[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.

[1] The emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air-particle flows generated from the release of fluidized and nonfluidized granular columns. Analyses of high-speed videos allowed for correlation of the pressure signal with the flow structure. The flows consisted of a sliding head that caused underpressure relative to the ambient, followed by a body that generated overpressure and at the base of which a deposit aggraded. For initially fluidized flows, overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation. Relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages. Furthermore, partial auto-fluidization, revealed in initially nonfluidized flows, also occurred and contributed to maintain high pore pressure, whereas dilation or contraction of the air-particle mixture with associated drag and/or pore volume variation transiently led the pressure to decrease or increase, respectively. The combination of all these processes resulted in long-lived high pore fluid pressure in the body of the flows during most of their emplacement. In the case of the initially fluidized and slightly expanded (∼3-4%) flows, (at least) ∼70%-100% of the weight of the particles was supported by pore pressure, which is consistent with their inertial fluid-like behavior. Dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas-particle mixtures consisting of a sliding head, possibly entraining basement-derived clasts, and of a gradually depositing body.Citation: Roche, O., S. Montserrat, Y. Niño, and A. Tamburrino (2010), Pore fluid pressure and internal kinematics of gravitational laboratory air-particle flows: Insights into the emplacement dynamics of pyroclastic flows,

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The effects of the ambient fluid on granular flow dynamics are poorly understood and commonly ignored in analyses. In this article, we characterize and quantify these effects by combining theoretical and experimental analyses. Starting with the mixture theory, we derive a set of two-phase continuum equations for studying a compressible granular flow composed of homogenous solid particles and a Newtonian ambient fluid. The role of the ambient fluid is then investigated by studying the collapse and spreading of two-dimensional granular columns in air or water, for different solid particle sizes and column aspect (height to length) ratios, in which the front speed is used to describe the flow. The combined analysis of experimental measurements and numerical solutions shows that the dynamics of the solid phase cannot be explained if the hydrodynamic fluid pressure and the drag interactions are not included in the analysis. For instance, hydrodynamic fluid pressure can hold the reduced weight of the solids, thus inducing a transition from dense-compacted to dense-suspended granular flows, whereas drag forces counteract the solids movement, especially within the near-wall viscous layer. We conclude that in order to obtain a realistic representation of gravitational granular flow dynamics, the ambient fluid cannot be neglected.

[1] Pore fluid pressure variations play an important role in the motion of natural granular flows like debris and pyroclastic flows. Pore pressure in a defluidizing air-particle bed was investigated by means of experiments and numerical modeling. Experiments consisted of recording the defluidization process, measured as the decay of the basal pore fluid pressure in initially aerated granular mixtures. Mixtures were aerated to different degrees of fluidization by introducing a vertical air flux at the base of a granular column. The degree of fluidization was characterized by the parameter bo (pore fluid pressure/lithostatic pressure). Bed expansion occurred for bo > 0.8-0.9, with maximum expansions near 8% at bo $1. Pore pressure diffusion in our mixtures was modeled by a simple diffusion equation, taking into account a variable diffusion coefficient. When mixtures were expanded (bo > 0.8-0.9), continuous consolidation introduced nonlinearities in the diffusion coefficients, which retarded the decay of pore pressure. In contrast, for non-expanded mixtures, the diffusion coefficient remained constant (linear diffusion). Our results highlight that mixture compressibility can effectively reduce the pressure diffusion coefficient in initially expanded granular mixtures, thus increasing the duration of pressure diffusion. In our experiments, as well as for most self-consolidating natural granular mixtures, changes in permeability due to mixture consolidation appear to be negligible for the defluidizing process, as they are counteracted by changes in porosity and because the fluid behaves as incompressible, even when the fluid is air.