Formation of aeolian ripples and sand sorting
  We present a continuous model capable of demonstrating some salient features of aeolian sand ripples: the realistic asymmetric ripple shape, coarsening of ripple field at the nonlinear stage of ripple growth, saturation of ripple growth for homogeneous sand, typical size segregation of sand and formation of armoring layers of coarse particles on ripple crests and windward slopes if sand is inhomogeneous. 
Edgar Manukyan and Leonid Prigozhin