Sediment transport by wind: Toward a general model

Abstract

A general model of eolian sediment transport by both saltation and suspension is developed. Saltation is analyzed through calculations of single trajectories, given initial conditions of liftoff speed, angle, and spin. Profiles of particle concentration and mass flux, generated by assuming that all trajectories are identical, display distinct maxima at the top of the trajectory. By incorporating the realistic distribution of trajectories that arises from the stochastic nature of grain impacts with a granular bed, the model yields monotonic decreases in particle concentration and mass flux with height, in accordance with published empirical data. The analysis of suspension, as the balance between downward advective flux as a result of the settling of grains and their upward diffusive flux as a result of turbulence, results in concentration profiles that fall off as power laws with height, also in accordance with empirical data. A relation for reference-level concentrations is presented for both blowing-snow and dust profiles. The solutions for saltation and suspension are combined to yield mass-flux profiles for the entire range of grain sizes in the bed. The saltation layer emerges naturally as that region where the total flux of saltating grains exceeds that of suspended grains. The model points clearly to the need for further research on the stochastic grain-bed interactions, which control both the probability distribution of liftoff velocities in saltation and the production of fine particles in suspension.