish the motion of the solid phase. Solid particle concentration in the reservoir dam also substantially influences the interaction between the submarine debris flow and the frontal wall of the dam, and the interaction between the tsunami and the submarine debris wave. The tsunami wave impact generates a largely amplified fluid level at the dam wall. Submarine debris shock waves are observed for small solid volume fractions in the reservoir. Another important aspect of the simulation is to investigate the complex interactions between the internal submarine debris wave and the surface tsunami wave. Three complex waves occur simultaneously: the subaerial debris flow in the upstream region, submarine debris flow in the reservoir basin, and a super tsunami wave on the surface of the reservoir. This helps to develop insight into the basic features of the complex nonlinear solid and fluid waves and their interactions.