Dune emergence: multidirectional wind regimes and boundary conditions

Abstract:

Dunes emerge from the erosion and deposition of grains carried by a fluid flow over a sedimentary bed. In turn, the flow adapts to the shape of the sedimentary bed. This coupling between topography, hydrodynamics and transport controls the dune size, orientation and morphology, particularly during the early stage of their growth. These processes occur under water or in aeolian conditions, and on Earth as well as on other planetary bodies such as Mars, Venus or Titan or even comets, as soon as a granuler bed is sheared by a flow. In this work, we highlight the influence of environmental parameters, such as wind regime, sand availability, and dune field boundary conditions on dune emergence. We use a theoretical approach, coupled with laboratory experiments, numerical simulations and field studies.

First, we validate the description of dune emergence in areas of high sediment availability as a linear instability. These dunes form as surface waves, whose amplitude increases temporally or spatially, depending on whether they form in the middle of the sedimentary bed, or at its upstream edge. Their orientation, wavelength and spatial/temporal growth rate are then mainly controlled by the angular distribution of sand fluxes.

On a non-erodible ground, dunes develop from various sources of sediments (other dunes, river and lakes deposits, etc.). Under specific wind regimes, they elongate to form linear ridges. We show that the spatial organization of these elongating dunes is then controlled by the distribution of sedimentary sources, and not by a length scale intrinsic to the elongation mechanism. Thus, they can form periodic dune fields at the downstream edge of sediment beds, or remain isolated structures in zones of localized deposition. In the latter case, they can reach a stable equilibrium state, whose morphology is controlled by the wind reorientation period.

These results can now be used on terrestrial but also extraterrestrial dune systems to interpret field or satellite observations, but also to infer information on winds or sedimentary material from dune pattern characteristics.