Risk Assessment And Engineering Impact Of Debris Flow In Dam Site Area Of Benzilan Hydropower Station On Jinsha River

Benzilan Hydropower Station is a large-scale hydropower station under construction in Deqin County of Yunnan Province and Derong County of Sichuan Province on the boundary of Jinsha River.The geological conditions of the dam area are complex,and there are often debris flows and collapses and other geological hazards.On August 14,2016,a large-scale debris flow broke out in Jiaxuegou,the first-class branch ditch on the left bank of Jinsha River in the dam site area,causing serious property losses.Due to the construction of Benzilan hydropower station and supporting projects,the geological environment in this region must be affected to a certain extent,and the risk of geological hazards will increase.In order to ensure the safe construction of the hydropower station and the safety of residents,it is necessary to evaluate the development characteristics and risk of debris flow in dam site area.Based on field investigation and data collection,the formation conditions,causes of disasters and basic characteristics of five key debris flow gullies in the study area are analyzed.Combining with the geological environment conditions and the development characteristics of each gully area in the study area,the risk assessment factors which are in line with the actual situation are selected.The AHP model is constructed to evaluate the risk of debris flow,and the weight calculation is allocated to the scheme layer.Then the calculated results of the risk degree are in good agreement with those of the grey correlation method and the Liu Xilin model.FLO-2D software is used to simulate the debris flow ditches in the study area at four different frequencies.The hazard assessment maps of the ditches are obtained by the simulation results.Finally,combined with the above results,the impact of debris flow on hydropower construction projects and the corresponding prevention and control suggestions are systematically analyzed.The research results and conclusions of this thesis are as follows:(1)Five key debris flow gullies in the study area have three necessary conditions for the development of debris flow disasters.(2)Large-scale debris flows have occurred in the region in history.Through field investigation and analysis,the recent debris flow outbreak in Jiaxue Gully was formed as follows: erosion-slump-blockage-collapse-formation of debris flow-later stage of debris flow,that is,due to the blockage caused by the start-up of material on the slope and the accumulation of undercurrent in the channel,when the bearing capacity reaches its peak value,it develops into a failure mode,which may cause other unstable sections to collapse in the subsequent process,and make the peak flow of debris flow increase continuously.(3)Five debris flow gullies belong to dilute debris flow,the susceptibility of Jiaxue gully,Alogong gully and Dongzhulin gully is "susceptible" level,and that of Puyung gully and Yagong gully is "mild susceptible" level;Jiaxue gully and Alogong gully are in the stage ofdevelopment,Dongzhulin gully is at the stage of development-recession,and Puyung gully and Yagong gully are at the stage of recession;(4)In the aspect of hazard analysis,the Jiaxue gully and Dongzhulin gully are highly dangerous,while the Alogong gully,the Puyung gully and the Yagong gully are moderately dangerous,that is,the possibility of large-scale and medium-scale debris flows occurring under rainstorm weather conditions.(5)Using FLO-2D software to simulate the process,the accuracy of the once-in-20-year outbreak and debris flow simulation data is verified,and satisfactory results are achieved.Based on the simulation results of four different frequencies(P = 0.5%,P = 1%,P = 2%,P =5%)and the combination of flow velocity and accumulation depth,the hazard classification standard of the study area is established and the hazard assessment map is obtained.(6)Jiaxue gully,Alogong gully and Dongzhulin gully are close to the proposed Alogong hydropower station and hydropower construction site,and the construction projects are distributed in the gully and at the edge of the gully stacking fan.Once the debris flow disasters occur,they will have an impact on them.Therefore,the prevention and control suggestions for the above three debris flow gullies are put forward.However,the Puyung gully and Yagong gully are far away from the dam site of the Alogong Hydropower Station and pose less threat.During the project operation period(according to 100 years),the amount of debris flow outbreak into the reservoir is less,which does not affect the normal operation of the reservoir.