Tsunamis generated by fast granular landslides: 3D experiments and empirical predictors

Landslides falling into water bodies can generate impulsive waves, which are a type of tsunamis. The propagating wave may be highly destructive for hydraulic structures, civil infrastructure and people living along the shorelines. A facility to study this phenomenon was set up in the laboratory of the Technical University of Catalonia. The set-up consists of a new device releasing granular material at high velocity into a wave basin. A system employing laser sheets, high-speed and high-definition cameras was designed to accurately measure the high velocity and geometry of the sliding mass as well as the produced water displacement in time and space. The analysis of experimental data helped to develop empirical relationships linking the landslide parameters with the produced wave amplitude, propagation features and energy, which are useful tools for the hazard assessment. The empirical relationships were successfully tested in the case of the 2007 event that occurred in Chehalis Lake (Canada).Peer ReviewedPostprint (author's final draft

A new predictive equation for estimating wave period of subaerial solid-block landslide-generated waves

In the aftermath of the deadly 2018 Anak Krakatau tsunami (Indonesia) and associated confusions over its modeling and generation mechanism, there has been an urgent need for further studies to improve our understanding of landslide-generated tsunamis. Two important factors in accurate modeling of landslide tsunamis are the wave period and the initial wave amplitude. Here, we apply a physical modeling approach and develop an empirical equation to predict the dominant wave period generated by solid-block subaerial landslide tsunamis. Fifty-one laboratory experiments are conducted at different water depths and using four different concrete blocks for the sliding masses. The results are consequently employed to derive a predictive equation for the wave period of solid-block subaerial landslide tsunamis. An innovation of this study is that we apply data from different scales (laboratory and field scales) to produce our predictive equation. For field data, the data from the 2018 Anak Krakatau event is used. We compared our predictive equation with other previously-published equations. To confirm the validity of our predictive equation, it is applied for the prediction of the wave period of an independent landslide tsunami event whose data was not used for the derivation of the equation

A universal parameter to predict subaerial landslide tsunamis?

The significance of the impulse product parameter P is reviewed, which is believed to be the most universal parameter for subaerial landslide tsunami (impulse wave) prediction. This semi-empirical parameter is based on the streamwise slide momentum flux component and it was refined with a multiple regression laboratory data analysis. Empirical equations based on P allow for a simple prediction of wave features under diverse conditions (landslides and ice masses, granular and block slides, etc.). Analytical evidence reveals that a mass sliding down a hill slope of angle 51.6° results in the highest waves. The wave height ―observed‖ in the 1958 Lituya Bay case was well predicted using P. Other real-world case studies illustrate how efficient empirical equations based on P deliver wave estimates which support hazard assessment. Future applications are hoped to further confirm the applicability of P to cases with more complex water body geometries and bathymetries. Keywords: hazar

The influence of landslide shape and continental shelf on landslide generated tsunamis along a plane beach

This work proposes an advancement in analytical modelling of landslide tsunamis propagating along a plane beach. It is divided into two parts. In the first one, the analytical two-horizontal-dimension model of Sammarco and Renzi (2008) for tsunamis generated by a Gaussian-shaped landslide on a plane beach is revised and extended to realistic landslide shapes. The influence of finiteness and shape of the slide on the propagating waves is investigated and discussed. In the second part, a new model of landslide tsunamis propagating along a semi-plane beach is devised to analyse the role of the continental platform in attenuating the wave amplitude along the shoreline. With these parameters taken into account, the fit with available experimental data is enhanced and the model completed

Composite modelling of the effect of the water body geometry on landslide-tsunamis

Subaerial landslide-tsunamis (impulse waves) are generated by mass movements such as landslides, rock falls or glacier calving interacting with a water body. Preliminary landslide-tsunami hazard assessment is commonly based on empirical equations derived from wave channel (2D) or wave basin (3D) experiments. It is crucial to select the most appropriate set of empirical equations for a particular case as the difference in the far-field wave height between 2D and 3D may exceed an order of magnitude. The present study systematically investigates the effect of the water body geometry on the wave characteristics. Physical model tests were conducted in 2D and repeated in 3D, involving two water depths, three rigid slides and different subaerial slide release positions. The waves were found to decay in 2D considerably slower with distance x ‒0.30 than in 3D with radial distance r ‒1.0. The 3D wave heights in the slide impact zone can be identical large as in 2D for a large slide Froude number F, relative slide thickness S and relative mass M. However, for small F, S and M, the 3D waves are considerably smaller, both in the near- and far-field. Empirical equations are presented to transform wave parameters from 2D to 3D. One 2D-3D test pair, involving a solitary-like wave, is investigated in detail regarding the slide kinematics, water surface elevations and slide-water interaction power. This power is derived from pressure measurements on the slide front and the slide kinematics. The identical test pair is then used to calibrate the Smoothed Particle Hydrodynamics SPH code DualSPHysics and to numerically investigate the wave features in five intermediate geometries between 2D and 3D. For a “channel” geometry with diverging side wall angle of 7.5°, the wave amplitudes along the slide axis were found to lie approximately halfway between the values observed in 2D and 3D. At 45°, the values are practically identical to those in 3D. These findings support preliminary landslide-tsunami hazard assessment

Tsunamis From Submarine Collapses Along the Eastern Slope of the Gela Basin (Strait of Sicily)

Geophysical surveys in the eastern slope of the Gela Basin (Strait of Sicily, central Mediterranean) contributed to the identification of several episodes of sediment mass transport, recorded by scars and deposits of various dimensions within the Pleistocene succession. In addition to a huge failure called Gela Slide with volume exceeding 600 km3, the most studied events show volumes estimated between 0.5 and 1.5 km3, which is common to many other submarine landslide deposits in this region and that can therefore be considered as a characteristic value. In this work, the tsunamigenic potential of two of such landslides, the so-called Northern Twin Slide and South Gela Basin Slide located about 50 km apart along the eastern slope of the Gela Basin, are investigated using numerical codes that describe the onset and motion of the slide, as well as the ensuing tsunami generation and propagation. The results provide the wave height of these tsunami events on the coast of southern Sicily and Malta and can be taken as representative of the tsunamigenic potential of typical landslides occurring along the slope of the Gela Basin

Statistical emulation of landslide-induced tsunamis at the Rockall Bank, NE Atlantic

Statistical methods constitute a useful approach to understand and quantify the uncertainty that governs complex tsunami mechanisms. Numerical experiments may often have a high computational cost. This forms a limiting factor for performing uncertainty and sensitivity analyses, where numerous simulations are required. Statistical emulators, as surrogates of these simulators, can provide predictions of the physical process in a much faster and computationally inexpensive way. They can form a prominent solution to explore thousands of scenarios that would be otherwise numerically expensive and difficult to achieve. In this work, we build a statistical emulator of the deterministic codes used to simulate submarine sliding and tsunami generation at the Rockall Bank, NE Atlantic Ocean, in two stages. First we calibrate, against observations of the landslide deposits, the parameters used in the landslide simulations. This calibration is performed under a Bayesian framework using Gaussian Process (GP) emulators to approximate the landslide model, and the discrepancy function between model and observations. Distributions of the calibrated input parameters are obtained as a result of the calibration. In a second step, a GP emulator is built to mimic the coupled landslide-tsunami numerical process. The emulator propagates the uncertainties in the distributions of the calibrated input parameters inferred from the first step to the outputs. As a result, a quantification of the uncertainty of the maximum free surface elevation at specified locations is obtained

On the effect of the water body geometry on landslide–tsunamis: physical insight from laboratory tests and 2D to 3D wave parameter transformation

Preliminary landslide–tsunami hazard assessment is commonly based on empirical equations derived from wave channel (2D) or wave basin (3D) experiments. The far-field wave in 2D can easily be an order of magnitude larger than in 3D. The present study systematically investigates the effect of the water body geometry on the wave characteristics in the near- and far-field. Subaerial landslide–tsunami tests were conducted relying upon both a 2D and a 3D physical model, undertaken with identical boundary conditions. The test parameters included two water depths, three rigid slides, as well as various slide release positions. Empirical equations for 3D offshore and laterally onshore wave properties are presented and compared with previous work. A direct comparison of the wave features reveals that the waves decay in 2D, 3D onshore and 3D offshore with x− 0.30, r− 0.67 and r− 1.0, where x (2D) and r (3D) describe the distance from the impact zone. In 2D four wave types are observed, whereas only the two least non-linear types were observed in 3D. This finding is further analysed with wavelet spectra. For a large slide Froude number F, relative slide thickness S and relative slide mass M, the 3D wave heights in the slide impact zone can be as large as in 2D. However, for small F, S and M, the 3D waves are considerably smaller both in the near- and far-field. A novel method is presented and validated to transform data from 2D studies to 3D. This method may have favourable implications on preliminary landslide–tsunami hazard assessment