Dynamics and hazards of stratified pyroclastic density currents

In the context of a study on pyroclastic flow hazards in caldera settings, we have analyzed the role of particle sedimentation and stratification on the kinematics of particle-driven gravity currents. By means of a two-dimensional multiphase flow numerical model we have simulated the propagation of monodisperse particle-laden currents triggered by the collapse of a dam. Numerical results indicate that the current evolves into a bipartite system, with a concentrated flow at the base (where particles accumulate due to gravitational settling) and a dilute, turbulent ash cloud. On a flat surface, the role of the basal concentrated layer on the current dynamics is negligible (as demonstrated by removing it by means of a depositional boundary condition) and the front kinematics is well reproduced by an integral "box-model", based on dimensional analysis and conservation laws. However, for initial volume concentrations above about 10%, particle-particle interaction become more important: a rheological model based on the kinetic description of a collisional regime predicts significant deviation from the box-model predictions in the final stages of the current propagation. Preliminary analysis shows that the main effect of particle-particle collisions is to increase the granular temperature (i.e., the magnitude of particle velocity fluctuations) in the flow head, with respect to the sedimenting body, and the current mobility. The calibrated box model is finally applied to pyroclastic density currents propagation in a caldera by applying an "energy-conoid" model to describe flow-topography interaction. Numerical results demonstrate that the adopted simplified approach can be useful in the context of probabilistic hazard assessment studies.