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The interaction between Pyroclastic Density Currents and waterbodies, first results from PELE large-scale experiment

Geoffrey (Jeff) Robert , Gert Lube, Ermanno Brosch, Anna Perttu

Abstract

Pyroclastic Density Currents (PDCs) can generate tsunamis and cross significant waterbodies posing high risk to populations living in near-shore volcanic areas. Global hazard assessments urgently require models for mitigating this risk, but the hostility of flows persistently defies any internal observation of this enigmatic hazard process. By synthesizing PDC-water interactions in large-scale experiments, we demonstrate that three fundamentally different types of water wave occur, depending on the vertical density stratification of the parental density current. Through direct measurements of the sub-aerial and sub-aqueous flow regions, we show that the wavelength, wave celerity, the ratio of wavelength and water depth, and the wave energy, increase by orders of magnitude with increasing concentration of the basal PDC flow region. Dilute turbulent surge-like PDCs generate low-energy shear-induced wind-waves. For more concentrated PDCs, a higher energy, soliton-type wave occurs through the instantaneous transfer of momentum from the dense underflow of PDCs to the near-shore waterbody. Importantly, for sustained eruption conditions, the prolonged momentum transfer from the underflow onto the water generates an extremely high-energy, long wavelength wave. This high-celerity wave can overtake earlier generated soliton-like waves and has never been reported in experimental and numerical simulations. The types and characteristics of real-world waves can be predicted by considering the momentum fluxes and duration of natural PDC events. This discovery suggests that tsunami hazard models should consider generation of multiple waves by a single PDC and must account for the vertical density stratification and for temporally variable momentum of PDCs to accurately predict hazard impacts.