Science Friday
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A sand dune is much more than just a big pile of sand. But how do they work? And what else do scientists want to know about sand?
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Hey, I'm Flora Lichtman, and you're listening to Science Friday.
Today on the show, the mysteries of sand dunes, including how to make them sing.
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.
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. I mean, 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. Natalie Fried. She's an associate professor in mechanical engineering at the University of Colorado in Boulder.
Natalie, welcome to Science Friday.
Thank you, Flora.
I feel like most of us have known a dune, have met a dune, but not like you know dune. So let's start with some 101 stuff. Like, what is the difference between a dune and a big pile of sand? Is there one?
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, right, with our shovel and our little sieve. And a sand pile is actually just forming almost like a triangle if you look from the side, right? The sand grains are kind of like 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 an gradual increase. So that's kind of a difference between our tiny pile in our sand pit and a large desert dune.
Are there varieties of dunes?
Absolutely. There are many different varieties of sand dunes. If you go to the desert and you would 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. Um. And that kind of creates certain shapes that are very characteristic for wind regimes that are just going in one direction. Um. 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, yeah, it's it's not one size fits all.
Does the type of sand make the dune too?
So if you think about the sand dune, it the sand itself is actually a grain size. We call it sand, but it's a very specific grain size.
Wait, are you telling me sand is not a substance, it's a size?
It is, it is. If you go smaller, you get clay, uh, 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 uh 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 um something called gypsum. And I don't know if you've ever been uh 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.
And physically, how do you think about a dune? Is it a solid? Is it?
No.
A solid on the verge of being a liquid or a non-Newtonian fluid? What what is it?
Yeah, so it's very interesting. It kind of depends on um how deep you look inside of the dune, like whether 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 kind of 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 kind of like make it a step bigger. So if I would have many grains in my hand, um, I can kind of pour them out and they form a little heap, like what we find in a sandpit, right? Um, 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 kind of like the intermediate step. But then if I take a step even further back, um, and I look at the big, big sand dune, the dune has physical behavior as well. But the, perhaps the information of the individual's sand grain is lost at that moment.
Is that why they're cool to you as a mechanical engineer?
Oh, I, I love sand dunes. I mean, it started when I was a kid. Um, 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.
We've all been there, yeah.
Yeah, exactly. And then, um, 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, um, in undergrad with some new techniques derived from geophysics and earth sciences. And I was fascinated. I was hooked.
We have a clip of one of those singing sand dunes. Let's listen to it.
I feel like I'm on the set of Arrakis.
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 at 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 fieldwork, 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.
So it's the sound of sand moving?
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 kind of amplifies this bouncing of sand grains at the surface in the avalanche. It resonates and it amplifies the sound to something that we can hear from miles away.
Wow. You know, we mentioned at the top this problem of moving dunes, dunes shifting. Tell us about that.
Yeah, so more recently, I've done quite a little bit of fieldwork in the Middle East, in Qatar. And we looked at these mobile bark and 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 kind of overtake each other every time they kind of... Rebirth. If you cut a trench in them, you can see these layers appearing from where the the dune was, let's say, a few weeks ago. It's very similar to tree rings. Like, 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 these dunes, if you cut a trench, um, straight through, you can also see kind of layers where there was more avalanching activity than in other times. And that's basically my my recent uh work, where we looked at sand dunes that move fast and what we can learn from the internal structure.
What what does fast mean in?
Oh, yeah.
In dune speed?
You can easily outrun it. So let's let's put some some length and time scales in it because it's it's difficult to conceive. So the dunes that we were looking at, um, the smaller dune was perhaps five meters high and perhaps one hundred meters across. And it moved about twenty meters per year, so that's not.
Twenty meters per year. That's not nothing, though.
It's not nothing. And then the larger dunes that we also climbed up. I mean, it took a little while before we got to the top. These dunes were thirty, forty meters high. They're about half a kilometer across, so perhaps a football field. And um, 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 a big dune, upwind of a big dune, it can chase it and it can interact. And we have a plethora of fun physical behaviour that results from that.
I want to see this time lapse.
Hmm, it's, you can actually nowadays with our satellite images, you can actually track it without being there. Because the disadvantage of fieldwork, I mean, 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 travelling 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.
Hmm, are you doing that? Are you bringing it down to a much smaller scale in the lab?
Yes, we are. We have our own, I call it a 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 you can do these experiments in the lab much faster and more controlled as well.
It's like the fanciest sandbox in the world.
I guess so. It's a little bit, so the experiment is two meters across, but how we set it up, you wouldn't be able to take a swim in it. But the sand grains are.
Okay, we have to take a break, but don't go away. Because when we come back, we're talking about travelling sand dunes and what they mean for people living nearby.
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.
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Where are these dunes that are moving most threatening to human infrastructure, to homes, to roads, to cities?
So, not all dunes move. I just want to make that clear as well. 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 calving away from uh storms that are coming to the coast that are taking sand away, digging away, and then these dunes kind of crumble and collapse. But most of the dunes at the at the coast are fairly stable due to the vegetation. Their roots are kind of helping it to stabilize the sand grains. If you go to much drier areas, let's say um some parts of the Sahara.
These dunes move fast and they keep going. Um, we have a a lovely picture from the dunes in um Mauritania. Um, and they're unfortunately uh encroaching on the capital of Nukashot. And slowly, surely, one by one, different areas of the city are just engulfed by the sand. And it's it's heartbreaking to look at that because entire neighborhoods are disappearing.
Wow, I mean, we hear a lot about desertification as a symptom of climate change. Does that mean this is gonna be a growing problem in our future?
I think so, um especially if there 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 um downstream riverbeds become drier and drier. Lake beds might dry out um 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 kind of cause and effect that you might not realize right away what 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.
Hmm. What are your big kind of burning sand questions? Like, what are the big things that haven't been answered that you're dying to know?
So, um, we've done a lot of work in kind of the last seven years with some of my students from my research group, where we were looking at sand dune interaction. So I've mentioned earlier that small dunes move much faster than large dunes. So if you have a small dune chasing a large dune, you know, you don't know exactly what what happens. They might actually merge into one dune, or they may actually kind of like exchange some sediment and actually coexist and then the front dune runs off again. So it's kind of like a game of chase, right? Um, 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 kind of 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, oh, I'm building a little like blockage, right? To stop the sand. But from a fluid mechanics point of view, that may actually aggravate the problem. Because that blockage, like that little wall, might actually capture sand 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.
Hmm. Fascinating. Thank you so much for making this subject, I mean, just the absolute opposite of dusty.
Fantastic. Thank you so much, Flora.
Dr. Natalie Vrint is an associate professor in mechanical engineering at the University of Colorado in Boulder.
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 Burquist. I'm Flora Lichtman. Thanks for listening.
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