  In the framework of the theory of partially fluidized granular flows we study
the formation of longitudinal structures observed experimentally by Forterre
and Pouliquen in a flow down a rough inclined plane. We show that the formation
of longitudinal structures is related to the positive feedback between the
fluidization rate and the lateral stress (side pressure), which leads to a
convective instability. Our theory explains main experimental features, such as
appearance and amplification of the structure at some distance from the outlet
and non-stationary behavior of the structures.
