| Gravitational erosion is the main form of soil erosion and one of the main types of soil erosion in the Loess Plateau region.Due to the many factors affecting gravitational erosion and the complex coupling relationship,the study of gravitational erosion mechanism lacks quantitative results.Therefore,the study of the development pattern of gravity erosion terrain and the critical characteristics of gravity erosion in the loess hills and gullies is of great significance to reveal the soil erosion mechanism.In this thesis,selecting the yellow mian soil in Dongzhuanggou watershed of Qingyang City as the research object,using the native soil samples,combining with the topography and rainfall characteristics of the research area,constructing the rainfall erosion process in the loess hilly gully area in the laboratory,using indoor artificial simulated rainfall,gravity erosion process topography change observation,soil moisture temperature online monitoring,soil shear strength measurement and other methods,selecting 32 mm/h,54 mm/h two kinds of the relationships between erosion surface length,width and area,erosion surface area and erosion body volume,erosion body sliding force and erosion body slump shear and shear strength reveal the critical characteristics of rainfall-triggered gravity erosion.The following study results are obtained:(1)A gravity erosion terrain change simulation monitoring system with a temporal resolution of 1 s and a spatial resolution of 1 mm is constructed.The theoretical spatial resolution of the system is higher than 0.8 mm/pix,the synchronisation speed between multiple cameras is 22/1000 s,and the time interval between each scene is taken continuously at 1 s intervals for 2 h.Basing on five scenarios with different rainfall intensities,the maximum error percentage between the real and measured values of wood blocks in the length direction is 10%,the root mean square error is around 1 mm,and the coefficient of variation is within 1%.The percentage error between the true volume values and the RC and ArcGIS calculated volume values,as well as the zonal and overall average,is 10%,which meets the current demand for accuracy in monitoring topographic changes during the study of gravity erosion.(2)Basing on the gravity erosion terrain change process simulation monitoring system,an indoor gravity erosion simulation experiment was carried out to analyse the change pattern of soil volumetric water content and the frequency of gravity erosion types at different rainfall events.The results show that with the increase of rainfall times,the soil volumetric water content in the initial state is small,and the water infiltration rate is large.The volumetric water content of the same water sensor shows an overall upward trend.When the surface soil is basically saturating,the soil moisture moves vertically to the deep soil under the action of gravity,and the volumetric water content of the lower soil shows an upward trend.Gravitational erosion mainly occurs in the middle and late stages of rainfall,mainly landslide,follows by mud flow,and the least collapse.(3)The influence of single(soil bulk weight or soil volumetric water content)and multi-factor(soil bulk weight and soil volumetric water content)coupling on the shear strength is analysed.The shear strength tends to increase and then decrease as the volumetric water content increases,with a non-linear relationship.The volumetric water content is consistent and the shear strength increases with increasing soil capacity,showing a linear relationship.As the number of rainfall fields increases,the effect of soil bulk weight and volume water content on shear strength is not independent,but an interactive process.The lower the soil bulk weight and the higher the volume water content,the lower the soil shear strength;conversely,the higher the soil shear strength.(4)The developmental characteristics of different gravity erosion types,the relationship between sliding surface length,width and area,area and erosion volume,and the dynamic coupling between sliding shear and shear strength of the erosion body are compared and analysed.The volumetric water content of landslides and mudflows is around 0.29 cm~3/cm~3,the sliding surface inclination is 60°～90°,the erosion body form is convex or linear,the length,width and gravity erosion surface area are linearly correlated,the correlation coefficient is more than 0.861;the volumetric water content of landslides and mudflows is around 0.3 cm~3/cm~3,the sliding surface inclination is mostly 50°～70°,the sliding surface form is concave or linear,convex,the length,width and gravity erosion surface area are exponentially correlated,the correlation coefficient is more than 0.930.The area of the erosion surface is positively correlated with the volume of the eroded body,with a coefficient of determination of 0.997.If the slip shear and shear strength calculated from the critical range of erosion occurring at the soil bulk weight and soil volume water content are just above the curve,it means that this is the critical state for erosion to occur in the soil. |
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