  We present a short review of recent theoretical descriptions of flows
occuring at the surface of granular piles, and focus mainly on two models: the
phenomenological ``BCRE'' model and the hydrodynamic model, based on
Saint-Venant equations. Both models distinguish a ``static phase'' and a
``rolling'' phase inside the granular packing and write coupled equations for
the evolutions of the height of each of these phases, which prove similar in
both approaches. The BCRE description provides a very intuitive picture of the
flow, whereas the Saint-Venant hydrodynamic description establishes a general
and rigorous framework for granular flow studies.
