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

Abstract

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.

Wave Generation by Fluidized Granular Flows: Experimental Insights Into the Maximum Near‐Field Wave Amplitude

“…However, multiple experiments were performed to ensure a consistent and suitable level of fluidization. A detailed discussion is included in Lipiejko et al (2022). The fluidizing ramp ends at the shoreline (x = 0 m) so that material is fluidized until it enters the water.…”

“…All the camera recordings were processed in MATLAB® using algorithms based on pixel intensity threshold methods to extract the impact velocity, the thickness of the fluidized flow and the free surface elevation. The data capture and processing details are provided in Lipiejko et al (2022).…”

“…A detailed discussion is included in Lipiejko et al. (2022). The fluidizing ramp ends at the shoreline ( x = 0 m) so that material is fluidized until it enters the water.…”

“…The wave generation process can be divided into three stages: the motion of the fluidized granular flow down the fluidizing ramp; the entrance of the flow into the water and subsequent wave generation; and the propagation of the underwater gravity current and the leading wave in the near-field region. The velocity profile of the fluidized flow down the ramp and the interactions between the fluidized granular flow and water are discussed in Lipiejko et al (2022). The present paper focuses on the effects of the flow mass and the water depth on the waves propagating in the near-field region.…”

“…Fluidized flows have been extensively studied in terms of the minimum gas velocity required to fluidize the granular material, the effect of the particle size on the fluidization rates or the effect of the fluidization on the velocity and runout distance of the flows (e.g., Geldart, 1973;Montserrat et al, 2012;Roche et al, 2002Roche et al, , 2004Roche et al, , 2008Rowley et al, 2014). However, significantly less attention has been paid to the wave generation by the entrance of fluidized granular flows into the sea (Battershill et al, 2021;Bougouin et al, 2020Bougouin et al, , 2021Lipiejko et al, 2022).…”