Sand dunes play a team game
That’s why they don’t collide.
By Mark Bruer
Sand dunes are able to “communicate” with each other to avoid colliding, according to new research.
It may not be communication quite as we understand it: they aren’t exactly whispering traffic instructions across the desert’s shifting sands. But it seems the air or water currents created by one dune will control the movement of another to ensure that both have enough room to move.
This finding will help us to understand long-term dune migration both on land and underwater, which threatens shipping channels, increases desertification, and can bury infrastructure such as highways.
As sometimes happens in science, researchers stumbled across this unexpected behaviour while looking for something else.
Desert, river and seabed sand dunes rarely occur in isolation, but usually form vast dune fields. The large-scale dynamics of these fields are poorly understood, largely due to the lack of long-time observations.
For example, the popular notion that large dunes are formed by smaller ones colliding has never actually been observed in nature.
So scientists at the University of Cambridge, Britain, built a device to study how dunes develop and move over an extended period.
Basically, it is a motorised, circular channel containing water which goes round and round. Into this the researchers put a pile of tiny glass beads to simulate sand so that they could measure how quickly the beads formed into a dune, and then study how the dune moved.
The team decided to create two dunes in the channel only to speed up their data gathering. And that’s when they got a surprise. The dunes interacted.
The two dunes started with the same volume and in the same shape. As the flow began to move across the two dunes, they started moving.
Research leader Nathalie Vriend says that because the speed at which a dune moves is known to be related to its height, the team expected the two dunes to move at the same speed.
But that isn’t what happened. Initially, the front dune moved faster than the back dune, and as the experiment continued, the front dune began to slow down, until the two dunes settled on exact opposite sides of the circle. From then on, they moved at almost the same speed.
In other words, the dunes had paced themselves to maximise separation and avoid running into each other.
Crucially, the pattern of flow across the two dunes was observed to be different: the flow is deflected by the front dune, generating “swirls” on the back dune and pushing it away.
"The front dune generates the turbulence pattern which we see on the back dune," says Vriend. "The flow structure behind the front dune is like a wake behind a boat, and affects the properties of the next dune."
"We've discovered physics that hasn't been part of the model before.”
The next step for the research is to work out how this new understanding of dune interactions might provide effective ways to divert large-scale dune migration.
The findings are reported in the journal Physical Review Letters.
CREDIT: University of Cambridge