Rheological Behaviour of Volcanic Granular Flows

Numerical simulations of volcanic granular flows are increasingly being used for hazard assessment on volcanoes and appear to be essential for future hazard mitigation. A potential problem of such an approach, however, is that the rheological behaviour of such flows is very complex and currently impossible to describe fully from a physical point of view. As we cannot, at present, simulate all the complexity of the interactions at a microscopic level, we have to use simplified rheological laws. The frictional behaviour is often considered as a model that well describes the first order behaviour of such flows. The aim of this paper is to compare numerical simulations to well constrained field examples to check the validity of this law. Results show that the frictional model appears to be too simple to be used for pyroclastic flow and long runout avalanche simulations. A simple empirical law, using a constant retarding stress, represents a better alternative.