Experimental observations of water-like behavior of initially fluidized, dam break granular flows and their relevance for the propagation of ash-rich pyroclastic flows
Résumé
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 μm 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 ∼(h0/g)1/2, where h0 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/(h0/g)1/2 ∼ 1.5. The flow velocity is then U ∼ equation image(gh0)1/2, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(h0/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 t1/3, as in dry flows. Motion ceases at t/(h0/g)1/2 ∼ 6.5 with normalized runout x/h0 ∼ 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.
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