How does grain size distribution impact the mobility of aerated granular flows?

Pyroclastic density currents (PDCs) are hot, density driven flows of gas, rock and ash generated during explosive volcanic eruptions or from the collapse of lava domes. They pose a catastrophic geological hazard, and have caused >90,000 deaths since 1600 AD1 . PDCs are able to travel for tens of km, traversing topographic barriers hundreds of metres high. They are notably more mobile than other gravity currents of comparable size2 . Gas fluidisation has been attributed as a major contributor to this high mobility. Experimentation on dry (i.e. non-fluidised) granular flows has assessed the influence of grainsize on mobility, finding that the finer the grains, the larger the mobility of the mass3 . Recent advances in analogue models of gas fluidised granular currents (where gas flowing through the material causes it to act like a fluid) have revealed the impact of aeration on current mobility, and how flow behaviour can control deposit architecture and morphology4 . However, these experiments have so far largely used only a single grain-size. The impact of grain size variations on the mobility of aerated granular currents remains untested. This project aims to investigate how grain size distribution affects current velocity and run-out distance, and how this distribution is preserved in the deposit. This will be tested in a series of analogue experiments using an aerated flume. Sediment of varying grain size and shape will be released from a hopper into a flume which can sustain fluidised currents, where a high-speed camera will be used to make observations and quantify velocity and run-out distance. Improved understanding of the factors governing how PDCs behave and deposit will improve our interpretations of ignimbrite deposits and contribute to more realistic hazard assessments.