Photographer Mikko Lagerstedt specializes in Nordic landscapes, like the windswept snow seen here. I love the way he’s captured the snow that gets picked up and blown by the wind. Notice the hazy layer of snow hovering over the foreground. This snow is saltating, just as sand does in the desert. When flakes get picked up by the wind, they follow a ballistic trajectory, much like a cannonball in a high-school physics class. As the snow crashes back down, its impact knocks up more flakes, and the process continues. Repeat enough times, and you’ll see this hazy layer of blowing snow blanketing a snowscape. (Image credit: M. Lagerstedt; via Colossal)
Tag: saltation
Unifying Sediment Transport Theory
On windy days, streaks of snowflakes snake in the air above a mountaintop snowfield. And when snorkeling in the surf, you can watch the inbound waves sculpt underwater ripples in the sand. Both are examples of sediment transport, and scientists have struggled to understand why the physics of these grains seems to differ between air and water. We observe certain behaviors, like saltation, in air and very different behaviors for grains underwater.
One of the key differences is how much erosion occurs for a given amount of shear. In air, the relationship is linear; double the shear stress and you double the sediment transport rate. But in water, the relationship is nonlinear, meaning a small change in the shear stress can have a much larger effect on the rate of transport.
A new study suggests that these differences are really only skin deep. Through detailed simulations, the researchers showed that what really matters is the energy dissipation caused by collisions between grains. Whether the medium is air or water, there are two important regions in the flow: the bed region where particles experience little movement, and the overlying region where grains are energized and lifted by the flow. In this framework, the researchers found no difference in how energy is dissipated, regardless of the medium.
So why do measured sediment transport rates vary between air and water? The authors concluded that the relationship between shear and transport rate is, indeed, nonlinear. It’s just that the wind here on Earth is too weak to reach that nonlinearity. (Image credit: snow – wisconsinpictures, sand – J. Chavez; research credit: T. Pähtz and O. Durán; via APS Physics; submitted by Kam-Yung Soh)
Dust Envelopes Mars
Day has turned into night for NASA’s Opportunity rover as a massive dust storm envelopes Mars. The first signs of the dust storm were reported May 30th, and over the last two weeks, the storm has grown to an area larger than North America and Russia combined. Despite the low pressure and density of Mars’ atmosphere, solar heating can create fairly strong winds – they don’t reach hurricane-force speeds, but they’d qualify as a very windy day here on Earth. With the lower gravity on Mars, this can lift dust well into the atmosphere, choking out the sunlight Opportunity needs to continue operating. The rover has entered a low-power mode and is no longer responding to communications. Martian dust storms have been known to last for weeks or even months, and this may be the last we hear from the intrepid rover on its fifteen year journey. Here’s hoping that Opportunity makes it through the storm and can eventually get the solar power needed to phone home again. (Image credit: NASA JPL)
An Armored Bed
A river’s flow constantly changes its underlying bed. The rocks and particulates beneath a flowing river can typically be divided into two zones: an upper layer called the bed-load zone where the flow moves particles with it and a lower layer where particles are mostly trapped but may creep over long periods. In gravelly river-beds this upper bed-load zone tends to accumulate more large particles, a phenomenon known as armoring. Experiments show that, in this region, large particles have a net vertical velocity moving upward, while smaller particles tend to move downward. Exactly why large particles are more prevalent in the bed-load zone in unknown; several theories have been offered. One suggests that the size segregation is similar to the Brazil nut effect and that smaller particles have a tendency to fall into gaps and sink more easily than larger ones. (Image and research credit: B. Ferdowsi et al., source)
Martian Ripples
Earth and Mars both feature fields of giant sand dunes. The huge dunes are shaped by the wind and miniature avalanches of sand, and their surface is marked by small ripples less than 30 centimeters apart. These little ripples are formed when sand carried by the wind impacts the dunes. But Martian dunes have a second, larger kind of ripple, too. These sinuous, curvy ripples lie about 3 meters apart and cast the dark shadows seen in the images above. On Earth we see ripples like these underwater, where water drags sand along the surface. On Mars, the same process is thought to play out with the wind, and so scientists have named these wind-drag ripples. (Image credit: NASA/JPL/MSSS; via APOD, full-res; submitted by jshoer)
Sand Ripples in Tidal Flats
Sand, winds, and waves can interact to form remarkable and complex patterns. These sand ripples from the tidal flats of Cape Cod are a testament to such interactions. When a fluid like air or water flows over a flat bed of sand, it can shear and lift grains of sand, moving them to a new location. Very quickly, turbulence within the flow disturbs the initially smooth surface and begins to form the wavelike crests we see. Because the change in surface shape alters the nearby air or water flow, there is a trend toward self-organization and persistence. In other words, once the ripples form, they’re reinforced by their effect on the wind or water that formed them. Once rippled, the surface does not tend to smooth back out. (Image credit: N. Sharp; research credit: F. Sotiropoulos and A. Khosronejad)
Snowy Deserts
Windblown snow bears a certain resemblance to desert sands or a Martian landscape. Many of the same aeolian processes–like erosion, transport, and deposition–take place in each. The animation above shows an example of suspension, where fine snowflakes are lifted and carried along near the ground. Larger snowflakes may bounce or skip along the surface in a process called saltation. For more, check out some of the crazy things snow does or learn about how dunes form. (Image credit: Redemption Designs, source video)
Barchan Dunes
Crescent-shaped barchan dunes are common on both Earth (top image) and Mars (bottom image). They form in areas where the wind comes predominantly from one direction. As the wind blows, it deposits sand on the gently sloping windward face of the dune. The leeward face of the dune is steeper; its shape is set by the sand’s angle of repose–essentially the steepest angle the sand can maintain without an avalanche. Barchan dunes are very mobile, moving between one and a hundred meters per year. They have also been seen moving through one another or moving along in formation. (Image credits: Google Earth, NASA/JPL/University of Arizona)
Engineering Sediment Transport
Sediment transport via fluid motion is a major factor in engineering, geology, and ecology. This video shows two common forms of sediment transport: particle suspension and saltation. Suspension, in which the fluid carries small solid particles, is visible high in the blue water layer. Saltation occurs closer to the surface when loose particles are picked up by the flow before being redeposited downstream. Watch some of the individual particles near the surface to see the process. Kuchta has several more demo videos of flow in this desktop flume, sold by Little River Research & Design. (Video credit: M. Kuchta; submitted by gravelbar)
