| Jinsha River is the mainstream of the upper reaches of the Yangtze River,which is rich in water resources.In recent years,with the implementation of "Western Development" and the deployment of "West-East electricity transmission project",the state has begun to plan a 13-level hydropower station in the upper reaches of Jinsha River in order to rationally utilize water resources and promote the social and economic development of the region.Jinsha River is located on the southeastern edge of the Tibetan Plateau.Influenced by the uplift,the tectonic movement are active and geological disasters are frequent.According to statistics,more than 60 paleo-landslide dam events have been identified.The landslide dam event is a very dangerous disaster event,which often brings serious disasters to the people near the upstream and downstream rivers.In recent years,the blockage of the Baige Landslide,for example,has seriously affected nearly 54,000 people in the upper and lower reaches of the river and caused economic losses of about 7.43 billion RMB.The Suwalong reach belongs to the upper reaches of the Jinsha River,where Suwalong Hydropower Station is being built.During the field investigation,four residual paleo-landslide dam were found within 20 km in this section.Because the area where the river-blocking event occurred may be blocked again,and the upper reaches of Jinsha River is also the area where landslide dam event frequently occur,it is necessary to study the paleo-landslide dam events in this river section,and find out the cause mechanism of landslide,as well as the destruction process of the dam and the movement process of the flood when the dammed lake outburst,so as to provide reference for the prevention,assessment and treatment of potential river-blocking event.However,because the paleo-landslide dam events occurred a long time ago and most of them have been destroyed,it is difficult to identify them in the field.Therefore,it is necessary to find an effective identification method in the research of paleo-landslide dam events.Based on the above problems,the main research contents of this paper are as follows:1.The identification method of paleo-landslide dam events from macro to concrete is put forward,which takes the residual landslide dam as the main identification basis.Based on this method,four paleo-landslide dam events in Suwalong reach were identified by analyzing the macro geological background of the Suwalong reach and the special topography and geomorphology(such as the large sliding arc terrain on the mountain,the sudden bending or narrowing of the river channel,and the special large-scale accumulation platform on both sides of the river channel)of the area around the residual landslide dam.By analyzing the genesis mechanism of the paleo-landslide,it is concluded that the blockage of the Gangda landslide was caused by the toppling deformation and failure of the reverse rock strata on the right bank under the action of gravity.Suoduoxi paleo-landslide dam event was mainly caused by the landslide on the left bank and affected by the debris flow on the right bank.The blockage of Rongcharong landslide was caused by the failure of reverse dipping strata on the left bank under the influence of gravity and fault.The Wangdalong paleo-landslide dam event was complicated in origin,which was caused by landslides and debris flows on the left and right banks,and was related to the activity of Quaternary glaciers.2.The OSL dating of lacustrine sediments in Suwalong reach was carried out to explore the formation of landslide and the burst time of landslide dam in Suwalong reach,and the evolution history of paleo-landslide dam events in Suwalong reach was obtained.The results show that the earliest landslide is the Suoduoxi landslide,which occurred at 6.3 ka BP and disappeared at 2.3 ka BP,and existed for about 4000 years.The most recent one was Wangdalong landslide,which formed at 2.1 ka BP and disappeared at 1.7 ka BP,and existed for about 400 years.3.The grain size analysis,SEM analysis and major element analysis of lacustrine sediments in Suwalong reach were carried out to explore the main input mode of lacustrine sediments after the formation of the dammed lake.The results of grain size analysis and SEM analysis show that these lacustrine sediments have the characteristics of aeolian sediments.The major element analysis results showed that the chemical composition of lacustrine sediments was different from that of local sediments,and the maturity of lacustrine sediments was higher than that of local sediments,indicating that lacustrine sediments did not come from the local area.It is concluded that the lacustrine sediments are mainly transported to the dammed lake by wind,and the effect of hydraulic transport is not significant.4.Taking the paleo-landslide of Rongcharong as an example,Rhino software was used to restore the original terrain,and discrete element PFC3D model was used to reproduce the process of blocking river by Rongcharong landslide.Simulation shows that the landslide movement time is about 65 s,and the maximum average velocity of landslide is 28 m/s,and the boundary of landslide dam is obtained.5.The empirical formula method and DB-IWHR model were used to calculate the peak discharge of overflowing burst flood of four ancient dammed lakes in the Suwalong reach.The results of empirical formula show that the peak discharge of four dammed lakes from large to small is Wangdalong,Rongcharong,Suoduoxi,and finally Gangda dammed lake.The calculation results of DB-IWHR model show that the peak flood discharge of Gangda dammedr lake outburst is 4974?6700 m~3/s,Suoduoxi 9963?14003 m~3/s,Rongcharong 28393?41195 m~3/s and Wangdalong 35328?51685 m~3/s,and the peak flood discharge is relatively large.Especially the Wangdalong and Rongcharong dammed lake,which should have caused great damage to the environment at that time.6.Taking Rongcharong ancient dammed lake as an example,and combining the computational fluid dynamics FLOW-3D model with the landslide dam obtained by the discrete element model,the evolution process of flood to the downstream when the top of the Rongcharong landslide dam burst was simulated,and the failure process of the landslide dam under flood scouring was also obtained.The results showed that the peak discharge was 64272 m~3/s,which appeared at the 5.0 h of simulation,and the maximum velocity was 20.3 m/s... |
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