Tsunamis generated by granular collapse

Although many tsunamis arise from tectonic events, various past situations showed that sudden large landslides can also lead to devastative events. In order to estimate the hazards induced by these tsunamis on coastlines we investigate this situation through model laboratory experiments. The experiments consists of the collapse of an initially dry column of grains into a shallow water layer and the subsequent generation of waves.

The experiments show that the collective entry of the granular material into water governs the wave generation process. The wave amplitude relative to the water depth is indeed driven by the Froude number based on the horizontal velocity of the moving granular front relative to the wave velocity. Three nonlinear wave regimes has been identified depending on the Froude number: transient bores for large Fr, solitary-like waves for intermediate value of the Froude number and nonlinear transition waves at small Fr. We showed that the wave amplitude and the wavelength, relative to the water depth, exhibit nonlinear scalings with the Froude number.

Moreover, we showed that the maximal amplitude reached near-shore by the generated wave in our experiments is linked to the instantaneous immersed volume of grains and to the ultimate immersed deposit. Despite the differences in scale and geometry between our small-scale experiments and larger-scale geophysical events, a rather good agreement is observed between the experimental law and the field data.