Size segregation during aeolian saltation on sand dunes

Recent Monte Carlo computer simulations have modelled the local, geometric mechanism by which mixed populations of differently sized particles order themselves when shaken, such that larger particles rise at the expense of the smaller. This process, called size segregation, is distinct from the mechanism of sifting where smaller particles fall between the interstices of larger particles. Size segregation is applied here to the sand grain populations found on the surfaces of dunes, which are involved in a sorting process driven by the wind. We present data from four barchan dunes located in the Eastern Province of Saudi Arabia. A simple model is used to demonstrate that size segregation, driven by saltation impacts, is energetically plausible at the wind velocities typically encountered on dune surfaces. Laboratory tests which support this model are also discussed.