Sand dunes migrate across landscapes by interacting with and repelling one another in what scientists have called "interesting and beautiful physics." These natural formations, which often cover thousands of square miles, can cause major problems for mankind, disrupting shipping channels and burying roads and buildings, so understanding how and where they are moving is hugely important.
Dune systems are formed in areas where there is a broad area of sand, either terrestrial or underwater. The movement of wind and water shifts the sand to create the dunes. But the exact mechanism driving their migration is debated.
Some suggest that dunes of different sizes will collide and grow until they form one enormous sand dune. Another suggestion is that they collide and exchange mass until they are all about the same size and move at the same speed. However, long term observations are lacking.
Now researchers from the University of Cambridge, U.K., have carried out laboratory experiments to show how dunes interact.
"Generally speaking, we were interested in sand dunes as a fundamental scientific problem," first author Karol Bacik told Newsweek in an email interview. "There are many open questions regarding their spontaneous formation, which contain interesting and beautiful physics. In this study we were specifically interested in the long-time evolution of desert landscapes."
In their study, published in Physical Review Letters, the researchers created a dune "racetrack" where they could observe two identical dunes migrating. The circular flume was filled with water and rotated, while high speed cameras tracked the movement of individual particles that made up the dunes. Their findings revealed an entirely new explanation for how the dunes move around, without exchanging mass.
The two dunes started out close together but over time they moved further apart. The swirls from the upstream dune appeared to repel the downstream dune. Because both dunes were the same height, the team thought they would move at the same speed, but this was not the case. The front dune initially moved faster before slowing down so they were both moving at the same speed. Once they reached a certain distance, they appeared to form an equilibrium on opposite sides of the racetrack.
"At first, the results were a big surprise," Bacik said. "However, when we understood their physical nature, we realised that our findings are consistent with previous experiments and field observations. Fingerprints of dune-dune interactions have been observed by other groups, but the resolution of their data did not allow to explain the underlying mechanism. With our unique circular geometry, we are able to acquire data from experiments at high resolution for the first time."
He said that large dunes in a desert evolve over decades or centuries. Their findings suggest that repulsion preventing the collisions of dunes may make the landscape more stable and robust, but more research will be needed to confirm this through satellite observations.
"Migrating dunes notoriously interact with the infrastructure by burying roads and buildings (on the ground), or filling and blocking shipping channels (underwater). In order to address these challenges, we need physical models which accurately predict the long-term evolution of sandy landscapes," Bacik said.
"We hope that our study will help to improve such models by incorporating turbulent fluctuations ("swirls in the flow.") Indeed, our study suggests that turbulence, which is often neglected, may be critically important for dune dynamics."
