Mechanism Of Debris-flow Dam Break And Characteristics Of Induced Flood Along South Section Of Sichuan-Tibet Highway

Owing to the unique and fragile natural environment in southeastern Tibet, debris flow in this area is active, which often blocks off the mainstream in a large scale, forming the huge dam, and causing serious harm when the dammed-lake breaks. This paper investigated the recent debris-flow dam in southeast Tibet. Depending on model test and theoretical analyses, we have studied the dam stability and its outburst mechanism, and the characteristics of the flood induced by the break. Main results are as follows:1. By discussing seven events of large-scale debris flows in four gullies along south section of Sichuan-Tibet highway, from which five blocking dams resulted, focusing on the major factors impacting the dam formation, we can see that the most effective factors are discharge, river width, gradient of channelbed, height of dam, slope and the grain composition of dam.2. Two break forms of debris-flow dam are put forward through research on process of dam break in the model tests. One is gradually eroding, and the other is deposits of debris flow initiating; the latter can be classified into two forms, the first is hydraulic initiation, and the other is gravitational initiation. Meanwhile, the different mechanisms of breach-development types are studied in detail respectively.3. By analyzing the break flood process of each experiment, we can find the most important factor which affects peak break flood is the form of dam break; break flood and discharge show the direct proportion relationship; the peak flood takes place when 30~50% of the water in the reservoir is effused; and the time of peak flood takes place on 1/3 of the whole flood duration with any kind of breach.4. Considering the relationship between peak break flood and the reservoir potential energy, the shape of reservoir area, the shape of breach and the effect of break form, we set up the formula which can be used to calculate peak break flood. In this formula, break duration T is used as parameter to express the break form. 1.402 w h w0.41data of Peilong Valley debris-flow dam break validated the reliability of the formula.5. We simulated the relationship between growth of breach and flood discharge and set up numercal model for debris-flow dam break, in which we made the shape of breach as trapezia and predigeste the development of breach. We assumed that breach developed as . Take the debris-flow dams from Peilong Valley and Zhamu Valley for example, we simulated the course of flood and found that the calculated results can match with the observed data very well. On this basis, some corresponding countermeasures are proposed for risk mitigation.The paper is supported by the National Science Foundation (Grant No.40771024) and Western Transportation Construction Project Foundation (Grant No. 200631879284).