  Inelastic grains in a flow under gravitation tend to collapse into states in
which the relative normal velocities of two neighboring grains is zero. If the
time scale for this gravitational collapse is shorter than inverse strain rates
in the flow, we propose that this collapse will lead to the formation of
``granular eddies", large scale condensed structures of particles moving
coherently with one another. The scale of these eddies is determined by the
gradient of the strain rate. Applying these concepts to chute flow of granular
media, (gravitationally driven flow down inclined planes) we predict the
existence of a bulk flow region whose rheology is determined only by flow
density. This theory yields the experimental ``Pouliquen flow rule",
correlating different chute flows; it also correctly accounts for the different
flow regimes observed.
