Non-linear regimes of tsunami waves generated by a granular collapse

TL;DR

This study experimentally characterizes nonlinear tsunami wave regimes generated by granular collapses, revealing three distinct behaviors depending on the Froude number, which enhances understanding of coastal wave hazards.

Contribution

It identifies and explains three nonlinear wave regimes from granular collapses, providing new insights into wave dynamics relevant for coastal risk assessment.

Findings

Three wave regimes: transient bores, solitary waves, and transition waves.

Wave amplitude scales differently with Froude number across regimes.

The granular front acts as a piston influencing wave generation.

Abstract

Tsunami waves induced by landslides are a threat to human activities and safety along coastal areas. In this paper, we characterize experimentally the waves generated by the gravity-driven collapse of a dry granular column into water. Three nonlinear wave regimes are identified depending on the Froude number $\mathrm{Fr}_f$ based on the ratio of the velocity of the advancing granular front and the velocity of linear gravity waves in shallow water: transient bores for large $\mathrm{Fr}_f$, solitary waves for intermediate values of $\mathrm{Fr}_f$, and non-linear transition waves at small $\mathrm{Fr}_f$. The wave amplitude relative to the water depth increases with $\mathrm{Fr}_f$ in the three regimes but with different non-linear scalings, and the relative wavelength is an increasing or decreasing function of $\mathrm{Fr}_f$. Two of these wave regimes are rationalized by considering that the advancing granular front acts as a vertical piston pushing the water, while the last one is found to be a transition from shallow to deep water conditions. The present modeling contributes to a better understanding of the rich hydrodynamics of the generated waves, with coastal risk assessment as practical applications.