It’s well known that sand dunes move around and that smaller dunes move faster than larger ones, said Nathalie Vriend, the paper’s senior author from Cambridge University’s BP Institute for Multiphase Flow. But researchers assumed that a pair of identical dunes would move together at the same speed.
Instead, identical dunes that start close repel each other, moving farther away over time.
“They’re definitely communicating,” Vriend said in an interview with The Washington 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.”
Here’s what’s happening: When a water or wind flow hits a dune, a disturbance is created — almost like the wake created by a boat — and that disturbance gives the neighboring dune a little push, causing that dune to accelerate.
As the flow continues, that same movement also continues until the downstream dune is far enough away not to be affected by the wake.
“Similar to if you’re far enough from the boat, you wouldn’t feel the wake as much,” Vriend said.
A small amount of sand may be exchanged as the dunes move. But that’s a consequence of the movement, and it’s the energy from the wind or water flow creating the disturbance.
In the lab, Vriend said Karol Bacik, the report’s lead author from Cambridge University’s GK Batchelor Lab, plopped sand heaps into a rotating tank with water. The experiment was meant to mirror what happens in nature, but much faster.
She described field work in the desert in Qatar, where an approximately 16-foot high dune may move at the speed of about 65 feet per year, or a 98-foot high dune moves at 16 feet per year. By comparison, the sand heaps in the lab moved at about 65 feet per hour.
“If you are using satellite images to look at these interactions, it would take a long time to see these interactions happening. That’s why we decided to bring it into the lab and put it in fluid,” she said.
On a warming planet, understanding how sand dunes move could become even more important for understanding their impact on dry climates.
Vriend said climate-fueled desertification and an increased prevalence of sand dunes can harm roads and infrastructure. She recalled reading about creeping sand dunes in Nouakchott, Mauritania, where “parts of the city were engulfed by sand dunes — that’s impacting people directly, destroying their livelihoods."
“I’m a physicist, a geoscientist, but I’m trying to contribute to these types of big questions,” Vriend said.