| Climate change around the world has detrimental impact on the survival of human beings and ecological environment. In order to deal with the floods that occur frequently owning to extreme weather, the programs of CPC Central Committee and State Council in the12th Five-Year Plan emphasize on facing the climate change around world actively, reinforcing the protection of ecology and the establishment of system of disaster prevention and mitigation and enhancing efforts on environmental protection and on resource conservation and management. Thus, it is a significantly practical and unavoidable problem that how to improve the engineering and non-engineering measures on flood control and the efficient utilization of water resource under adverse weather conditions and then to realize the coordinative development of economy, society, resources and environment.The controlling projects of reservoirs on large river basins is an important part of the socio-economic, ecological environment, thus, the significance of its operation management risk is particularly noteworthy. With the gradual formation of large-scale reservoir group, integrated management in watershed has become the inevitable trend of development. This paper, for need of the development of national economy and society, chooses the controlling projects of reservoirs on Yangtze River basin as the object of study. According to main route of risk analysis, it makes a systemic and in-depth study, separately from the identification and quantization of risk factors, risk estimate and evaluation, multi-objective decision, coordination optimization between risk and benefit, construction of coordination mechanism and so on, to establish the basic theory and systemic methods of risk analysis of the multi-purpose reservoir operation. The achievements facilitate decision-making on disaster prevention and mitigation of the large river basin, help to improve the overall efficiency of water resources utilization and promote the benign cycle and sustainable development of engineering technology, social economy, and the ecological environment. The main achievements made in the full paper are as follows:(1) The quantitative method and evaluation index system of risk factors. For the numerous uncertain factors confronted with reservoir operation such as hydraulic and hydrological factors, engineering state, human management and so on, we analyze the main risk factors that possibly affect the reservoir operation; For the ones probably referred such as randomness, fuzzy factor, runoff forecast error, and runoff and the flood of random simulation, we put forward the corresponding processing methods and form a quantitative method system for single risk factor; Based on the specialty of Copula function that joint distribution of several variables can be obtained through the marginal distribution of each variable, we put forward the combining quantitative method of multiple risk factors. Furthermore, based on the study on the method of identification and quantization of risk factors, we propose the generalized risk evaluation index system, providing an important basis for subsequent job on the risk analysis of reservoir operation.(2) Risk estimate based on improved monte carlo method. Based on the method of monte carlo and maximum entropy, making full use of specialty of the monte carlo method that it can avoid describing the complex internal characteristics of the study object and the difficulties on analytical analysis of the system behavior or benefit owing to those characteristics, combining with the maximum entropy method which can use a small amount of known information to analyze and then determine the best distribution form of the variables, we put forward the improved monte carlo method on the risk calculation of reservoir operation, offer the mathematical proof on convergence of the proposed method, explain why the proposed method can improve the convergence rate, thus avoiding the shortcoming of the monte carlo method that calculation of small probability events needs a great quantity of simulation calculation, and realize the fast calculation of multiple risk indicators for complex engineering system.(3) Random multi-objective decision-making model for describing the multiple minimum risk problem of reservoir operation. For various uncertainty and multiple goals of multi-purpose reservoir operation, with the randomness of the incoming runoff as the key point, we set up random multi-objective decision-making model for reservoir operation. And then transforming it to the multiple minimum risk problem with the target value drawn up as the level value, we propose the runoff simulation based on considering frequency diversity and the sequence solution of seeking best compromise solution to random multi-objective decision by progressive optimization algorithm. Then, it is convenient for decision makers to make an optimal decision according to the different requirements and goals of reservoir operation,and it offers a new idea to random multi-objective decision-making of the reservoir operation.(4) Coordination optimization model between risk and benefit based on the probability optimization method. Based on the random multi-objective decision-making of reservoir operation and improved monte carlo method, for the demand of flood control, power generation, water supply, shipping and such kind of operation requirements and object, we establish the multi-objective operation risk analysis model of multi-purpose reservoir, and make a in-depth study on the method of determining weight to the risk evaluation index system of multi-objective operation and the calculation method of composite risk. Then, based on this, we propose probability optimization method, and verify the feasibility and good effectiveness of the method and model in the practical project by multi-purpose operation risk analysis of the Three Gorges Reservoir.(5) Joint flood control operation risk analysis of Xiluodu Reservoir and the Three Gorges Reservoir. First, we establish the risk evaluation index system of the joint flood control operation of the cascade reservoirs. Secondly, based on normal quantization of accidental error in flood forecasting, we put forward the random simulation method for incoming flood process with the standard deviation in the allowable range. Furthermore, based on the idea considering flood frequency diversity, we set up the risk analysis model of the joint flood control operation of the cascade reservoirs and the solving method based on the improved monte carlo method. Through the application in the project joint flood control operation of Xiluodu Reservoir and the Three Gorges Reservoir, this method and model prove reasonable,feasible and of quite wide applicability. Under the condition that when cascade reservoirs suffer the typical flood of certain frequency, it can not only offer the relationship between forecast error and joint operation risk, but also estimate the maximum and minimum critical value of the standard deviation of flood forecasting errors according to the capacity of flood downstream area and the preference of decision makers, so as to provide the important basis for operation decision-making.(6) Joint utilizable regulation risk analysis of Xiluodu Reservoir and the Three Gorges Reservoir. Considering joint utilizable regulation of the cascade reservoirs involve the aspects like power generation, shipping, water supply and ecological, we establish the value function respectively, and present a risk evaluation index system of the joint utilizable regulation. On this basis, we construct composite risk calculation model for joint utilizable regulation. And we apply it in the joint utilizable regulation of Xiluodu Reservoir and the Three Gorges Reservoir, make the risk assessment and analysis of dispatching schemes of different preferences and obtain the risk evaluation results for various dispatching schemes, providing the reference for joint operation decision-making with various preferences.(7) Coordination mechanism betwwen flood control and benefit of controlling cascade reservoirs on the upper reaches of Yangtze River. Based on analysis of characteristics of flood of Yangtze river basin and the flood control capacity of its trunk stream and tributaries, for15reservoirs on trunk stream and tributaries of the upper reaches of the Yangtze River with the Three Gorges as the center, in view of analysis of compensation principle of hydrology and storage capacity and the characteristics of floods on upper reaches, we propose flood control operation compensation mechanism of the middle and lower reaches of the Yangtze River for the large-sized flood. The compensation mechanism includes the principle, model and mode of the compensation operation, so as to reduce the flood loss downstream and increase joint flood control benefit of reservoir group; In view of the small and medium-sized flood, we establish the coordination mechanism with the flood control and benefit as the target, including the coordination of flood control and later water storage and the coordination mode of flood control and power generating, to increase the degree of flood utilization and improve benefits. Moreover, with a comparative analysis of the existing dispatching schemes for flood, we illustrate the feasibility of the thought in this paper and provide a theoretical support for joint operation of the cascade reservoirs on the trunk stream and tributaries of the upper reaches of the Yangtze River in the flood season.
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