The speed at which a dune migrates depends on its size; smaller dunes move faster than larger ones. That speed differential implies that small dunes should frequently collide into and merge with larger dunes, eventually forming one giant dune rather than a field of smaller separate ones. But that’s not what we observe in nature.
To figure out why dunes aren’t colliding that often, researchers built a dune field of their own in the form of a rotating water tank. Inside the tank, their two artificial dunes can chase one another indefinitely while the researchers observe their interactions. What they found is that the dunes “communicate” with one another through the flow.
As flow moves over the upstream dune, it generates turbulence in its wake, which the downstream dune then encounters. All that extra turbulence affects how sediment is picked up and transported for the downstream dune, ultimately changing its migration speed. For two dunes of initially equal size and close spacing, these interactions push the downstream dune further away until the separation between the dunes is large enough that they both migrate at the same speed. Even between dunes of unequal sizes, the researchers found that these repulsive interactions force the dunes away from collision and into migration at the same speed. (Image credit: dune field – G. Montani, others – K. Bacik et al.; research credit: K. Bacik et al.; via Cosmos; submitted by Kam-Yung Soh)