| Purple soil is one of the main soil types in southwest area of China,which has strong physical weathering and good infiltration performance.But the parent material layer has weak water permea-bility and infiltration is blocked,so the surface soil is easy to be saturated during continuous rainfall condition.Comparing with unsaturated soil,the cohesive force between particles in saturated soil is weak,and its corrosion resistance is poor,which will aggravate soil erosion.As two important ero-sion forms of soil erosion on the slopes,sheet and rill erosion are influenced by the hydrodynamic characteristics of runoff.It is of great significance to deeply understand the hydrodynamic mecha-nism of sheet and rill erosion and improve the accuracy of soil erosion prediction model by distin-guishing the sheet and concentrated flow and erosion form and study the their characteristics and differences under different factors.With this in mind,the experiment was conducted to measure the runoff velocity,sediment yield and critical slope length of sheet erosion transfer to rill erosion(sheet flow transfer to concentrated flow)by indoor artificial simulated rainfall test under different slope gradients(2°,5°,10°,15°,20°)and rainfall intensities(30,60,90 mm/h)combined with saturated water supply device.Hydrody-namic parameters such as runoff depth,Reynolds number,Froude number,Darcy-Weisbach re-sistance coefficient,runoff shear force,flow power,unit flow power at different slope length were calculated based on average velocity.Sheet and rill erosion sections were distinguished according to critical slope length,and based on this,the hydrodynamic parameters of sheet and concentrated flow and the sheet and rill erosion rate were distinguished and quantified.The hydrodynamic characteris-tics of sheet and concentrated flow and the change characteristics of sheet and rill erosion rate on saturated purple soil slopes under different slope gradients and rainfall intensities were analyzed,so with the correlation between sheet and rill erosion rate and different hydrodynamic parameters.Ac-cording to the relevant analysis results,the prediction equations of sheet and rill erosion rate on sat-urated soil slope were constructed.The main research conclusions are as follows:(1)The critical slope length decreases with the increase of slope gradient and rainfall intensity.The average velocity of concentrated flow on slope is 2.01～2.77 times that of sheet flow,and their both increase with the increase of slope gradient and rainfall intensity.Under the same working con-ditions,the runoff depth of concentrated flow is deeper than that of sheet flow,and the runoff depth of sheet flow and concentrated flow decreases with the increase of slope gradient,and increases with the increase of rainfall intensity as a whole.The sheet flow pattern is laminar throughout the exper-iment.The concentrated flow pattern is laminar under the condition of light rainfall intensity(30mm/h),and transitional under the conditions of moderate(60 mm/h)and heavy(90 mm/h)rainfall intensity.Under the condition of low slope gradients(2°,5°and 10°),the sheet flow pattern is slow flow,while under the condition of hige slope gradients(15°and 20°),the sheet flow pattern is rapids.The concentrated flow is mostly rapids under different conditions.Under the same conditions,the Darcy-Weisbach resistance coefficient of sheet flow is larger than that of concentrated flow,which decreases with the increase of slope gradient and rainfall intensity,while that of concentrated flow increases with the increase of slope and rainfall intensity.The shear force,flow power and unit flow power of concentrated flow on saturated soil slope are higher than that of sheet flow under the same slope gradient and rainfall intensity,and their values increase with the increase of slope gradient and rainfall intensity.Rainfall intensity has greater influence on runoff shear force than slope gradient,while slope gradient has greater influence on unit flow power than rainfall intensity.For the flow power,rainfall intensity has greater influence on sheet flow power,and slope gradient has greater influence on concentrated flow power.(2)The soil erosion rate on saturated slope increases with the increase of slope gradient in loga-rithmic function and with the increase of rainfall intensity in power function.Rill erosion rate in-creases linearly with the increase of slope gradient and with the increase of rainfall intensity in pow-er function.Under the condition of 2°slope gradient and 30 mm/h rainfall intensity,the rill erosion rate on the saturated soil slope is 0 times that of the sheet erosion rate,but with the increase of slope gradient and rainfall intensity,the gap between them gradually widens.And when the slope gradient reaches 2°and the rainfall intensity reaches 90 mm/h,the rill erosion rate is 6.05 times that of the sheet erosion rate.The contribution rate of rainfall intensity to sheet erosion rate and rill erosion rate is always greater than that of slope gradient.(3)The sheet erosion rate is positively correlated with the average velocity,Froude number,runoff shear force,flow power and unit flow power of sheet flow,and negatively correlated with runoff depth and Darcy-Weisbach resistance coefficient of sheet flow,among which the flow power has the best correlation with sheet erosion rate.The rill erosion rate is positively correlated with the average velocity,Reynolds number,Froude number,Darcy-Weisbach resistance coefficient,runoff shear force,flow power and unit flow power,among which the average velocity has the best correla-tion with rill erosion rate.The results of multiple stepwise regression analysis shows that the sheet erosion rate can be well predicted by flow power.The determining coefficient(R~2)of the prediction equation is 0.96,the Nash effective coefficient(NSE)is 0.96,and the average absolute percentage error(MAPE)is 6.2%.Rill erosion rate can be predicted by runoff average velocity and the R~2 of the prediction equation is 0.91,NSE is 0.91 and MAPE is 19.5%. |
