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

Volcanic tsunamis can expand the radius of hazards posed by a volcano well beyond the reach of the eruption itself; however, their source mechanisms are poorly understood. The tsunamigenic potential of pyroclastic density currents was studied experimentally by releasing a fluidized column of glass beads from a reservoir; the beads then ran down an inclined ramp into a water‐filled flume and generated waves. The effects of the particle size distribution on the generated waves were analyzed by comparing the waves generated by flows comprising different proportions of particles of diameters 63–90 μm and 600–850 μm. The flows comprising higher proportions of large particles travel more slowly down the ramp; however, all the flows impact the water at velocities greater than the shallow water wave speed, gh $\sqrt{gh}$, where h is the still‐water depth. The entrance of the fluidized flow into the water generates a solitary‐like leading wave followed by a smaller trough and trailing waves. Upon impact, the flow separates into a part advected with the wave crest and a part which turbulently mixes with water and propagates along the bottom of the flume, forming an underwater gravity current. A higher proportion of large particles makes the flows more porous, allowing the water to penetrate through the flows more easily, slightly decreasing the efficiency of the energy transfer. While this affects the celerity of the waves, the results show that, over the studied range of particle size distributions, all the flows can generate waves of similar amplitudes regardless of the particle size distribution. Plain Language Summary Volcanic eruptions can produce mixtures of volcanic particles and hot air that travel horizontally driven by gravity, which are called pyroclastic density currents (PDCs). PDCs can travel large distances at high speeds. When PDCs reach the sea, they displace the water and can generate large waves, which are called tsunamis. This paper explores PDC generated tsunamis by performing experiments of the entrance of a granular flow into water. The granular material, made up of various proportions of glass beads of diameters 63–90 μm and 600–850 μm, was released from a reservoir. The beads then ran down the porous ramp into a water‐filled flume and generated waves. A compressed air supply was introduced below the porous ramp to fluidize the material. The fluidization causes granular mixtures to behave like a fluid. Flows with higher proportions of large p