Continuum modelling of pressure‐balanced and fluidized granular flows in 2D: comparison with glass bead experiments and implications for concentrated pyroclastic currents

E.C.P. Breard , J. Dufek , Olivier Roche (1)

1 LMV - Laboratoire Magmas et Volcans

Granularflows are found across multiple geophysical environments and include pyroclasticdensity currents, debrisflows, and avalanches, among others. The key to describing transport of thesehazardousflows is the rheology of these complex multiphase mixtures. Here we use the multiphase modelMFIX in 2‐D for concentrated currents to examine the implications of rheological assumptions and validatethis approach through comparison to experiments of both frictional andfluidizedflows made of glassbeads (Sauter mean grain‐size of 75μm). Because the rheology of highly polydisperse, highly angular,polydensity granular mixtures is poorly known, we focus on simplified monodisperse or bidisperse mixturesdescribed by the frictionalflow theory of Schaeffer (1987, https://doi.org/10.1016/0022‐0396(87)90038‐6)and Srivastava‐Sundaresan, often referred to as the Princeton model (Srivastava & Sundaresan, 2003,https://doi.org/10.1016/S0032‐5910(02)00132‐8). We show that simulations including the latter modelreplicate well theflow shape, kinematics, and porefluid pressure that match well‐constrained dam‐breakexperiments of initiallyfluidized or pressure‐balanced granularflows. Simulations reveal that porefluidpressure is intrinsically modulated by dilation and compaction of theflow and hence can be generated inconcentrated pyroclastic density currents. We use these simulations to interpret basal pore pressure signalsfrom localflow properties (mixture density, solid velocity, and porefluid pressure). Rheological changesretard these simulatedflows considerably near the predicted runout, but most continuum models cannotinherently predict a zero velocity. We suggest an inertial number parameter that can be used to approximatedeposition, and this approach could be a valuable tool used to validate simulations against naturalpyroclastic current examples.

Domaines: Volcanologie

E.C.P. Breard, J. Dufek, Olivier Roche. Continuum modelling of pressure‐balanced and fluidized granular flows in 2D: comparison with glass bead experiments and implications for concentrated pyroclastic currents. Journal of Geophysical Research : Solid Earth, 2019, 124, pp.5557-5583. ⟨10.1029/2018JB016874⟩. ⟨hal-02277041⟩