Development Features And Evolution Mechanism Of Sinkholes Along The Margin Of Loess Tableland

The hilly and gully region is located in the Loess Plateau,where the surface is fragmented and land resources are extremely scarce.As a complete field with a relatively flat terrain,the loess tableland is the most valuable land resource suitable for farming and living in the Northwest region.With the frequent occurrence of extreme weather in the Northwest region and the implementation of a large number of water diversion irrigation projects,new erosion around the tableland keep the erosion of the tableland toward the center.As a typical eroded landform,sinkholes on the edge of the loess taleland constantly cut the complete plot of the plateau,causing a large amount of soil erosion.Therefore,identifying the temporal and spatial development characteristics of sinkholes and revealing their evolution mechanism and dynamic driving mechanism are of great scientific and engineering significance for the protection of the Loess Plateau and soil and water conservation.On the basis of the principle of “typical tablelad,well-developed strata,and clear phenomena,” this study selected the Haojiagou gully in Jinzhong City,Shanxi Province,as the research area.With sinkholes in the study area taken as the object,spatially geometric sinkholes in the study area were accurately extracted with the help of engineering geological survey methods such as unmanned aerial vehicle(UAV)aerial photography survey,remote sensing interpretation,and ground investigation.Morphological analysis and time series analysis were conducted to systematically analyze the spatiotemporal development characteristics of single and group sinkholes.Meteorological monitoring,in-situ test,physical modeling,and numerical analysis were synthesized to investigate the developmental mechanism and evolution model of sinkholes around the loess tableland,and the evolution mechanism was revealed.The following four research achievements were made:(1)Geological survey and monitoring of sinkholes aroud the loess tableland.On the basis of meteorological and environmental monitoring data in the study area,a remote sensing survey plan for the sinkholes at the plateau,which integrates UAV time-series aerial photogrammetry and ground survey,was developed.During the study period(2017～2019),14 UAV aerial photogrammetry and 2 ground surveys were conducted.On the basis of the aforementioned investigations,the time-history curves of atmospheric rainfall,surface temperature,and ambient temperature and humidity in the study area were formed.Then,a series of regional digital orthophotos,a digital orthophoto map,digital elevation model images with 2 cm spatial resolution,and large-scale topographic maps were constructed.In addition,an aerial image radiation normalization method based on spatiotemporal fusion and a sinkhole interpretation method based on spatial change detection were proposed to achieve high-precision interpretation of the spatial shape and distribution information of sinkholes.These methods achieve accurate interpretation(better than 10 mm)of sinkholes’ spatial morphology under the condition of low time-spectral radiation difference(<10%).(2)Analysis of the developmental characteristics of single sinkholes.A method for characterizing the spatial morphological parameters of a single sinkhole combined with the entrance plane feature and vertical section feature was proposed,and five evolution stages—incubation,formation,development,destruction,and extinction—were introduced.In combination with the remote sensing survey data of UAV and the MATLAB program design,the basic morphological parameters of a single sinkhole were extracted rapidly,and a comprehensive morphological evaluation index that can indicate the development order of sinkholes was constructed.The sinkholes in the study area were then divided into three evolution stages: formation period,development period,and destruction period.Statistical calculation and analysis showed that during the evolution process of the formation period,the shape of the sinkhole entrance gradually flattened,the edge profile became rougher,the diameter and depth of the cave continued to increase,and the ratio of diameter to depth increased.The development rate of the long axis of the cave mouth is 12.5% slower than that of the short axis in the stage of the formation period(toward a near-circular shape),10.9% faster than the short axis in the stage of the development(significant directivity)period,and 4.3%faster than the short axis in the stage of the destruction period(weak directivity).On this basis,the correlation between the development rate of sinkholes and atmospheric rainfall is more than75%,and the later the development stage,the better the correlation.(3)Analysis of the developmental characteristics of group sinkholes.A method to characterize the spatial distribution parameters of group sinkholes on the basis of topographic features was proposed.From an analysis of the cluster development characteristics and time series development characteristics,the cluster development trajectory of sinkholes was further analyzed by defining the parameters of the centroid of the entrance.The number of sinkhole clusters is related to the inclination of the slope that the number of sunny slopes(135°～225°)=55% > semi-shady slopes(45°～135°)> semi-sunny slopes(225°～315°)and shady slopes(315°～360° and 0°～45°)= 0;the main development direction of sinkholes is related to the aspect that semi-sunny slopes(49%)> shady slopes(33%)> semi-shady slopes(18%)> sunny slopes(0),and the development rate of sinkhole clusters tends to increase with the increase in the slope and aspect gradient.The average movement rate of the centroid of the group sinkholes is nearly 26 mm/100 mm(rainfall).The later the evolution stage,the slower the movement rate of the centroid.The specific manifestation of this condition is that it changes from developing along the direction line of the gullies to spreading evenly around the area.The development of sinkholes with cluster symbiosis is mainly controlled by the slope micro-topography and its catchment conditions.The symbiosis direction of the cluster is distributed along the slope runoff direction line,and three basic symbiosis modes exist: series,parallel,and mixed.(4)Development mechanism of sinkholes around the loess tableland.Physical simulation of rainfall erosion,in-situ testing of sinkholes around tableland,and numerical analysis of seepage evolution clarified that atmospheric rainfall and topography are the main environmental factors that drive the formation and evolution of sinkholes.Moreover,soil structure is the key to controlling the generation and evolution of the fissure system in slopes,which influences the development of sinkholes around the loess tableland.Group sinkholes are mainly distributed in the valley area around the tableland in the symbiotic form of series,parallel,and mixed connection.Sinkholes with different symbiotic forms lead to soil erosion at the tableland during the continuous succession process of gestation-demise.The hydraulic channel of the series of sinkholes has a certain degree of connectivity,and its linear connection constitutes the basic prototype of the gully or gully,which eventually destroys the topography of the tableland in the form of cutting.The horizontal arrangement of parallel sinkholes at the same altitude constitutes the basic prototype of the plateau erosion terrace.Multiple series sinkholes can also form a parallel relationship,resulting in a larger area of tableland degradation.