The effect of periodic changes in wind direction on the deformation and morphology of isolated sand dunes based on flume experiments and field data from the Western Sahara

Sand dunes are shaped by the effect of wind on sand particles; consequently, their morphologies are strongly influenced by complex seasonal variations in wind direction. To improve our understanding of these effects we conducted flume experiments that generated periodically bidirectional and oblique flows, and found that dune topographies were controlled by two elements under these conditions. The most important factor was the angle between the two flow directions, which determined both the nature of change in the crest line following the change in flow direction, and the resultant topography. The second factor was the event duration ratio of the two flows, which influenced the curvature of the crest line. A new phase diagram of the morphological development of isolated dunes under bidirectional flow conditions is proposed, driven by differences in flow angle and duration. This diagram is in good agreement with data from a natural dune field in the Western Sahara where the wind regime is well understood.