  Multi-phase flows encountered in nature or in industry, exhibit non trivial
rheological properties, that can be understood better thanks to model materials
and appropriate rheometers. Here, we use model unsaturated granular materials:
assemblies of frictional spherical particles bonded by a small quantity of a
wetting liquid, over a rough inclined plane. Our results show steady uniform
flows for a wide range of parameters (the inclination angle and the mass
flow-rate). A theoretical model, based on the Mohr-Coulomb yield criterion
extended to inertial flows: $\tau$ = $\tau$c + $\mu$(I)P, in which $\tau$c and
$\mu$(I) are the cohesion stress and the internal friction coefficient
respectively, gives predictions in quantitative agreement with experimental
measurements only when one considers that dry and wet samples have
straightforwardly different internal friction commonly described by the
so-called $\mu$(I)-rheology. The liquid bridges bounding grains not only induce
cohesion, but modify the internal friction of the wet assemblies.
