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

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 mu m 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 similar to(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) similar to 1.5. The flow velocity is then U similar to root 2(gh(0))(1/2), larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(h(0)/g)(1/2) similar to 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 similar to 6.5 with normalized runout x/h(0) similar to 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.