Study On Rill Erosion Of Purple Soil Under The Effect Of Subsurface Water Flow

The young purple soil formed on sand shale has high infiltration performance,especially the large pores formed by root holes,insect holes or straw rot in purple soil slope farmland,which will become the preferential flow channel for rainwater to enter the soil.When rainwater seeps into the weak permeable layer at the interface between the cultivated layer and the underlying parent layer of soil,the infiltration is blocked and it is easy to form a subsurface water flow.Subsurface water flow will change the pore water pressure of the soil and reduce the critical shear stress of the soil,thereby increasing the erodibility of the soil.The coupling of surface and subsurface water flow will cause strong soil erosion,and the study of its process and mechanism is of great significance.However,due to the difficulty of observing the subsurface water flow in the soil and the complex and variable of erosion process,it has always been a difficult problem regarding the influence of the subsurface water flow on the soil erosion process.In this paper,the purple soil is taken as the research object.Through the limited narrow trench simulation test,the subsurface water flow is constructed by supplying water from the bottom.On this basis,Sediment transport capacity and sediment content of rill flow under different saturation depths（5,10,15 cm）and differen hydraulic conditions(2,4,8 L·min^-1 and 5°,10°,15°)are measured.The erosion process of purple soil under the action of subsurface water flow was systematically studied,the related erosion process model was established,and the effects of different saturation depths,slopes,flows and their interactions on the erosion process of small grooves were analyzed.This study provides data reference for the parameters of the purple soil rill erosion model,and provides a method and theoretical basis for studying the coupled erosion under the action of surface runoff and subsurface water flow.The study mainly reached the following conclusions:（1）The sediment transport capacity of purple soil fine ditches under different conditions of subsurface water saturation depth,slope and flow rate is between 0.159to 0.983 kg·s^-1·m^-1.Under different slope and flow conditions,the increase in sediment transport capacity caused by the saturation depth of subsurface water flow is between 0.99%and 43.24%.The sediment transport capacity of purple soil is positively correlated with the subsurface water flow saturation depth,slope and flow,but the growth rate of sediment transport capacity gradually decreases with the increase of slope and flow.Using multiple regression equations to analyze the impact of the interaction between the three on the sediment transport capacity,the results show that the interaction between subsurface water flow and discharge,and slope and discharge will increase the sediment transport capacity.The change of sediment transport capacity with the subsurface water depth can be well modeled by an exponential function（R² is between 0.87 and 0.99,P<0.01）.The sediment transport capacity and slope are in a logarithmic function relationship（R² is between 0.87 and0.99,P<0.05）,and are in an exponential function relationship with flow rate（R² is between 0.98 and 0.99,P<0.05）.Regression fitting of the relationship model between sediment transport capacity and slope and discharge under different water saturation depth conditions,the results show that the discharge coefficientβis between 1.14 and1.16,and the slope coefficientγis between 0.54 and 0.68,which shows Than the flow has a greater impact on the sediment transport capacity.Using the parameters fitted by the relationship model between the erosion sediment content of the rill and the rill length,according to the definition of the critical slope length of the sediment transport,the critical slope of the purple soil sediment transport under different subsurface water flow saturation depths and surface hydraulic conditions is calculated It is between 9.1and 21.46 m.（2）The sediment content of the rill flow increases with the growth of the rill length,but the growth rate of the sediment content gradually decreases,and finally the sediment content approaches the sediment content when the water flow reaches the sediment transport capacity.Substituting the measured maximum sediment content into the relationship between the sediment content of the rill and the rill length regression regression fits the rill erosion process at different saturation depths of subsurface water flow,which can well solve the problem of rill flow under small slope and small flow can not reach the sediment transport capacity.The fitting results show that the sediment content of the rill increases exponentially with the rills’length at different subsurface saturation depths.Linear regression analysis was performed on the analytical calculated value and the experimental value.The slope of the regression equation was between 0.90 and 1.19,indicating that the analytical value and the experimental value had a good consistency,which proved the feasibility of the above numerical analysis method.（3）The effects of subsurface saturation depth,slope and flow on the erosion and sediment content of purple soilrill were evaluated through indoor experiments.The measurement results show that the sediment concentration is positively correlated with the subsurface saturation depth,slope and flow,and the interaction of the saturation depth of the subsurface water flow with the slope and the flow will also increase the sediment concentration of the flow.Regression fitting was carried out on the relationship model between the sediment concentration of the water flow and the slope and discharge under different conditions of subsurface water saturation.The results showed that the slope coefficient m was between 0.83 and 0.94,and the discharge coefficient n was between 0.19 and 0.28.The effect of flow on the sediment content is not as great as that of slope,and this effect decreases with the increase of slope.