The Erosion Morphology And Hydraulic Evolution Process Of The Rill Erosion On The Steep Slope Of The Benggang Area Under Runoff Scouring

Rill erosion is a common soil erosion phenomenon in residual soil areas of granite,and previous studies have pointed out that gully erosion can lead to a series of serious water and soil loss phenomena,such as Benggang.In this study,the residual soil of granite in Wuli Town,Tongcheng County,Hubei Province was selected as the research object.Topsoil soil layer（B）and red soil layer（H）soils were selected to prepare reshaped soil for indoor erosion simulation experiments.Based on the climatic conditions of the study area and previous research results,slope angles of 25°and 45°,and erosion flow rates of 4.5 L/min and 7.5 L/min were set.The changes in runoff sediment concentration were determined by grab sampling,while soil stress and moisture content were measured using soil pressure and moisture sensors in the channel.In addition,internal channel measurements such as gully width,gully depth,and runoff velocity were also taken.The morphology of gully erosion and the evolution of hydraulic parameters were studied under the conditions of single topsoil layer（B）,single red soil layer（H）,and upper surface and lower red soil layer（BH）.The main research conclusions are as follows:（1）In terms of rill morphology changes,rill erosion can be divided into three stages:uniform incision stage,headward erosion stage,and ephemeral gully transition stage.Initially,the depth of erosion along the entire channel is similar,but as the gully develops,the depth becomes significantly different in different parts of the gully,with the central part experiencing the most severe erosion.The cross-section of the gully is more likely to become a"trapezoid"shape,increasing the probability of rockfall.The bottom of the gully experiences sediment accumulation with shallower depth.There is a good functional relationship between the total incision depth of the rill and the average incision rate（adjusted R²=0.49～0.90）,with the incision rate slowing down continuously as the rill develops,until it approaches zero,and the rill as a whole transitions to a ephemeral gully.The average incision rate of the red soil layer is greater than that of the upper surface and lower red soil layer and the topsoil layer.The strong anti～collapse ability of the topsoil layer under high moisture content is the main reason for its slow incision.（2）In terms of hydrodynamic parameter changes,throughout the entire process of rill erosion,the flow velocity and Froude number decrease continuously,while the Reynolds number shows a trend of increasing first and then decreasing.There is a significant correlation between flow velocity and other runoff parameters.Under the condition of a flow rate of 4.5 L/min,there is an exponential function relationship between the Froude number and flow velocity（adjusted R²=0.83～0.97）,while under the condition of a flow rate of 7.5 L/min,there is a quadratic function relationship between the two（adjusted R²=0.79～0.99）;the Reynolds number always has a quadratic function relationship with flow velocity（adjusted R²=0.41～0.87）.（3）Regarding changes in erosion sediment yield,there is a power function relationship between the runoff sediment concentration R_sand the flushing duration t during rill erosion,expressed as R_s=At^-b（adjusted R²=0.23～0.97）.The occurrence of collapse in the channel will increase the runoff sediment concentration by18.57%～541.93%.The average runoff sediment concentration is greater in the red soil layer than in the upper surface and lower red soil layer and the topsoil layer.At the same time,there is a good linear relationship between runoff sediment concentration and flow velocity during the first stage of rill erosion（adjusted R²=0.69～0.91）.（4）During the period before rill head collapse,the total soil pressure shows a continuous upward trend and reaches its peak at the moment of collapse.As a result of soil collapse,the total soil pressure decreases.The increase in soil pressure before collapse is 1.05%～6.94%,and the decrease after collapse is 15.31%～44.23%.The greater the increase of soil pressure before collapse,the greater the decrease after collapse.After the collapse occurs,the runoff velocity and Froude number briefly decrease,while the Reynolds number briefly increases.The runoff inside the channel tends to become slow and more disorderly,and then returns to a normal changing trend.The conclusion of this study indicates that in the process of rill erosion in residual soil of granite under steep slope conditions,the central part of the slope is the most severely eroded,and the larger gully depth and width create conditions for Benggang development.The head of the gully collapses when the stress exceeds the critical value,and the occurrence of gully collapse can be inferred by changes in hydraulic parameters such as flow velocity,Froude number,and Reynolds number.Overall,the topsoil layer has a stronger resistance to erosion than the red soil layer,and the undercutting rate and sediment yield of the topsoil layer are smaller than those of the red soil layer.Protecting the topsoil layer can reduce soil loss to a certain extent.