Influence of slope angle on pore pressure generation and kinematics of pyroclastic flows: insights from laboratory experiments
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The influence of slope angle on pore pressure generation and kinematics of fines-rich pyroclastic flows was investigated through laboratory experiments. Granular flows were generated by the release of a column of fine glass beads (d = 0.08 mm) in an inclined channel (0–30°). The granular column could be fluidized while the channel base was either smooth or made rough by glued beads of 3 mm diameter. Pore pressure measurements reveal that the degree of autofluidization, caused by air escaping from the substrate interstices into which flow particles settled, was high at all slope angles. Flow runout increase due to autofluidization, however, was reduced at slope angle higher than ∼12° because of the occurrence of a strong deceleration phase that limited the flow duration. This is probably caused by the combination of flow head thinning at increased slope angle and settling of particles into the substrate interstices until the flow ran out of mass. Analysis of high-speed videos suggests that ingestion of ambient air at the flow front did not occur, even on steep slopes of 30°. Experiments at inclinations close to (25°) or slightly higher (30°) than the repose angle of the granular material (28.5°) revealed the formation of a thin basal deposit that was then eroded as the flow thickness and velocity gradually decreased. Our study suggests that air escape from substrate interstices in nature can be a significant external cause of pore pressure generation that favors low energy dissipation and long runout distances of pyroclastic flows on moderate topographies.
KeywordsPyroclastic flows Fluidization Pore pressure Analog modelling Inclined substrate Substrate roughness
We are grateful to Greg Valentine and David Jessop for the interesting discussions and comments that helped to improve the manuscript. We thank the associate editor Gert Lube, Ben Andrews, and Eric Bréard for the careful and constructive reviews. This work was financed by the Laboratoire Mixte International SVAN of Institut de Recherche pour le Développement (IRD, France) and by the French Government Laboratory of Excellence initiative n°ANR-10-LABX-0006, the Région Auvergne and the European Regional Development Fund. This is Laboratory of Excellence ClerVolc contribution number 169.
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