Field evidence for surface-wave-induced instability of sand dunes

The dune thing Barchans are crescent-shaped dunes that move faster over desert surfaces than most other dune types. A three-year field study shows how they do it. Collisions and changes in wind direction destabilize the dunes, generating surface waves that form new, smaller barchans. This prevents the formation of a single giant dune, so plays a fundamental role in the development of dune patterns. Field studies of barchans—crescent-shaped dunes that propagate over solid ground under conditions of unidirectional wind 1 —have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography 2 , 3 , 4 , which are thought to control dune morphology and kinematics 5 , 6 , 7 . Because of the long timescale involved, however, the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood 8 . Here we combine data from a three-year field study in the Moroccan Sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves, as suggested previously 9 , 10 , 11 , 12 . We find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans. Because the resulting surface waves propagate at a higher speed than the dunes themselves, they can produce a series of new barchans of elementary size by breaking the horns of large dunes. The creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns.