Modelling combined flow-like mass movements: the case study of an Italian Alps catchment with protection works

In high-mountain contexts, rainfall can trigger various flow-like mass movements, from debris flows to hyperconcentrated flows and flash floods. Despite similar runout and velocity, propagation mechanisms are different. In such complex phenomena, also the existing protection structures play a fundamental role. In this paper, a multi-phase mathematical framework is adopted to simulate the propagation of a mixture of soil and water along a 3D terrain model. The mass and momentum conservation equations are solved including the rheological behavior models for the materials involved: frictional for soil, Newtonian for water. Some selected scenarios are discussed for a site-specific case study in Northern Italy. The controversial role played by two storage basins located at the toe of the gully is explored numerically and compared to the field evidence. The novelty of the paper is to show how the water temporarily impounded in the basins enhanced the mobility of an incoming debris flow, which turned into an hyperconcentrated flow and went out of the protections structure.