Rainfall-induced runoff and debris flows: modelling the Cervinara 1999 cascade flow-like mass movements

Different types of mass flow-like movements are often triggered by rainfall in the same mountain basin in different seasons of the year, ranging from debris flows to hyper-concentrated flows and flash floods. Despite some similarities, such as large runout and high velocity, these natural hazards are different in their propagation mechanisms. Landslide mass and materials eroded along the path may be deposited along the channel(s) and subsequently remobilised; in other cases, runoff and debris mix inside the channels or nearby the protective structures. Such combined processes are typical along the northern Italian Alps but also in steep catchments in Liguria, Campania and Calabria regions. In this work, a two-phase mathematical framework is adopted to simulate the propagation of solid and water mixtures along a 3D terrain model. The mass and momentum conservation equations are solved by including the rheological behaviour models of the materials involved: frictional for soil, Newtonian for water. Selected scenarios are presented for a case study in Southern Italy with a discussion provided on how solid concentration of flow-like mass movements evolves in a mountain catchment. Numerical results show that at first, the runoff water accumulated within the natural channels and then a debris flow propagated rapidly down the slope meanwhile the concentration of solid material decreased due to the addition of runoff water and a hyperconcentrated flow reached the foothill area, later even more diluted and capable to move several kilometres far until it almost reached a railway line.