Alpine landslide risk scenario: run-out modelling using a 3D approach

Rockslides of considerable volume developing on the flanks of narrow alpine valleys represent a significant hazard for the potential of river damming and consequent sudden discharge of water. A limestone rock mass of about 650,000 m3 endangers a portion of the upper course of Tagliamento River valley in north-eastern Italy. This section of the valley is prone to detachment of rock landslides due to its structural setting and it has a history of landslide damming: in the literature two huge events are widely documented and a third event is identified and described in this paper. The ancient detachment crown and the corresponding deposit can be observed in the portion of slope adjacent to the unstable rock mass. This paper deals with the run-out model of the unstable rock mass using a Smoothed Particles Hydrodynamics (SPH) model implemented in the DAN3D code. In order to select the appropriate material parameters, the ancient rockslide was back-analysed. Subsequently, the parameters were applied to the present unstable portion of the slope to determine the location and dimensions of the deposit that could be formed. It is demonstrated that the deposit has the potential to create a natural dam across the Tagliamento River that would form a 500,000 m3 lake. Although the lake will have limited dimensions, this would represent a very high risk for the downstream villages and infrastructures in case of a sudden collapse of the natural dam.