Analysis Of The Main River Flood Triggered By The Tributary Dammed Lakes Outburst

China is a country which has various barrier lake with the most widely distributed and the moraine dammed lake is one of the most threatening type. Moraine dammed lakes mainly developed in the high mountains,for example, Tibet has developed 925 moraine dammed lakes.The stability of moraine dams which are main make up of moraine loose deposits is poor,and compared with other types of moraine lake are easier to break.In recent years,a spate of main river extraordinary flood disasters which are caused by breaking of tributary moraine dammed lakes happened in China,they caused great loss of life and property. Outburst flood of tributary moraine dammed lakes is a new risk and the railway and highway construction have not dealt with it. In addition, the Tibetan river channel have complex shapes with wide and narrow,so calculating the evolution of the outburst flood under complicated conditions and forecasting the possible damage to the line project is a subject with a clear application background.The flume experiment which is designed according to characteristics of the Palongzangbu basin, simulated the main river outburst flood and the tributary outburst flood. We found that when the tributary flood flowed into the main river, it was shunted and diffused, and the local resistance loss increased because of the disturbance flow pattern when the flood arrived the sudden contraction section in the channel.The Palongzangbu basin is developed with many tributaries, and feathery arranged on both sides of the main river and tributaries.Intersections of the tributaries and the main river are densely arranged,and a large number of high risk moraine lakes were distributed in the tributaries.The paper used Fluent to simulate inflow process in the intersection of Midui moraine lake outburst flood,Studied on the flow characteristics in the flood at the confluence and plotted the velocity, pressure and angular velocity nephograms.Finally, the area of flood scouring area, the area of the impact damage and the area of solid deposition are summarized,and analyzed the hazards caused by the flood at the confluence and the corresponding countermeasures of disaster reduction.The error is too big when using peak flattening method directly on outburst flood in the complex river channel. So the flow and peak water level attenuated in that two sections Subsequently the Lee Stevan formula and Xie Renzhi formula are modified for the attenuation of flow and depth of water, adding the attenuation coefficient to original formulas to apply to complex situations of flood calculation. Through the case of Midui moraine lake outburst, it illustrated the modified formula has the higher accuracy. It can be a method for rapid assessment of risk on outburst flood, which can match the accuracy requirements of the principle scheme of the railway line selection.The paper describes the water damage situation of line projects in Tibet and its preventive and controlling principle. Summarized the main hazards of outburst flood on the line project.As for the high grade line project to build, disaster reduction countermeasures were proposed by using the high line position based on the highest debris flow blocking water is 25m and the highest outburst flood water is 10m;As for the the existing line project, disaster reduction countermeasures were proposed by building the subgrade protection structure.