Dune migration and morphological changes during the incipient phase of stabilization and transformation in the northern Ordos Plateau, China
Eerdun Hasi, Meiping Liu, Yu Sun, Chao Guan, Dan Liu
College of Resources Science & Technology, Beijing Normal University, Beijng, China
It is a common phenomenon that barchan dunes and parabolic dunes coexist and advance along the prevailing wind in the shrub-encroached grassland of the northern Ordos plateau, northern China. However, the environmental factors controlling the movements and transformation between these two types of dunes are not clear. Here we present a 6-year (2009-2015) field observatios on one barchan dune with trailing arms and two parabolic dunes (U-shaped with slipface on dune apex and V-shaped with tonguing apex) in this region. Three dimensional topographic surveys of these dunes were measured with RTK GPS and the horizontal displacement and morphological changes of the dunes were analyzed. Results show that the migration and morphology of the dunes varies depending on the regional precipitation, wind speed and dunes shapes. From 2009 to 2015, the migration rates of the barchan dune, U-shaped and V-shaped parabolic dunes were 13.1, 10.3 and 4.8 m year-1, respectiely. The mean direction of dune migration ranged from 92°to 106°(E to ESE), close to the resultant sand drift direction in the study area. During the same period, the dune height decreased by 0.6, 0.7 and 0.4m, the volume of dune sand reduced by 4800, 14000 and 11910m3,while the dune area increased by 8700, 33700 and 9800m2, indicating the dunes had been flattening and losing large amount of sand material. Due to increased vegetation cover and limited sand supply, dunes migration rate decreased year by year since 2009, proved by a special case in 2012 when regional precipitation increased substantially and the dune movement rate slowed down significantly. The 3-year mean annual precipitation between 2009 and 2012 was 223mm and the migration rates of the dunes were 17.0, 16.1 and 6.4 m year-1. The outline of the crescent dune was relatively stable, but several bar-shaped erosion grooves appeared on the windward slope of the U-shaped parabolic dune and blowouts developed on the two arms of the V-shaped parabolic dune. Between 2012 and 2014, the average rainfall was 364mm year-1 and the dune movement rate dropped to 7.9, 4.2 and 2.9 m year-1. After 2013, the U-shaped parabolic dune was almost covered by vegetation and the slipface disappeared, only two blowouts on arms remained in a bare state on the V-shaped parabolic dune, and the crescent dune transformed to a parabolic with blowout developed on windward due to shrub growth on the upwind base. Our observational results suggest that increased precipitation leads to increased vegetation cover, thereby promoting rapid stabilization of parabolic dunes and crescent-parabolic transformation.