  We develop a continuum description of partially fluidized granular flows. Our
theory is based on the hydrodynamic equation for the flow coupled with the
order parameter equation which describes the transition between flowing and
static components of the granular system. This theory captures important
phenomenology recently observed in experiments with granular flows on rough
inclined planes (Daerr and Douady, Nature (London) v. 399, 241 (1999)): layer
bistability, and transition from triangular avalanches propagating downhill at
small inclination angles to balloon-shaped avalanches also propagating uphill
for larger angles.
