Factors controlling the dynamics of volcanic granular mass flows: insights from experimental measurements of the basal forces of granular flows in an inclined channel
Abstract
Coarse-grained pyroclastic flows and debris avalanches are examples of volcanic granular mass flows whose emplacement is controlled fundamentally by their basal forces, which dissipate energy through particle-substrate interactions. We addressed the issue of basal forces through direct measurements of the force components of granular flows in an inclined channel with smooth basal and lateral boundaries. The channel was 1.5 m-long, 0.3 m-wide, and had slope angles A=13°-84°. Glass beads of grain size 1.5 mm were released from a reservoir to generate granular flows in the channel. The aperture of the reservoir gate (2-6 cm) controlled the flow thickness. We did force measurements at frequency of 3 kHz at the base of the flows, using a 15x15 cm flush-fitting plate connected to a 3-component sensor that measured the basal normal (N) as well as the shear longitudinal (S) and transverse components of the particles weight. Hence, the effective basal friction coefficient equal to tan(S/N) could be determined readily. The flows were accelerating over a distance increasing with the slope angle, until they reached a steady state. Hence, at a given distance for the reservoir, we measured the basal forces of either steady fully developed flows a low slope angles <18-25° or accelerating immature flows at steeper inclinations. The basal friction coefficient varied systematically with both the flow height and the channel inclination, hence confirming earlier findings from discrete element simulations. The coefficient was close to tan(A) in the steady flow regime, as imposed by force balance. In contrast, for accelerating flows, it was nearly constant and equal to ~0.3-0.4 at intermediate inclinations (~25-50°) and it decreased with the slope angle in the steepest configurations. An analysis revealed three main issues. First, the decrease of the basal friction coefficient with the flow height at given slope angle was due to non-negligible side-walls friction. Second, the steady flows had front velocities scaling with the particle mass holdup H (the mass of particles per unit basal area expressed as an equivalent grain height) to the power 1/4. Third, the basal friction coefficient of steady to moderately accelerating flows at A<50° scaled with a modified Froude number that considers H as the typical length scale.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V23G0292R
- Keywords:
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- 4314 Mathematical and computer modeling;
- NATURAL HAZARDS;
- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8428 Explosive volcanism;
- VOLCANOLOGY;
- 8445 Experimental volcanism;
- VOLCANOLOGY