Experimental Studies On The Processes Of Gravity Erosion And Sediment Yield On The Steep Loess Slope

Gravity erosion is not only a hazard in almost all countries,causing billions of dollars of damages and many casualties,but also contributes to landscape evolution and erosion in mountainous regions.Mass movement-driven sediment yields are important issues in the field of soil and water processes.Such process-based data are hard,if not impossible,to obtain under natural rainfall conditions due to the unpredictable nature of gravity erosion in timing and location.However,this kind of knowledge is very important to linking erosion processes and sources to sediment transport in rivers.So,a representative study area is selected and a novel method is adopted here based on field experiment,direct shear test and field investigation.The objectives of this study are to track the migration trace of sediment under extreme rainfall,to study the responses of sediment yields to mass movement,to reveal the effect of gravity erosion on particle size distribution of sediment,and to analyze the distribution regularities of mass movement.As the most important trigger,rainfall induced 40%of the catastrophic mass movements.In some catchments of the Loess Plateau,mass movements contributed over 50%of the total sediment discharge.The dry density markedly affects shear strength by changes in cohesive force and internal friction angle,while the moisture content markedly affects shear strength by changes in cohesive force.A novel topography meter based on a structural laser was used to observe the mass failures in the experiments.The experimental results are shown as follows in the Liudaogou Catchment,Shenmu County:(1)The mass movements have characteristic of occurrence universally.Averagely twelve mass movements occurs on the steep slope in the each rainfall event for the F1?F7 with the height of 1 m and 1.5 m,the steep lower slope of 60,70,and 80°,and the intensity of 0.8 mm/min and 2.0 mm/min.The distribution regularities of mass movement were explored based on 499 mass movements from the field experiments.A power function decrease in the mass movement occurrence frequency was observed as the volume size increased.Probability density of mass movement was modelled using a power function of the form:P=1256.4v-1.525.(2)Mass movement-driven sediment yields were related to the mass movement volume,number and type.Our results revealed that sediment concentration in the failure events was 3 times on average higher than pre-failure erosion,and that the sediment transport rate in failure experiments increased 250%on average compared with pre-failure sediment transport rate for the F1?F7.These findings imply that the control of gravity erosion is more effective than any other solution in reducing sediment production in the watershed on Loess Plateau.(3)Gravity erosion has significant influence on the sediment particle size.As a result of gravity erosion,median particle size was decreased from 0.084 to 0.051 mm,sediment heterogeneity was increased from 5.6 to 26.8,and fractal dimension was increased from 2.60 to 2.78 for the F4-F6 with the height of 1.5 m,the steep lower slope of 60,70,and 80°.The result implies that gravity erosion makes the sediment particle size more nonuniform and irregular.The temporal changes of the sediment particle size during the experiments reflect a shift from the particle-selective detachment during the water erosion process to particle-selective transport during gravity erosion process.Thus,results will be of theoretical and scientific significance in understanding the gravity erosion mechanisms and soil erosion regularities in the steep loess slope of the contiguous area of Shanxi-Shaanxi-Inner Mongolia on Loess Plateau.