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The dynamics of volcanic biphasic flows – VOLBIFLO

Submission summary

Abstract Biphasic flows are commonly produced by volcanic eruptions, from the gravitational collapse of eruptive columns or from parts of the edifices. These flows, consisting of volcanic particles and gas, have temperatures of several hundreds of degrees °C and propagate at velocities of up to several hundreds of meters per second. During their eruptive history, volcanoes in Antilles have produced numerous volcanic biphasic flows of various types. Active volcanoes such as la Soufrière (Guadeloupe) and la Montagne Pelée (Martinique) present strong volcanic hazards linked to such flows and are considered as potentially highly destructive. We shall recall that the 1902 eruption at la Montagne Pelée killed nearly 30000 people, and future eruptions will certainly produce flows that will represent major hazards for the local population. One of the main objective of the international volcanological community is to develop numerical models that are able to simulate volcanic biphasic flows propagating on real topographies, in order to establish detailed hazards maps. The objective of the present project is to study the dynamics of volcanic biphasic flows. These flows are of various types (pyroclastic flows, debris avalanches) but are probably governed by the same physical laws. Our goal is to better understand the flow mechanisms and to determine rheological laws that could be taken into account in the numerical models. We propose a multidisciplinary research project combining analogue laboratory experiments, the development of numerical codes, and quantitative field analyses. The objective of the experimental work is to determine the basic mechanisms of biphasic flows, by studying particularly the gas-particles systems. The goal is to bring the information required to develop appropriate numerical models, and to help the interpretation of the characteristics of the deposits in the field. Very few works have been carried out on such types of flows, and despite recent advances they are still poorly understood. The dynamics of biphasic systems have to be explored in details, as their behaviour differ radically from that of granular flows for which the interstitial fluid has a negligible influence. Experiments will need the use of a high speed video. The development of numerical codes able to simulate volcanic biphasic flows is a crucial objective for hazards assessment and risks mitigation. For the numerical results to be used in case of eruptive crises, in Antilles for instance, it is necessary to ensure the validity of the models, which depends fundamentally on the numerical scheme and on the rheological laws used. The results of these models will be compared to the experimental results in order to validate them, and then to natural cases in order to estimate the rheology controlling the different types of flows by reproducing their deposits as best as possible. In order to constrain the models by comparing them to the natural cases, we need to collect precise and quantified information in the field (extension, volume, morphology, and structure of the deposits, inferred initial velocity, interaction with topography…). For this reason, the main targets chosen are volcanoes presenting very well preserved deposits (in Chile), in contrast to volcanoes in Antilles. The objective is to characterise quantitatively the deposits, using geophysical methods (stereo-photogrammetry, Lidar, differential GPS). The experimental and field studies will allow to bring the information necessary to develop the numerical models aiming to simulate volcanic biphasic flows. Morphology and structure of the deposits will be quantified precisely. The experiments will allow determining the mechanisms governing the flow dynamics, and the regimes will be defined using appropriate dimensionless numbers. Interstitial air pore pressure measurements will clarify the way internal friction is controlled, and video analyses will characterise the internal velocity field. The numerical models will be improved and validated from the field and experimental works and will allow to determine, from the characteristics of their deposits, the rheological laws acting in natural flows.

Project coordination

Olivier ROCHE (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Help of the ANR 149,055 euros
Beginning and duration of the scientific project: - 36 Months

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