Propagation and deposition mechanisms of dense pyroclastic density currents: insights from analogue laboratory experiments. (Invited)

Analogue laboratory experiments on air-particle flows represent a useful tool to investigate the mechanisms of propagation and deposition of dense (or the dense part of) pyroclastic density currents. In this context, we carried out experiments in the dam-break configuration and studied the emplacement processes of analogue biphasic currents generated from the quasi-instantaneous release of fluidized columns of fine (80 µm) particles. The low permeability of the granular material permitted relatively slow diffusion of the initial pore pressure within the flows until they came to halt. Analysis of the flow kinematics and comparison with flows of water in the same apparatus revealed that the air-particle currents propagated in two distinct stages. They behaved as their inertial water counterparts for most their emplacement, as both types of flows had the same morphology and propagated at constant front velocity U~√(2gh), h being the initial height of the granular column. This occurred as long as the height of the collapsing fluidized columns was higher than the that of the resultant flows, thus generating a driving pressure gradient. This fluid-inertial behavior suggested that the pore fluid pressure was high during propagation of the mixture. In order to check this hypothesis, we carried out non invasive measurements of the pore fluid pressure at the base of the air-particle flows and made correlation of the pressure signal with the flow structure from analyses of high speed videos. The flow structure consisted of a sliding head that caused underpressure relative to ambient conditions and whose magnitude correlated with the flow velocity. The flow head was followed by a body that generated overpressure and at the base of which a deposit aggraded at a nearly constant rate. Both the flow head and body were sheared pervasively as the internal velocity increased upwards. The combination of pressure advection from the source and relatively slow pressure diffusion resulted in long-lived high pore fluid pressure in the body of the flows during most their emplacement, which is consistent with their inertial behavior. When the pressure had sufficiently decreased at late stage, the flows entered a granular-frictional regime and stopped. Complementary experiments on the release of dry granular columns evidenced partial auto-fluidization of the flow body, as some pore pressure was generated at early stages, which might have occurred in initially fluidized flows as well. These experimental studies suggest that dense pyroclastic density currents on subhorizontal slopes can propagate as inertial fluidized gas-particle mixtures consisting of a sliding head and of a gradually depositing body. They also indicate that analyses combining resistance terms of inertial-turbulent and then (Coulomb) frictional form appear to be appropriate tools for simulating the emplacement of these density currents.