A theoretical model for aeolian polydisperse-sand ripples

A continuum model is constructed to investigate the effects of sediment heterogeneity on the formation of aeolian sand ripples and the grain size sorting over the bed forms. The evolutions of small perturbations of both bed elevation and bed composition are studied analytically by a linear stability approach. In this model the transport of polydisperse sediment is considered as the integrated processes of saltation, reptation caused by impact-driven and creep caused by gravity-driven and drag-driven. Theoretical analysis indicates that size-selective sand transport leads to grain sorting phenomena, which appreciably affect the dynamics of the formation of aeolian sand ripples. The sediment heterogeneity is beneficial to the stability of sand bed, and that the wavelength is lengthened compared with the case of uniform sand. Longer ripples appear when the sediment heterogeneity is characterized by a poorly sorted. On the other hand, the relative volume fraction of the different grain size in the sediment heterogeneity appreciably affects the characteristics of sand ripples. Moreover, it is found that the theoretical model successfully reproduces the observed tendency of coarser grains to accumulate towards the crests of sand ripples, while finer grains accumulating towards the troughs. Lastly, a physical interpretation of the theoretical model results which qualitatively agree with field observations and wind tunnel experiments is given. (C) 2019 The Authors. Published by Elsevier B.V.