The marbles that fall into the water predict tsunamis

Crack! The rope grabbed by Wladimir Sarlin broke. But the experiment was successful. His gesture, which consisted of quickly pulling a rope, opened the shutter of a silo and released a tall column of silica balls, which slipped into the water and caused… a tsunami (in miniature). This PhD student at the end of his thesis at the FAST laboratory of the University of Paris-Saclay reproduces one of the many experiments he conducted with his two directors, Cyprien Morize and Philippe Gondret (University of Paris-Saclay), as well as in the role of Alban Sauret (professor at the University of California, Santa Barbara), to solve a geophysical mystery.

Can we predict the height of a wave caused by a landslide, such as the collapse of a cliff, or the crumbling of the slope of a volcano? Conversely, from the analysis of the submerged ground after a disaster, is it possible to estimate the height of the wave generated? To these two questions the team answered affirmatively on 13 September a Physical Review Fluids. Their formula, a blend of geometry, fluid mechanics and the physics of dry granular media, links the width of a wave to the height of the water and the height and width of the ground that will collapse. The properties of the rock ultimately matter little.

Comparison with historical cases

This result is the result of ten years of research started in Marseille, when Alban Sauret and Sylvain Viroulet (not co-signer of the article), looking at the famous cliff of Cap Canaille, in Cassis, wondered what would happen if a part fell into the water. . The problem was finally simplified by transforming the cliff into a column of marble about fifty centimeters high and the Mediterranean into a basin about two meters long and twenty centimeters deep.

Piston waves are observed in shallow water: the earth pushes the water. In deeper waters, the earth “falls” vertically into the water, in a large “splash”

A first result in 2021 links the amplitude of the wave to the speed of the grains and to that of a wave that propagates in a shallow basin. Another, the same year, compares his experimental results with historical cases of tsunamis linked to collapses, estimating the volumes of land involved, such as that of the Lituya Bay in Alaska, in 1958, and its wave that submerged a high hill. at 500 meters, or that of the Taan fjord which suffered a 100 meter high tsunami in 2015.

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