Destructiveness of pyroclastic surges controlled by turbulent fluctuations

Ermanno Brosch,Luke Fullard,Matteo Cerminara,Tomaso Esposti-Ongaro,Gert Lube,Betty Sovilla,Josef Dufek,Eric C P Breard

doi:10.1038/s41467-021-27517-9

Abstract

Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 100–102 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.

Highlights

Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 100–102 kPa inside flows capable of obliterating reinforced buildings
To study the generation of high dynamic pressure inside pyroclastic surges we synthesized them in large-scale experiments using the international eruption simulation facility PELE in New Zealand (Pyroclastic flow Eruption Large-scale Experiment)[24]
Our results demonstrate that during pyroclastic surge propagation, the dynamic pressure generated by the conversion of potential energy is distributed across a wide range of frequencies

Summary

Introduction

Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through dynamic pressures of 100–102 kPa inside flows capable of obliterating reinforced buildings. 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 realworld flows through the development of c. 100 million people worldwide are potentially endangered by these fast moving (10s to 100s of m s−1), fully turbulent and groundhugging gravity currents of hot volcanic particles and gas[4,5] Their significant threat to life and their ability to catastrophically damage reinforced buildings, infrastructure[6,7,8,9] and forests[10] results from internal dynamic pressures of 10s to 100s of kPa and remains poorly mitigated globally. Volcanologists can capitalize on the wealth of pyroclastic surge deposits and use these estimates to reconstruct local timeaveraged conditions of past eruptions to inform hazard assessments for future events[17]

Methods

Results

Conclusion