Extraction And Morphological Characteristics Analysis Of Karst Depressions In Fengcong Area Based On DEMs

The Fengcong-depression area in Guilin is one of the most typical tropical karst landforms in the world.It has the most diverse negative karst landforms.After the landform evolve into the stage of karst depression,the surface,underground crevices and cave systems begin to connect to each other.From this stage,the karst erosion by underground water becomes more important to shape the landform.The karst depression in the Fengcong area is regarded as a sign of the maturity of karst landform,the most typical surface karst form,and the most important basic form in the karst area.It is a unique status of the karst morphology.In karst geomorphology,different negative topography concepts are often confused with each other.The definition of karst depressions in existing studies is not clear enough.Therefore,the scope of karst depression is often simplified.Or the traditional methods are used to extract depressions in Karst geomorphic areas.But these methods are not accurate enough.To get accurate boundaries of karst depressions,the accurate concept of depression is needed.This study is based on the positions of saddle points and improves the extraction results by topographic openness.As for the extracted depressions,the contour trees are constructed based on openness to realize the extraction of nested secondary depressions as well as the construction of nested relationships in composite depressions.The main works and conclusions of this study are as follows:(1)This paper summarizes the conceptual differences of different types of karst negative topography,and distinguishes the differences between karst depression and other negative topography,as well as the different concepts of depression in different contexts.It is recognized that the differences in concepts of different karst negative topography are mainly due to the differences in scale and hydrological characteristics.Since Doulin(single depression),and then composite depressions,sloping valleys,surface water and groundwater are interconnected.In addition,these negative topographic structures are closed,so the karst depressions involved in this study include three levels.(2)Based on the position of saddles,a method of extracting the karst depressions in Fengcong landform is put forward.For the karst depression is defined as the concave part below the saddles,the saddles are the key points in the extraction of karst depressions.Through the statistical analysis of the topographic openness value of saddle points,their distribution pattern could be obtained,so that the automatic extraction of karst depression can be realized.This method solves some shortcomings in extracting closed depression of karst landform,such as manual mapping,approximation of polygon depression,hydrological analysis filling depression and contour tree,and improves the extraction effect.(3)The individual morphological characteristics and group characteristics of karst depression are analyzed.The result shows that the depressions with smaller roundness are always located in the area of geological fault line,and its direction can reflect the direction of geological activity;the index of fissure point percentage and index of node fission are strongly correlated with the development degree of karst depression;the area of leaf node depressions can reflect the geological conditions of karst landform evolution.