Crack! The string grabbed by Wladimir Sarlin has broken. But the experiment succeeded. His gesture, consisting of pulling quickly on a rope, opened the shutter of a silo and released a high column of small silica balls, which slid into the water and caused… a (miniature) tsunami. This doctoral student at the end of his thesis at the FAST laboratory of the University of Paris-Saclay reproduces there one of the many experiments he carried out with his two directors, Cyprien Morize and Philippe Gondret (University of Paris-Saclay), as well as 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 submerged land after a disaster, is it possible to estimate the height of the wave generated? To these two questions, the team answered in the affirmative on September 13 in Physical Review Fluids. Their formula, a mixture of geometry, fluid mechanics and physics of dry granular media, links the amplitude of a wave to the height of water, and to the height and width of the land that will collapse. The properties of the rock ultimately count for little.
Comparison with historical cases
This result is the fruit of ten years of research which began in Marseilles, when Alban Sauret and Sylvain Viroulet (not co-signer of the article), wonder, while looking at the famous cliff of Cap Canaille, in Cassis, what would happen if a part fell into the water. The problem was finally simplified by transforming the cliff into a column of marbles 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 relates the amplitude of the wave to the speed of the grains and to that of a wave propagating in a shallow basin. Another, the same year, compares their experimental results to historical cases of tsunamis linked to collapses, estimating the volumes of land involved, such as that of Lituya Bay in Alaska, in 1958, and its wave which submerged a hill up to 500 meters high, or that of the Taan fjord which experienced, in 2015, a tsunami 100 meters high.
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