TWO MODES FOR DUNE ORIENTATION

Most Earth sand deserts experience multidirectional winds that shape long, regularly spaced and parallel linear dunes. These dunes often exhibit complex and compound patterns with different length scales and orientations, which seem difficult to relate to a single wind cycle. This complexity is commonly ascribed to changes in wind regimes since many dunes date back to Last Glacial Maximum. We present results of underwater experiments where a single wind regime can lead to two different dune orientation depending on sediment availability. Sediment availability selects the overriding mechanism for the formation of dunes: increasing in height from the destabilization of a sand bed or elongating in a finger on a nonerodible ground from a localized sand source. These mechanisms drive the dune orientation. Therefore, dune alignment maximizes dunes orthogonality to sand fluxes (or gross bedform-normal transport) in the bed instability mode, while dunes are aligned with the sand transport direction in the fingering mode. The corresponding orientations can almost be orthogonal to each other so that a dune field can rapidly switch its orientation in reaction to a change in sand supply. Then, we derive a model for dune orientation, which explains the coexistence of bedforms with different alignments, and the spreading of sand seas through arid regions. Applied to Earth deserts, the model quantitatively predicts both the orientation of rectilinear dunes, and of their superimposed patterns in agreement with the wind data of the last decades. These results suggest that most linear dunes on Earth sand seas are elongating and simply aligned with the integrated sand transport direction.