Observed avalanche flows of dense granular material have the property
to present two possible behaviours: static (solid) or flowing (fluid). In such situation,
an important challenge is to describe mathematically the evolution of the physical interface
between the two phases. In this work we derive analytically a set of equations
that is able to manage the dynamics of such interface, in the thin-layer regime
where the flow is supposed to be thin compared to its downslope extension.
It is obtained via an asymptotics starting from an incompressible viscoplastic model
with Drucker-Prager yield stress, in which we have to make several assumptions.
Additionally to the classical ones that are that the curvature of the topography,
the width of the layer, and the viscosity are small, we assume that the internal friction angle
is close to the slope angle (meaning that the friction and gravity forces compensate at leading order),
the velocity is small (which is possible because of the previous assumption),
and the pressure is convex with respect to the normal variable.
This last assumption is for the stability of the double layer static/flowing configuration.
A new higher-order non-hydrostatic nonlinear coupling term in the pressure allows us to close the asymptotic system.
The resulting model takes the form of a formally overdetermined initial-boundary problem
in the variable normal to the topography, set in the flowing region only.
The extra boundary condition gives the information on how to evolve the static/flowing interface,
and comes out from the continuity of the velocity and shear stress across it.
The model handles arbitrary velocity profiles, and is therefore more general than depth-averaged models.

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