The Effect Of Subsurface Flow Along A Hillslope Of Benggang And Its Hydrodynamic Characteristics On Benggang Erosion

Benggang is the most serious soil erosion landform in the tropical and subtropical hilly areas of southern China,which seriously hinders local agricultural production and economic development.For a long time,the research on the formation mechanism of Benggang erosion has paid more attention to the temporal and spatial variation of soil water on slope,the erosion of soil on slopes by surface runoff,and the influence of changes in soil shear strength under different soil water conditions on the Benggang erosion.In fact,in the process of Benggang erosion development,in addition to factors such as surface runoff and soil water content,subsurface flow is also an important part,and it participates in all stages of Benggang erosion.Therefore,the purpose of this study is to explore the impact of different types of subsurface flow on the occurrence of Benggng erosion and the reasons for this impact.This paper measured the the soil water content,pore water pressure and soil matrix potential of the Benggang catchment(two parallel monitoring zones,each monitoring zone contains the upper,middle and lower three part of the slope)and four soil layers(0-20,20-40,40-60 and 60-80 cm layers)in the different soil profiles of Benggang wall.This study explores the impact of subsurface flow on Benggang erosion by the differences in soil physical and chemical properties,the temporal and spatial distribution of soil water,the temporal and spatial distribution of different types of subsurface flow and the hydrodynamic characteristics,the soil erosion capabilities of subsurface flow.The main results are as follows.(1)On the Benggang water catchment slope,from the top to the bottom,the surface to the deep,the soil types gradually changed from clay soil to loamy soil,and the sand particles in the soil gradually increase,and the clay particles gradually decrease.In different positions and soil layers on the slope,there are more macropores in the lower part of the slope and the deeper soil layer,and the number of macropores in the lower part of the slope is twice that of the upper part of the slope.A large number of macropores in the lower part of the Banggang water catchment slope greatly promote the development of Benggang erosion.(2)Soil macropores and soil texture have an important impact on the changes of soil water content at different locations and depths.On the upper part of the slope,because of the high content of soil clay particles and few macropores,the soil water content is high with little variation.In the lower part of the slope,due to the abundant macropores,the soil water content is relatively low,but the soil water content changes greatly.The soil water content in the study area is irregularly distributed with"high on the top and low on the slope",resulting in a"front-heavy and back-feet"state of the slope.The special state of soil water promoted the occurrence of Benggang erosion.(3)High-frequency monitoring of soil water content and soil temperature changes determined the occurrence of different types of subsurface flow on the slope.The different types of subsurface flow have different manifestations of soil water and heat changes.In the soil layer where preferential flow occurs,the change of soil water content and soil temperature is earlier than that of other soil layers.Therefore,the change of soil temperature is first controlled by rain temperature,and then by the corresponding upper soil temperature.In the matrix flow,the soil temperature is initially controlled by the temperature of the adjacent upper soil layer and then by the temperature of rainwater.(4)The spatial distribution of different types of subsurface flow result in the temporal and spatial differentiation of soil water content on the slope.The occurrence frequency of preferential flow is different at different locations on the slope and soil layers.The closer to the Banggang wall,the deeper the soil layer,the higher the frequency of preferential flow.The distribution of preferential flow results in special distribution of soil water on the slope.When the frequency of preferential flow doubles,the average soil water content of the slope decreases by 10%,and the coefficient of variation increases by 70%.The higher the occurrence frequency of preferential flow,the lower the average soil water content and the stronger the variation of the soil layer.(5)By improving the calculation method of hydraulic factors of uniform pipe flow,the hydraulic factors(Reynolds number,pore water pressure and water flow shear force)of different types of subsurface flow(preferential flow and matrix flow)in the two profiles are calculated.When matrix flow is the main form of subsurface flow,the Reynolds number is less than 0.1,and the water flow shear force is also less than1.5N/m~2.When preferential flow is the main form of subsurface flow,the Reynolds number is greater than 0.8 and the water flow shear force generally greater than 3 N/m~2.Different types of subsurface flow have different control factors for water flow shear force.For preferential flow,the pore water pressure(generally close to 2KPa)caused by the rapid accumulation and leakage of soil water inside soil cracks is dozens of times that of matrix flow(less than 0.5 KPa).Pressure potential is a key factor to control the water flow shear force of preferential flow.In the matrix flow,the water flow shear force caused by the matrix potential is dozens of times that caused by the pressure potential.The matrix potential is the key factor to control the water flow shear force of the matrix flow.The calculation and comparison of the Reynolds number,pore pressure and water flow shear force of different types of subsurface can be widely used in future research.(6)Considering the pore pressure,the Reynolds numbers of different types of subsurface flow during the Benggang collapsed process and the starting forces on soil particles of different types of water flow shear force on Benggang wall were calculated.On Benggang slope,the Reynolds number of preferential flow is ten times higher than that of matrix flow.On Benggang wall,for same diameter soil particles,the starting force of preferential flow is also dozens of times that of matrix flow.The Reynolds number and the starting force on the soil particles of the subsurface flow can be used as important indicators to measure the erosion ability of the subsueface flow.During the movement of the subsurface flow,the preferential flow scoured more soil particles(3 times that of matrix flow),and the average particle diameter is larger(3 times that of matrix flow),the damage to soil cracks is stronger.This means that the preferential flow has stronger soil erosion ability.The erosion of soil particles by the subsurface flow,the changes in the soil microstructure(such as the generation of cracks),and the destruction of soil particles are the important was that the subsurface flow promotes the development of Benggang erosion.