Heidelberg geographers use new methods to capture large dune shapes in time and space and to explain their origin
Star dunes are among the largest dune formations on Earth and - due to their changing shape over time - they can be important indicators for understanding the effects of climate change. Scientists from Heidelberg University’s Institute of Geography have examined such a dune in the Erg Chebbi sandy desert in Morocco by means of state-of-the-art laser scanning. For the first time, they linked highly accurate 3D measurements of changes in the dune surface with a continuous collection of wind data. Surveying the surface dynamics of the star dune was made possible through cooperation between two research groups, one on geomorphology around Olaf Bubenzer and the other on geoinformatics around Bernhard Höfle.
The large star dune formations can be found worldwide in almost all sand seas, which are called ergs. The morphology of these dunes is highly complex, being the result of past and present wind dynamics and their sand movements. Their basic structure is often several thousand - even up to ten thousand - years old. Hence, they also reflect the respective climate conditions and related factors over time, such as vegetation cover. The change in vegetation cover and the transportation of sediments can enable a deeper understanding of the growth and decline of sandy deserts, and therefore also provide insights into the impacts of climate change in arid regions. However, there has not yet been sufficient explanation of how star dunes evolve and change. New detailed and repeated measurements of wind and surface dynamics now enable a better understanding of the way these complex dune formations develop.
The sandy surface in the Erg Chebbi desert in Morocco was surveyed by means of terrestrial laser scanning. That produces a 3D model of a landscape using hundreds of millions of measuring points. From the repeated measurement data the Heidelberg scientists were able to derive changes taking place in the dune shape between October 2018 and February 2020. In addition, the changes in the course of the complex dune crests were captured as horizontal movement. They represent the sand transport and can be related to the hourly measurements of the direction and speed of wind. The sand on the dunes is only transported at wind speeds exceeding about six metres per second, which the researchers say corresponds roughly to the speed of the fastest 5,000-metre runner.
"To our surprise, we noticed a shielding effect when analysing the star dune in Morocco. At places where a loss of sand is usually to be expected the dune we were investigating had an accumulation of sand," said Manuel Herzog, a doctoral candidate in Prof. Bubenzer’s research group. Such sand accumulation leads to growth at the dune arms, which seems to lead to large dunes like the one studied in the Erg Chebbi sustaining themselves. This suggests that, besides their height, the steep slope angles at the dune arms have greater significance for sand transport than assumed to date. "An important factor in observing the dynamics of star dunes is to survey these dunes more frequently using 3D laser scanning. That is the only way we can improve our estimates of how the sand transport takes place on different time scales, for instance considering seasonal effects," says Olaf Bubenzer.
The Heidelberg scientists from geomorphology and geoinformatics want to transfer their analytical approach - linking 3D measurements of surface changes with continuous capture of wind data - to other geographical areas, in order to better understand these complex mega-dune formations and the underlying wind-related dynamics. That way, and going beyond certain sand dune formations, they hope to come up with new findings on climate change issues concerning, in particular, assumptions about the spread of arid regions.
Prof. Bubenzer heads two projects that, inter alia, explore the geomorphology of deserts with the aid of interdisciplinary approaches. Prof. Höfle leads the research group "3D Geospatial Data Processing (3DGeo)". The results of the research arising from their cooperation were published in the journal "Earth Surface Processes and Landforms".
M. Herzog, K. Anders, B. Höfle, O. Bubenzer: Capturing complex star dune dynamics - repeated highly accurate surveys combining multitemporal 3D topographic measurements and local wind data. Earth Surface Processes and Landforms, 47 (11), 2726-2739. (18 May 2022)
O. Bubenzer: Entwicklung von Dünenlandschaften. Ruperto Carola research magazine, 12/2018, 106’113 . (3 July 2018).