Impact of Fluidized Granular Flows into Water: Implications for Tsunamis Generated by Pyroclastic Flows

Abstract

Novel laboratory experiments of fluidized granular flows entering water are reported, for the purpose of investigating tsunamis generated by pyroclastic flows. Qualitatively, the impact of a fluidized granular flow into water leads to (i) an initial vertical granular jet over water, (ii) a leading and largest wave, and (iii) a turbulent mixing zone forming a turbidity current. The present study focuses on the leading wave features in the near‐field region, as a function of the mass flux per width qm and the volume per width υ of the flow, the maximum water depth Ho, and the slope angle θ of the inclined plane. The obtained waves are of Stokes and cnoidal types, for which the generation is mostly controlled by qm and υ. By contrast, Ho plays no role on the wave generation that occurs in the shallowest region. Moreover, a comparison between fluidized granular, dry (nonfluidized) granular, and water flows entering water is addressed under similar flow conditions. The dimensionless amplitude scales as A/Ho=f(ζ), where ζ=FrSMsinθ is a dimensionless parameter depending on the Froude number Fr, the relative slide thickness S, the relative mass M, and the slope angle θ. Data of fine fluidized granular, fine dry granular, and water flows collapse on a master curve, which implies that the nature of the flowing material is of lesser importance in the current setup. By contrast, coarse granular flows generate lower amplitude waves, which is attributed to the penetration of water into the porous granular medium.

Experimental Insights on the Propagation of Fine‐Grained Geophysical Flows Entering Water

“…One of the relevant macroscopic analyses to compare the dynamics of granular flows is the tracking of the front position x f and the front-velocity u f with time and position (Figure 4). The presence of a water body significantly affects the temporal evolution of the flow front beyond the initial height H i , the latter being already reported for water dam-break flows (e.g., Ancey et al, 2008;Laubert & Hager, 1998;Ritter, 1892) and subaerial granular flows (e.g., Bougouin et al, 2020;Farin et al, 2014;Mangeney et al, 2010;Roche et al, 2008). At early times, all curves collapse together corresponding to the propagation of granular flows down the subaerial inclined plane.…”

“…The impact between the granular flow and the water body generates (i) an initial granular jet corresponding to grains ejected above the water surface, (ii) a leading and largest wave, and (iii) a turbulent mixing zone from which a particle-driven gravity current is spontaneously formed (Figure 3). These observations are direct consequences of the presence of a water body, which have already been observed for fine particle-fluid mixtures entering water (Allen et al, 2012;Bougouin et al, 2020;Freundt, 2003;Mcleod et al, 1999). By contrast, both the granular jet and the gravity current have never been reported in the literature for coarse granular materials (i.e.,…”

“…On the left, the subaerial granular flow is relatively thin, dense and rapid before evolving to a thick and dilute particle-driven gravity current underwater. An impulse wave is also generated on the water surface, which has already been studied in Bougouin et al (2020).…”

Wave Generation Across a Continuum of Landslide Conditions From the Collapse of Partially Submerged to Fully Submerged Granular Columns

“…To compute this, we note that the elevation of the center of mass of the dry granular material is C s = H 0 /2 and the elevation of the center of mass of the fluid is C w = H w /2. For a saturated column, the water in the granular pore space adds a buoyant force that reduces the gravitational force on the column, resulting in a compound center of mass: where the second term in the numerator accounts for the influence of the granular wedge packing fraction (ϕ) on the momentum transfer, controlling the ability of the fluid to percolate the granular wedge (Bougouin et al., 2020). Therefore, denser granular wedges lead to higher values of C , resulting in higher wave amplitudes.…”

“…The present analysis is valid for the collapse of a vertical column or the fall of material down a steep slope, where granular particles may have air or fluid in the void space. The results are not intended for prediction of waves generated by earthquakes or other types of mass movements including flows along flat or very low slopes, flows with a very small density difference (e.g., turbidity currents [Meiburg & Kneller, 2010]), cases with fluidization or high rates of air entrainment (e.g., pyroclastic flows [Bougouin et al., 2020]), highly turbulent flows, or complex and transitional mass flow events such as landslides that transitions into a debris flow. The theoretical expression derived here is in good agreement with the present experimental data and the results of two other laboratory studies, indicating robust predictions for the maximum wave amplitude generated by the vertical collapse of granular material.…”