Avalanches of granular materials

Our starting point is the description of surface flows constructed by Bouchaud et al. In the present text, we show a) how their equations are derived b) how they can be applied to typical examples : filling of silos, rotating drums (in an idealized form), and various transient regimes. Finally, we discuss their view, where the difference between critical angles for inset (theta(m)) and for repose (theta(r)) is interpreted in terms of a spinodal instability. We present an alternative view, based on a nucleation process.