Transport and sedimentation of Pyroclastic Density Currents across topographic obstacles

Abstract

Pyroclastic density currents (PDCs) can cross significant topographic obstacles. The processes that govern the interaction of PDCs with obstacles remain poorly understood leaving uncertainty in hazard planning and mitigation. Here, we report the results of large-scale experimental PDCs comprising hot volcanic particles and gas propagating across ridge-shaped obstacles. Observations from high-speed video and measurements of the velocity, density and temperature structure of the flows are used to identify the flow processes that occur when PDCs propagate across and become partially blocked by hill-shaped topographic obstacles; and how these characteristics are recorded in PDC deposits. The experiments show that the interaction of PDCs with ridges generate strong local perturbations to the internal flow velocity, density and temperature structure. These flow changes are linked to three main processes: the blocking of the lower, concentrated flow region in front of the obstacle; the compression and acceleration of the non-blocked flow regions on the stoss side; and the flow detachment behind the crest and formation of a turbulent wake before flow re-attachment downstream. Flow-topography interactions result in deposition and erosion rates that vary by three and two orders of magnitude, respectively, which explain the strong asymmetry of PDC deposits across topographic obstacles. The facies architecture of experimental deposits across ridges resembles those of natural PDC deposits from Te Maari and Taupō volcanoes (New Zealand). The findings of this study can guide the interpretation of PDC deposits or be taken into consideration in numerical models simulating the propagation of PDCs across complex topography for hazard forecast.