| When debris flow, which contains large quantities of stone, sand and clay particles, enters into a Main River, the river is frequently blocked, and farmland, railways and roads, and all of the construction along the river upstream will be ruined. Also, stone and sediment deposited in the Main River, it has an important influence on the movement of sediment and flow, and river channel may change. As a consequence, the changing river will affect construction of overall the railway and highway situation.From mechanics point of view, debris flow is usually classified as Non-Newtonian flow, and water in river as Newtonian flow. Thus, there is a complex interaction between Non-Newtonian flow and Newtonian flow, when a debris flow joins into a Main river. The transportation and deposit of sediment and water flow within the junction area is much more complex then between water flows. In this paper, first we investigate the mechanism of confluence between debris flow and the main river, Based on field observation and experiment, then deduce an equation for flow and sediment in the junction area with different conditions of confluence, and study a solution of the equation.First, we investigated Xiaojiang River, a well known area of rainfall debris flow occurrence, located in Yunnan Province in China. From field investigation, we gathered useful data about confluence between debris flow and the main river, and the effect of debris flow action railway and highway.To study the mechanism of confluence between debris flow and the main river, we carried out experiments. First, model similarity laws for model tests are studied. Flow phenomena at the junction area and experimental data are also reported. Based on the experiment, two patterns in confluence between debris flow and Main River, stratification flow and submerging flow, are put forward. The velocity changes and shape of debris flow head submerging into water, and mechanism of stratification flow in the confluence are examined.Based on the field observation and the experiment, the theoreticalequation to describe the movement of debris flow head in the submerging flow pattern is established, and the calculation parameters are studied in turn.The following conclusions can be drawn from the simulation:1. In the case of a low Froude number (Fr<0.35), the velocity of debris flow head submerging into water exhibits an approximately linear decrease, which is in good agreement with the experimental data.2. The viscosity ( ) and drag coefficient (CD) have a large influence upon the speed of debris flow head.3. Sediment settling velocity (wS) has pronounced effect on the concentration (Cv) in the body of debris flow head.With regard to stratification flow, based on theories of both classic fluid mechanics and non-Newtonian fluid mechanics, this thesis has first established a two-dimensional and three-dimensional governing equation with the tensor form, to describe the confluence of debris flow and main channel.Using the present rheologic equation and parameters of both debris flow and flow with hyperconcentration of sediment, and the MAC and PIC methods, a coupling numerical simulation which can solve this equation has been proposed.Calculated results show that this method is really feasible for analysis of the movement characteristic of flow and sediment in a conjunction area. The 2-D and 3-D calculating model can depict details of different aspects, from different angles.Considering that the characteristics of debris flow body are many and varied, this paper discusses the conventional Discrete Element Method (DEM). A three-dimensional model for simulating debris flow is put forward first. Using this model, the debris flow deposit process and the accumulative fan form are simulated under both conditions, the small water flow in the main channel and no water action.Then, a coupling model is established, in which the calculation alternates between debris flow with DEM and the main channel with the hydromechanics method. The sediment deposit an...
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