Characteristics,Distribution And Geographical Environmental Factors Of Collapsing Gully In Granite Region Of Southern China

Collapsing gully is the most serious and special soil erosion landform in the tropical and subtropical granite red soil areas of southern China.For a long time,collapsing gully erosion has not only been widely concerned by researchers,but also attracted the attention of governments and local people.People's knowledge of collapsing gully erosion has been raised to a new height,and a series of control measures have been put forward.However,collapsing gully erosion is still well developed in South China,and the effect of pilot treatment is not obvious,because the erosion mechanism of collapsing gully erosion is lack of systematic research.Therefore,the study on the distribution characteristics and influencing factors of collapsing gully erosion in different latitudes is of great significance to further study the mechanism and the control of collapsing gully.This study selects the typical granite collapsing gully distribution area of Tongcheng?TC?in Hubei,Ganxian?GX?in Jiangxi,Changting?CT?in Fujian and Wuhua?WH?in Guangdong as the research object.We used field investigation,sampling and indoor analysis methods to study the physical and chemical properties and fertility of farmland soil in the collapsing gully area.The number,area,shape parameters and activity types of the collapsing gully areas in the typical area were investigated and the evaluation of the erosion strength of the collapsing gully at different latitudes was conducted.We combined the geological background data and the field measurements of granite joints to find out the impact of geological factors on collapsing gully.Meanwhile,we selected typical collapsing gullies in different areas and sampled soil profiles according to A layer,B1layer,B2 layer,BC layer,C1 layer and C2 layer.By analyzing the differences of basic physical and chemical properties,oxides,micro-morphology and water characteristics,the intrinsic material basis of collapsing gully erosion was clarified.The factors reveal the external driving mechanism of erosion and the cause of the difference of erosion intensity.The main results are as follows:The scale and activity intensity of collapsing gully erosion increases with latitude.There are 128 collapsing gullies in Tongcheng sample area with a total area of 8.27 hm²,256 collapsing gullies in Ganxian sample area with a total area of 21.47 hm²,169collapsing gullies in Changting sample area with a total area of 17.72 hm²,and 306collapsing gullies in Wuhua sample area with a total area of 37.07 hm².In the four sample areas,the number of collapsing pits is the largest,and the proportion of active collapsing gullies increases gradually from north to south.Soil in catchment area,collapsing gully body and alluvial fan area of collapsing gully is seriously degraded,soil sand content increases,acidity increases,and nutrient loss.The grade coefficient of soil fertility in the alluvial fan area showed a gradual decrease from the control area,the edge of the fan,the middle of the fan to the top of the fan.Soil degradation has a direct impact on crop yields,and it is necessary to take measures to restore soil quality.The morphological parameters of the collapsing gully have obvious variation law from north to south with the latitude zone.From north to south,the area,height,slope,and bevel length of the collapsing gully,the height,width,inclination angle and length of the main trench,the width of the furrow and the number of branching grooves are all gradually increasing.There was a significant correlation between morphological parameters?p<0.05?.According to the principal component analysis morphological parameters,the results show that the height of the collapsing gully,the height of the collapsing gully and the weight of the area of the collapse are large,and the erosion intensity of the collapsing gully can be highly indicated.According to the principal component-cluster evaluation method,the collapsed hills in the four sample areas were divided into ?,?,? and ? erosion intensities.The erosion intensities of collapsing gully increased gradually from ? to ?.Collapsing gully ? accounted for 69.53%,collapsing gully ? for 25.00%,collapsing gully ? for 3.91%,collapsing gully ? for 1.56%in Tongcheng;Collapsing gully ? for 42.97%,collapsing gully ? for 43.36%,collapsing gully ? for 10.55%,collapsing gully ? for 3.13%in Ganxian;Collapsing gully ? for39.64%,collapsing gully ? for 33.73%,collapsing gully ? for 13.02%;collapsing gully ? for 13.40%in Changting and collapsing gully ? for 13.40%,collapsing gully ? for 38.89%,collapsing gully ? for 31.70%and collapsing gully ? 16.01%.It can be seen that the erosion intensity of type I and type II is the main erosion intensity in each collapsing gully sample area.However,with the latitude zone from north to south,the proportion of erosion intensity of type III and type ? increases gradually,indicating that the erosion intensity of collapsing gully increases gradually from north to south.The hilly area formed by the geological tectonic movement provides the topographic conditions for the collapsing gully erosion of various areas.The granites in Tongcheng,Changting and Wuhua sample areas were all exposed in Yanshanian period.The granites in Ganxian sample area were exposed in Caledonian period.The loose weathering crust of granite is the material basis for the collapsing gully erosion.Granite joints have high density and complex orientation,all of which are mainly of high dip angle and have high inheritance.The dominant directions of joints in Tongcheng sample area are NE and NW,the dominant directions of granite joints in Ganxian sample area are NNE and NW,the dominant directions of joints in Changting sample area are NNE and NW,and the dominant directions of joints in Wuhua sample area are NNE and NNW.The joints of granite are correlated with the channel of collapse and the erosion intensity to a certain extent.The trend of joint has a good correlation with the trend of main gully and branch ditch,especially the branch gully of collapsed hill is controlled by the trend of joint.The greater the inclination of joint,the greater the average depth of branch ditch,promote the development of collapsing gully and branch ditch,and accelerate the collapsing gully.The gradation difference of granite soil properties is the internal cause of collapsing gully.With the increase of soil depth,soil organic matter and cation exchange capacity gradually decreased,soil sand content increased,clay and kaolinite content decreased,soil properties of the coefficient of variation is larger.Soil spectral characteristics showed hierarchical differences,soil boundary moisture content had a decreasing trend,cohesion and internal friction angle decreased with the increase of soil depth.Soil oxides mainly consist of SiO₂,Al₂O₃ and Fe₂O₃.From C layer to A layer,SiO₂ decreases gradually,while Al₂O₃ and Fe₂O₃ increase gradually.This indicates the process of desilication and enrichment of aluminum in granite soils.The contents of Al₂O₃ and Fe₂O₃ are positively correlated with cohesion and internal friction angle?p<0.01?,and SiO₂ content is positively correlated with cohesion and internal friction.There was a significant negative correlation between rubbing angle?P<0.01?.Soil micro-morphological characteristics showed that matrix cementation was dominant in upper soil layer,and pore cementation was dominant in lower soil layer.The area,perimeter,length and width of soil coarse bone particles increased gradually with the increase of soil depth,while roundness decreased gradually.The suction stress of each soil layer increases with the increase of matrix suction.Under the same water content,the suction stress of upper soil layer is larger,while that of lower soil layer is smaller.Therefore,the underlying layer of granite soil is loose,the mechanical stability is poor,while the upper layer is compact,and the mechanical stability is better is the intrinsic cause of the collapse erosion development.The main disturbance modes of slope include deforestation,surface grassland destruction and improper mountain development;the main disturbance modes of slope foot include the exploitation of soil and rock resources,transportation and industrial construction,and different land use patterns.The average annual rainfall ranged from1446 mm to 1712 mm,but the variation of latitude zone was not obvious,and the average annual temperature ranged from 17.1 to 21.4?,and gradually increased with the latitude zone from north to south.The vegetation coverage of Tongcheng,Ganxian,Changting and Wuhua sample areas were 14.7%,22.1%,39.7%and 13.1%respectively.Low vegetation coverage promoted the formation and development of collapsing gully,and collapsing gully would still develop under better vegetation conditions.The area is mainly distributed in the elevation range of 40³30 m.Landslides in Tongcheng,Ganxian and Changting sample areas are mainly developed in the range of 15-25 degrees,while those in Wuhua sample area are mainly developed in the range of 20-30 degrees.Ganxian,Changting and Wuhua plots distribute more collapse hills on the sunny slope than on the shady slope,while Tongcheng plots distribute less collapse hills on the sunny slope than on the shady slope.From north to south,the slope,slope length and relative elevation have a significant increase.Correlation analysis showed that environmental factors were highly correlated with morphological parameters of landslide.Grey correlation analysis showed that slope length was the most important factor affecting the latitude difference of erosion intensity,followed by relative elevation,annual average temperature and annual maximum continuous rainfall.