Numerical simulations of a fluidized granular flow entry into water: insights into modeling tsunami generation by pyroclastic density currents

Abstract

The tsunami generation potential of pyroclastic density currents (PDCs) entering the sea is poorly understood, due to limited data and observations. Thus far, tsunami generation by PDCs has been modeled in a similar manner to tsunami generation associated with landslides or debris flows, using two-layer depth-averaged approaches. Using the adaptive partial differential equation solver Basilisk and benchmarking with published laboratory experiments, this work explores some of the important parameters not yet accounted for in numerical models of PDC-generated tsunamis. We use assumptions derived from experimental literature to approximate the granular, basal flow component of a PDC as a dense Newtonian fluid flowing down an inclined plane. This modeling provides insight into how the boundary condition of the slope and the viscosity of the dense granular-fluid influence the characteristics of the waves generated. It is shown that the boundary condition of the slope has a first-order impact on the interaction dynamics between the fluidized granular flow and water, as well as the energy transfer from the flow to the generated wave. The experimental physics is captured well in the numerical model, which confirms the underlying assumption of Newtonian fluid-like behaviour in the context of wave generation. The results from this study suggest the importance of considering vertical density and velocity stratification in wave generation models. Furthermore, we demonstrate that granular-fluids more dense than water are capable of shearing the water surface and generating significant amplitude waves, despite vigorous overturning.

Experimental Modeling of Tsunamis Generated by Pyroclastic Density Currents: The Effects of Particle Size Distribution on Wave Generation

“…This agrees with the numerical modeling of Battershill et al. (2021), which treats the arriving flow as a continuous Newtonian fluid.…”

“…Battershill et al. (2021) also highlights the importance of the boundary conditions between the flow and the inclined plane on the wave generation. These boundary conditions control the velocity profile of the flows moving down the inclined plane.…”

“…This separation has typically been attributed to the differences in density within the flow. However, numerical simulations, where the PDC was approximated as a Newtonian fluid, show that even PDCs denser than water are capable of shearing the water surface (Battershill et al., 2021). Subaerial and underwater parts of the flow entrain air and water, respectively, by vertical motion of the flow.…”

Scenario–based Modelling of Waves Generated by Sublacustrine Explosive Eruptions at Lake Taupō, New Zealand

“…Progress in recent years has been sparked by the attention gained by the recent tsunamigenic events and research such as Ward and Day (2001) on flank collapses at La Palma. The resulting debates (Ward and Day, 2005;Pararas-Carayannis, 2002), additional research and improvements in modelling assumptions and techniques have helped improve comprehension of the hazards associated with volcanic tsunamis, particularly flank collapses (Abadie et al, 2012;Tehranirad et al, 2015); however, far more work is needed on the remaining possible mechanisms to build a more complete model of what wave hazards different volcanoes can truly pose (Paris et al, 2014Battershill et al, 2021). This work presents a scenario-based case study of waves produced by subaqueous explosive eruptions under Lake Taupō, simulated using numerical methods.…”