| Along Duwen freeway in "5.12" Wenchuan meizoseismal area 2008, there are two main types of debris flow channels, that is, wide-gentle channel and narrow-steep channel. The wide-gentle channel has the feature of gentle longitudinal slope, large mean width and catchment area. Whereas the wide-gentle one generally shows opposite features with narrow longitudinal slope, small mean width and catchment area. Under the heavy rainfall, these channels are subject to large-scale debris flows due to huge amount of earthquake-induced collapsing and sliding materials. Generally, studies are mainly focused on the debris flows with wide-gentle channel,where the gentle longitudinal slope and large catchment area are beneficial for engineering activities (for example, hydropower stations, industrial and living area) and human habitation. However, little attention has been paid to the studies on the narrow-steep channel where engineering activities are much less due to its narrow longitudinal slope,small mean width and catchment area.In this thesis, we focus on the extra-large group of debris flows occurred on July 10th 2013 along Duwen freeway in Sichuan province, in particular the wide-gentle channel and narrow-steep channel formed in Taoguan gully and Mozi gully, respectively. With field investigation on the formation conditions and development characteristics of these two types of debris flows, we analyzed the resulting impaction, burying and blockage to the Duwen freeway and the Minjiang River. The formation mechanism behind the hazard of debris flows occurred in the above-mentioned two types of channels was intensively studied.Large-scale fluid dynamics software CFX was used to simulate the dynamics of the two channels under 50-year storm event. We investigated the distribution of the velocity field and its influence on buildings along the debris flow gully. Particularly, we studied its impact force and the corresponding risk on the bridge pier of Duwen freeway and evaluated the destructive effects on the Duwen freeway bridge, blocking of the Minjiang River and the flooding of residents of settlements. Finally prevention measures of the debris flows were proposed based on the features of the two channels.We systematically studied the wide-gentle and narrow-steep channel of debris flows in terms of the formation mechanism, risk prediction, dynamical process simulation, disaster chain and effectiveness analysis of the treatment. The main achievements are summarized as follows:(1) Based on the field investigation and data collection, the term "wide-gentle" and"narrow-steep" channels were proposed according to the different terrain and geomorphic conditions, drainage area and mean width. The definition and the classification standard of the wide-gentle and narrow-steep channel debris flow were given based on the statistical analysis of the parameters obtained from 13 typical debris flows occurred on July 10th 2013(2) With the optimal combination of empowerment method based on the optimization theory and the maximum entropy principle, the mathematical model of the risk evaluation was established, and the risk of the 13 typical debris flows occurred on July 10th 2013 along Duwen highway were evaluated. The evaluation results agree well with the current situation and development trend of the debris flow.(3) The formation mechanism of hazard from the wide-gentle channel were studied by analyzing the loose materials, terrain and rainfall conditions in Taoguan gully. The results show that the loose materials from collapse and landslides directly take part in the material supply by lateral erosion of the flood. The deposition on the gully was down cut and undermined by flood and the debris flow. However, a new kind of material supplying was emerged from the shattering materials located on the slope crest participated in the debris flow activity by means of source tracing erosion under the rainfall.(4) For narrow-steep channel, taking Mozi gully for example. We focuse on Mozi gully and analyzed the loose materials, rainfall and terrian condition, the hazard mechanism.It turned out that, the Mozi gully debris flow was induced by the blocking and burst of the collapsing accumulation body under the heavy rainfall, which has a significant amplification effect and transitive persistence.(5)In the dynamics process research of these two types of debris flow channel,large-scale fluid dynamics software CFX was applied to simulate and reproduce the dynamical motion and hazardous zone of the debris flow occured in Taoguan and Mozi gully on July 10th 2013. Using the dynamics parameter such as velocity and the flow obtained in the numerical simulation, We calculated the impact force of the Taoguan gully debris flow and the corresponding risk on the bridge pier of Duwen freeway. Furthermore, We also predicated the probability of blocking the Minjiang river and the backwater and submerging range of the Mozi gully debris flow when the barrier dams failed.(6) Based on the hazard mechanism and the resulting risk of the wide-gentle and narrow-steep channels, we demonstrated the clear types and suitability of regular engineering treatment. Moreover, we proposed effective treatment engineering measures of these two types channel of the debris flow based on Taoguan and Mozi gully by considering the channel features and treatment object. |
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