Granular flows driven by gravity

Two-phase fluids, consisting of solid particles in water or in the air, are present in many fields of hydraulics and specifically in the morpho-hydrodynamics of rivers and mountain streams. In particular, when these mixtures contain a high concentration of sediments, as in debris flows or in snow avalanches, they must be treated with the methods of granular fluid mechanics. Most recent theories of granular fluid mechanics, applied to the two-phase flows driven by gravity, are discussed first. Then some of the many unsolved issues on which presumably the research effort will focus in the forthcoming years will be identified and outlined.