The Shape-Shifting Science Of Sand Dunes

FLORA LICHTMAN: Hey, I’m Flora Lichtman, and you’re listening to Science Friday.

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Today, on the show, the mysteries of sand dunes, including how to make them sing.

NATHALIE VRIEND: We create this very tiny sand avalanche. And at that moment, if the conditions are right, your entire body starts to vibrate. It’s a very imposing feeling.

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FLORA LICHTMAN: With September upon us, maybe you’re thinking, it’s time to leave the sun and sand behind. But we are not ready. So today, we’re spending a moment on sand. In particular, dunes. From beaches to deserts, sand dunes are this important geological feature. In some places, dunes protect the shore and, of course, the beach houses behind them from the onslaught of ocean waves. In other places, the dunes themselves are on the move, threatening homes and roads and even entire cities.

Joining me now, to sift out some grains of truth about sand, is Dr. Nathalie Vriend. She’s an associate professor in mechanical engineering at the University of Colorado in Boulder. Nathalie, welcome to Science Friday.

NATHALIE VRIEND: Thank you, Flora.

FLORA LICHTMAN: I feel like most of us have known a dune, have met a dune, but not like you know dunes. So let’s start with some one-on-one stuff. What is the difference between a dune and a big pile of sand? Is there one?

NATHALIE VRIEND: Ooh, I’m not sure if there’s a huge difference. Like a pile of sand, we’ve all been building them in our sand pits, when we were little kids, with our shovel and our little sieve. And a sand pile is actually just forming almost a triangle, if you look from the side. The sand grains are kind of rolling down, and those sand grains are stable when they’re about an angle of 30 degrees. So anything steeper, and they will just avalanche down. So that’s our typical sand pile that we know.

Now, if we scale it up from the sand pit to a large dune, either at the beach or in the desert, we still have that pile, but it’s just much, much larger. And of course, one important difference is that if we are in a desert, we might have some wind blowing. And that wind will actually make that symmetrical pile asymmetrical. And we still have that steep side on one side. But on the other side, there is, perhaps, a gradual increase. So that’s a difference between our tiny pile in our sand pit and a large desert dune.

FLORA LICHTMAN: Are there varieties of dunes?

NATHALIE VRIEND: Absolutely. There are many different varieties of sand dunes. If you go to the desert and you actually look at it from a satellite image, you can see different shapes. And it all depends on how much sand is available in that specific regional spot and how the wind is blowing.

If the wind is always blowing from, let’s say, the northern direction, then the sand dune is quite uniform, and it just always travels south. And that creates certain shapes that are very characteristic for wind regimes that are just going in one direction. However, if you are, let’s say, in a place where there are mountains and the wind is sometimes, in the summer, coming from the north and sometimes, coming from the south, in the winter, you can get very different shapes. So it’s not one size fits all.

FLORA LICHTMAN: Does the type of sand make the dune, too?

NATHALIE VRIEND: So if you think about a sand dune, the sand itself is actually a grain size. We call it sand, but it’s a very specific grain size.

FLORA LICHTMAN: Wait, are you telling me sand is not a substance, it’s a size?

NATHALIE VRIEND: It is. It is. If you go smaller, you get clay, which we find in our soil. If you go bigger, you get your gravel, which we might find in riverbeds. So sand is actually a size, but we know it as a substance, right? But the substance has a chemical composition. So most of the sand grains we know are made of quartz and feldspar. These are chemical substances, but it’s not the only one.

We can also find sand dunes and sand made of something called gypsum. And I don’t know if you’ve ever been to the White Sands National Park, Flora, but those dunes are really white. They look different. Like, the sand looks very different. And those sand grains are made from gypsum.

FLORA LICHTMAN: And physically, how do you think about a dune? Is it a solid? Is it–

NATHALIE VRIEND: No.

FLORA LICHTMAN: –a solid on the verge of being a liquid or a non-Newtonian fluid? What is it?

NATHALIE VRIEND: So it’s very interesting. It depends on how deep you look inside of the dune, what kind of magnification you use. So if you would go all the way down, to the individual sand grain, then those sand grains are quite solid, and they bounce against each other, perhaps deform a little bit, roll over each other. But that is just the grain interaction.

And then let’s actually make it a step bigger. So if I would have many grains in my hand, I can pour them out, and they form a little heap, like what we find in a sand pit. And at that moment, that heap is solid. But as I pour it out of my hand, it’s a fluid. And then, if I toss up the grains into the air, it flies away in the wind, and it’s more like a gas. So that is like the intermediate step. But then if I take a step even further back and I look at the big, big sand dune, the dune has a physical behavior as well. But perhaps the information of the individual sand grain is lost at that moment.

FLORA LICHTMAN: Is that why they’re cool to you, as a mechanical engineer?

NATHALIE VRIEND: I lost sand dunes. It started when I was a kid. So I grew up in the Netherlands, and my parents took me, sometimes, to the coast. And there was this large dune. At least, for a kid, it was a large dune. And I climbed up to the top, and I would run down, as fast as I could.

FLORA LICHTMAN: We’ve all been there.

– Exactly. And then, when I was starting grad school, I discovered this fantastic topic that my PhD supervisor was already investigating with undergraduate students, and that was on the so-called booming sand dunes or singing sand dunes. And I found it so mystical, and I was able to connect my fluid and solid mechanics that I learned in undergrad with some new techniques derived from geophysics and Earth sciences, and I was fascinated. I was hooked.

FLORA LICHTMAN: We have a clip of one of those singing sand dunes. Let’s listen to it.

[SAND DUNE HUMMING]

I feel like I’m on the set of Arrakis.

NATHALIE VRIEND: [LAUGHS] It’s even more impressive if you’re there, on the dune. So we create this sound by just sliding down the steep face of the dune, which is the angle of repose, which is about 30 degrees. And we create this very tiny sand avalanche. And at that moment, if the conditions are right, which typically means in the middle of the summer, your entire body starts to vibrate. It’s a very imposing feeling, to create this booming sand dune sound.

And I’ve taken classes of undergraduate students for field work, where we were actually a few miles away, and a second group was creating this sound on the sand dune, and we could hear it so clearly from so far away. So it’s quite an experience.

FLORA LICHTMAN: So it’s the sound of sand moving?

NATHALIE VRIEND: It’s the sound of sand moving, but it’s not only the sound of sand. You also need some kind of amplification. Because what you heard, it’s quite loud. So in my PhD research, we discovered that there is also a resonator within the dune that amplifies this bouncing of sand grains, on the surface, in the avalanche. It resonates, and it amplifies the sound to something that we can hear from miles away.

FLORA LICHTMAN: Wow. We mentioned, at the top this problem of moving dunes, dunes shifting. Tell us about that.

NATHALIE VRIEND: So more recently, I’ve done quite a bit of fieldwork in the Middle East, in Qatar. And we looked at these mobile barchan dunes, which are swiftly moving dunes that exist in an area of the desert where the wind is blowing mainly from one direction. And these dunes are traveling quite fast. They overtake each other every time they rebirth. If you cut a trench in them, you can see these layers appearing from where the dune was, let’s say, a few weeks ago.

It’s very similar to tree rings. If you cut a tree, you can see the tree rings, and you can see certain years where the tree was growing faster and slower. So in these dunes, if you cut a trench straight through, you can also see layers where there was more avalanching activity than in other times. And that’s basically my recent work, where we looked at sand dunes that move fast and at what we can learn from the internal structure.

FLORA LICHTMAN: What does fast mean in dune speed?

NATHALIE VRIEND: You can easily outrun it. So let’s put some length and time scales in it, because it’s difficult to conceive. So in the dunes that we were looking at, the smaller dune was perhaps 5 meters high and perhaps 100 meters across, and it moved about 20 meters per year.

FLORA LICHTMAN: 20 meters? That’s not nothing, though.

NATHALIE VRIEND: It’s not nothing. And then the larger dunes that we also climbed up– it took a little while before we got to the top. These dunes were 30, 40 meters high. They’re about half a kilometer across, so perhaps a football field. And these dunes only move a few meters per year. So different-sized dunes actually coexist in that dune field.

And the funny thing about dunes is that small dunes actually move much faster than big dunes. So if you have a small dune behind the big dune, upwind of a big dune, it can chase it, and it can interact. And we have a plethora of fun, physical behavior that results from that.

FLORA LICHTMAN: I want to see this time lapse.

NATHALIE VRIEND: Nowadays, with our satellite images, you can actually track it without being there. I love fieldwork. Don’t get me wrong. But the disadvantage is that you’re there for a week, and you can only get a snapshot of what’s going on at that moment. But if you want to track these longer term processes of dunes traveling across hundreds of meters, you either need to use satellite images, or you need to bring that experiment down to a much smaller scale in the lab.

FLORA LICHTMAN: Are you doing that? Are you bringing it down to a much smaller scale in the lab?

NATHALIE VRIEND: Yes, we are. We have our own dune machine. So we are running an experiment in our lab, where we create dunes in water. And in water, the dunes are moving much faster than they do in air. And as a result, we can speed up the process. And with the right scalings, we can still learn lessons about what’s happening in the field. But we can do these experiments in the lab much faster and more controlled as well.

FLORA LICHTMAN: It’s like the fanciest sandbox in the world.

NATHALIE VRIEND: I guess so. So the experiment is 2 meters across. But how we set it up, you wouldn’t be able to take a swim in it, but the sand grains are.

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FLORA LICHTMAN: We have to take a break, but don’t go away. Because when we come back, we’re talking about traveling sand dunes and what they mean for people living nearby.

NATHALIE VRIEND: Slowly, surely, one by one, different areas of the city are just engulfed by the sand. And it’s heartbreaking to look at, because entire neighborhoods are disappearing.

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FLORA LICHTMAN: Where are these dunes that are moving most threatening to human infrastructure, to homes, to roads, to cities?

NATHALIE VRIEND: Not all dunes move. I just want to make that clear as well. And most coastal dunes are quite wet, and they have vegetation growing. And most of these dunes are not moving at all or just moving a little bit. And the biggest change at coastal dunes is perhaps carving away from storms that are coming to the coast, that are taking sand away, digging away. And then these dunes crumble and collapse.

But most of the dunes that are at the coast are fairly stable, due to the vegetation. Their roots are helping it to stabilize the sand grains. If you go to much drier areas, let’s say, some parts of the Sahara, these dunes move fast and they keep going. We have a lovely picture from the dunes in Mauritania. And they’re, unfortunately, encroaching on the capital of Nouakchott. And slowly, surely, one by one, different areas of the city are just engulfed by the sand. And it’s heartbreaking to look at that, because entire neighborhoods are disappearing.

FLORA LICHTMAN: Wow. We hear a lot about desertification as a symptom of climate change. Does that mean this is going to be a growing problem in our future?

NATHALIE VRIEND: I think so, especially if there are a lot of cause and effect that you might not realize initially. So, for example, if rivers are dammed upstream to preserve water for agriculture, for example, then downstream riverbeds become drier and drier. Lake beds might dry out and create a lot of dust and sand that can then be brought into the landscape and form dunes. So there is a lot of cause and effect that you might not realize, right away, what’s happening. And if there’s more sand, more sediment transport, we call it, then we have more dunes appearing. And they just keep going.

FLORA LICHTMAN: What are your big burning sand questions? Like, what are the big things that haven’t been answered that you’re dying to know?

NATHALIE VRIEND: So we’ve done a lot of work over the last seven years with some of my students from my research group, where we were looking at sand dune interaction. So I mentioned earlier that small dunes move much faster than large dunes. So if you have a small dune chasing a large dune, you don’t exactly what happens. They might actually merge into one dune, or they may actually exchange some sediment and actually coexist, and then the front dune runs off again. So it’s like a game of chase.

So that’s what we’ve done a lot of work on recently. And another bit of research that we’ve done is putting obstacles in front of dunes and seeing what happens. And this is a direct application for resilience in dry environments.

Because you can imagine, if you’re a farmer and there is a sand dune approaching your agricultural field, you might think, I’m building a little blockage to stop the sand. But from a fluid mechanics point of view, that may actually aggravate the problem, because that blockage, that little wall, might actually capture scent behind it and make the problem much worse. So there’s this interesting interplay between Earth science, sediment dynamics, fluid mechanics. And we really need to understand the physics in order to understand cause and effect.

FLORA LICHTMAN: Fascinating. Thank you so much for making this subject just the absolute opposite of dusty.

NATHALIE VRIEND: Fantastic. Thank you so much, Flora.

FLORA LICHTMAN: Dr. Natalie Vriend is an associate professor in mechanical engineering at the University of Colorado, in Boulder.

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Thanks for listening. Don’t forget to rate and review us, wherever you listen. It really does help us get the word out and get the show in front of new listeners. Today’s episode was produced by Charles Bergquist. I’m Flora Lichtman. Thanks for listening.

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