  Granular flows down inclined channels with smooth boundaries are common in
nature and in the industry. Nevertheless, the common setup of flat boundaries
has comparatively been much less investigated than the bumpy boundaries one,
which is used by most experimental and numerical studies to avoid sliding
effects. Using DEM numerical simulations with side walls we recover
quantitatively experimental results. At larger angles we predict a rich
behavior, including granular convection and inverted density profiles
suggesting a Rayleigh-B\'enard type of instability. In many aspects flows on a
flat base can be seen as flows over an effective bumpy base made of the basal
rolling layer, giving Bagnold-type profiles in the overburden over that layer.
We have tested a simple viscoplastic rheological model (Nature 2006, vol 441,
pp727-730) in average form. The transition between the unidirectional and the
convective flows is then clearly apparent as a discontinuity in the
constitutive relation.
