Multiphase flow behaviour and hazard prediction of pyroclastic density currents

Abstract

Pyroclastic density currents (PDCs) are hot, ground-hugging flows of ash and debris that may occur when a volcano erupts. They can travel at speeds of hundreds of metres per second, reaching many tens to hundreds of kilometres from the source. They cause more than a third of volcanic fatalities globally. This means development of robust PDC hazard models is a priority in volcanology and natural hazard science. However, the interaction of gases and particles inside PDCs is complex. PDCs are also very dangerous. This makes it challenging to measure internal flow properties and validate hazard models. Within the last decade, major advances from large-scale experiments, field observations, and computational and theoretical models have provided new insights into the enigmatic internal structure of PDCs. These have identified key processes behind their fluid-like motion. Recent developments have also revealed important links between newly recognized processes of mid-sized turbulence and PDC behaviour. In this review, we consider how recent advances in PDC research bring us closer to more robust hazard modelling. We outline the need to measure the internal properties of natural flows using geophysical methods. We also identify critical future research challenges. Greater understanding of PDCs will also provide insights into the dynamics of other natural gravity currents and high-energy turbulent multiphase flows, such as debris avalanches and turbidity currents.