Two Dune Growth Mechanisms in a Landscape Scale Experiment

By leveling neighboring parcels of a dune field in a landscape-scale experiment at the edge of the Gobi desert, we show that various dune types develop simultaneously under natural wind conditions. Using 4 years of high-resolution topographic data, we demonstrate that, depending on sand availability, the same wind regime can lead to two different dune orientations, which reflect two independent dune growth mechanisms. In the flat sand bed experiment, as periodic oblique dunes emerge and develop to 2 meters in height, we analyze the mechanism of dune-size selection that finally leads to the prevalence of a 15 m-wavelength pattern. We quantitatively compare our experimental results to the prediction of the dune instability theory using transport and flow parameters independently measured in the field (Lü et al., 2021). The remarkable agreement between theory and observations demonstrates that the linear regime of dune growth is permanently expressed on low-amplitude sand bed topography. To analyze at the same time dune growth in areas with limited sand availability, we start another experiment with conical sand heaps deposited on gravels. We observe the transition from dome to barchan and asymmetric barchan shapes. We identify a minimum size for arm elongation and evaluate the contribution of wind reversals to its longitudinal alignment. Thus, the elongation of a longitudinal linear dune from a fixed source of sand placed on a nonerodible bed is synchronous with the emergence and the coarsening of periodic oblique dunes in an area of unlimited sediment supply (Lü et al., 2022). This shows that two independent dune growth mechanisms coexist and regulate dune shape, orientation and dynamics according to the boundary conditions and the nature of the bed (i.e., an erodible sand bed or a nonerodible ground). These experimental field observations support existing theoretical models of dune dynamics boosting confidence in their applicability for quantitative predictions of dune evolution under various wind regimes and bed conditions.

References:Lü et al., Direct validation of dune instability theory, Proceedings of the National Academy of Sciences USA, 18 , e2024105118 (2021).Lü et al., Coexistence of two dune growth mechanisms in a landscape-scale experiment, Geophysical Research Letters, 49, e2021GL097636 (2022).