Study On Rill Erosion Of Several Surface Soils Under Simulated Rainfallin Typical Granite Collapse Area

Granitic red soil is mainly distributed in the high temperature and rainy areas of southern China.It is one of the main eroded soils in the south of the Yangtze River in China,and the collapse is the main erosion form in this area.The rill erosion causes the erosion and loss of surface soil in the water-collecting slope of the collapsed area,which is the beginning of the soil erosion in the collapsed area,and is also one of the main ways of slope erosion in the collapsed area.In this paper,red surface soil developed from granite in Tongcheng,Changting,Wuhua and there eroded degrees of layer A,layer B and layer BC in Changting were selected to carry out simulated rainfall experiment.On the basis of field investigation,continuous experiments were carried out on red surface soil developed from granite in Tongcheng,Changting,Wuhua under 15°slope and two experimental conditions:rainfall(90 mm h^-1)and rainfall plus scour(90 mm h^-1+2 L min^-1).red soil in Changting with different eroded degrees of layer A,layer B and layer BC were tested by rainfall plus scour(90 mm h^-1+2 L min^-1,120mm h 2.66 L min^-1).Characteristics of runoff and sediment yield,distribution of sediment particle,rill morphological characteristics and hydraulic mechanism were studied in order to clarify the erosion mechanism of granite red soil.The main conclusions are as follows:?1?In terms of characteristics of runoff and sediment yield,the runoff rate of granite red surface soil in different collapsed areas increases with runoff time and tends to be stable.The runoff rate in different erosion stages is characterized by rill stability stage>rill development stage>Sheet erosion stage.The concentration of sediment and sediment yield increased at first and then decreased slowly and finally stabilized with the duration of runoff.The significant difference between soils were marked as CTA<TCA<WHA.The concentration of sediment and sediment yield in different erosion stages were Interrill phase>Rill advance phase>Rill stable phase,sediment yield and the concentration of sediment increased with flow.The soil erosion rate,sediment concentration and runoff yield of soil with different erosion degrees increased with the intensity of rainfall and scour.CTA is most affected by the intensity of rainfall and scour.The characteristics of soil loss rate and runoff rate in three erosion degrees soil were also different,soil loss rate is CTA<CTB<CTC,and the runoff rate is CTB>CTA>CTC.The multivariate stepwise regression analysis found that the change of soil loss rate and sediment concentration was mainly attributed to the difference of clay and silt content,and the difference of runoff rate was mainly attributed to the difference of soil bulk density.?2?In terms of the distribution of sediment particles,the sediment particle content of granite surface soil and different erosion degree soil in different collapse areas showed a decreasing trend with fine particles?clay,silt?,and a increasing trend whit the coarse particles?gravel,sand?.The enrichment rate of sediment particles was characterized by the enrichment rate of clay andsilt was more than 1,and the enrichment rate of gravel and sand is less than 1.As the scouring flow increases,the enrichment rate of cohesive particles decreases,and grit gravel The enrichment rate increases and approaches 1.The fractal dimension of sedimentation increased with the intensity of rainfall and scour,and the fractal dimension was significantly positively correlated with the content of clay,which was significantly negatively correlated with the content of sand.The relatively high fractal dimension driven by heavy rain reflected the decrease of sand and the increase of clay.It can be used as an evaluation index for the distribution of sediment size in the soil erosion.?3?In terms of hydraulic mechanism,the average velocity of water flow,Reynolds number,Froude number,runoff shear force,flow power and unit flow power increased with the increase of rainfall scour intensity.The resistance coefficient decreased with the increase of rainfall scour intensity.The average flow velocity,Reynolds number and Froude number of soils with different erosion degrees were all expressed as CTA<CTB<CTC.From the fitting effect,the average flow velocity,Reynolds number,Froude number,runoff shear force,water flow power and unit water flow power were high with sand loss rate,which can be used to predict the interaction of granite red soil in rainfall runoff.Characteristics of runoff and sediment yield in the erosion process of the downslope.?4?In terms of morphological development of rills,rill headward erosion rate of granite surface soils in different collapsed areas was CTA<TCA<WHA.With the increase of scouring flow,rill headward erosion rate increased,and the increase of WHA was more obvious,tca is the second,and cta is the smallest.The ratio of width to depth of rills is CTA>TCA>WHA.The ratio of rill width to depth was cta>tca>wha,which decreased with the increase of scour intensity.The energy required for the development of the rill to the same depth is CTA>TCA>WHA.Under the same energy consumption,the rill depth was WHA>TCA>CTA.With the increase of scour flow,the energy required decreased when the rill is cut down and the rill head retreated to the same position under the condition of single rainfall.