3D Simulation of Vajont Disaster. Part 1: Numerical Formulation and Validation Alessandro Franci a , Massimiliano Cremonesi b , Umberto Perego b , Giovanni Crosta c , Eugenio Oñate a a International Center for Numerical Methods in Engineering (CIMNE), Universitat Politècnica de Catalunya (UPC), Carrer Gran Capitán, UPC Campus Nord, Barcelona b Politecnico di Milano, Department of Civil and Environmental Engineering, Piazza Leonardo da Vinci 32, Milan, Italy c Università degli Studi di Milano Bicocca, Department of Earth and Environmental Science, Piazza della Scienza, 4, Milan, Italy Abstract This work presents a numerical method for the simulation of landslides generated impulse waves and its application to the historical Vajont case study. The computational tool is based on the Particle Finite Element Method (PFEM), a Lagrangian strategy that combines the finite element solution of the governing equations with an efficient remeshing strategy to deal with large deformation problems. After presenting the numerical formulation, different landslide impulse wave problems with Froude number ranging from 0.5 to 2.8, are analyzed to validate the proposed methodology. The computational method is shown to be able to reproduce accurately the landslide runout, the momentum transfer between the sliding material and the impounded water, and the consequent wave propagation observed in experimental physical models. Then, the PFEM model is applied to the numerical simulation of the Vajont disaster, which is analyzed with a fully-resolved three-dimensional model. The numerical results are discussed and compared to the post-event observations and the numerical results of other computational methods. The results in terms of landslide velocity and runout, geometry of the deposit, maximum water runup, dam overtopping wave, and water discharge in the downstream valley are in good agreement with observations and reconstructions. The calibration and validation performed for this study form the basis for the PFEM analyses presented in a companion paper finalized to simulate different scenarios of the Vajont rockslide considered in the experimental tests done a year before the disaster. Keywords: Rockslide, Rock Avalanche, Vajont, Particle Finite Element Method, PFEM, Impulse Wave 1. Introduction Landslides are responsible for significant human and eco- nomic losses worldwide (Froude and Petley, 2018). Between 1995 and 2014, landslides caused over 160,000 deaths and 11,000 injured worldwide (Haque et al., 2019) and, only con- sidering 27 European countries, an approximated economic loss of 4.7 billion Euros (Haque et al., 2016). According to World Bank data (Dilley et al., 2005), about 300 million people live in landslides-prone areas. Global warming effects, the intensi- fication of extreme rainfalls and new settlements in risk areas, driven by the rise of the world population, are contributing to increasing the number of deadly landslides worldwide (Haque et al., 2019). This critical scenario puts in the foreground the urgency of improving the current predicting techniques for this major natural hazard. Due to the complexity of landslide dynamics and the diffi- culty in defining the material properties and behavior, predict- ing landslide effects on natural and human environments is a hard endeavor. This task is even more complicated when land- slides are accompanied by other natural hazards in a cascad- ing mode, as in the case of landslides falling into water reser- voirs and generating impulse waves. Depending on the slide initial position, geometry, material, velocity, evolution, and wa- ter reservoir characteristics, tsunami-type waves may form and affect the shoreline of the reservoirs. The fjord area of western Norway is one of the world re- gions most susceptible to this type of natural hazard (Harb- itz et al., 2014). In this area, rock avalanches and associated tsunamis caused more than 170 casualties during the last 100 years (Blikra et al., 2005). The most tragic event occurred in Tajford in 1934, when a massive rockslide of around 3 million cubic meters dropped into the fjord resulting in a devastating flood wave that washed over 60m on the opposite shorelines and the nearby Tafjorden communities (Braathen et al., 2014). More recently, landslide induced tsunami waves were recorded in Greenland (Gauthier et al., 2018; Paris et al., 2019; Bloom et al., 2019), where the seriousness of this type of phenomena can become more and more relevant considering present-day climate changes, especially at high latitudes. The highest recorded wave runup produced by a landslide oc- curred on July 9 th 1958 at the head of Lituya Bay on the south- ern coast of Alaska (Miller, 1960; Fritz et al., 2009). On that occasion, an 8.3 magnitude earthquake along the Fairweather fault triggered a major rockslide which, after impacting the wa- ter of the lake, generated a giant tsunami and a water runup of 524m (Fritz et al., 2001). The landslide impulse wave event of Vajont (northern Italy) was the one with the highest number of associated casualties (around 2000). On the night of October 9 th 1963, about 275 million cubic meters of rock detached from the northern side Preprint submitted to Elsevier October 2, 2020