Physical modeling of tsunamis generated by three-dimensional deformable granular landslides

Tsunamis are gravity water waves that are generated by impulsive disturbances such as submarine earthquakes, landslides, volcanic eruptions, underwater explosions or asteroid impacts. Submarine earthquakes are the primary tsunami source, but landslides may generate tsunamis exceeding tectonic tsunamis locally, in both wave and runup heights. The field data on landslide tsunami events are limited, in particular regarding submarine landslide dynamics and wave generation in the impact region. The objective of the present study is to physically model tsunamis generated by three-dimensional deformable granular landslides. Predictive equations for tsunami wave and runup characteristics are obtained which may be used for initial rapid hazard assessment and mitigation.;The physical model was setup in the NEES 3D tsunami wave basin at Oregon State University in Corvallis, Oregon. A pneumatic landslide tsunami generator was deployed to simulate natural landslide motion on a hill slope. The instrumentation consists of four underwater and above water cameras, a particle image velocimetry (PIV) camera, twenty five wave and runup gauges and a multi-transducer acoustic array (MTA). The subaerial landslide shape and kinematics on the hill slope and the surface elevation of the offshore propagating tsunami wave and runup on the hill slope are measured.;The evolution of the landslide front velocity, maximum landslide thickness and width are obtained along the hill slope. The landslide surface velocity distribution is obtained from the PIV analysis of the subaerial landslide motion. The shape and the size of the submarine landslide deposit are measured from the MTA data. The subaerial landslide impact on the water surface displaces water away from the impact region. The leading tsunami wave crest and trough are generated by the water displacement and the subsequent drawdown. The trailing waves are generated by the subsequent oscillating shoreline runup and drawdown on the hill slope until the still water surface is restored. The 3D tsunami waves propagate away from the landslide source as radial wave fronts.;The amplitudes of the first 3 waves decay in the radial and the angular direction. The rate of radial decay is primarily dependent on the landslide width and Froude number at impact. The angular decay follows cos theta and cos2 theta for the first and the second wave, respectively. The first wave amplitudes depend primarily on the landslide Froude number and relative thickness at impact. The landslide volume mainly influences the amplitudes of the second waves. The wave celerity of the leading tsunami wave may be approximated by the solitary wave speed while the trailing waves are slower due to the dispersion effects caused by decay in wave period and wavelength from the front to the back of the wave. The wave periods and wavelengths are dependent on the landslide Froude number, landslide thickness, width at impact and the landslide volume. The wave periods and wavelengths increase with the propagation distance due to dispersion effects but vary minimally in the angular direction.;Between 1--15% of the landslide kinetic energy is converted into the wave train energy in the experimental study. The efficiency of wave generation is relatively low in 3D as compared to 2D due to energy dissipation by frictional losses and internal deformations during the landslide motion and the distribution of the unidirectional landslide energy by the radial wave. The landslide generated waves are weakly nonlinear in nature and span from shallow to deep water depth regimes with the bulk of the waves in the intermediate water depth regime. Analytical wave theories describing the 3D tsunamis in the present study are limited to segments of the wave train or specific cases, but certain wave profiles may be approximated by Stokes theory or cnoidal wave theory depending on the water depth regime. The experimental data serves as benchmark for numerical simulations and model advancement.