  Velocity fluctuations of grains flowing down a rough inclined plane are
experimentally studied. The grains at the free surface exhibit fluctuating
motions, which are correlated over few grains diameters. The characteristic
correlation length is shown to depend on the inclination of the plane and not
on the thickness of the flowing layer. This result strongly supports the idea
that dense granular flows are controlled by a characteristic length larger than
the particle diameter.
