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Article Dans Une Revue Bulletin of Volcanology Année : 2010

Velocity fields and depositional processes in laboratory ash flows, with implications for the dynamics of dense pyroclastic flows

L Girolami
  • Fonction : Auteur
Olivier Roche
T Corpetti

Résumé

We conducted laboratory experiments on dam-break flows of sub-250-µm volcanic ash, generated by the release of gas-fluidized and variably non-expanded to expanded (up to 35%) beds, in order to gain insights into the kinematics of pyroclastic flows. The flows were typically several cm thick, had frontal speeds of up to ~2 m s-1 , and were non-turbulent on scales larger than the constituent particles. High-speed videos taken through the transparent sidewall of the 3-m-long channel were analyzed with a particle-tracking algorithm, providing a spatial and temporal description of transport and sedimentation. The flows deposited progressively as they traveled down the flume, being consumed by sedimentation until they ran out of volume. Deposition commenced 5-20 cm rearward of the flow front and (for a given expansion) proceeded at a rate independent of distance from the lock gate. Deposit aggradation velocities were equal to those inferred beneath quasi-static bed collapse tests of the same ash at the same initial expansions, implying that shear rates of up to ~300 s-1 have no measurable effect on aggradation rate. The initially non-expanded (and just fluidized) flow deposited progressively at a rate indicative of an expansion of a few percent, perhaps due to Reynolds dilation during initial slumping. The fronts of the flows slid across the flume floor on very thin basal shear layers, but once deposition commenced, a no-slip condition was established at the depositional interface. Velocity increased with height above the depositional interface, reached a maximum, then declined slightly towards the flow surface, perhaps due to air drag. At a given location, the velocity profiles were translated upwards as the deposit aggraded. The results show that even cm-thin, non-turbulent and poorly expanded flows of ash deposit progressively, as inferred for many pyroclastic flows. The change from (frontal) slip to (rearward) no-slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits.
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hal-02169477 , version 1 (01-07-2019)

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L Girolami, Olivier Roche, T. Druitt, T Corpetti. Velocity fields and depositional processes in laboratory ash flows, with implications for the dynamics of dense pyroclastic flows. Bulletin of Volcanology, 2010. ⟨hal-02169477⟩
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