Numerical modeling of self‐channeling granular flows and of their levee‐channel deposits

Abstract

[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.

A depth-averaged debris-flow model that includes the effects of evolving dilatancy. II. Numerical predictions and experimental tests

“…This well-known difficulty has given rise to 'well-balanced schemes', which can accurately resolve balanced steady states, or small perturbations to such (e.g. [39,[42][43][44][45][46][47]). Prevalent equilibria of interest depend on the particular application.…”

“…In static cases, net driving forces must exceed frictional bounds (2.15) in both cells (cf. [46]). To account for the additional source term for p b , at each cell centre the system,…”

Can the collapse of a fly ash heap develop into an air-fluidized flow? — Reanalysis of the Jupille accident (1961)

“…Such a model induces a spatial distribution of the resistance to flow, which has been identified by many authors as a fundamental feature of water-or air-fluidized granular flows (Iverson, 2005;Ancey, 2007;Girolami et al, 2010) and appears to be of importance for the Jupille fly ash flow (lateral levees, steep flow front). Even nonfluidized granular flows display such a behavior, because of the dependence of the friction coefficient on the shear rate (Félix and Thomas, 2004;Mangeney et al, 2007). However, as this dependence has been identified as negligible in the present case study, pore pressure effects are thought to prevail.…”

Stopping dynamics of a steady uniform granular flow over a rough incline

“…A possibility would be to measure velocity profiles through transparent lateral walls, to design a two dimensional flow or to use discrete element methods. As the local frictional rheology μ(I) does not predict a static deposit or a height threshold, that is one major limitation of μ(I) until today, alternatives would be to consider a non-local rheology [20,21], or to write the friction law as a function of I and h: μ(I, h) [7,22]. We made some experiments with a smooth bottom plane.…”