Morphological maintenance mechanism of the complex longitudinal dunes in the Kumtagh Desert

The dune morphology and its formation and evolution are controlled by regional wind conditions and sand source sediments. Due to the combination of multi-directional winds and sediment grain size, the complex longitudinal dunes morphology maintained in the Kumtagh Desert. The Kumtagh Desert is known for its distinctive feathery-like dunes, which are actually complex longitudinal dunes composed of barchan dunes and linear dunes, i.e., raked dunes, and the interdune is a gently undulating coarse-grained undulating dunes, i.e., zibar dunes, without obvious slip surface. Through systematic analysis of field investigation, measured data, remote sensing images and sediment characteristics, the formation and morphological maintenance mechanism of the complex longitudinal dunes in the Kumtagh Desert were revealed. The results showed that there were three directions of wind as dynamic condition in the complex longitudinal dunes distribution area in the Kumtagh Desert, with high wind energy environment, with the main wind being the north-northeast wind and the secondary wind being the eastly and westly winds, and the angle between the north-northeast wind and the eastly wind was 74.64°, the angle between the north-northeast wind and the westly wind was 100.40°, which was between 90° and 135°, and the ratio of sediment transport rate was 2.2:1. According to the image analysis, the main dunes of the raked dunes were SW-NE longitudinal dunes and extended obviously along the ridge line. The secondary dunes developed on the northwest slope of the main dunes were crescent-shaped under the main wind, while the larger dunes still maintained the crescent shape under the secondary winds. Therefore, it is believed that the raked dunes in the Kumtagh desert is in fact a kind of complex longitudinal dunes with barchan dunes superimposed on top of the longitudinal dunes as the main dunes. According to the field investigation and sampling analysis, the sand dunes and interdune sediments had a wide range of particle size distribution, poor sorting, and the frequency curve were obviously bimodal or multi-peak, including very coarse sand, even fine gravel and medium fine sand components. The special complex longitudinal dunes developed under the multi-directional winds in the Kumtagh Desert is completely different from the complex longitudinal dunes in Namibia, i.e., large longitudinal or linear dunes with barchan dunes on both slopes. The effective sand sources for the formation of sand dunes are scarce, and their morphological maintenance is controlled by sediment particle size in the Kumtagh Desert. The north-northeasterly wind caused the coarse particles in the interdune area climb to directly to the main dunes, and even to the middle and lower part of the windward slope of the barchan dunes on the northwest slope of the main dunes, resulting in the stabilization of the main sand dunes body under the alternating action of multi-directional winds, presenting a landscape of barchan dunes stacked on longitudinal dunes.

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