Avalanche experiments on an erodible substrate are treated in the framework
of ``partial fluidization'' model of dense granular flows. The model identifies
a family of propagating soliton-like avalanches with shape and velocity
controlled by the inclination angle and the depth of substrate. At high
inclination angles the solitons display a transverse instability, followed by
coarsening and fingering similar to recent experimental observation. A primary
cause for the transverse instability is directly related to the dependence of
soliton velocity on the granular mass trapped in the avalanche.Comment: 3 figures, 4 pages, submitted to Phys Rev Let