Characteristics and controls of the runout behaviour of non-Boussinesq particle-laden gravity currents – A large-scale experimental investigation of dilute pyroclastic density currents

Abstract

One of the most dangerous aspects of explosive volcanism is the occurrence of dilute pyroclastic density currents that move at high velocities of tens to about a hundred of metres per second outwards from volcanic vents. Predicting the runout behaviour of these turbulent flows of hot particles and air is complicated by strong changes in the flow density resulting from entrainment of ambient air, sedimentation of particles, as well as heating and expansion of the gas phase. Consequently, their behaviour remains relatively poorly understood.

We investigate the runout behaviour of dilute pyroclastic density currents in large-scale experiments using hot volcanic material and gas. We demonstrate that the flows transition through four dynamic regimes with distinct velocity, density, and force characteristics. Regime transitions are marked by the intrusion and cessation of fast-moving internal gravity waves into the flow head, while the main flow retarding force is the pressure drag force when ambient air is forced to move around the advancing front. The Froude number--a measure of inertial over gravitational forces--is not constant and assumption of a constant Froude number results in underprediction of the flow velocity. These results are not only important for hazard mitigation of pyroclastic density currents but are also relevant for other turbulent gas-particle gravity currents, such as powder snow avalanches and dust storms.