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

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 q(m) and the volume per width nu of the flow, the maximum water depth H-o, and the slope angle theta of the inclined plane. The obtained waves are of Stokes and cnoidal types, for which the generation is mostly controlled by q(m) and nu. By contrast, H-o 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/H-o = f(zeta), where zeta = FrSM sin theta is a dimensionless parameter depending on the Froude number Fr, the relative slide thickness S, the relative mass M, and the slope angle theta. 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.