Bedload transport of mixed-size sediments by wind

Natural sediments contain a range of particle sizes. The physics of bedload transport of natural sediment, therefore, is complicated by variability in response to the fluid of the different grain-size fractions in motion, and by the nature of the collisions between moving grains and particles of disparate mass composing the sediment bed. This work presents a theoretical model capable of predicting the mechanics of motion of poorly sorted sediments in the bedload layer and the evolution of particle-scale topography of the sediment surface. The model consists of four components that are coupled to predict the bedload-transport rates of grain-size fractions within a mixture. They are: (1) a computer-generated sediment bed that reproduces the grain size-frequency distribution and fully characterizes the physical properties of both the bed surface and subsurface; (2) a saltation component that describes the ballistic trajectories of moving grains; (3) a collisional algorithm that treats the behavior of grain-grain interactions at the bed in the presence of fluid forces on the grains, by tracking the position of each moving grain relative to the sediment surface, such that the exact location and geometric configuration of the collision site is known; and, (4) a fluid-mechanical element that predicts the extraction of momentum from the wind by each grain-size fraction and by the entire saltating mass, as a function of position in the flow. The model can be used to predict bedload-discharge rates for a large range of flow conditions, sediment properties, and grain-size frequency distributions, and, thereby, permits reliable computation of sediment fluxes beyond the range of available experimental data.; Bedload flux of the sediment mixture is determined by calculating the product of particle velocity times grain concentration for each size fraction and integrating over the height of the bedload layer to yield volume discharges of each fraction; the total bedload flux is the sum of the fluxes for each grain-size fraction. Model predictions compare well with measurements made by Yamasaka and Ikeda (1988) for the same grain-size frequency distribution. A bedload-transport equation, derived from regressions on model-predicted, fractional transport rates, is presented for use in calculating size-by-size and bulk sediment fluxes.; Model predictions of grain saltation parameters, including particle lift-off speeds and angles, agree with empirical data on particle motion in granular mixtures. The model quantifies some of the experimental observations of mixed-size sediment transport that have been discussed qualitatively in the literature, including modifications of grain-collision behavior with variations in surface sorting, and it lays the foundation for examination of the origin and characteristics of eolian sedimentary structures. It also predicts significant variations in transport rates and collision histories between homogeneous and inhomogeneous sediment mixtures, particularly at low transport rates. The demonstrated sensitivity of grain-bed and grain-fluid interactions to the size-frequency distribution of the sediment mixture emphasizes the importance of accurately accounting for grain-size variations in numerical models of processes involving heterogeneous sediments.