Numerical simulations of the 1963 Vajont landslide, Italy: application of 1D Lagrangian modelling

On 9 October 1963, a rock mass of about 250 millions m³, with a front width of 1,850 m, a mean thickness of about 200 m and a length of 350–400 m, detached from Mt Toc, on the left side of the Vajont valley, NE Italy, and collapsed into the reservoir created by a 261 m high dam. About 40 millions m³ of water was displaced, climbed up 200 m on the opposite side and then destroyed the little town of Longarone causing more than 2,000 casualties. This event is well known and was the object of numerous technical and scientific studies, especially in the two decades following the disaster. This work proposes a re-examination of the matter, focusing on the numerical modelling of the landslide, and is carried out by using a 1D numerical code based on a Lagrangian approach. The model is properly conceived for cases with slide length prevailing on width. When the slide width is comparable or prevailing on length, as is the case of the Vajont slide, the mass is first partitioned into a number of longitudinal long and narrow sub-slides. The Vajont mass was subdivided into six strips and the motion of each sub-slide was computed. The knowledge of the final deposit position allowed us to constrain the value of some relevant parameters characterizing the slide motion, the most interesting being the friction coefficient μ, that we varied in order to obtain the best-fit between calculated and observed deposits. Taking into account the additional constraint that all sub-slides move with the same or at least with similar speed, and introducing a global misfit based on deposit and velocity misfits, we were able to show that solutions minimizing the global misfit exhibit a relevant difference between the basal friction coefficients of the western and of the eastern sides of the sliding surface, with the former being significantly smaller (0.14–0.16) than the latter (0.32–0.34).