Volcanogenic tsunami and wave hazard remains less understood than that of other tsunami sources. Volcanoes can generate waves in a multitude of ways, including subaqueous explosions. Recent events, including a highly explosive eruption at Hunga Tonga-Hunga Ha’apai and subsequent tsunami in January 2022, have reinforced the necessity to explore and quantify volcanic tsunami sources. We utilise a non-hydrostatic multilayer numerical method to simulate 20 scenarios of sublacustrine explosive eruptions under Lake Taupō, New Zealand, across five locations and four eruption sizes. Waves propagate around the entire lake within 15 minutes, and there is a minimum explosive size required to generate significant waves (positive amplitudes incident on foreshore of >1 m) from the impulsive displacement of water from the eruption itself. This corresponds to a mass eruption rate of 5.8x10^7 kg s^-1, or VEI 5 equivalent. Inundation is mapped across five built areas and becomes significant near shore when considering only the two largest sizes, above VEI 5, which preferentially impact areas of low-gradient run-up. In addition, novel hydrographic output is produced showing the impact of incident waves on the Waikato river inlet draining the lake, and is potentially useful for future structural impact analysis. Waves generated from these explosive source types are highly dispersive, resulting in hazard rapidly diminishing with distance from the source. With improved computational efficiency, a probabilistic study could be formulated and other, potentially more significant, volcanic source mechanisms should be investigated.

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.

Laboratory experiments were performed to study the impact of the relative particle protrusion P/D (P is the protrusion height and D is the diameter of the target particle) on the mechanism of entrainment of sediment particles from a spherical rough bed. The target particle to be entrained was instrumented with electronic sensors, which can measure the tri-axis linear acceleration, and consequently the inertial hydrodynamic forces, during the entrainment process. The velocity field was obtained using the two-dimensional Particle Image Velocimetry (PTV) technique and the velocity data were synchronised with the force data relative to the entrainment time. Experimental results show that the magnitudes of inertial drag force and lift force have a decreasing trend as particle protrusion increases. The ratio of inertial lift force to drag force reveal that drag force slightly dominates the entrainment process at P/D > 0.7 while lift force slightly dominates at P/D < 0.62. Also, the inertial drag and lift coefficients were computed by the force data and velocity data. The inertial drag coefficient was found to be independent of P/D when P/D < 0.62 but declined with increasing P/D for P/D > 0.62. Similarly, the variation of inertial lift coefficient with P/D reversed at P/D = 0.7. This variation of force coefficients with P/D is consistent with the independence of inertial forces with respect to P/D when P/D > 0.62-0.7. In summary, the inertial forces demonstrate that the impact of protrusion on the particle entrainment becomes less important when P/D > 0.62-0.7.