Risk Analysis Of Landslide Generated Impulse Waves In The Wu Gorge Of Three Gorges Reservoir Area

Since the Three Gorges project was completed and put into operation,the stability of a large number of landslides developed along the reservoir area has been one of the main problems during the operation of the reservoir.Once these landslides lose their stability and slide into the reservoir at a certain speed,the generated impulse waves can spread upstream and downstream along the river,which extremely expanding the impact area of landslides from around the landslide body to several kilometers or even tens of kilometers along the river,greatly increasing the scope and number of threatened objects of landslide hazards themselves and raising the uncertainty and difficulty of landslide risk prevention and control work.Therefore,it is of great practical significance to carry out the research of landslide generated impulse waves risk.The risk analysis framework of landslide generated impulse waves in the Three Gorges reservoir area was proposed by integrating the landslide risk evaluation framework and tsunami risk evaluation framework,considering the characteristics of generated impulse waves in the Three Gorges reservoir area.The quantitative evaluation process of generated impulse waves risk for the Three Gorges reservoir area are composed of a landslide generated impulse waves hazard calculation formula including the spatial probability,temporal probability and intensity of impulse waves,and a landslide generated impulse waves vulnerability evaluation method based on the vulnerability of the physical damage form of the elements in risk.Firstly,The physical modeling method and numerical simulation method were used to predict the intensity of landslide generated impulse waves hazard in the Three Gorges Reservoir area.Then,the quantitative evaluation of landslide generated impulse waves hazard and vulnerability under various conditions was carried out on a local scale and a regional scale.The form of risk expression of dynamic elements in risk was explored.The main research contents and results are as follows:(1)A quantitative landslide generated impulse waves hazard and vulnerability evaluation process were proposed,and a framework of landslide generated impulse waves risk analysis for the Three Gorges reservoir area was constructed.The framework includes four modules: risk investigation,hazard evaluation,vulnerability evaluation and risk evaluation.(1)The basic geological data,meteorological and hydrological data,landslide survey and monitoring data,bathymetry data and elements in risk survey data of the study area should be collected and analyzed before carrying out the risk survey in order to determine the source of impulse waves and its deformation and damage characteristics,and to determine the evaluation conditions of landslide generated impulse wave risk.(2)The quantified hazard calculation formula of landslide generated impulse waves with the spatial and temporal probability and disaster intensity was proposed,in which the spatial and temporal probability calculation of landslide generated impulse wave can be divided into three steps with progressive relationship: the spatial and temporal probability calculation of landslide,the probability calculation of landslide entering water body and the spatial probability calculation of impulse wave propagation.Then the disaster intensity index considering the damage pattern of the elements in risk was selected.The method including the empirical formula based on the physical model test or the numerical simulation method can be used to predict the intensity of the landslide generated impulse wave.(3)A quantitative vulnerability assessment model based on hazard intensity and vulnerability was established,and vulnerability assessment were proposed for the three common types of elements in risk for landslide generated impulse waves: ships,piers and coastal engineering facilities and residential buildings,based on their potential physical damage patterns.Based on the intensity of hazard obtained from the hazard analysis and the vulnerability index,the potential damage of the three types of elements in risk can be calculated.(2)A generalized physical model approach is used to analyze the characteristics of landslide generated impulse wave hazards in the Three Gorges reservoir area.The empirical formulas to prediction the landslide generated impulse waves in the Three Gorges reservoir area were recommended by comparing with the experimental results.A generalized river channel model based on the statistical analysis of the bank angle and the width of the river bed in the area with high density of landslide of the Three Gorges reservoir and a landslide model based on the statistical analysis of the scale parameter are established.The attenuation pattern of the landslide generated impulse wave propagation along the river channel was analyzed through the wave surface data recorded in the experiment.It is found that the river channel-type of landslide generated impulse wave in the Three Gorges reservoir propagates in the wave series with different propagation directions due to the reflection of the river channel on both sides will be superimposed at a certain place,which increasing the uncertainty in the process of propagation attenuation;The experiment plan was designed by using the control variable method and comparing the influence weight of each control factor on the intensity of the impulse wave.and it was concluded that the thickness of the slide body and the angle of slope have the most significant influence on the intensity of the impulse wave,while the impact velocity,slide length,density and still water depth have the modern influence.And the slide width has the lowest impact on the intensity of the impulse wave.The wave crest of the leading wave in the generation area and propagation area is recommended to be predicted by Bregoli et al.,and Evers et al.,and the propagation attenuation of the maximum wave crest is recommended to predict by the empirical formula proposed by Yin et al.,.(3)Based on the Tsunami Squares method,an algorithm suitable for the simulation of the landslide runout and impulse wave generation process in the Three Gorges reservoir area was proposed and calibrated based on the analytical formula,experimental data and historical cases in the Three Gorges reservoir area and the scope of application of the method was pointed out.Firstly,combining the characteristics of landslides in the Three Gorges reservoir area,the algorithm of landslide motion process considering gravity,friction and water resistance and the algorithm of landslide generated impulse wave generation and propagation considering gravity and three wave generation methods were proposed.Secondly,By simulating the propagation process of solitary waves of different initial wave heights along a two-dimensional pool,it is found that when the initial wave height exceeds 0.4,the Tsunami Squares method will overestimate the propagation process of the propagation By simulating the runup process of solitary waves with different initial wave heights along different slope angles and comparing the simulated maximum runup heights with experimental data and empirical formula predictions,it was concluded that the Tsunami Squares method is suitable for simulating the runup process along steep bank slopes or waves with wave heights less than 0.4 in front of the bank along gentle slopes.Then,the generation and propagation of landslide generated impulse waves with different wave types and conditions were simulated using Tsunami Squares.Finally,two historical cases of landslide generated impulse waves in the Three Gorges reservoir area were back analyzed,and it was found that the simulation results of the Tsunami Squares method could achieve good agreement with the survey data.(4)A quantitative risk assessment of landslide generated impulse wave on a local scale was carried out for the Ganjingzi landslide in the Wuxia bank section of the Three Gorges reservoir area.Based on the structural characteristics and deformation status of the Ganjingzi landslide,the Ganjingzi landslide was divided into four sub-areas;five calculation conditions were determined considering the reservoir water level changing conditions and rainfall,and the spatial and temporal probabilities of the landslide were obtained by simulating the stability and damage probability of the landslide using the numerical simulation method Geo-Studio.After that,the probability of the landslide entering the river was obtained by simulating the runout process of the landslide under various conditions through the Tsunami Squares method.The susceptibility to damage under different conditions in the study area was assessed;the risk curves of the landslide generated impulse wave under different conditions was obtained by taking the distribution properties of the dynamic elements in risk into account.(5)A quantitative risk assessment of regional landslide generated impulse waves was carried out with the Wu gorge in the Three Gorges reservoir area.According to the characteristics of the geological environment,slope structure and external environment of the study area,the potential impulse wave sources during different reservoir water levels were identified;the Tsunami Squares method was used to predict the intensity of landslide generated impulse waves generated by every sources when they occurred independently under different reservoir water conditions;the potential vessels in the study area were classified and their overturning damage and risk curves under different reservoir water conditions were calculated separately.Also,combining the landslide generated impulse wave intensity and the potential damage of the piers from the potential impulse wave sources,the risk curve of the probability of damage of the vessel in these piers was plotted.