The transformation between different modes of motion of particles in steady aeolian transport

Abstract

Aeolian sediment transport is an important morphodynamic process that drives geomorphological evolution, affects the climate system, and induces various natural disasters. The creep load involves both particles of a sufficiently large size that cannot be easily lifted to the upper air, as well as smaller particles that can transition between different modes of motion, although the contribution of the latter has rarely been investigated. In this study, we investigate the features of the saltation cycle, namely the time period from the moment a particle is entrained into the air to when the particle is again trapped in the bed, and the transformation mechanism between creep, reptation, and saltation by directly simulating the steady-state sand transport of particles within the saltation regime. The results indicate that a saltation cycle generally contains one to tens of individual hop processes, with the mean and standard deviation of the hop times decreasing with the increasing friction velocity, owing to the presence of more energetic particles at the bed surface. Over half of the transported particles move in creep or reptation model because a moving particle generally experiences frequent transition between creep, reptation, and saltation. Furthermore, the fundamental reason for the change in the mode of motion is the strong impact velocity dependence of the total coefficient of restitution during the rebound process. Current rebound models predict a reasonable coefficient of restitution only if the particle impacts the bed at a moderate velocity; however, their predictions deviate from the actual situation at relatively low or high impact velocities. These findings enhance our understanding of the motion dynamics of particles within wind-blown sand movements, and further reduce the uncertainty in predicting their mass flux in different modes of motion.