Heterogeneity Of Granite Soiland Hydrological Process On Benggang Slope

A special type of special soil erosion landform,associated with a permanent collapsing gully on a low hillslope developed from granite red soil,is mainly triggered by water erosion and gravity erosion,is present in in the tropical and subtropical granite red soil areas of southern China,and locally called Benggang.In the field,the drainage area of a Benggang can be very small,and the headwall can retreat（collapse）regardless of the upper slope drainage area.Thus,the influence of the alteration of the soil water content has an important influence on the triggering of Benggang erosion.The alteration of soil water content has an important impact on shear strength,which is an important component influencing a soil’s resistance to erosion and the headwall stability.An increase in the soil water content leads to a decrease in the shear strength and is thus a driving force for the formation of mass movements and headwall retreat,affects the process of gravity erosion,which can lead to the development of Benggang erosion.Therefore,it is essential to pay attention to the impact of the alteration of the water content on Benggang erosion.At present,most of the research on Benggang erosion has focused on the water erosion process,with emphasis on the surface water sediment movement and runoff characteristics.The influence of the alteration of the soil water content remain poorly constrained and most of these studies focused on a certain soil property,without considering the impact of the spatial distribution of these properties on Benggang erosion,the comprehensive impacts of different soil configurations on the triggering of Benggang erosion are unclear.Therefore,the study on the influence of slope soil properties on soil water content and explored the relationship between soil properties and the development of slope erosion can provide useful information on the mechanism of Benggang erosion,and is of great significance to further study the control of Benggang erosion.The experimental site is located in Tongcheng County,Hubei Province.According to transitions in pedogenic process,the five soil layers on the granite slope of the test site are further divided into 10 sublayers from top to bottom:surface layer（Ⅰ,Ⅱ）,red soil layer（Ⅰ,Ⅱ）,transition layer（red-transition layer,transition layer,transition-sandy layer）,sandy soil layer（Ⅰ,Ⅱ）,and detritus layer（>400 cm）.)We measured the soil basic physical,hydraulic properties in different soil layers and slope positions to analyzed the spatial heterogeneity of soil properties of granite Benggang slope,and monitored the soil water content by TDR on typical Benggang,recorded a Benggang erosion event and simulated the effect of alterations of soil water content on different soil configurations of a Benggang slope by using the VADOSE/W model to find out the impact of heterogeneity of granite soil on the hydrological process of Benggang slope soil and the trigger conditions of Benggang erosion in different soil configurations.The main results are as follows:（1）The basic physical properties,hydraulic properties,shear strength and limit water content of the soil in the vertical profile of the granite slope of Benggang show obvious variability.With an increase in depth,the coarse particles content gradually increases and the fine particles decreases,as the soil texture transitions from clay loam to sand.The surface layer,red soil layer and transition layer contain higher contents of fine particles,and more small pores are distributed within the fine-grained soil,generating a higher water retention capacity;The sandy soil layer and detritus layer contents of coarse particles,and large particles among the coarse particles result in larger gaps between particles and therefore a lower water retention capacity.Saturated hydraulic conductivity decreases and then increases with increasing soil depth,the saturated hydraulic conductivity values are higher in the surface layer（1.01 mm/min）but lower in the transition layer（0.02 mm/min）.With the increase of soil water content,the shear strength decreases and the shear strength of sandy soil layer is less than those of red soil layer and the transition layer.when the soil water content of red soil layer and the transition layer is higher（e.g.,0.42 cm³/cm³）,the corresponding shear strength is greater than that of sandy soil layer at a lower soil water content（e.g.,0.32 cm³/cm³）.And the liquid limit and plastic limit of red soil layerⅠand the transition layer are higher than those of sandy soil layer,that is to say,with increasing soil water content,sandy soil reaches its liquid limit faster than red soil layer.This property suggests that although the red soil layer and the transition layer have high soil water content,,they will not necessarily cause slope instability,and Benggang erosion may be more easily triggered by an increase in the soil water content of the deep sandy layer.（2）In the direction along the Benggang slope,the saturated hydraulic conductivity decreases with an increasing distance from the headwall while thesoil water retention capacity increasing,and there is significant difference in saturated hydraulic conductivity between different slope positions（P<0.05）.This phenomenon does not disappear with an increase in the soil depth.There are no significant change in the soil bulk density and texture of the same soil layer with different slope positions and the dominant factor contributing to this phenomenon is the cracks distribution.The cracks are induced by Benggang erosion in the lower slope,under the action of gravity,there is generally tensile stress in the sloping soil,which can cause gradual soil tensile deformation and rupture,increase the development of soil pores into cracks,and form a macropore-preferred flow channel.The soil with a closer distance to the collapsing wall is subjected to a downward force,which facilitates the formation of cracks,a macropore size of>1 mm is 3.52%in the lower slope and greater than that in the middle slope（1.52%）and the upper slope（1.46%）.Therefore,the saturated hydraulic conductivity values are larger on the lower slope than those on the middle and upper slopes.Due to the influence of the macropores on the slope,the saturated hydraulic conductivity increased with increasing inner ring diameter from 10 cm to 40 cm but that no significant difference existed between inner diameters of 30 cm and 40 cm,indicating that an internal diameter of 30 cm is the most reasonable for saturated hydraulic conductivity measurements.When using smaller samples,some of the smaller pores are squeezed and blocked during the sampling process,resulting in the measured soil saturated hydraulic conductivity being less than the actual value.The soil infiltration process applied to different slope positions and soil depths of a Benggang slope can be simulated by the Kostiakov and Philip equations.（3）The heterogeneity of granite soil affects the distribution of soil water content.The four-year soil water content monitoring shows that the distribution of soil water content in different slope positions and soil layers has a similar trend.In the rainy and dry season,the distribution features of soil water content are as follows:upper slope>middle slope>lower slope,topsoil>deep soil.The process of soil water distribution and redistribution on the slope is related to the soil properties and crack distribution.Compared to the soil at the upper slope,the soil at the lower slope contains denser and longer cracks,which makes the soil water more likely to percolate into the deep soil.Therefore,the soil water content in the soil（0-80 cm）of the lower slope near the headwall with more cracks was lower than that in the upper and middle slope.In the vertical profile,the water content of 0-40 cm soil depth is lower than that of 60-80 cm soil depth.（4）The VADOSE/W model adequately simulated the change in the soil water content along a slope under different climatic conditions without the addition of cracks,there were obvious deviations between the simulated and measured results,especially at the lower slope and headwall,where the R²,RE,RMSE range was 0.19-0.95,3.80-21.20%,0.98×10^-2-7.21×10^-2,respectively,and these deviations could not be eliminated by adjusting saturated hydraulic conductivity.After equivalent cracks were added by adjusting the material properties,the simulation results for the lower slope and collapsing wall showed a significant reduction relative to the real value,where the R²,RE,RMSE range was 0.81-0.89,-0.37-4.57%,0.41×10^-2-2.50×10^-2,respectively.Thus,the effect of cracks on the soil water content was different from that of saturated hydraulic conductivity.We propose that the distribution of cracks should be considered when analyzing the change in the soil water content in a simulation mode.The cracks have obvious influence on the water distribution of the Benggang slope,especially under rainfall conditions（stages III and IV）.In the vertical profile,with cracks the water content decreased in the topsoil（red soil layer and transition layer）and increased in the deep sandy layer relative to the situation without cracks.In the direction along the slope,the modeled soil water content in the topsoil of the lower position near the headwall with more cracks was lower than that in the upper slope position during stages III and IV.This means that the distribution of cracks has an important influence on the soil water content,cracks can accelerate the downward movement of water and increase leakage,which results in an increase in the water content and pore water pressure in deep soil.（5）Benggang slope generally has three different soil configurations（SCs）:red-transition-sandy（SCⅠ）,transition-sandy（SCⅡ）and sandy（SCⅢ）.There is a difference in the distribution of soil water content on slopes with different soil configurations（SCs）.For SCⅠand SCⅡ,the soil water content is higher in topsoil than in deeper soil,while for SCⅢ,the opposite trend is observed.For SCⅠ,a positive pore pressure first appears in the transition layer,and for SCⅡand SCⅢ,a positive pore pressure first appears in the deep sandy layer.The limit water content and shear strength decreased with increasing granitic profile depth,topsoil（red soil layer and transition layer）has higher limit water content and shear strength than the sandy soil layer,which is the unique material basis for the occurrence of Benggang erosion.The high soil water content in deep sandy soil and the buildup of positive pore water pressure and reduce shear strength result in the triggering of Benggang erosion.Our study reveals that the Benggang erosion is triggered by an increase in the soil water content in the deep sandy layer.Among different SCs,if there is still topsoil,it can reduce the infiltration of rainfall into the deep sandy layer,thus slowing the rate of increase of the soil water content in the deeper layer,which can reduce the possibility of collapse.The construction of drainage ditches and horizontal terraces may make the red soil layer shallower or expose the sandy layer,thereby increasing precipitation infiltration and the water content of the deeper sandy soil layer,which can easily cause Benggang erosion.Therefore,the key to preventing the Benggang erosion is to protect the topsoil（red soil and transition layer）.