Sand dunes form under the pressure of wind or water and move downstream when the direction is regular. We know surprisingly little about this process, however, considering how important it can be. Not only that, but a new study reveals some of what we thought we knew is wrong.
Observations of single sand dunes have revealed larger dunes move more slowly. However, it's been an open question whether nearby dunes interact with each other, something that would affect our models of how dunes move. Such movement can pose a threat to villages or farmland on the edge of sandy deserts, or to shipping channels crossed by strong currents. Tidal and wave energy generators may face similar dangers from moving underwater dunes.
Physicists contemplating this problem have imagined smaller sand dunes catching up with larger ones and combining to form a single dune. This has never been observed, however, and others have suggested that when dunes collide, they bounce off each other, perhaps transferring some of their sand in the process, explaining why we have never seen dunes as large as the first scenario would imply.
However, when modeling sand dunes in a water-filled flow tank, Cambridge University PhD student Karol Bacik found something unexpected. The front dune created a turbulent wake, including swirls that pushed the back dune away from it. He reports in Physical Review Letters that when two dunes were initially the same size, the downstream one would move faster, expanding the distance between them. The lead dune needed to be 2.5 times the mass of its follower for their speeds to match. Although the experiment was done underwater, Bacik expects wind-driven dunes will behave similarly.
Previous dune tracks were too short to allow multiple dunes to be observed over long periods of time, but Bacik and colleagues built a circular track and watched the dunes circle it for hours. The gap between the dunes grew until it stabilized with the two 180-degrees apart. In the wider world, where dunes move more or less in a straight line, no such balance would occur, and the gap would be expected to expand until other forces intervene. This could explain why large dune fields are usually so regularly spaced, barring sandworm interference.
Exactly what this means for trying to fight dune migration remains to be seen, but it is likely some modification to existing strategies will be required.
Bacik was not initially interested in the interactions between dunes. "Originally, I put multiple dunes in the tank just to speed up data collection, but we didn't expect to see how they started to interact with each other," he said in a statement.