[1] 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 the empirical variable friction coefficient proposed by Pouliquen and Forterre (2002) 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 two-dimensional 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 of the stress tensor would be expected to change the balance of forces. The front's shape appears to be constant during propagation. 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.

Experiments on dry granular matter flowing down an inclined plane are performed in order to study the dynamics of dense pyroclastic flows. The plane is rough, and always wider than the flow, focusing this study on the case of laterally unconfined (free boundary) flows.We found that several flow regimes exist depending on the input fluxand on the inclination of the plane. Each flow regime corresponds to a particular morphology of the associated deposit. In one of these regimes, the flow reaches a steady state, and the deposit exhibits a levée/channel morphology similar to those observed on small pyroclastic flow deposits. The levées result from the combination between lateral static zones on each border of the flow and the drainage of the central part of the flow after the supply stops. Particle segregation featuresare created during the flow, corresponding to those observed on the deposits of pyroclastic flows. Moreover, the measurements of the deposit morphology (thickness of the channel, height of the levées, width of the deposit) are quantitatively related to parameters of the dynamics of the flow (flux, velocity, height of the flow), leading to a way of studying the flow dynamics from only measurements of the deposit. Some attempts to make extensions to natural cases are discussed through experiments introducing the polydispersity of the particle sizes and the particle segregation process

Mixtures of two types of glass beads have been sheared in a chute flow, in a half-filled rotating drum, and placed in a funnel to form a pile. In the three experimental devices, for small size ratios, there is a segregation of the large beads at the surface of the flowing phase (usual case), but for high size ratios (above about 5) the large beads segregate inside (reverse segregation). Precise measurements show that the segregation drives the large beads to an intermediate level inside the bed. In all devices, there is a continuous evolution of the location of the segregated beads from the surface to deep inside, when increasing the size ratio between the beads. The location of the segregated beads at intermediate levels is well defined both for high size ratios (above 5) and for very small size ratios (about 2), the level being very close to the surface in that case. The reverse and intermediate segregations are masked when using high fractions of large beads in the experiments. Their interpretation involves the high mass of the large particles balancing geometrical effects at a particular intermediate level inside the flowing layer.