Sand Dunes ‘Communicate’ as They Migrate
Wind or water flow deflecting off one dune will shove its neighbor away
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Sand dunes are among nature’s loveliest wonders, forming undulating mounds in deserts, on beaches, and even underwater. Dunes are also known to migrate—and now a new study suggests that they “communicate” as they move.
Of course, as inanimate objects, the dunes are not speaking; the new research, published in the journal Physical Review Letters, focuses on how the mounds behave as they get whipped about by wind and water flow. Dunes rarely occur on their own, but instead “form striking collectives known as dune fields or dune corridors,” the study authors write. These clusters move downstream with wind and water flow, and smaller dunes are known to move faster than larger ones. But how—or even if—the dunes interact with one another has been the subject of debate.
“There are different theories,” explains Karol Bacik, a PhD candidate at Cambridge University and first author of the paper. “[O]ne is that dunes of different sizes will collide, and keep colliding, until they form one giant dune, although this phenomenon has not yet been observed in nature.” Another theory posits that dunes collide and exchange mass, “sort of like billiard balls bouncing off one another—until they are the same size and move at the same speed.”
Led by Nathalie Vriend of Cambridge’s BP Institute for Multiphase Flow, the researchers constructed a water-filled “racetrack,” or circular flume, that rotated two piles of sand—mini dunes, if you will. The experiment was meant to simulate what happens in nature, but at a much quicker speed. In Qatar, as an example, a 16-foot dune might move at a speed of around 65 feet per year, Vriend tells Paulina Firozi of the Washington Post; the mounds in the lab moved 65 feet per hour.
The little dunes, though they were the same volume and shape, did not move together at the same rate, which surprised the researchers, since “the speed of a dune is related to its height,” Vriend explains. Instead, the mounds moved away from one another, “as though they didn't like each other,” Bacik tells CNN’s David Williams. Eventually, once the mounds had reached the opposite sides of the flume, they formed an equilibrium.
This unexpected phenomenon seemed to be caused by the pattern of flow across the dunes. As the flow was deflected off the front dune, it generated swirls of turbulence—similar to what you would see in the wake of a boat—that pushed the downstream dune away and caused it to accelerate. The movement continued until the second dune was far enough from the turbulence to stop feeling its effects.
Understanding how dunes migrate over time is particularly important as the Earth’s deserts expand due to climate change. Large masses of sand could threaten coastlines, infrastructure and buildings, the researchers say, and being able to predict dunes’ movement could help reduce some of the damage. But in a more immediate sense, the experiment showed how dunes convey their need for space, without making a sound.
“They’re definitely communicating,” Vriend tells the Post. “If I give my neighbor in front of me a push, it’s something I do. But we’re not talking about humans with brains, we’re talking about sand dunes that communicate—inanimate objects communicating information.”