| In the Three Gorges Reservoir area,there are many landslides developing along the river banks.More than 2500 of the landslides are influenced by the reservoir water fluctuation.Once the landslides fail and slip into the water body,they may trigger secondary wave hazards,which can significantly expand the influence scope and result in more serious consequences,compared with independent landslide hazards.However,traditional research on geological mechanism and engineering mitigation technology is not enough to meet the needs of comprehensive prevention and mitigation of the potential hazard chains triggered by the reservoir landslides.In general,the study on geological hazard risk includes both hazard analysis of the hazard and vulnerability analysis of the potential elements at risk,and this comprehensive perspective helps to highly efficient and cost-effective prevention and mitigation works.The article takes the risk of landslide triggered hazard chain in the Three Gorges Reservoir area at a specific-site scale as the research object.The risk management system of an individual landslide and its triggered hazard chain in the Three Gorges Reservoir area is constructed through the integration and supplementation of the research frameworks of landslide and tsunami risk,based on the improved equation calculating landslide direct and indirect risk and the successful risk management of the Liangshuijing landslide;taking the Liangshuijing landslide in Yunyang County,Chongqing City as an example,the spatial and temporal trends and triggering factors of the landslide deformation are summarized,the direct risk of the landslide and the indirect risk of its secondary wave hazard under different scenario combinations are calculated quantitatively,then based on the landslide risk characteristics,four risk control plans are proposed and the optimal one is selected through the cost-benefit analysis,after that the feedback of potential hazard chain risks on control measures is discussed.The main contents and results are as follows:(1)Through the theoretical study of hazard risk management,the research framework of landslide and tsunami risk is integrated,and the quantitative risk management system applicable to the landslide triggered hazard chain in the Three Gorges Reservoir area is constructed based on the improved equation of classical landslide risk calculation.The system mainly includes five steps: data collection,hazard analysis,consequence analysis,risk calculation,and risk control.(1)Before the research of risk management,the basic geological data,meteorological and hydrological data,landslide investigation and monitoring data,and investigation and statistics data of potential elements at risk in the study scope should be collected.Then,the development,deformation characteristics and failure mechanism of the landslide should be analyzed,and the influence of triggering factors on landslide surface displacement should be studied,so as to determine the scenario combinations for the quantitative risk management.(2)The occurrence probability is used to represent the hazard of the landslide and secondary wave.Based on the time sequence and chain effect of hazard occurrence,the calculation of occurrence probability of the potential landslide-triggered wave is simplified.Then the landslide runout analysis and secondary wave prediction should be carried out,which will be used to outline the impact area of the hazard chain and determine the movement parameters for the hazard intensity calculation in subsequent step.(3)The hazard consequence is represented by the sum of the vulnerability of each type of element at risk multiplying its economic estimates.The elements at risk are divided into four types according to their relative positions to the hazard: on the landslide body,static on-shore,static on-water and dynamic on-water.Based on the study of normative design values,empirical relationships and numerical simulation results,the corresponding estimation equations to quantify vulnerability are proposed for all types of elements at risk.(4)The indirect risk caused by secondary wave is added to the classical landslide risk calculation equation.The magnitude and distribution of the comprehensive risk of multiple types of hazard and elements at risk can be shown on risk maps.(5)The practical application of risk results is studied,and the applicable conditions of different risk decisions are summarized.The residual risk can be used to measure the feedback of the landslide hazard chain on the risk control plan.Then based on the service period and cost of main project,the optimization of risk control plan is carried out through the improved cost-benefit analysis where the risk benefit is represented by difference between the original and residual risk values.Thus,a relatively complete risk management research system of landslide triggered hazard chain in the Three Gorges Reservoir area at a specific-site scale has been constructed.(2)Taking the Liangshuijing landslide in Yunyang County,Chongqing City as an example,a relatively complete risk management research of its triggered hazard chain is carried out,which has verified the effectiveness of the improved quantitative risk management framework to a certain extent.(1)The analysis of the development characteristics and deformation trends of the Liangshuijing landslide.The landslide is located in the interbedded sandstone and mudstone of the middle Jurassic Shaximiao formation which is one of the typical rock formations being prone to slip.Based on the geological data,investigation and monitoring data,the development,deformation characteristics and failure mechanism of the landslide are analyzed.The landslide boundary had been generally formed,the cumulative surface displacements of the middle front and right part near the landslide head were larger than those of other parts,and during 2009-2014 the landslide deformation was obviously affected by the reservoir level and rainfall.The theoretical analysis of the cumulative displacement vectors indicates that the landslide follows a rotational failure mechanism.Based on the correlation analysis between landslide displacement and triggering factors,the time-series characteristics of the landslide deformation were analyzed.The results show that the averaged reservoir water level in the current month contributes about 51 %(the most)to the displacement in the current month,while the cumulative precipitation in the current month contributes about 29 %.The lower the reservoir level,the faster the reservoir level change and the larger the precipitation,the greater the increment of the landslide displacement.(2)The hazard analysis of the Liangshuijing landslide and its potential secondary wave hazard.Based on the response regulations of the landslide deformation to the triggering factors,the scenario system for quantitative risk management is constructed,where the reservoir level is divided into five stages(high water level,drawdown,rapid drawdown,low water level,and rising stage)and the rainfall conditions are corresponding rainfall extremes of 10,20,and 50 years.The failure probability of the landslide under different scenarios is calculated by the finite element numerical simulation software Geo Studio,and the results show that in the rapid drawdown stage the failure probability has a maximum and the greater the rainfall the larger the failure probability.Based on the runout analysis of the landslide and the prediction of the potential landslide-triggered wave simulated by the Tsunami Squares software,the landslide movement characteristics and wave propagation are analyzed,and the results show that in the combination scenario of rainfall + rapid drawdown stage,the peak values of landslide velocity,the run-ups on the shore directly opposite the landslide and the wave heights in the navigation channel are all reached the maximum.(3)The quantitative vulnerability analysis of the elements at risk and consequence analysis of the Liangshuijing landslide and its potential secondary wave.According to the prediction of the impact area of the landslide and its secondary wave in the earlier hazard analysis,the investigation of the type,quantity,location and economic value of the potential elements at risk are conducted.There are a piece of forest land and a road on the landslide body.Two temporary piers and the moored vessels are static on-water,and the vessels moving on the river with uncertain spatial and temporal distributions are dynamic on-water.To quantify the vulnerability of the elements at risk,that of the elements at risk on the landslide body is regarded as 1,while that of other types are estimated through the corresponding equations calculating the resistance and hazard intensity based on the prediction of the wave propagation.The direct consequence of the landslide is about 2.35 million RMB,and the maximum indirect consequence of its potential secondary wave is about 33.21 million RMB.(4)The risk calculation and mapping of the Liangshuijing landslide and its secondary wave.The total risk of the landslide triggered hazard chain is calculated by the improved risk equation.The results show that the indirect risk due to the secondary wave is about11 times of the direct risk due to the landslide itself,and the risk maps also clearly show that the risk of the hazard chain is mainly concentrated in the Yangtze River and along the river bank.The potential secondary wave can greatly expand the influence area of the landslide and increase it threat level.The fluctuation of the reservoir level and the existence of moving elements at risk make the risk of the landslide triggered hazard chain change dynamically.The total risk has a maximum of 22.65 million RMB in the combined scenario of rainstorm once in 50-year and rapid drawdown stage of the reservoir water level.Considering the temporal probability of rainfall and periodic characteristics of the reservoir water level in the risk analysis can reflected the long-term trend of hazard chain risk to a certain extent.The total risk of the landslide triggered hazard chain continues to rise over time,but at a smaller rate.(5)The re-analysis of the landslide triggered hazard chain under different risk control plans and plan comparison through a cost-benefit analysis.According to the risk characteristics of the Liangshuijing landslide triggered hazard chain,i.e.,the main risk is indirect risk,the risk decision of decreasing the occurrence probability,avoiding the risk and reducing the hazard consequence is made,and four mitigation projects(anti-slip piles,cutting,drainage,and monitoring)are proposed to control the risk.The results of the re-analysis of the hazard chain risk show that,the residual risk of hazard chain after being taken the cutting project is reduced by about 69 %compared with the original total risk is the lowest,while that after being taken the antislip piles project is reduced by about 67 %.However,increasing the number of pile rows does not significantly reduce the risk,indicating that the traditional mitigation plan with anti-slip piles as the main project may have a bottleneck of risk control effect.Through the cost-benefit equation,the benefit-cost ratio of each plan is calculated.Conducting the cutting project can improve the landslide stability,reduce the volume sliding into the water body,and transform the slope shape almost permanently,meanwhile the cost is relatively low,thus it is the optimal plan whether navigation closure is considered or not.Additionally,the residual risk of the hazard chain is the lowest after being taken the combination of the four independent projects,with a reduction of about 89 % and 65 ?88 % compared to the original total risk and the residual risk under the four independent projects,respectively.In fact,the Liangshuijing landslide was conducting cutting project in 2014,which saved about 80 % of the cost compared with the original anti-slip project.After mitigation,the surface velocity is reduced by about 47 %.Therefore,it is of practical significance and economic benefits to carry out landslide prevention work in the reservoir area from the perspective of risk management,and the effectiveness and practicality of the improved risk management framework are verified to some extent. |
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