This paper presents results from one hundred and twenty samples which were collected from sand ripples in the Algerian Sahara. Ripples were found that are transitional between small aeolian ripples (wavelength 1 cm-20 cm) formed in fine unimodal sand, and larger ripples (wave length 20 cm-20 m) formed in coarse bimodal sand; these are commonly distinguished as two distinct types. The formation of the first type is usually explained by a grain impact mechanism in which the ripple wavelength corresponds to the mean path length of the saltating sand grains. A mathematical model is described which simulates the saltation mechanism of bimodal sand mixtures; the spread of grain impact velocities is taken into account by means of an empirically determined rebound probability matrix. Results of computations using this model are presented which demonstrate that the mechanism of saltation can also account for the largest ripples. These results which explain the presence of the transitional ripples, are attributed to the improved rebound of the fine fraction off the coarse fraction in bimodal sand.

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