A new generation of models for Debris Flow

Aronne Armanini, University of Trento

Debris flows are catastrophic events that affect mountain regions all over the world. Debris flows are two-phase flows, composed by water and sediments, mobilized and driven by gravity, which occurs after heavy intense rainfalls in relatively small catchments. These events are characterized by the fact that they travel along very long distances (several kilometers), with high velocities (up to several m/s), and are composed by poorly sorted sediments (ranging from few millimeters up to meters). These phenomena are known since centuries, but just in the last decades they have received the attention of the scientific community, because in many countries the urban development and the spread of the tourist facilities in mountainous areas have increased their potential hazard.
Debris flows normally are triggered in the upper part of the basin, because of the destabilization of the debris deposits and along the way they increase their global discharge, which overcomes even of one order of magnitude the liquid discharge that has them triggered. This is one of the most peculiar aspects of the debris flows.
Numerical models of these flows are becoming the most reliable and diffused tool to guide the mapping operations and the assessment of efficiency of the control structures and of the prevention systems. However, in the technical literature a clear vision of the models suited to this purpose, does not exist yet.
Generally speaking, the shallow water hypothesis is widely accepted in these models. A one-dimensional approach (1DH) is suitable for reproducing the canalized parts of the flow upstream the alluvial fans, while, in order to simulate the spreading of the flow inside the fans, a two-dimensional depth integrated scheme (2DH) is recommended.
Scientific and commercial codes are available for this purpose, but the uncertainties of these models regard the limit of the rheological schemes adopted to simulate these flows. In many models debris flows are treated as non Newtonian homogenous fluid. This kind of models does not grasp the two-phasic features of such kind of phenomena.
A two-phase approach, suitable to modeling such phenomena, will be presented.
The approach is based on the microstructural characteristics of the granular flows that furnish a realistic interpretation of the rheological properties of the mixture, necessary to set the hydrodynamic equations of the flow. The equations are further integrated on the depth in the form of shallow water equations.
A series of applications to different situations of hazard mapping will be presented.