Mechanics of bedload transport in the saltation and sheetflow regimes

Bedload transport over a mobile plane bed occurs by saltation at low to medium shear stress and by sheetflow at high shear stress. Saltation is broadly defined as the sliding, rolling, and jumping of grains over the bed, whereas sheetflow refers to the motion of the entire bed as a carpet or sheet of colliding grains.; A general bedload transport model is developed to describe the mechanics of bedload transport in both the saltation and sheetflow regimes. This model is based on the investigation of two important coefficients in Bagnold's [1966] energy equation: the dynamic friction coefficient tan α and the efficiency coefficient e_b. Tan α is a measure of the rate at which tangential shear stress T is converted to normal dispersive stress P, and e_b reflects the efficiency with which flow velocity averaged over entire flow depth is converted to grain velocity within the bedload layer. Their functional relations are obtained from a statistical analysis of two independent samples of experiments covering a wide range of hydraulic and sediment conditions. A functional relation for a third coefficient i_b/ω, where i_b is immersed bedload transport rate and ω is flow power, is derived using Bagnold's [ 1966] energy equation and the functional relations for tan α and e_b. These three functional relations constitute a general bedload transport model that describes different aspects of bedload transport in both the saltation and the sheetflow regimes. More specifically, the relations for tan α, e_b, and i_b/ω reflect the change in the relative frequency of grain-to-grain collisions and grain-to-bed collisions as dimensionless shear stress &thetas; changes.; High-speed video reveals that the mode of bedload transport in the sheetflow regime is fundamentally different from that in the saltation regime. The boundary between these two modes of bedload transport is derived theoretically. This theoretical criterion is then corroborated by a statistical analysis. High-speed video also reveals the internal structure of the bedload layer in the saltation and sheetflow regimes. These internal structures are quite different and help to explain why resistance to flow imparted by the upper-regime plane bed is the less than that by low-regime plane bed even though rates of bedload transport are higher.; Two bedload transport equations are developed using shear stress and flow power as the measures of flow intensity that drive the bedload downstream. Although the two equations, which are algebraically equivalent, give identical predications of the bedload transport rate, the simplicity of the flow power equation implies that flow power rather than shear stress is the fundamental variable determining the bedload transport rate.