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The birth of dunes under a bi-directional wind regime

Most terrestrial sandy seas form at subtropical latitudes, where wind direction exhibits seasonal variations. In this work, we extend the two-dimensional linear stability analysis of a sand flat bed, associated with a unidirectional wind, to the three-dimensional case in order to take into account the variability of winds, in terms of strength and direction. It has been observed (experimentally, numerically and in the field) that the orientation of the dunes depends on the wind regime.

The birth of dunes under a bi-directional wind regime

Publication date: 12/02/2019

Press, Research

Related teams :
Geological Fluid Dynamics

Related themes : Earth System Science

Focusing on the simple case of bidirectional flow regimes, we explicitly show how this orientation is controlled by the angle between the two wind directions, as well as by the transport ratio between them. In particular, we find the transition from patterns perpendicular to the flow to patterns aligned with it when the angle between the two winds exceeds 90°. This analysis also predicts the initial wavelength of the pattern, which decreases near the transition angle due to a geometric effect. For constant wind directions, this initial length increases when the wind speed is close to the transport threshold.

Dune patterns for multidirectional wind regimes in (a) the Namib Desert, Angola (16°27'S 11°58'E), and (b) the Taklamacan Desert, China (37°25'N 81°50'E). The flux roses are calculated using wind data from the ERAInterim project. Single arrow: direction of the resulting flow. Double arrow: prediction of the initial orientation of the dunes using linear stability analysis. © DigitalGlobe

This theoretical analysis is complemented by similar underwater experiments, where the sediment bed is subjected to water flows changing direction over time. For angles smaller than 90°, the experimental data validates the model, providing evidence of the predicted geometric effect, but also of the increase in wavelength close to the transport threshold. For angles greater than 90°, we observe that the wavelength of the dunes is systematically lower than that predicted. This is interpreted as a sign of the increased non-linearity induced by the direct contact of the flow with the avalanche phases of the dunes when the flow blows alternately from both sides of the dune.

Dune patterns forming from a flat bed of sediment in underwater experiments, from a bi-directional wind regime. The angle between the winds is (a) 30°, forming dunes transverse to the resultant flow, and (b) 150°, forming dunes aligned with the resultant flow. © Sylvain Courrech du Pont and Cyril Gadal

Ref : C. Gadal, C. Narteau, S. Courrech du Pont, O. Rozier, P. Claudin, Incipient bedforms in a bidirectional wind regime, Journal of Fluid Mechanics, 862, 490-516, DOI : 10.1017/jfm.2018.978

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