Granular flow rheology; the key to understanding the exceptional mobility of pyroclastic density currents

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Pyroclastic density currents (PDCs) are hot avalanches of volcanic rock, pumice, ash and gas that descend the flanks of volcanoes. They can destroy and bury 100's km2 of terrain. Their high temperatures, inherent mobility and unpredictable nature render them one of the most hazardous volcanic phenomena. Since 1600AD, pyroclastic flows have resulted in over 90,000 deaths, 33% of all volcanic fatalities recorded, making them the single biggest cause of death at volcanoes. Forecasting the flow paths and the extent of inundation by pyroclastic density currents at a given volcano depends on our understanding of (i) the flow mechanisms involved (ii) developing models that can faithfully capture the dynamic nature of those flows and accurately simulate past events, and (iii) applying those models probabilistically, so that all possible future scenarios at a given volcano can be considered in order to generate probabilistic hazard maps. Here we will tackle (i) and (ii), but our track history in (iii) demonstrates our longer-term intention. The rationale for this research therefore stems from both a strong end-user defined need, as well as motivation to advance the science of these complex multiphase (particle and gas) natural flows. The aim of this research is therefore to improve the capability of forecasting pyroclastic density current inundation zones around volcanoes by making breakthroughs in understanding the interplay between flow behaviour and how the rheological nature of the flow changes as it propagates.During flow, pyroclastic density currents progressively develop regions that vary in their physical nature and flow mechanisms. Typically, the flows develop high particle concentrations at the base, with frictional or collisional contacts between the particles. An overriding ash cloud develops above this, where particle concentration is low and most particles are supported by turbulent convection of hot gases. As the flows propagate over topography, these upper and lower regions respond differently to changes in slope and valley confinement. Acceleration, deceleration and spreading of the upper and lower units occur at different points, and flow separation can be induced. The propensity for these upper ash clouds to separate from the parent basal flow and travel in unexpected directions often results in lethal consequences. This research will focus on understanding the rheological variations in the basal granular flow and will consider how it may, in turn, modulate mass flux into the overriding ash cloud. We will test the hypothesis that variations in the basal undercurrent rheology, in part induced by topography, result in pore fluid pressure fluctuations that feed the generation and separation of upper turbulent ash clouds from their parent undercurrents. We will achieve this by integrating data obtained from complementary field, geomorphological, experimental and computational studies, in particular utilising cutting-edge modelling tools developed for engineering applications. We will build on important new advances in the understanding of industrial granular flows to characterise how flow rheology varies (through time and space), and what controls those variations. Our results will form the basis for a new constitutive rheology description, providing a fundamental step forward by allowing advance from flow-averaged rheology laws currently employed in flow simulation tools used for hazard quantification. Extensions of this work, in particular the application of the new generation simulation tools will produce hazard maps that have lower associated uncertainties. Using methods we have already developed for probabilistic hazard mapping, we will quantify that degree of improvement. The project is timely and will benefit from synergy with a major Edinburgh-based initiative on industrial granular flows, as well as ongoing research by project partners.