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

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.

We identify 4 principal socio-economic beneficiaries of this research programme: volcanic hazard practitioners, policy makers, industry, and the general public in the UK.

1. Volcanic hazard practitioners: Volcanic hazard practitioners are individuals at government agencies around the world responsible for undertaking volcano monitoring and hazard assessments. This work will achieve impact through facilitating substantial improvements of the most widely used pyroclastic density current simulation tool used to undertake hazard assessments, TITAN2D. TITAN2D, is a shallow water depth averaged model, which simulates granular flows over natural terrains. We will work with project partners and TITAN2D developers at the University of Buffalo to translate our new variable rheology constitutive description into the TITAN2D code. Once a new rheology substitution for TITAN2D is formulated, we will work with collaborators at geological surveys and participants at Hazard Assessment workshops to implement and showcase the resultant new tool for hazard assessment.

2. Policy makers: Decision makers utilize hazard assessments and information about the state of unrest of a particular volcano to make hazard management decisions for short term hazard mitigation as well as long term planning. Through facilitating major improvements to robust volcanic hazard assessment, this project offers a pathway to reducing risk. As well as addressing NERCs increasing resilience to natural hazard impact area, this research also follows the UN International Strategy for Disaster Reduction guidelines for natural hazards and risk management by addressing the volcanic phenomena with the highest impact on lives, and potentially livelihoods and property assets. We will engage directly with UNISDR Science and Technical Advisory group and contribute to their case study documents. These activities will be facilitated by the BGS who have a world-leading track record in research impact.

3. Industry: This project involves interdisciplinary research, which can lead to innovative approaches and results affecting diverse academic fields as well as industry. Free-surface gravity currents are one of the most common particle transport mechanisms in the handling of industrial bulk solids (powders and granular materials), across diverse sectors such as the chemical, pharmaceutical, food and personal care industries. It is of mutual benefit to interact with industry and we plan to participate in the Annual Industry Club events to be organised through Co-I Sun's EPSRC project and host a joint NERC/EPSRC workshop in 2019 to promote synergy, inviting both academic and industrial partners and stakeholders.

4. General public in the UK: Increasing awareness of natural geophysical processes will help attract students into Earth science related degrees. In addition, Edinburgh is home to two excellent museums that cover geoscience topics, with which we plan to collaborate: the National Museum of Scotland (free entry, 1.9 million visitors/year), and Dynamic Earth (330K visitors/year). We will develop a mobile, interactive gravity current exhibit based on a recently constructed mini flume tank coupled with interactive computer-based activities and we will tour the exhibit around dedicated science outreach and open days throughout the duration of the project and beyond.