Morphological Characteristics And Dynamic Evolution Process Of Crescent-shaped Sand Dunes In The Alashan Gobi Region

The study area is the Suhutou Gobi area in Alashan,on the northeast edge of Badain Jaran Desert.Based on many years of meteorological data,field observation results,multiperiod crescent-shaped three-dimensional laser point cloud data and wind tunnel simulation data,this study is based on long-period monitoring and short-period observation,quantitative analysis and qualitative analysis,in order to reveal the morphological characteristics and dynamic evolution process of crescent-shaped dunes in Alashan Gobi area.The main conclusions are as follows:(1)From 2016 to 2020,the annual sediment transport potential of the Su Hongtu Gobi area is between 591.0 VU and 656.6 VU,and the composite sediment transport potential is between 402.8 VU and 456.8 VU.WNW and ENE winds prevail throughout the year in this area,and the wind direction change rate RDP/DP is about 0.67,which is a typical blunt double-peak wind condition and belongs to a high wind energy environment.Wind erosion is the main wind-blown activity.(2)There is an obvious correlation between crescent dunes in Gobi,and the correlation coefficients(R2)between symmetry of two wings and other morphological parameters of dunes are all below 0.6.In addition,there is a strong correlation between other morphological parameters,and the correlation coefficient(R2)is above 0.8.The migration law of crescent dunes in Alashan Gobi area is as follows: the number of crescent dunes shows a decreasing trend,and the migration direction of the dunes moves from NW to EN,in a forward way,with an average azimuth of 141.25° and an average annual movement speed of 22.36 m/a.The movement trajectory of crescent dunes in this study area is curved rather than straight,and the dunes move clockwise or counterclockwise while curving.(3)According to the change data of dune morphology from 2015 to 2021,the overall elevation of some dunes in the study area decreased,but the slope of regional dunes tended to slow down;The surface area and volume of sand dunes are generally decreasing,so it can be inferred that crescent sand dunes in this study area are in the process of reverse evolution.(4)According to the fitting relationship between the morphological parameters of crescent dunes and the movement rate,the movement rate of dunes gradually slows down with the increase of the inner arc length Li,the outer arc length Lo,the ridge line length Lr,the dune length L,the dune height H and the dune chord length Lc;With the increase of dune surface area and volume,the movement rate of dune gradually decreases.(5)From the changes of surface sediments in different parts,it can be seen that the grain size of surface sediments in different crescent dunes in Gobi area is widely distributed,ranging from extremely fine gravel to coarse silt(-2φ ~ 5φ),among which the percentage of extremely coarse sand,coarse sand and coarse silt is relatively small,and mainly medium sand,fine sand and extremely fine sand.(6)The horizontal and vertical airflow field and airflow velocity changes are different between windward side and leeward side of sand dunes with different incoming winds.With the increase of the incoming flow angle,the size of the separation vortex formed on the leeward side of the dune will be different.When the incoming flow direction is 90° and270°,the trajectory circle of the near-surface airflow is basically symmetrical,and the flow around the two wings is basically the same.When the incoming wind speed is 135°,180°and 225°,the track circle of near-surface airflow mainly distributes in 0 ~-40 cm.The changing trend of air velocity is basically the same for different incoming wind directions of135° and 225°,180° and 270°.(7)During a dust storm,the surface flow fields of crescent dunes are highly similar,but there are differences in spatial scale.Taking the wilderness wind speed of 13.09 m/s and the wind direction angle of 260° ~ 270° as an example,the overall change form of the airflow field on the dune surface is as follows: when the airflow is blocked,the airflow slows down at the foot of the windward slope of the dune,forming an airflow deceleration zone,where the airflow drops to 8.7 m/s,and a part of the compressed airflow accelerates up along the windward slope of the dune,gradually rising from 8.7 m/s to 9.3 m/s.When the airflow reaches the top of the dune,its speed reaches its maximum.A part of the airflow passes through the leeward slope of the dune along the central axis.Because of the height of the leeward slope,a transverse pressure difference is generated,resulting in secondary flow.The vortex speed generated on the leeward slope of the dune is reduced to a minimum,and finally merges with the mainstream.The airflow starts to increase again,and gradually returns to the wind speed in the wilderness.Because the airflow is no longer bound by two wing angles.The other part gradually stretches the streamline along the two wing corners of the dune.