Sand dunes are constantly moving. Grains of sand shifting from windward to leeward side, and thus gradually the whole dune wanders. Smaller dunes are faster, as less grains need to move. Still, it is very seldom that a small dune catches up with a large one—owing to the newly-discovered drift mechanism.
A team led by the physicist Karol Bacik at the University of Cambridge discovered the origins of this effect and published their findings in February 2020.
During the experiments, two artificial dunes made up of tiny glass pellets moved in a circle within a ringlike groove filled with water (look up “dune tunnel”). The current was produced by rotating paddles on the surface as well as the counter-rotating groove.
The researchers determined that the upstream dune was moving in a circle at a constant pace, while the one in its wind shadow first increases the distance swiftly, and then gradually adjusts its tempo to match that of the first dune until they are located at the opposite sides of the groove.
This behavior is caused by the swirls on the upstream dune reaching all the way to the dune behind it and displacing numerous grains of sand at its summit. These are pulled into the water current, float with it for a short while, then settle down on the leeward side of the dune. In this way, the swirls on the upstream dune promote the sand transport of the downstream one. The reach of those turbulences is limited, reduced by the increasing distance, until a specific spacing is achieved. This is how dunes of different forms stabilize each other. If at the beginning a small and a large dune are situated next to each other, the first, faster dune would sooner or later catch up with the slower one. However, as they get closer, the effect of swirls from the smaller dune on the big one increases, accelerating the sand transport—ever more as the gap closes. As long as the size difference is contained within a limit, the pair of dunes falls into a pattern of equal, constant speed.
Similar correlations may be applied to wind-driven dunes moving over land.
Source: Spektrum
