Herrmann explained how dunes are formed, move, interact, shift from one type to another, and can be anchored by vegetation (photo: Daniel Antônio/Agência FAPESP)

Published on 03/11/2024

By José Tadeu Arantes  |  Agência FAPESP – Nouakchott, the capital of Mauritania, sits between the ocean and the Sahara Desert and is being swallowed up by dunes. Brazil has coastal dunes from Pará state in the Amazon all the way down to Rio Grande do Sul, a state that borders Uruguay. Management of these large sand formations is important to conserve the landscape, which has economic significance for tourism, among other activities, and to protect towns, villages and roads. Natural movement of dunes can be a threat, as it is in Mauritania, but dunes themselves can afford protection against extreme weather events. Knowledge of the dynamics of dunes is being taken to outer space: while dunes on our planet can be as long as 100 m or more, Martian dunes can be over 1 km in length.

“The Life of Dunes”, the first event in the series FAPESP Lectures 2024, explored their formation, motion, interactions, transitions between types, and anchoring by vegetation.

The lecture was delivered by Hans Jürgen Herrmann, theoretical physicist and professor emeritus at ETH Zurich’s Institute of Building Materials in Switzerland. Currently, he is working at ESPCI Paris’s Laboratory of Physics and Mechanics of Heterogeneous Media in France, and the Federal University of Ceará’s Department of Physics (DF-UFC) in Brazil.

Herrmann spoke mostly, but not only, about the dunes of Jericoacoara (Ceará) and Lençóis Maranhenses (Maranhão), both in Northeast Brazil. Jericoacoara has the world’s largest barchan dune. Barchans are crescent-shaped dunes with horns pointing downwind and marking the lateral advance of the sand.

“Barchans are the dunes that move fastest,” he said, presenting aerial photographs of the Jericoacoara dunes taken in 1958 and 1988 so that the audience could see how they had moved 500 m in 30 years.

Dune dynamics are driven basically by the volume of sand and the wind regime, he explained. In the case of barchans, the crescent shape is formed by the wind blowing regularly from a single direction, with a gentle convex slope and clearcut crest on the windward side, and the two extremities projecting forward on the leeward side. Sand transported by gravity from the crest to the foot and the swirling of the wind as it blows over the dune build the concave slip face, which faces downwind (see the slides presented during the lecture at about 28:30). 

Barchans gradually migrate with the wind as a result of erosion on the windward side and deposition on the leeward side. “The speed of their motion is inversely proportional to their height. The taller the dune, the less quickly it moves,” Herrmann said.

Wind transports sand in three main ways, he explained. The first is reptation, which occurs when the grains are large, with diameters greater than 0.4 mm, and “creep” over the ground but do not take off. When the grains are lighter (medium-sized, between 0.3 and 0.1 mm), the wind may be strong enough to pick them up so that they bounce along in a motion termed saltation. Very fine sand (less than 0.06 mm) can be borne over great distances in dust storms; this is suspension.

Wind also moves dunes by erosion, deposition, and splashing, which is when the wind lifts sand particles with a great deal of energy and they kick up others as they hit the ground, producing a cascade of trajectories. All these processes can be simulated in a computer, helping scientists understand what happens when dunes collide.

Interaction with vegetation is another important factor. It can stop dunes from moving or determine an inversion of their shape, when vegetation brakes the horns of a barchan, since they are lower than the main body of the dune. The main body continues to move forward, and the concave shape becomes convex, changing the morphology from barchanoid to parabolic. “Without vegetation, barchans move like barchans. If vegetation anchors the horns, the shape is inverted,” Herrmann said, adding that there are around 150 different dune morphologies.

“Studies show that fossil dunes that have been stopped by vegetation have three times less plant and animal biodiversity than migratory dunes,” which should be taken into account when formulating management strategies to stop dunes from moving in order to protect dwellings and roads, he explained.

Unlike on Earth, barchans on Mars are shaped like a chevron pointing into the wind. This observational finding is reproduced by simulations when Earth-based parameters are replaced by Martian ones. The lower gravitational acceleration and thin atmosphere are crucial in this case.

The first FAPESP Lecture 2024 was moderated by Oswaldo Baffa Filho, a professor at the University of São Paulo’s Ribeirão Preto School of Philosophy, Sciences and Letters (FFCLRP-USP), and attended by Marco Antonio Zago, President of FAPESP, and Fernando Ferreira Costa, coordinator of the scientific committee that is organizing the lecture series. According to Zago, among other aims the series is designed to contribute to the construction of a Brazilian research agenda committed to sustainable development.

A recording of the first event in the series FAPESP Lectures 2024, entitled “The Life of Dunes”, can be watched at: www.youtube.com/watch?v=nsNQrsYRvNY. 

 

Source: https://agencia.fapesp.br/51074