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[1] The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at 170°C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3-m-long, horizontal lock-exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h 0 were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with ffiffiffiffiffiffiffi gh 0 p , like other types of dam-break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s À1 , aggradation rates were identical to those determined independently, at the same value of E, in quasi-static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h 0 /x 0 in the lock reservoir and (2) the ratio t sett /t grav of two timescales: a particle settling time t sett and a gravitational acceleration time t grav .Citation: Girolami, L., T. H. Druitt, O. Roche, and Z. Khrabrykh (2008), Propagation and hindered settling of laboratory ash flows,

In this paper, we used a 3-D discrete-element model, Grains3D, which allows the simulation of unsteady granular flows of monodisperse soft spherical particles in a common situation (i.e., down a rectangular channel). A series of numerical dam-break experiments was performed to predict the behavior of granular columns that propagate down a rough horizontal surface from different initial conditions (varying the initial aspect ratio). Numerical results were compared to those obtained experimentally by Lajeunesse et al. (Phys Fluids 17:103302, 2005) from a similar configuration. Runout distance, temporal flow evolution, deposit morphology and internal flow structures of similar laboratory experiments were quantitatively reproduced as well as prediction of empirical and theoretical scaling laws. This paper highlights that such fully 3-D simulations of soft-spheres can remarkably capture dam-break collapses performed in a rectangular channel. Moreover, Grains3D can provide a complete physical description of such complex unsteady systems which will be the topic of future on-going studies.