When not laterally confined in valleys, pyroclastic flows create their own
channel along the slope by selecting a given flowing width. Furthermore, the lobe-shaped deposits
display a very specific morphology with high parallel lateral levees. A numerical
model based on Saint-Venant equations and Coulomb type behavior is used to simulate
unconfined granular flow over an inclined plane with a constant supply. Numerical simulations
successfully reproduce the self-channeling of the granular lobe and the levee-channel
morphology in the deposits without having to take into account mixture concepts or polydispersity.
Numerical simulations suggest that the quasi-static shoulders bordering the
flow are created behind the front of the granular material by the rotation of the velocity
field due to the balance between gravity, the 2D pressure gradient and friction. For
a simplified hydrostatic model, competition between the decreasing friction coefficient
and increasing surface gradient as the thickness decreases seems to play a key role in
the dynamics of unconfined flows. The description of the other disregarded components
or the stress tensor would be expected to change the balance of forces. The shape and
velocity of the front appear to be constant during propagation along the inclined plane.
The width of the flowing channel and the velocity of the material within it are almost
steady and uniform. Numerical results suggest that measurement of the width and thickness
of the central channel morphology in deposits in the field provides an estimate of
the velocity and thickness during emplacement.
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