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Abstract Pyroclastic density currents (PDCs) are the most lethal volcanic process on Earth. Forecasting their inundation area is essential to mitigate their risk, but existing models are limited by our poor understanding of their dynamics. Here, we explore the role of evolving grain-size distribution in controlling the runout of the most common PDCs, known as block-and-ash flows (BAFs). Through a combination of theory, analysis of deposits and experiments of natural mixtures, we show that rapid changes of the grain-size distribution transported in BAFs result in the reduction of pore volume (compaction) within the first kilometres of their runout. We then use a multiphase flow model to show how the compressibility of granular mixtures leads to fragmentation-induced fluidisation (FIF) and excess pore-fluid pressure in BAFs. This process dominates the first ~2 km of their runout, where the effective friction coefficient is progressively reduced. Beyond that distance, transport is modulated by diffusion of the excess pore pressure. Fragmentation-induced fluidisation provides a physical basis to explain the decades-long use of low effective friction coefficients used in depth-averaged simulations required to match observed flow inundation. 

Abstract Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 10 0 –10 2 kPa inside flows capable of obliterating reinforced buildings. However, to date, there are no measurements inside these currents to quantify the dynamics of this important hazard process. Here we show, through large-scale experiments and the first field measurement of pressure inside pyroclastic surges, that dynamic pressure energy is mostly carried by large-scale coherent turbulent structures and gravity waves. These perpetuate as low-frequency high-pressure pulses downcurrent, form maxima in the flow energy spectra and drive a turbulent energy cascade. The pressure maxima exceed mean values, which are traditionally estimated for hazard assessments, manifold. The frequency of the most energetic coherent turbulent structures is bounded by a critical Strouhal number of ~0.3, allowing quantitative predictions. This explains the destructiveness of real-world flows through the development of c. 1–20 successive high-pressure pulses per minute. This discovery, which is also applicable to powder snow avalanches, necessitates a re-evaluation of hazard models that aim to forecast and mitigate volcanic hazard impacts globally. 

Abstract Geophysical fluid‐granular flows, such as pyroclastic currents and debris flows, owe much of their runout and hazard behavior to the occurrence and time‐variant decay of a flow‐internal fluid pore pressure. However, modeling the effects of fluid pore pressure to forecast hazards is challenging because a unified method in Earth Sciences to quantitatively determine the permeability of these natural mixtures is currently missing. Here we combine experiments on fluidization and defluidization of pyroclastic materials, eolian sediments, and glass beads mixtures with numerical multiphase simulations to compare previous attempts to compute the permeability of complex natural particle‐fluid mixtures. In analogy to particle‐engineering studies on simple gas‐particle mixtures, we demonstrate that the effective length‐scale in the characterization of the fluid‐particle interaction of complex natural mixtures is the product of the Sauter mean diameter and the particle sphericity. Its use in the Kozeny‐Carman equation allows accurate prediction of mixture permeability, and we suggest the routine calculation of the Sauter mean from grain size distributions of the deposits of geophysical mass flows in Earth Sciences. We also show, through defluidization experiments, that the duration of gas retention in natural mixtures is well described when using the Sauter mean as the effective particle size. Further, we show through multiphase simulations that initial bed expansion extends the pore pressure diffusion timescale up to nine times. These results can be applied to small‐to‐large volume dense pyroclastic currents where the ranges of Sauter mean diameter predict gas retention for long duration and to debris flows and snow avalanches.