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

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(2-4), which are thought to control dune morphology and kinematics(5-7). 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(9-12). 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.