Dunhuang Yardan Landform Morphology And Evolution Research Based On Remote Sensing Images

The Yardang in Dunhuang City,Gansu Province is the representative of this kind of landform in China;therefore it is the main research area of Yardang.Previous studies have focused on the qualitative classification of Yardang geomorphology and the formation of environmental conditions,and lacks the study of Yardang geomorphology from morphological point of view.Based on the theories and methods of Quaternary geology,geomorphology,remote sensing,geographic information system and spatial statistics,this paper studies the morphological research of Yardang geomorphology from the perspective of Yardang's classification and distribution.Furthermore,the evolution process of Yardang is analyzed from the point of view of interior and exterior forces.On the basis of establishing the sign of Yardang interpretation,the object-oriented method is used to interpret SPOT6 remote sensing data,and 5548 pieces of Yardang units are translated.Furthermore,the kernel density analysis method was used to find that the whole distribution of Yardang was uneven,mainly in the northern,center and southern of the study area.In addition,the two major river confluence regions of the study area are obtained by retrieving the digital elevation model(DEM)of the research area through the stereoscopic pair of the Resource No.3 satellite imagery.Accordingly,the research area is divided into the north?middle and south partitions,owning 1640,1260 and 2648 Yardang units,respectively.The morphological parameters of the Yardang units were obtained by “rotation method” and “standard deviation ellipse” method,and the database was established.The parameter values of the area and length of Yardang units reach the maximum in the north,then reduce in the central area and finally increase again in the south,however,in the east-west direction,the Yardang units distribute uniformly.In the north,the Yardang units present in the north-south trend,but in the south,they are NE trending.No obvious rules in the central area.Through the quantitative assessment combined with the field observation and verification,the Yardang units are divided into 5 types as the following: mound-shaped,wall-shaped,long strip-shaped,tower-shaped and column-shaped.The column-shaped is the largest in the quantity,and the mound-shaped is the least.Nonetheless,the area of mound-shaped is the largest,accounting for 19.10% of the total Yardang area.The mean nearest neighbor index indicates that the plane distributions of Yardang shows better agglomeration,and the agglomeration of the mound-shaped is the strongest.Local indicators of Spatial association(LISA)obtained by cluster and outlier analysis indicates that the Yardang units in the northern area are young,belonging to the early stage of Yardang evolution.The southern and central areas are mainly occupied by middle-aged and old age,which correspond,respectively,to the middle(and/or late)and late(and/or decline)stages of Yardang evolution.In terms of vertical distribution,the quantity,type and scale of Yardang units show a decreasing law with increasing elevation.Moreover,with the evolution of Yardang landform,the slope of Yardang unit changed from steep to gentle.Yardang is the results of combination of interior and exterior forces: the interior forces control Yardang in large scale,and then the exterior forces further shape it and accelerate its development.Through the neighborhood analysis of each type of Yardang units,it is found that there are two ways of Yardang evolution:(1)mound-shaped to wall-shaped to tower-shaped to column-shaped;(2)mound-shaped to long strip-shaped to column-shaped.The two methods basically correspond to the above-mentioned landform evolution,reflecting the development process of Yardang.