Emergence of a Barchan Belt in a Unidirectional Flow: Experiment and   Numerical Simulation
  We observed time evolution of dune fields in a water tank experiment and simulated it by using a simple model without taking complex fluid dynamics into account. The initial sand bed changed its form into transverse ripples, that is, dunes with straight crest lines perpendicular to the flow direction. Then the crescentic shaped dunes called barchans emerged from transverse ripples. 
Atsunari Katsuki, Macoto Kikuchi, Noritaka Endo