How pyroclastic flows attain high velocities– new insights from large-scale experiments

Abstract:

Explosive volcanic eruptions are most deadly when highly mobile pyroclastic density currents (PDCs) sweep down mountain flanks after eruption columns or lava domes collapse. An important goal in volcanic hazard assessment is to predict the velocity of PDCs in numerical hazard models. To date, this has only been partially successful for dilute PDCs, and major uncertainties remain with regard to the dynamics of PDCs that have coupled dense underflow and dilute ash-cloud regions. Interrogating these complex multiphase processes is only practical in the laboratory, but this introduces the problem of scale where it is difficult to minimise the boundary layer effects which, in small flows, can dominate behaviour. To bridge the gap, we synthesized large-scale PDCs by gravitational collapse of 1300 kg of a natural pyroclastic mixture and examined flow front velocity fields on inclined slopes.

We show that, proximally, PDCs experience rapid expulsion of gas that entrains particles and yields a fast-moving turbulent and dilute cloud that becomes the flow head and moves at 200-250% of the impact velocity. Downstream, the turbulent cloud moves ahead of the underflow. We show that the velocity-dependent friction coefficient (which represents the major dissipative form of energy) directly relates to the kinematics of the underflow front. In experimental situations where the dilute flow front velocity exceeds at all times the underflow velocity, we propose a new analytical model to quantify the asymptotic waning of the flow front velocity with distance. Using measurements of the flow head geometry and density, we show that our model successfully predicts the flow front kinematics including a long-lasting period of deceleration. Importantly, the interplay between the turbulent flow front and the presence of an underflow leads to the formation of a wedge-shaped head characterized by unexpectedly high coefficients of entrainment of ambient air.