%0 Journal Article
%9 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES
%A Girolami, L.
%A Roche, Olivier
%A Druitt, T. H.
%A Corpetti, T.
%T Particle velocity fields and depositional processes in laboratory ash flows, with implications for the sedimentation of dense pyroclastic flows
%D 2010
%L fdi:010049707
%G ENG
%J Bulletin of Volcanology
%@ 0258-8900
%K Pyroclastic flow ; Fluidized granular flow ; Laboratory experiment ; Velocity profile ; Progressive aggradation
%M ISI:000280564500009
%N 6
%P 747-759
%R 10.1007/s00445-010-0356-9
%U https://www.documentation.ird.fr/hor/fdi:010049707
%> https://www.documentation.ird.fr/intranet/publi/2010/08/010049707.pdf
%V 72
%W Horizon (IRD)
%X We conducted laboratory experiments on dam-break flows of sub-250-A mu m 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 internal kinematics of pyroclastic flows. The flows were typically several cm thick and had frontal speeds of up to similar to 2 m s(-1). 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 similar 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 shear-induced 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. Within the flows, the trajectory of the constituent particles was linear and sub-horizontal. The velocities of the particles 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, 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.
%$ 066