February 1, 2023

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In a small river that he built himself, a geologist from Zurich saw that round grains are faster

Sedimentation is an important landscape formation process. Grains break through rocks and are transported by wind, water and ice, after which they settle somewhere. There is a lot of uncertainty about the rate at which these sediments are transported.

Grain size and density have been the most important variables in sedimentation models so far, but researchers at Eidgenössische Technische Hochschule Zürich in Switzerland Posted last week in nature A study showing that grain shape also has a significant effect on sediment transport. “These new findings may contribute to a better understanding of historical and future sediment formation,” said Eric Dale, geologist and lead author of the paper.

Dale’s research focuses on sediment transport in rivers. Here, he explains, grains face two forms of resistance. The river bed itself provides a prevention for grain jumping over it. In addition, water provides resistance. Over time, these grains end up in deltas, or farther offshore. This proportion is called sedimentation.

Cubes and pictures

Dale says it’s difficult to determine the effects of grain shape on the speed and distance of sediment transport. This is why he experimented with a self-built small river in the laboratory. Use different grain shapes, from balls and cubes to angular slabs and ovals with rough surfaces, all of the same size and density. These “test pellets” were shot and filmed with high-speed cameras.

For example, Dale and his colleagues found that round grains are transported 2.5 times faster than angled grains, because they encounter more resistance because they easily get stuck to the river bed. As a result, you would expect round grains to be transported further and longer distances in rivers than angled grains.

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It was different. “Slanting pellets have flat sides, and we’ve seen that these pellets float on those sides because of the water resistance and can still travel a long distance despite their low speed,” Dale explains. “In addition, the angled grains orient themselves to the most appropriate position, which reduces water resistance over time.” The exact difference in velocity between round and angled pellets is still not clear, but one thing is certain, according to Dale: “Shape matters.”

In current models, only round grains are used

In current sedimentation models, only round grains are used. According to Deal, the results of studies using these models are often translated directly into the course of natural processes, but he believes this is unjustified.

Models of historical sedimentation processes are used to predict sediment buildup, for example in delta regions such as the Netherlands: The deal: “There are different grain shapes because sediment from the sea—usually round—and rivers—often angular—collect here. Sediment supplies can now be predicted for coastal defenses more accurately using our method.”