Images capturing five moments in the sequence after a fluidized granular flow enters water
Snapshots of a fluidized granular flow entering water, at five different times, after gate opening from which the granular mixture is released. The initial conditions of mass flux, water depth, and slope angle of the inclined plane varied during the experiments. White arrows represent the air flux perpendicular to the inclined plane for the fluidization process. Credit: Bougouin et al. [2020], Figure 2
Source: Journal of Geophysical Research: Solid Earth

The sudden displacement of water that commonly generates tsunamis can have various triggering mechanisms. Although earthquake-induced tsunamis are by far the best known, and warning systems are mainly structured to deal with this category, there is a plethora of mechanisms related to volcanic edifices and their activity that are also are able to trigger tsunamis. These include underwater explosions, slope instabilities causing submarine and subaerial landslides, flank and caldera collapses, and pyroclastic flows. All of these make coastal communities living close to active volcanoes particularly vulnerable.

Laboratory experiments carried out by Bougouin et al. [2020] on fluidized granular flows impacting the water after travelling along an inclined plane show how the sustained high gas pore pressure of the mixture ensures the dynamic similarity with natural pyroclastic flows entering the sea. The main characteristics of the generated waves are controlled by mixture dynamics, mass flux and volume of the fluidized granular flow. It is also shown how in the near field region the maximum water depth did not affect wave features and can thus be disregarded.

In addition to the macroscopic dynamics of the process (formation of an initial single vertical granular jet over water followed by the leading wave and by low-amplitude waves, coupled with a turbulent mixing zone forming an underwater turbidity current), it is also shown that grain-size and nature of the moving flow (dry, non-fluidized granular flows, fluidized granular flows or water flows) can unexpectedly play a major role in wave generation.

Citation: Bougouin, A., Paris, R., & Roche, O. [2020]. Impact of fluidized granular flows into water: Implications for tsunamis generated by pyroclastic flows. Journal of Geophysical Research: Solid Earth, 125, e2019JB018954.

—Marco Pistolesi, Associate Editor, JGR: Solid Earth

Text © 2020. The authors. CC BY-NC-ND 3.0
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