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Elk graze in Great Sand Dunes National Park and Preserve. (Credit: National Park Service)

Last summer, Stephanie McNamara got her first glimpse of in southern Colorado. The park is a monument to sand, where dunes stretch across 30 square miles and tower nearly 750 feet high, making them the tallest such formations in North America.

McNamara, a graduate student in geophysics at �鶹��Ѱ�����Boulder, had recently joined a project studying features like these. But it was one thing to read about dunes and another to experience them in person.

“We have what we call saltating grains, which are small sand grains that skip across the surface of the dune. You could feel those on your legs, then, all of a sudden, they’re in your mouth,” McNamara said. “Unless you’re in a dune field, it’s hard to think about everything that’s blowing around.”

The graduate student is part of a research effort led by Nathalie Vriend, associate professor at the Paul M. Rady Department of Mechanical Engineering at �鶹��Ѱ�����Boulder. Among other questions, the lab explores how dunes evolve over time—shifting and surging across the landscape through processes like those that were scraping McNamara’s legs.

The sci-fi classic book Dune, and its recent film adaptations, takes place mostly on the fictional world of Arrakis—a desert planet where sand stretches endlessly in every direction. But dunes abound even on a water world like Earth. Sand dunes occupy an estimated 5% of our planet’s land area. They sit on six continents and come in all shapes and sizes. The most common dunes, barchans, are shaped like crescent moons, while others look like stars or castle walls. Some dunes even sing.

Just like the dunes in the national park, these formations are relentlessly, sometimes stubbornly, alive.

Understanding exactly how dunes move has become an urgent pursuit, Vriend said. The world is growing hotter and dryer, and deserts across the globe are spreading, swallowing homes, farmland and even entire villages.

Vriend’s research group uses a variety of tools, including ground-penetrating radar and computer simulations, to reveal the physics and mathematics that govern dunes. The team ultimately wants to answer a pressing question: Can humans efficiently shift or even halt the flow of the planet’s largest dunes?

“Here in the U.S., we encountered this problem 100 years ago when we had the Dust Bowl, when dust and sand was ravaging through farms and completely destroying harvests,” Vriend said. “We’re still facing this issue today, and, as a result, we need to understand how sediment moves, how it transports.”

Stephanie McNamara at Great Sand Dunes National Park and Preserve. (Credit: Stephanie McNamara)

Desert songs

Vriend had her own moment of discovery early during her time as a doctoral student at the California Institute of Technology in Pasadena. As part of a research trip, the mechanical engineer hiked to the top of one of the Mojave Desert’s famous singing dunes. These dunes create an eerie humming noise that can echo for miles in every direction.

“I remember going down this dune, and this sound started to boom out on the desert floor, and my whole body started to vibrate,” said Vriend, who came to �鶹��Ѱ�����Boulder in 2022. “Being a tiny part in a big, big landscape like a dune landscape is very humbling.”

Years later, Vriend and her advisor at Caltech, Melany Hunt, helped uncover what makes singing dunes sing—it has to do with how small avalanches of sand vibrate as they slide down the face of a dune.

Vriend and her former graduate student, Karol Bacik, summarized the state of science on the dynamics of dunes in a paper in the Annual Review of Fluid Mechanics.

She noted that the physics that underlie large dunes is surprisingly complex.

“If [sand] rests on your hand, it forms a little heap, and it’s solid. Then you pour it out, and it acts like a fluid and flows,” Vriend said. “If you throw it up in the air, and the wind is blowing, it flies away like a gas.”

Which all makes tracking the flow of dunes a tricky undertaking. Depending on their size and the prevailing winds, dunes can move anywhere from a few feet to dozens of feet every year.

In a , Vriend and her colleagues set out to discover what might happen when the winds blowing on a dune suddenly reverse direction. She led the experiment as a Royal Society University Research Fellow at the University of Cambridge in the United Kingdom.

To simulate the complicated dynamics of a dune field, the team turned to a massive apparatus called a circular flume. The machine is, essentially, a ring of water that measures more than 6 feet across and spins like a turntable. The spinning flume creates underwater currents that wash over piles of sand, not unlike winds blowing in the desert.

The researchers discovered that nearly two-thirds of the dune never moves at all during such a reversal. Instead, only the top layer of sand seems to shift, flowing from one direction to the other—a bit like a baker icing, then re-icing a cake.

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Grains of sand

Winds blowing over a dune can move grains of sand in three main ways. What path these grains take depends on their size, although the exact size in each category can change from site to site, said Stephanie McNamara, graduate student in geophysics at �鶹��Ѱ�����Boulder.

Credit: Nathalie Vriend

Creep
The largest grains roll and bounce over the face of a dune—a process known as creep.

Saltation
Medium-sized grains undergo a phenomenon called saltation. They jump into the air then land back down, knocking into more grains in the process, somewhat like a game of billiards.

Suspension
Then there are the smallest grains. Winds suspend these particles high in the air like a gas. “They can even leave the dune field,” McNamara said.

Spreading sand

For millions of people around the world, these dynamics aren’t just the stuff of scientific papers or sci-fi blockbusters. Rather, they can become a matter of survival, McNamara said.

In the United States, for example, sand dunes around Lake Michigan , roads and other infrastructure. In Mauritania, sand from the Sahara Desert has and is threatening the nation’s capital of Nouakchott, home to roughly 1.5 million people.

Those trends will likely only get worse in the decades to come, the researchers added.

That’s because, in many regions of the world, deserts are growing and spreading at an alarming rate—the result of a confluence of factors, from warming temperatures due to climate change to human land-use practices like deforestation. According to , this rapid “desertification” may have already affected the lives of more than 200 million people.

But can humans efficiently alter the flow of dunes, a process that has continued unabated for countless eons?

Humans have tried various approaches over the years to doing just that, from putting up fences to planting vegetation. But those strategies have limitations, and some can even be counterproductive. McNamara, Vriend and their colleagues want to use their knowledge of dune dynamics to improve the process.

Earlier this year, McNamara traveled to Chile where she joined researchers led by Tomás Trewhela at the Universidad Adolfo Ibañez. They worked on sand dunes around Viña del Mar, a community on the Pacific Ocean. The team planted several large obstacles, including poles with arms made from pool noodles, into the dune. The group is now monitoring the experiment to investigate how various obstacles may aggravate or stall sand transport.

Vriend is currently reinstalling the circular flume in her lab on the �鶹��Ѱ�����Boulder campus—an infrastructural challenge since the machine weighs 400 pounds.

“Can we actually affect where a dune is going to migrate?” McNamara said. “What happens if we have a dune, and we put rocks in front of it, for example?”

The graduate student saw first-hand why questions like these are so important during a research trip in June to the Indonesian island of Java. There, she met people whose lives had been overturned by geological disasters, including residents who had lost their homes to repeated flooding.

“The human impact is what drives my research,” McNamara said. “It’s really going to stick with me—talking to these families and seeing where they’re living, hearing about the impossible decisions they have to make.”