Transverse instability of avalanches in granular flows down an incline

Avalanche experiments on an erodible substrate are analyzed using the "partial fluidization" model of dense granular flows. The model identifies a family of propagating solitonlike avalanches with shape and velocity controlled by the inclination angle and the depth of the 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 the soliton velocity on the granular mass trapped in the avalanche.