Experimental observations of water‐like behavior of initially fluidized, dam break granular flows and their relevance for the propagation of ash‐rich pyroclastic flows

Abstract

[1] The physics of ash-rich pyroclastic flows were investigated through laboratory dam break experiments using both granular material and water. Flows of glass beads of 60-90 mm in diameter generated from the release of initially fluidized, slightly expanded (2.5-4.5%) columns behave as their inertial water counterparts for most of their emplacement. For a range of initial column height to length ratios of 0.5-3, both types of flows propagate in three stages, controlled by the time scale of column free fall $(h 0 /g) 1/2 , where h 0 denotes column height and g denotes gravitational acceleration. Flows first accelerate as the column collapses. Transition to a second, constant velocity phase occurs at a time t/(h 0 /g) 1/2 $ 1.5. The flow velocity is then U $ ffiffi ffi 2 p, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(h 0 /g) 1/2 $ 4. Granular flow behavior then departs from that of water flows as the former steadily decelerates and the front position varies as t 1/3 , as in dry flows. Motion ceases at t/(h 0 /g) 1/2 $ 6.5 with normalized runout x/h 0 $ 5.5-6. The equivalent behavior of water and highly concentrated granular flows up to the end of the second phase indicates a similar overall bulk resistance, although mechanisms of energy dissipation in both cases would be different. Interstitial air-particle viscous interactions can be dominant and generate pore fluid pressure sufficient to confer a fluid-inertial behavior to the dense granular flows before they enter a granular-frictional regime at late stages. Efficient gas-particle interactions in dense, ash-rich pyroclastic flows may promote a water-like behavior during most of their propagation.

Frictional velocity-weakening in landslides on Earth and on other planetary bodies

“…Experimental studies 4 show that granular collapse over beds with slope angles up to 22°have a maximum velocity of about 2 m s À 1 , that is, o4 m s À 1 , so that equation (3) 4,51 show that the maximum velocity is a function of ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi gH 0 cos y p . As a result, on slope angles yC30°typical for these rockfalls, the threshold velocity would be higher than 4.8 m s À 1 .…”

Granular collapse in a fluid: Role of the initial volume fraction

“…15 The case of the collapse in presence of an interstitial fluid has been less studied. Roche et al 5,20 have investigated the role of air in the propagation of the surge in the dam break configuration. They have shown that air can strongly enhance the mobility of the mass when the particles are fine enough and when the initial pile is fluidized by an injection of air.…”

“…However, they can be useful to precisely study physical mechanisms, which may play a role in real flows. [2][3][4][5] The study presented in this paper is carried out in this spirit. The goal is to explore the role of the coupling between the grains and the interstitial fluid during the collapse of a granular mass fully immersed in a liquid.…”

Can the collapse of a fly ash heap develop into an air-fluidized flow? — Reanalysis of the Jupille accident (1961)

“…The results showed that initially fluidized particles behave like an inviscid fluid for most of their propagation, which means that energy dissipation is really low (Roche et al, 2008). Pore pressure measurements showed that dynamic pore pressures persist in the flowing mass.…”

“…The assumed heap undermining by a rising water table that led huge volumes of ash fall onto an important layer of air is an air-fluidization process that shares similarities with experiments by Roche et al (2008Roche et al ( , 2010 and Roche (2012).…”