| The Coarse Sediment Region at the Middle Reaches of the Yellow River is the area with the most severe soil erosion in loess plateau, and it is also the main source of coarse sediment which is the most serious threat to the security of Yellow River. But because of the environmental varieties of Yellow river basin and the complexity of regional environmental combination, there is great dispute on the dynamics of erosion sediment. So studies on the dynamic processes and mechanism of hydrodynamics of soil erosion in this region have both theoretical and practical value for effective control measures and sustainable development.On the basis of field investigating data, the dynamic processes of shallow flow erosion in the coarse sediment region of the Yellow River middle reaches were studied under different rainfall intensity and slope gradients with the method of simulated rainfall experiment. The study focused on the processes of slope erosion and sediment yield, the hydraulic properties of shallow flow, the dynamic mechanism of slope erosion and sediment yield processes, and the erosion processes on the downslope segment. The following results were obtained:1. Based on the analysis of runoff and sediment yield processes of slope erosion, and the relationship between them, the variation processes of slope erosion were researched. (1) The runoff rate increased rapidly with rainfall process at first and got steady later, and it could be described with a logarithmic equation. Runoff depth positively related to rainfall intensity as well as slope gradients, and their relationship both could be described with power equations. A binary linear equation could be used to describe the variation of runoff depth with rainfall intensity and slope gradients, and the influence of rainfall intensity on runoff depth was more significant than slope gradient. (2) A logarithmic and linear combined equation could be used to describe the variation of slope erosion rate with rainfall process. 4-5min after rainfall began were turning points of erosion rate variation with rainfall process; Slope erosion modulus positively related to rainfall intensity as well as slope gradient, and their relationship could be described with an exponential equation and a logarithmic equation respectively. A binary power equation could be used to describe the variation of erosion modulus with rainfall intensity and slope gradients, and the influence of slope gradient on erosion modulus was a little more important than rainfall intensity; (3) Under different rainfall intensity, different slope gradients, and different rainfall intensity and slope gradients, slope erosion modulus all increased with runoff depth, and their relationship could be described with a power equation, a linear equation, and a linear equation respectively.2. Based on the analysis of the velocity, the average depth , the Reynolds number, the Froude number and the resistance coefficients of shallow flow, hydraulic properties of shallow flow were discussed. (1) The velocity of shallow flow increased with rainfall process, and it could be described with a logarithmic equation. About 5min after rainfall began was the turning point of the velocity from increasing rapidly to slow down. The velocity of shallow flow was positively related to rainfall intensity and slope gradients. (2) The average depth of shallow flow increased steadily with rainfall intensity but decreased with slope gradient. A power equation and a linear equation could be used to describe them respectively. A binary power equation could be used to describe the variation of average depth with rainfall intensity and slope gradients. (3) The Reynolds number of shallow flow increased with rainfall process, and the influence of rainfall intensity on Reynolds number was more significant than slope gradient. The shallow flow in this experiment was not the traditional laminar flow but in the state of instability; The Froude number increased steadily with rainfall process. The influence of slope gradient on Froude number is more important than rainfall intensity. The shallow flow in the experiment was in the state of supercritical flow. (4) The resistance coefficients of shallow flow decreased with rainfall process at first and became steady later. It negatively related to slope gradient but varied little with rainfall intensity. A binary power equation could be used to describe the decline trend of the average resistance coefficients with rainfall intensity and slope gradients.3. Based on the analysis of the relationship in flow shear stress, flow power, unit flow power, section unit energy and slope erosion modulus, the dynamic mechanism of slope erosion and sediment yield were studied. The results showed that slope erosion modulus positively related to flow shear stress, flow power, unit flow power and section unit energy respectively, which could be described with linear equations. Under the experimental conditions, section unit energy was the closest hydrodynamic parameter related to slope erosion. The corresponding soil erodibility coefficient was 3.1604g/cm, and the critical average unit energy was 0.0243cm.4. Based on the analysis of the erosion processes on the downslope segment, the influence of runoff from both of the upslope and downslope and the sediment from upslope on the downslope erosion were clarified. (1) The downslope erosion rate varied with rainfall process and characterized with up-down fluctuations. A linear equation could be used to describe the variation of downslope erosion modulus with rainfall process. (2) Downslope erosion modulus positively related to rainfall intensity and slope gradients respectively with up-down fluctuations, which could be described with logarithmic equations. A binary linear equation could be used to describe the variation of downslope erosion modulus with rainfall intensity and slope gradients. The influence of rainfall intensity on downslope erosion modulus was more significant than slope gradient. (3) The influence of runoff from both of the upslope and downslope and the sediment from upslope on the downslope erosion could be described with a binary linear equation, with the former contribution being 56.9% and the latter 25.4%. And downslope erosion had a positive relationship with the former and negatively related to the latter. It can be concluded that the influence of runoff on downslope erosion was more significant than sediment. The less of the sediment content, the stronger of the downslope erosion. Taking soil and water conservation measures to reduce flow concentration from upslope and increase rainfall infiltration can effectively control soil erosion on downslope.
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