SEDIMENT TRANSPORT BY WIND: SALTATION, SUSPENSION, EROSION AND RIPPLES

A general model for sediment transport by wind is developed and tested against available data sets on the vertical variation of sediment concentration, mass flux, wind velocity and erosion by windblown particles. The full spectrum of particle behavior, from saltation through suspension is addressed.; Saltation is treated through calculation of single trajectories that are fully determined by the initial conditions of liftoff speed, angle and spin. By specifying the ejection rate of grains from the bed and the broad distribution of liftoff velocities that arise from the stochastic nature of grain impacts with a granular bed, vertical profiles of mass flux can be calculated and are in accord with measurements.; An existing diffusion model of suspension is calibrated with available blowing snow and dust profiles using reference level concentrations. The suspension phenomenon is further explored to define more explicitly the coupling of the saltation and suspension processes at the bed. Individual suspension trajectories are calculated by incorporating turbulent fluctuations using a modified Langevin equation. Ensembles of trajectories are summed to yield concentration profiles that agree well with expected power law behavior.; The horizontal force on the wind due to the downwind acceleration of particles acts to decrease wind velocity gradients near the ground. Force profiles are calculated using ejection rates and initial velocity distributions calibrated with empirical mass flux data. The resulting reduction in stress available to shear the air, when coupled with an eddy viscosity closure hypothesis, yields modified wind velocity profiles that capture well the near-bed reduction in velocity gradients.; Erosion by wind-blown particles is shown to result from the flux of kinetic energy to an obstacle surface by grains entrained by the wind. Kinetic energy fluxes due to both saltating and suspended grains peak well above the ground in strong winds, in accord with observed abrasion profiles for vertical cylindrical obstacles and for ventifacts. Abrasion due to suspended particles should dominate well above the ground, and should sensitively record the deflection of air flow around the obstacle.; New insights into the grain-bed impact process arising from experiments involving single grain impacts lead to a simple model of eolian impact ripples. A stability analysis demonstrates that a flat bed is unstable, and that the fastest-growing undulations have a wavelength an order of magnitude greater than the mean hop length of the numerous grains mobilized by energetic saltation impacts.