Migration Law And Morphological Evolution Of Barchan Dunes In Hunshandake Sandy Land

Hunshandake Sandy Land is located in the southern end of Xilinguole Grassland in Inner Mongolia.It is an important part of desert/sandy land in eastern China and one of the sources of dust storms in northern China.As one of the main forms of sandstorm hazards,dune migration destroys and burys facilities,buildings,roads and farms,causing farmland desertification and grassland vegetation degradation,which poses a serious threat to regional ecological security,infrastructure construction and people’s lives and property.The migration rate of dunes can characterize the transport amount of sand source materials,and the moving direction of dunes can indicate the direction of sediment transport.The development of sand resources and the construction of windbreak and sand fixation projects in sand areas need to master their migration laws.At the same time,the morphological changes in the process of dune migration are also of great significance to improve the understanding of dune formation and evolution and enrich the research of aeolian landform.In this paper,the Esri World Image historical image is used to dynamically monitor the dunes in the selected area of Hunshandake Sandy Land,summarize the migration law and morphological evolution process of dunes,then analyze and discuss the influencing factors.The following conclusions are drawn:(1)The average annual migration rate of dunes in the study area is 0.74～9.92 m/a,and the medium moving dunes are dominant(85.04%),followed by fast dunes(13.39%)and slow dunes(1.57%),and there is no extremely fast dune type.The moving direction of the dune is between 79°and 123°,mainly E(68.50%),which is in good agreement with the multi-year synthetic sand transport wind direction,but there is still a certain angle.There are spatial and temporal differences with dune migration.In space,the migration rate of dunes in area I is the fastest(2.71～9.92 m/a),followed by area III(2.17～6.87 m/a),and the slowest in area II(0.74～5.78 m/a).The moving direction changes from E to ESE from west to east.The migration rate of dunes tends to accelerate over time,and there is no uniform change in the moving direction.(2)Dune migration is affected by many factors,including wind conditions,dune size,vegetation coverage,precipitation,temperature and dune density.Through linear fitting,it is found that there is a positive correlation between dune migration rate and sediment transport potential(R~2=0.19)and annual average temperature(R~2=0.26),while it is negatively correlated with vegetation coverage(R~2=0.40),dune bottom area(R~2=0.30)and annual average precipitation(R~2=0.21).The fitting degree of vegetation coverage and migration rate is the best,which is the main factor causing the difference of dune migration rate.(3)During the migration of dunes,the morphological changes are complex.Among the parameters,the length of windward slope,the length of south wing and the bottom area tend to increase,while the width,the length of leeward slope and the length of north wing tend to decrease.The influence of the two-way wind on the south wing is significantly different from that on the north wing.There is a certain correlation between dune morphological parameters,but due to the complexity of the environment,it has not reached a good theoretical dependence.(4)The standard barchan dunes have a tendency to evolve into parabolic dunes,and the plane shape transits from crescent-cake-parabola.The evolution process is as follows:the two wings of barchan dunes are low and susceptible to groundwater and vegetation.The migration rate of the two wings is slower than the top of the dunes and even in situ retention.The dunes evolve from crescent-shaped to cake-shaped.If further evolved,the two wings of the dunes point to the upwind direction and merge on the windward slope,and the two wings are elongated to nearly parallel to form a U-shaped parabolic dune.Vegetation coverage plays a key role in this process.Comparing the migration rate of dunes in different evolution stages,the standard barchan dune is the fastest,the transitional dune is the second,and the primitive parabolic dune is the slowest.