| Loess Plateau of China is the most serious soil erosion area in the world,causing the Yellow River becoming a world-famous"suspend river"which is dangerous to both sides of the lower river.Severe soil erosion on the Loess Plateau has posed a threat to the sustainable development of the area and the security of the banks of the downstream of Yellow River.Sheet erosion process,especially on the steep slopes,involves many propounded scientific issues of soil erosion processes and hence,it is the most important erosion process which existing a large area on the loess hillslope.The sheet flow from the large areas is the main power of serious soil erosion on the Loess Plateau.The control of the sheet erosion can effectively control the sediment yield on the Loess Plateau.Studying the dynamic process of sheet erosion on loess hillslope could not only reveal the mechanism of the dynamic process of sheet erosion,deepen the study of sheet erosion process,but also lay the solid foundation for the process-based sheet erosion model and hence,it could promote the further development of soil erosion theory and provide an scientific basis for the management of soil and water loss and the ecological environment construction on loess hillslope.Dynamic processes of sheet erosion on loess hillslope were studied under simulated rainfall experiments with complete combinations of six rainfall intensities(0.8,1.0,1.5,2.0,2.3 and 2.5mm min-1)and six slopes?7°,10°,15°,20°,22°and 25°?.The study focus on raindrop detachment?capacity?process and its hydrodynamic mechanism,sediment transport?capacity?process and its hydrodynamic mechanism,sheet flow detachment process and its hydrodynamic mechanism,and the effects and contributions of the driving force and sediment load in sheet flow to sheet flow detachment.The main conclusions are as follows:1.The raindrop detachment?capacity?process and its hydrodynamic mechanism on loess hillslope were studied.?1?The variations of raindrop detachment capacity with rainfall time under different rainfall intensities and slopes are similar,the trends of which are decreasing sharply at beginning and then having a slow decrease or tending to be steady,which could be well described by power equations.The variations of raindrop detachment capacity with rainfall intensity under different slopes are all increasing rapidly at first,then increasing slowly and finally tending to be steady,which could be described by exponential equations.While with the slope increasing,the raindrop detachment capacity decreased slightly under different rainfall intensities,which could be described by linear equations.?2?Raindrop detachment capacity could be described by a binary power equation of slope and rainfall intensity,which is positively correlated with rainfall intensity and negatively correlated with slope,and the influence of rainfall intensity is greater than that of slope.?3?The variations of raindrop detachment capacity with rainfall kinetic energy,raindrop terminal velocity and raindrop median volume diameter under different slopes are all increasing rapidly at first,then increasing slowly and finally tending to be steady,which could be well described by exponential equations.Rainfall kinetic energy is the best rainfall parameter to describe the raindrop detachment capacity under the experiment condition.The best response equation of raindrop detachment capacity to rainfall parameters is the exponential equation of raindrop detachment capacity to raindrop kinetic energy.?4?The comprehensive response of raindrop detachment capacity to raindrop kinetic energy and water depth,raindrop kinetic energy and flow velocity,kinetic energy,water depth and flow velocity could be described by power-exponential combination equations,respectively.?5?The hydrodynamic equation of raindrop detachment capacity on loess hillslope is the power-exponential combination equation of raindrop detachment capacity to raindrop kinetic energy and water depth.2.The sediment transport?capacity?process by sheet flow and its hydrodynamic mechanism on loess hillslope were revealed.?1?The sediment transport capacity increased with rainfall intensity under different slopes.When under the slopes of 7°and10°,the increase rate gradually decreased with the increasing rainfall intensity,and the relationship between sediment transport capacity and rainfall intensity could be well described by logarithmic equation.While when under the slopes of 15°,20°,22°and 25°,the increase rate gradually increased with the increasing rainfall intensity,and the relationship between sediment transport capacity and rainfall intensity could be well described by power equation.The sediment transport capacity increased with slope under different rainfall intensities.When under the rainfall intensities of 0.8 and 1.0 mm min-1,the increase rate is relatively great with the increasing slope,and the relationship between sediment transport capacity and slope could be well described by linear equation.While when under the rainfall intensities of 1.5,2.0,2.3,and 2.5 mm min-1,the increase rate is relatively slow with the increasing slope,and the relationship between sediment transport capacity and slope could be well described by power equation.?2?Sediment transport capacity could be described by a binary power equation of slope and rainfall intensity,which are positively correlated with both rainfall intensity and slope,and the influence of rainfall intensity is greater than that of slope.?3?Sediment transport capacity increased with the increase of stream power,unit stream power,shear stress,and unit energy under different slopes and rainfall intensities,which could be described by power equations,respectively.Stream power is the best hydrodynamic parameter to describe the sediment transport capacity under the experiment condition.The best response equation of sediment transport capacity to hydrodynamic parameter is the power equation of sediment transport capacity to stream power.?4?The comprehensive response of sediment transport capacity to stream power and raindrop kinetic energy,stream power and raindrop terminal velocity,stream power and raindrop median volume diameter could be described by power-exponential combination equations,respectively.Stream power together with raindrop kinetic energy was the best hydrodynamic and rainfall parameter to describe the sediment transport capacity.?5?The hydrodynamic equation of sediment transport capacity by sheet flow on loess hillslope is the power-exponential combination equation of sediment transport capacity to stream power and raindrop kinetic energy.3.The sheet flow detachment process and its hydrodynamic mechanism on loess hillslope were clarified.?1?The detachment rate of sheet flow increased with rainfall intensity under different slopes.When under the slopes of 7°,10°,15°and 20°,the increase rate is relatively slow and uniform with the increasing rainfall intensity,and the relationship between detachment rate of sheet flow and rainfall intensity could be described by power equation.When under the slopes of 22°and 25°,the increase rate is relatively slow with the rainfall intensity increasing from 0.8 to 1.5 mm min-1,and the increase rate is relatively great with the rainfall intensity increasing from 1.5 to 2.5 mm min-1.The relationship between detachment rate of sheet flow and rainfall intensity could be described by exponential equations.The detachment rate of sheet flow increased with slope under different rainfall intensities.When under the rainfall intensities of 0.8,1.0 and1.5 mm min-1,the increase rate is relatively slow and uniform with the increasing slope,and the relationship between detachment rate of sheet flow and slope could be described by power equation.When under the rainfall intensities of 2.0,2.3,and 2.5 mm min-1,the increase rate is relatively slow with the slope increasing from 7°to 20°,while the increase rate is great when the slope steeper than 20°.The relationship between detachment rate of sheet flow and slope could be described by exponential equation.?2?Detachment rate of sheet flow on loess slope could be described by a binary power equation of slope and rainfall intensity,which are positively correlated with both rainfall intensity and slope,and the influence of rainfall intensity is greater than that of slope.?3?The detachment rate of sheet flow increased with the increase of stream power,unit stream power,shear stress,and unit energy under different slopes and rainfall intensities,which could be described by linear equations,respectively.Stream power is the best hydrodynamic parameter to describe the detachment rate of sheet flow under the experiment condition.The best response equation of detachment rate of sheet flow to hydrodynamic parameter is the power equation of detachment rate of sheet flow to stream power.?4?The comprehensive response of detachment rate of sheet flow to stream power and raindrop kinetic energy,stream power and raindrop terminal velocity,stream power and raindrop median volume diameter could be described by binary power equations,respectively.Stream power together with raindrop kinetic energy was the best hydrodynamic and rainfall parameter to describe the detachment rate of sheet flow.?5?The hydrodynamic equation of detachment rate of sheet flow on loess hillslope is the binary power equation of detachment rate of sheet flow to stream power and raindrop kinetic energy.4.Effects and contributions of the driving force and sediment load in sheet flow to sheet flow detachment were identified.?1?The detachment rate of sheet flow increased with sediment load under different rainfall intensities and slopes,and it could be described by a power equation.?2?Under different slopes,the response relationship of detachment rate of sheet flow to stream power,raindrop kinetic energy and sediment load could be well described by ternary logarithmic equations;under the different rainfall intensities,the response relationship of detachment rate of sheet flow to stream power and sediment load could be well described by binary logarithmic equations.?3?The response equation of detachment rate of sheet flow to driving force and sediment load on loess hillslope is the ternary power equation of detachment rate of sheet flow to stream power,raindrop kinetic energy and sediment load.?4?Under different slopes,the contribution rate of stream power,raindrop kinetic energy and sediment load to detachment rate of sheet flow is 51.57%-81.11%,12.30%-37.94%and 0.41%-9.72%,respectively.Under different rainfall intensities,the contribution rate of stream power and sediment load to detachment rate of sheet flow is 55.97-89%and 0.6%-38.75%,respectively.Under different slopes and rainfall intensities,the contribution rate of stream power,raindrop kinetic energy and sediment load to detachment rate of sheet flow is 65.69%,18.41%and 6.5%,respectively.Detachment rate of sheet flow is mainly affected by stream power and raindrop kinetic energy,and less by sediment load.Stream power and raindrop kinetic energy have a positive effect on the detachment rate of sheet flow,which stimulated the occurrence and development of sheet flow detachment,and the influence of stream power is greater than that of raindrop kinetic energy.While sediment load has a negative effect on the detachment rate of sheet flow,which impede the occurrence and development of sheet flow detachment. |
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