Surface velocity distribution and rheological proprties estimation of debris flows

Debris flows are among the most dangerous natural hazards in mountainous areas. In European Alps, they are associated to summer heavy rainfalls and can be extremely destructive. Nevertheless, their rheology, at field scale, is still not well known. These mixtures of debris, water and air behave as non- Newtonian fluids, with a plastic yield strength, a high bulk density and a high dynamic viscosity. Indeed, these parameters are difficult to measure in the field. In this paper, data from a monitoring system are used to measure the surface velocity of debris flows, through image analysis, following the trajectories of single particles on the flow surface, and to infer shear strength and viscosity of flows. The surface velocity distribution shows at times either rigid plugs of different width or plug does not exist at all. Our observations indicate the existence of both Bingham and Newtonian behaviour even in a single surge, as the consequence of significant changes in stage, solid concentration and in particle- size. Shear strength and viscosity, can be evaluated through appropriate relationships. The analysis and processing of surface velocity distribution allow realistic estimations of crucial aspects of flow behaviour like impact forces and run-out, parameters essential for the designing of effective countermeasures.