Research On Landslide-induced Surge In Three Gorges Reservoir Area

The water level in Three Gorges Reservoir was firstly reached to175m in2011, which indicated that the operation of Reservoir is reached to the designed criterion. Landslides are largely distributed in Three Gorges Reservoir. According to the incomplete statistics, totally3028landslides are monitored in the Second and Third Stage of the Reservoir operation. Influenced by the fluctuation of the water level and high water pressure, many landslides become obviously active along the600miles river bank from Yichang to Chongqing, and old and potential landslides failure probabilities are greatly increased. Landslide generated waves is the most severe hazard chain of landslides, which can pose serious threat to the safety of navigation, docks, structures and residents. The occurrence of all such events as Tangyanguang landslide in Zhexi reservoir in1961, Italy Vajont landslide in1963and Xintan landslide in Three Goreges in1985roused severe consequence for the wave hazard.The research of landslide surge is a complicated geotechnical engineering and hydraulic engineering problems, also the factor of reservoir bank landslide surge complex, including landslide size,shape,speed of sliding into the water, dipping angle of the slide surface,water depth, width of river course,channel cross-sectional shape,river bend degree and so on.At present, domestic and foreign scholars simplified the factors influencing landslide surge and the shape of river course, also ignored the geological model of reservoir bank when they make assessment of landslide surge in the quantitative, thus there’s a large gap between the actual value and the calculation value.So it makes sense to carry out research in the field of landslide surge prediction in reservior area, which will contribute to the navigation safety and strategies planing of disaster reducing and avoiding. The research needs the combination of landslide motion process, producing surge and propagation attenuation process, the study of wading landslide motion speed calculation model, Combination with the physical model test and study of landslide surge in the Three Gorges reservoir formation and propagation.There are500large built reserviors in china. Some like baihetan and xiluodu which building are going to be used, their safety of navigation are also threatened by the landslide and surge. The study result of landslide surge theoretical calculation and technology method which can provide some effective technical support for some large resevoirs has significiant application value.Taking Xintan landslide, Qianjiangping landslide and Gongjiafang landslide as examples, this paper summarized geological model and river channel model for typical landslide-surge disaster thoroughly analysising their causes and surge features. Then we captured nondimensional forms of landslide surge height by using dimensional analysis method. Combined with the existing geological data of potential landslides in the Three Gorges Reservoir area, the orthogonal text program and single-factor test program have been designed for landslide-surge experiment.Based on the1:200scale physical river channel model of Baishuihe landslide, precisely controling3D landslide-surge experiment have been conducted. The landslide surge calculation model considering landslide scale and sliding surface angle is presented in the paper after comparing the results got from Noda and Panjiazheng formula. Underwater block resistant force experiments have been done to calculate the landslide speed and hydraulic resistance. Incorporated this model into the study of Xintan landslide, Qianjiangping landslide and Gongjiafang landslide, the results are more reasonable. In analytical solutions for landslide propagation wave along the channel, we deduced the continuity equation and motion equation considering channel corss-section shape, then with the help of separation variable method and linear approximation, we obtain the attenuation law of surge considering different water channel shape in sharp attenuation stage. At last. Xintan landslide is taken as an example to calculate landslide propagation waves by combing the calculation formula considering frictional head loss in slow attenuation stage. Through the system study of landslide surge hazard, some research conclusions have been made in this paper as follow:(1) Typical landslide surge analysis in the Three Gorges ReservoirTaking Xintan landslide, Qianjiangping landslide and Gongjiafang landslide as examples, by analyzing their topography and geomorphology, genetic mechanism, motion process and surge feautres, we summarized geological model and river channel model of these three landslide surge hazaed examples. Xintan landslide stands for the geological model which landslide slides along the bed rock, and the channel model which landslide thickness is close to the river water depth under the condition of natural river. Qianjiangping landslide stands for the geological model which landslide has the characteristics of interlaminar shear in back and shear layers in front, and the channel model which landslide thickness is close to the river water depth under impoundment condition. Gongjiafang landslide stands for the geological model which slumping mass slides along the bed rock, and the channel model which landslide thickness is far less than the river water depth under impoundment condition.(2) Landslide surge physical model experiment equpiment and method are studiedThrough the statistical analysis of geological data about the potential landslides in TGRA, adopting the orthogonal experimental design method, we formulate the experiment scheme; it include landslide scale, speed entering into the water, sliding plane obliquity, water depth, and slope angle. According to the experiment program, taking the channel of Baishuihe landslide in TGRA as prototype, Based on the premise of geometric similar,dynamic similar, movement similar, we establishe the river physical model in map scale1:200, and thus develop experimental control system including the sliding control equipment, cargo block and crane equipment, then adopt capacitance wave probe, high-speed camera and other measuring instruments, forming experimental measurement system, at last we develope landslide surge three-dimensional physical model experiment.(3) morphological analysis of landslide surge and definition of the largest leading waveThrough analyzing morphological changes of landslide surge when landslide entering into the water, the production process of landslide surge can be divided into three stages:the first stage is landslide entering into water stage. When landslide slide into the water along the sliding surface, water attaining surface pushes the water, making the the upper water body in semifloret shape jump to the water surface or the other side of the bank slope. The second stage is landslide surge formation stage. When landslide continuously downslide, the cavity forming in its tail is bigger and bigger, the water around the cavity quickly enter into the empty cavity and produce a great splash due to collisions, splash regiment fall in the near place very soon. At the same time, the energy which landslide carry continuously passed to the surrounding water, then forming surge around them. Among them the surge height is the biggest in landslide sliding direction, we define this is the largest leading wave. The third stage is surge propagation and attenuation stage. When landsldie surge propagate to the surrounding area, its height gradually decay. On one hand, it propagate to the opposite bank, then forming the climbing wave; On the other hand, it propagate along the river channel, and produce climbing waves along the bank.(4) Landslide surge calculation model in TGRA is put forwardBased on the classical landslide surge formulas proposed by Noda and Pan Jiazheng, by analyzing the measured data, using the dimensional analysis method and multivariate nonlinear regression analysis method, we deduced the landslide surge calculation formulas in TGRA, including the largest leading wave height, the opposite bank climbing wave and the propagation wave along the channel. Besides, the sensitivity analysis of the largest leading wave’s influencing factors were performed, and it is included that landslide speed is most sensitive to the leading wave height, the sensitivity order of other factors is landslide length,landslide width, sliding plane obliquity, landslide thickness and water depth.(5) Speed model of fording landslide baesd on water resistance experimentThe experiment system of moving block’s resistance in water is designed. We select area of and speed as the influencing factors to develop the pressure head experiment. Through the data analysis, we establish the the water resistance model of block’s attaining surface in water. Adopting slice method, we establish motion equation along the sliding direction and the direction perpendicular to the sliding surface, combing with the water resistance model, we obtain the speed model of fording landslide.(6) Calculation and back analysis of typical landslide surgeCalculation and back analysis to the typical landslide surge should consider landslide speed, landslide volume entering into the river and the opposite bank climbing wave. Due to the calculation method is one approach of sloving practical landslide surge problem, we can not just use one singule reduction factor, because it can’t react the kinetic frictional shear strength parameters. Therefore, reduction factor should be within a certain range.According to the speed model based on slice method and the landslide surge calculation formulas in TGRA based on physical model experiment, we calcluate the largest leading wave, the opposite bank climbing wave and the propagation wave along the channel of Xintan landslide, Qianjiangping landslide and Gongjiafang landslide in Three Gorges Reservoir Area. Combining with the field survey data, we obtain that when the kinetic frictional shear strength parameter’s value is between0.8-0.95, the calculation results of landslide velocity, the opposite bank climbing wave and the propagation wave along the channel is basically reasonable. Landslide surge calculation formulas in TGRA based on physical model experiment can provide some scientific basis for prediction and early warning of landslide surge in Three Gorges Reservoir Area.When calculating landslide speed and its surge, in addition to consider the reduction factor, we also should pay attention to the stability and deformation of landslide, the steepness of sliding surface, The height between landslide center and water level, the distribution of the groundwater level, channel depth, etal..(7) Research on propagation height of landslide surge considering channel cross-section shape coefficientCombining the Saint-Venant equations in hydrodynamics and Fluid mechanics, we define the function of landslide’s river channel cross sectional shape, deduce the continuity equation and movement equation including channel shape coefficient.Then, the decay rule of surge in sharp attenuation stage is gained by separation variable method.Based on physical model experiment, we concluded that the propagation wave along the channel haves a rule which first sharpe decay then slow deacy. In a sharp attenuation stage, by adopting the method of linear approximation perturbation, landslide surge equation including continuity equation and motion equation are inearized for the zero order and first order. At the same time,we acquired the analytic expression of propagation wave along the channel in steep attenuation phase using the method of separation of variables.From the decay rule of surge in sharp attenuation stage, we know landslide surge attenuation under V-shaped channel cross-section is the fastest, secondly, U-shaped channel, and the last is rectangular channel cross-section.Combing the calculation formula considering frictional head loss in slow attenuation stage, Xintan landslide is taken as an example to calculate landslide propagation waves. By analyzing the calculated results of theoretical formula and experimental formula, we think that the theoretical formula considering channel shape coefficient is suitable for narrow channel.