Autofluidization of pyroclastic flows propagating on rough substrates as shown by laboratory experiments

Abstract

This study investigates the influence of the substrate surface roughness on the emplacement mechanisms of pyroclastic flows. We carried out laboratory experiments on gravitational flows generated from the release of initially fluidized or nonfluidized columns of fine particles (diameter d = 0.08 mm) in a horizontal channel. The roughness of the channel base was uniform in each experiment, created by gluing particles of diameter d 0 = 0.08 to 6 mm to the base. Other things being equal, the flow runout distance increased with the channel base roughness (d 0 ) to a maximum of about twice that of flows on a smooth substrate when d 0 = 1.5-3 mm, before decreasing moderately at higher roughness values of d 0 = 6 mm. Long runout originated mainly during the late stages of emplacement as flow deceleration was strongly reduced at high substrate roughness. This was caused by (partial) autofluidization due to an upward air flux escaping from the substrate interstices in which flow particles settled. Autofluidization was evidenced by high pore fluid pressure measurements at the base of initially nonfluidized flows and also by reduced flow runout when the interstices were initially partially filled so that less air was available. Furthermore, the runout distance of flows of large particles (d = 0.35 mm), which could not be fluidized by the ascending air flux, was independent of the substrate roughness. This study suggests that autofluidization caused by air escape from the interstices of a rough substrate is one important mechanism to explain the common long runout distance of pyroclastic flows even on subhorizontal topographies.

Grain size and flow volume effects on granular flow mobility in numerical simulations: 3‐D discrete element modeling of flows of angular rock fragments

“…The importance of the friction coefficients (for both clast‐clast and clast‐boundary interactions) is shown by Girolami et al [] as well. Also, the fact that the flows (see Figure ) travel at a constant maximum speed for a while (such as in the dam‐break experiments by Chedeville and Roche []) does not indicate a lack of basal friction which is always set to a value different from zero in our simulations. This constant speed is the result of an overstretched inflection between the acceleration and the deceleration of the granular masses (whose initial and final speeds are zero) in a plot of distance versus time.…”

“…These effects need to be understood before more complexity is added to the modeling. For example, in gas‐fluidized granular flows (but only with grain size finer than 350 µm), high pore pressure generates an upward gas percolation that is hypothesized to be able to reduce the overall basal friction in natural pyroclastic flows [ Sparks , ; Chedeville and Roche , ]. However, our numerical simulations demonstrate that fine grain size flows do not need gas intervention to have higher mobility because there is neither interstitial air nor any other interstitial gas in the model.…”

Enhanced Mobility in Concentrated Pyroclastic Density Currents: An Examination of a Self‐Fluidization Mechanism

“…Nonetheless, the presence, mechanism, and effect of elevated gas pore pressure have remained obscured in natural flows. Typically, fluidization is envisaged as the result of internal processes through degassing of clasts (Wilson, ) or external due to combustion of vegetation, entrapment of gas at initial flow generation, entrainment at the front Wilson, , or by settling of basal particles into substrate interstices and subsequent upward air advection (Chedeville & Roche, ). Using the ratio H / L of PDCs and debris avalanches, Hayashi and Self () argued that gas‐fluidization was unnecessarily invoked as a friction‐reduction mechanism for PDCs, as they hypothesized that a single mechanism could explain the long‐runout of debris avalanche and dense PDCs.…”

The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review

“…In addition to these works, the effect of fluidization on the dynamics of granular flows was addressed in various experimental configurations. The key physical parameter of fluidization is the interstitial pore fluid pressure that arises as a consequence of (i) vertical gas-particle differential motion and associated drag, which occurs when a gas of internal or external sources percolates upwards and/or when a granular mixture deflates and expels the interstitial gas upwards (Bareschino et al 2008, Chédeville and Roche 2014, Breard et al 2018, or (ii) sustained gas-particle relative oscillations that cause steady rotational fluid currents across a boundary layer around the particles, a phenomenon called acoustic streaming Soria-Hoyo 2015, Soria-Hoyo et al 2019). Pore pressure reduces particle interactions and thus favors propagation of dense gas-particle mixtures.…”