New model could help block shifting sand dunes, protecting infrastructure and ecosystems


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Cambridge scientists used scaled-down laboratory models to show how sand dunes move across a landscape, revealing the conditions that determine whether they will cross obstacles in their path, like pipelines or walls, or whether they will be stopped in their tracks.

The team’s experience, which featured mock-ups of obstacles of varying size and shape, shows that large obstacles are most effective in stopping dune migration, especially when they are ridge-shaped. , like a wall, rather than smooth and cylindrical, like a pipeline.

The model, published in Physical examination fluids, is the first to describe the interactions between sand dunes and obstacles.

By analyzing how currents are deflected in the presence of an obstacle, they were also able to develop an effective, data-driven tool that aims to predict how a dune will interact with its environment.

Research could help design more effective barriers that can, for example, prevent sand dunes from invading farmland. It could also be used to protect sand dunes and their unique ecosystems from damage.

“The displacement of sand dunes has a direct impact on people and their livelihoods; across the world and in various environments,” said lead author Karol Bacik, who conducted the experiments as a doctoral student. student at the Department of Applied Mathematics and Theoretical Physics (DAMTP) at Cambridge. “By revealing the physics behind the interactions between dunes and obstacles, this work gives us the guiding principles we need to divert or stop dunes, mitigating damage.”

As deserts continue to expand, sand dunes pose a growing risk to the built environment: they engulf roads and entire homes as they engulf the land. Likewise, dunes on the seabed can block shipping routes and even compromise the safety of submarine cables and pipelines.

But in some places, rather than stopping the movement of the sand dune, it may be better for a dune to cross an obstacle as quickly as possible. Take pipelines, for example, which can be damaged if buried under the weight of a fixed dune for too long.

Bacik’s work shows how obstacles of varying design are to be selected to suit the desired result, “If you want the dune to pass, make the obstacle as smooth and rounded as possible – if you want to stop it, make the dune pass. – as neat as possible, “Bacik said.

The research is part of a series of experiments that Nathalie Vriend – who is jointly based at the BP Institute for Multiphase Flow in Cambridge, the Department of Earth Sciences and DAMTP – conducted experiments to understand why sand dunes move as they do. “The sand is fascinating: pour a little of it with your hand and it sinks like a liquid… then when it lands it makes a solid pile,” she said. “But throw it in the air and it blows like gas. Its ability to transform between states like this makes it a real challenge to model the movement of sand.”

The team made a ring-shaped reservoir to contain their sand dunes, which can move around in circuits, almost like a “merry-go-round”. By plunging the dunes into the water and disrupting the flow with paddles, they were able to reconstruct how the dunes are moved by water currents. They then place obstacles of varying sizes and shapes on the path of the moving dunes to observe their effect.

“We can see evidence of sand dunes moving right in front of us, but what is fascinating is that their movement is entirely due to the hidden flow of water currents or winds,” Bacik said, “You can’t see the curled tails of the turbulence until you use a visualization technique … and only then, after you’ve performed a fluid analysis, can you really understand why sand dunes move the way they do.

The ultimate goal of the researchers is to model the movements of sand dunes in more complex and realistic three-dimensional landscapes, in addition to exploring the wind-blown dunes found in deserts. Ideally, they would like to be able to pinpoint a location on a map, enter information about weather, drafts or water, and predict whether a dune would pass over a specific obstacle. Although these numerical simulations are more complex, their new experiments serve as an important validation benchmark for further exploration.

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More information:
Karol A. Bacik et al, Dynamics of migrating sand dunes interacting with obstacles, Physical examination fluids (2021). DOI: 10.1103 / PhysRevFluids.00.004300

Provided by the University of Cambridge

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