Morphodynamics of pure elongating dunes: Implications for sediment flux in the Ténéré desert

Abstract:

Although they are a very common bedforms in terrestrial sand seas, description of linear dune growth, either by extension or lateral migration is still hindered by our limited understanding of the underlying mechanisms. Therefore, sand flux estimates in arid regions relies essentially on the migration speed of barchan dunes but not on the dynamics of linear dunes despite their abundance. Here we use half a century of high-resolution aerial and satellite imagery of the Ténéré desert, the world’s largest source of mineral aerosols, to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration. As they form away from topographic obstacles in a zone of low sediment availability experiencing multimodal winds, these elongating structures are ideal to isolate and quantify dune growth by pure extension. Using similar conditions in a numerical model, we show how deposition downstream of low hills may result in nucleation and development of bedforms. From pure elongation, we derive the local sand flux parallel to the linear dune crests. Together with sand flux estimates perpendicular to the barchan dune crests, this study indicates that two independent dune growth mechanisms can now be used for reconstructing sediment transport and wind conditions from dune morphodynamics.