The Impact Of Slope Length And Rainfall Intensity On Runoff And Sediment Yield Processes

Impact of slope length on runoff erosion and sediment yield under rainfall condition is the important part of water erosion mechanism,it can provide theoretical basis for the establishment of watershed erosion model and scientific prevention and control of soil erosion.The Lishi loess selected as research object has been obtained in Wangjiagou Watershed in the west of Shanxi Province,based on the laboratory simulated rainfall experiment,this study discussed two main issues.First,studied the effects of slope length and rainfall intensity on runoff and sediment yield in the loess slope,and put forward the runoff erosion model suitable for the watershed.Second,analyzed the influence of slope length and rainfall intensity on variation characteristics of runoff dynamics parameters.The main conclusions were as follows:(1)Studied the effect of slope length on runoff erosion and sediment yield.Within the range of 50~120mm/h rainfall intensity,the runoff and sediment yield were increased linearly with increasing slope length,and the greater the rainfall intensity,the more significant the slope length effect.The variation trend of runoff hydrodynamic parameters were different with increasing slope length,it led to runoff velocity,runoff depth,runoff shear stress and Reynolds number were increased,the Froude number were showed three different trends:increased,decreased and fluctuated,the flow patterns of all slope length were laminar flow and jet flow,which indicated that the runoff of the slope were more severe.The sediment yield under 2~5m slope length increased exponentially with increasingrunoff(R~2=0.647~0.996),the sediment transport rate increased linearly with increasing runoff shear stress(R~2=0.896~0.997),it indicated that runoff was an important factor for causing sediment yield and sediment transport.The increment of runoff and sediment yield have not apparent changing rules when the slope length increases with the same length,but the runoff and sediment yield reached the maximum at 4~5m slope length.Accordingly,in order to shorten the slope length and reduce the loss of soil and water,suggested that soil and water conservation were measured at every 4m slope length,such as planting Caragana microphylla and Seabuckthorn as plant fence,and combining afforestation with planting grass to increase vegetation coverage,reduce surface runoff and slow down runoff collection speed,by planting Platycladus orientalis,Pinus tabulaeformis,Wild apricot and other plants to reduce raindrop splash and intercept part of the rainfall,and use root to contain water and sands to reduce soil erosion,by setting up the drain to collect runoff and diversion irrigation,or level terrace to change the route of runoff and storage water and sediment.(2)Compared the effect of different rainfall intensities on runoff erosion and sediment yield.Within the range of 2~5m slope length,the runoff of each period increased with increasing rainfall duration,the sediment yield of each period was showed two different trends: the increased trend when the rainfall intensity was 50~80mm/h,the fluctuated trend when the rainfall intensity was90~120mm/h.Within the range of 2~5m slope length,the runoff and sediment yield were increased exponentially with increasing rainfall intensity,and the greater the slope length,the more significant the rainfall intensity effect.The runoff and sediment yield curves increased steeply at 80mm/h and 60mm/h rain intensity respectively,the rainfall intensities were the lower limit rainfall intensity with a large increase of the runoff and sediment yield in this area.Accordingly,it is suggested that the 60mm/h rainfall intensity should be taken as the warning point in the Wangjiagou Watershed in order to avoid serious soil erosion.Similarly,with the increased of rain intensity,the flowvelocity,runoff depth,Reynolds number and runoff shear stress were increased,the Froude number was fluctuated.The change of runoff hydrodynamic parameters influences the erosion degree of slope,the correlation order of sediment yield and parameters was Reynolds number > flow velocity >runoff depth > Froude number,it indicated that runoff velocity and turbulence had great influence on erosion and sediment yield.Runoff shear stress and sediment transport rate were very significant linear positive correlation,and the correlation coefficient was 0.926,it indicated that the sediment transport rate can be predicted by runoff shear force in the watershed.(3)The Analyzed the interactive effects of slope length and rainfall intensity on slope erosion.Correlation between rainfall intensity and runoff,sediment yield and hydrodynamic parameters bigger than the slope,partial correlation analysis showed that the correlation between rainfall intensity and erosion were improved,slope length was also,the results showed that the slope length and rainfall intensity interact with each other to some extent.The relationship between slope length and sediment yield was close to that of runoff,and the relationship between rainfall intensity and runoff was close to sediment yield.The correlation between slope length and rainfall intensity and runoff depth and runoff shear stress were the highest,and the correlation with the Froude number was the lowest,the results show that slope length and rainfall intensity were not the main factors affecting Froude number.Based on the 2~5m slope length and 50~120mm/h rainfall intensity,runoff and sediment yield model were established for the Loess slope in the west of Shanxi Province,the coefficient R~2 of the equation were 0.812 and 0.763,and the model fitted well.