High-speed motion pictures document a series of essentially two-dimensional, free-surface flows of 6-mm diameter plastic spheres generated in an inclined glass-walled chute 3.7 m long and 6.7 mm wide. Flows exhibit a diverse suite of structural features, depending on particle flux, chute inclination, and geometry of the fixed bed. Flows at sufficiently low inclination or high particle flux can generally be divided into frictional and collisional regions. In the frictional region, intergranular momentum transfer occurs by enduring, frictional contacts; kinetic theories of granular flow do not apply. The frictional region typically consists of a quasi-static zone extending up from the fixed bed and an overlying block-gliding zone, in which coherent blocks of grains move parallel to the bed. The collisional region overlies the contact region in sufficiently deep flows; otherwise it extends to the bed. In it, momentum transfer is dominated by collisions. It typically consists of three zones: a lower grain-layer-gliding zone, in which grains appear to slide over one another: a middle chaotic zone, in which grain motions are highly random, as in a dense gas; and an overlying saltational zone, in which grains trace long, curved paths. Zones grade smoothly into one another, and some are missing in certain flows. In none of the flows does a frictional region overlie a collisional region. Zone thicknesses and block dimensions are previously unrecognized length scales in granular flows.

The no-slip velocity boundary condition applies only when a frictional region adjoins a geometrically rough fixed bed, otherwise significant slip occurs (up to 50% of the mean flow velocity). Binary collisions dominate only in the collisional region and there perhaps only in the chaotic and saltational zones. Because many collision-dominated geophysical flows start from friction-dominated ones and end by reverting to the frictional regime as the energy supply diminishes, kinetic theories of granular flow, which unanimously employ the binary-collision assumption, can at best describe only a portion of the phenomena. Microstructural descriptions of the relatively slow, compact, and frictional regions evident in most geophysical flows are required to complement existing kinetic theories. ¿ American Geophysical Union 1990