  The surface shape of a spinning bucket of granular material is studied using
a continuum model of surface flow developed by Bouchaud et al. and Mehta et al.
An experimentally observed central subcritical region is reproduced by the
model. The subcritical region occurs when a metastable surface becomes unstable
via a nonlinear instability mechanism. The nonlinear instability mechanism
destabilizes the surface in large systems while a linear instability mechanism
is relevant for smaller systems. The range of angles in which the granular
surface is metastable vanishes with increasing system size.
