| Debris flow is a special torrent which occurs between mountain valleys and contains a large number of loose solid debris and uneven substances.The outbreak of debris flow is caused by rainstorm,rapid melting of ice and snow,volcanic eruption,earthquake and dyke break.The outbreak of debris flow has the characteristics of suddenness,short duration,harmfulness and periodicity.When it happens,it will cause great danger to the personal and property safety of local residents and affect the normal production and life of the people.The study object of the thesis is the Nesap Gully Debris Flow.Nesap Gully located in Zhaxi Village,Nielamu Town,Nielamu County,Rigeye City,Tibet.Affected by the "4.25" earthquake in Bobokla,Nepal in 2015,because the source of material in the Gully increased and especially the source of the Gully Bed and Gully Bank was affected by the earthquake,the stability of gully's solid decreased,which led to the outbreak of small and medium-scale debris flows in different degrees in recent years.A total of 265 villagers from 74 households were threatened lived in Zhaxi Village.On both sides of the debris flow around the middle and lower areas,Cultivated land was about 2500 Mu and grassland was 5000 mu.Meanwhile,along G318 National Highway about 500 meters roads and A culvert were threatened as well as power and communication facilities,which could cause potential economic losses about 60 million yuan.Through the collection,analysis,collation of geological data and on-site investigation,the engineering geological conditions of the debris flow in Naisap Valley are identified,and the influence of rainfall and seismic factors on the debris flow is analyzed.Based on the analysis of the characteristics of debris flow and its flow dynamics,the formation and starting mechanism of debris flow is deeply studied.The extension method combined with grey correlation method was used to evaluate the risk of debris flow tributaries in Naisap Valley.The movement characteristics of debris flow were simulatively analyzed by FLO-2D software.According to the intensity division and the period of debris flow outbreak,the river basin risk of debris flow was evaluated.The main contents of this thesis are as follows.(1)The study on the starting mechanism of debris flow in Naisap Valley: Firstly,the geological environment of the study area is identified,including the engineering geological conditions and hydrogeological conditions of the Naisap Valley.Then,the basin characteristics and dynamic characteristics of the provenance,circulation and accumulation areas of the Naisap Valley debris flow are analyzed.Finally,the starting mechanism of the debris flow in the study area is analyzed and expounded.(2)The risk assessment of debris flow in Naishapu gully: The matter element of debris flow gully is established by extension method,and the weight of different influencing factors is calculated by grey correlation method.It is concluded that the two branches of the gully are in a high dangerous state.(3)Numerical simulation.FLO-2D numerical simulation software was used to simulate the movement of debris flow in Naisap Valley.The erosion and siltation of debris flow in Naisap Valley were simulated under different rainfall frequencies(10%,5%,2%,1%).The process and characteristics of debris flow outbreak in Naishapu Valley are analyzed and the intensity of debris flow is divided,and the risk assessment of the basin is carried out through intensity and rainfall cycle.The regional maps of debris flow intensity and risk in the study area were obtained by assessing the risk of the basin through intensity and rainfall cycle.It provides a theory for monitoring and control of debris flow in Naisap Valley in the later stage. |
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