Abstract

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 ∼(ho/g)1/2, where ho 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/(ho/g)1/2 ∼1.5. The flow velocity is then U ∼ 2(gho)1/2, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(ho/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/(ho/ g)1/2 ∼ 6.5 with normalized runout x/ho ∼ 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. Copyright 2008 by the American Geophysical Union.