Look beneath the waves on a beach or in a bay, and you’ll find ripples in the sand. Passing waves shape these sandforms and can even build them to heights that require dredging to keep waterways passable to large ships. To better understand how the sand interacts with the flow, researchers build computer models that couple the flow of the water with the behavior of individual sand grains. One recent study found that sand grains experienced the most shear stress as the flow first accelerates and then again when a vortex forms near the crest of the ripple. (Image credit: D. Hall; research credit: S. DeVoe et al.; via Eos)
Tag: sand ripples
What Makes a Dune?
Wind and water can form sandy ripples in a matter of minutes. Most will be erased, but some can grow to meter-scale and beyond. What distinguishes these two fates? Researchers used a laser scanner to measure early dune growth in the Namib Desert to see. They found that the underlying surface played a big role in whether sand gathered or disappeared from a given spot. Surfaces like gravel, rock, or moistened sand were better for starting a dune than loose sand was. Each of these surface types affected how much sand the wind could carry off, as well as whether grains bounced or stuck where they landed. Every trapped sand grain made the surface a little rougher, increasing the chances of trapping the next sand grain. Over time, the gathering sand forms a bump that affects the wind flow nearby, further shaping the proto-dune. As long as the wind isn’t strong enough to scour the surface clean, it will keep gathering sand as the process continues. (Image credit: M. Gheidarlou; research credit: C. Rambert et al.; via Eos)
Fediverse Reactions
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“Emerald Roots”
As charged particles from the solar wind bombard the upper atmosphere, a glowing plasma forms and dances in the sky. The green light of the plasma reflects off moistened sand, rippled by the passage of wind and tide. Each component seems simple, but this striking image contains hidden depths of fluid dynamics. Magnetohydrodynamics govern the aurora’s dance; the sand’s self-organization mirrors dune physics; and even the rocky outcropping in the background was carefully shaped by erosive forces from wind and water. Truly, fluid dynamics are found everywhere. (Image credit: L. Tenti; via 2023 Astronomy POTY)
“Fade”
Stingrays swim over rippled sands in this award-winning image from photographer Gregory Sherman. Above this peaceful vista, waves break and a dramatic skyscape looms. It’s the regular back-and-forth of the waves that shapes the ripples of sand. The rays can also use the sand for cover; note the remnants of sand atop a few of these rays, and then see how they hide themselves. See more winners from the Underwater Photographer of the Year awards in this gallery. (Image credit: G. Sherman/UPY 2023; via Colossal)
Shaping the Earth Through Cataclysm
Though we often think of the Earth as changing slowly, some events are so catastrophic that they change the landscape irrevocably. Some 15,000 years ago, a massive lake covered what is now Missoula, Montana. Dammed in by a 2,000-foot-tall wall of glacial ice, this lake contained more water than Lakes Ontario and Erie combined. But when the ice dam broke, the lake drained in days, sending a deluge across the Pacific Northwest.
The floodwaters carved new canyons and waterfalls, left massive ripples in the landscape, and deposited rocks from thousands of kilometers away as they raged their way to the sea. It was one of the most massive floods the Earth has ever seen. And, incredibly, it happened over and over as the lake refilled and broke again. Check out this Be Smart video for even more of this incredible story. (Image and video credit: Be Smart)
How Dunes Form
On its face, the idea that sand and wind can come together to form massive mountainous dunes seems bizarre. But dunes — and their smaller cousins, ripples — are everywhere, not just on Earth but on other planetary bodies where fine particles and atmospheres interact. In this video, Joe Hanson gives a great overview of sand dynamics, beginning with what sand is, how it moves, and what it can ultimately form. It’s well worth a watch, even if you know a little about dunes already; I know I learned a thing or two! (Image and video credit: Be Smart)
Martian Flyover
Fly over a Martian crater in this incredibly detailed 8K video built from Mars Reconnaissance Orbiter imagery. Like Earth’s deserts, Mars is largely shaped by wind, and we get some fantastic views of sand ripples in this flyover. For reference, the vertical scale covered in the video image is roughly 1 kilometer. It’s pretty astounding to see this kind of detail from a spacecraft 250 kilometers away! (Video and image credit: S. Doran/NASA; via Colossal)
Sandy Wrinkles
Water flowing back and forth over sand quickly forms a field of dune-like wrinkles. On the upstream side, the flow is a little faster, and it picks up grains of sand. When the flow slows on the downstream side of a bump, the sand gets deposited. In this way, small bumps in the sand continue growing larger. A similar process between wind and sand forms enormous dunes here on Earth and on Mars. These smaller water-driven wrinkles are very common in tidal areas and in sandy creeks. They can even build up and break down such that they create periodic waves that surge down the stream. (Image and video credit: amàco et al.)
Surge Flows
Sandy beaches can be a great place to play with neat flows. In a recent video, Frank Howarth describes playing with beach rivers on the Oregon coast and observing a surge flow there. Under the right conditions, a current flowing over sand will build up sand ripples large enough that they form miniature dams in the flow. This traps additional water, which eventually collapses the sand ripples, releasing a surge of water. The surge tends to smooth out the sand and cause the ripple-making process to start over. It’s a fairly unusual phenomenon, but it’s one known to happen seasonally in a few specific places, like at Medano Creek in Colorado’s Great Sand Dunes National Park. There the snowmelt-fed creek surges during the late spring and early summer, releasing a fresh wave every 20 seconds or so. (Image credit: F. Howarth, source; h/t to Sebastian E.)
The Winds of Mars
The Martian atmosphere is scant compared to Earth’s, but its winds still sculpt and change the surface regularly. The average atmospheric pressure on Mars is only 0.6% of Earth’s, and the density is similarly low at 1.7% of Earth’s. Despite this thinness, Martian winds are still substantial enough to shift sands on a daily basis, as shown above. These two images were taken one Martian day apart, showing how sand ripples moved and how the Curiosity rover’s tracks can be quickly obscured. Part of the reason Mars’ scant atmosphere is still so good at moving sand is that Martian gravity is roughly one-third of ours; if the sand is lighter, it doesn’t take as much force to move! (Image credit: NASA/JPL-CALTECH/MSSS)
