The not so simple life of barchans

Sand dunes aren’t just simple piles of sand, as we’ve noted before in Cosmos.

Earlier this year we reported on research suggesting they can “communicate” with each other to avoid collisions; currents created by one will control the movement of another.

Now Brazilian researchers have found that barchans, the distinctive crescent-shaped dunes with horns, in fact have five basic types of interaction: chasing, merging, exchange, fragmentation-chasing, and fragmentation exchange. And collisions are involved.

The good news, according to Erick Franklin from the University of Campinas, is that the same formation and interaction dynamics apply regardless of size.

That means what he and a colleague have learnt studying small aquatic barchans in the lab, which form in just minutes, can inform our understanding of big ones in deserts and even the really big ones on Mars (up to a kilometre in length and centuries in the making).

Barchans are created by the interaction between the flow of a fluid, such as gas or liquid, and granular matter, typically sand, under predominantly unidirectional flow conditions. The horns point in the direction of the fluid flow.

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Small barchans in water. Credit: Agência FAPESP

They typically form corridors in which their sizes are similar. Analysis of individual dunes suggests they should grow indefinitely, Franklin says, but they don’t. One explanation is that binary interactions redistribute the sand: instead of growing continuously, dunes subdivide.

“This has been proposed in the past, but no one had extensively tested and mapped these interactions, as dune collisions take decades to happen in terrestrial deserts,” he says.

“Taking advantage of the fact that underwater barchans are small and move much faster, we conducted experiments in a hydrodynamic channel made of transparent material, with turbulent water flow forming and transporting pairs of barchans…”

Franklin and Willian Righi Assis ran 120 experiments with barchans of up to 10 centimetres, varying parameters such as grain diameter, density and roundness, water flow velocity, and initial conditions. The images acquired were processed by computer using a numerical code.

“Our experiments showed that when a binary collision occurs, the barchan that was originally downstream… expelled a dune of an approximately equal mass to that of the barchan upstream…” Franklin says.

“The first impression was that the upstream barchan passed over the other barchan like a wave, but the use of coloured grains helped us show this didn’t happen. Actually, the upstream barchan entered the downstream barchan, which became too large and released a mass more or less equal to the mass received.”

Interactions between the two barchans basically involved two mechanisms. One was the disturbance caused in the fluid, which bypassed the upstream barchan, accelerated and impacted the downstream barchan, which eroded. This is termed the “wake effect”. The other was the collision in which the colliding barchans’ grains merged.

Which of the five interactions occur is related to dune size and velocity. Chasing and merging are the simplest, exchange is a little more complicated, and the other two only happen when fluid flow is very strong.

The findings are reported in the journal Geophysical Research Letters.

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Barchan dunes on Mars’s Hellespontus region as seen by the Mars Reconnaissance Orbiter. Credit: NASA / JPL / University of Arizona

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