| Flood disaster is one of the natural disasters most closely related to human production and life.It has the characteristics of wide range of damage,large disaster loss,short formation time,strong unpredictability and obvious periodicity.For example,during the No.5 flood of 2020 in the Yangtze River Basin,the upper reaches of the Yangtze River collected a large amount of floods in a relatively short period of time.The flood peak flow of the Three Gorges Reservoir reaches 75,000m~3/s,which is the largest since the construction of the Three Gorges Reservoir,and has brought huge challenges to the cascade reservoir system in the Yangtze River Basin.Thanks to the accurate investigation and judgment of the flood process and the reasonable dispatch of the cascade reservoirs,the Three Gorges Reservoir has successfully resolved the No.5 flood with the remarkable flood control effect of the maximum discharge of 49,400m~3/s,the peak cutting rate of 34.1%,and the flood retention capacity of 3.113 billion m~3.The fine overall planning and joint dispatch of the reservoir group system give full play to the flood control function of the cascade reservoir system in the basin,minimize the flood control pressure on the middle and lower reaches of the Yangtze River,and ensure the safety of life and property.Therefore,accurately and timely flood prediction and finely coordinated reservoir operation are indispensable,and the two can effectively reduce or even completely avoid the occurrence of flood disasters when they work together.However,the overall management and operation of cascade reservoirs in the basin not only brings into play huge comprehensive benefits,but also increases the complexity and strong coupling of operation and operation system,which brings a series of academic frontier and engineering practical problems that need to be solved,mainly reflected in the following three aspects:(1)The cascade reservoir groups are managed by different dispatching entities,and there are certain information barriers between different river basin management agencies,which intensifies the difficulty of early study and judgment of the flood process.Existing river flood simulation technology is still relatively expensive in terms of time.How to develop a more efficient,accurate and adaptable runoff forecasting simulation model in response to the problems of low runoff simulation accuracy and poor timeliness is one of the current hot issues in the joint operation of cascade reservoirs.(2)Rainfall runoff is affected by many factors such as climate change,basin topography,and storage and control of main and tributary reservoirs.In addition,the completion and commissioning of large-scale water conservancy projects have also changed the flow of water in the reservoir area.How to clarify the impact of changes in rainfall and runoff on the safe and economic operation of cascade reservoirs,and to develop a fine-grained forecast theory of rainfall and runoff in the reservoir area under the operating conditions of cascade reservoirs is another hot issue currently studied.(3)The hydraulic connections among cascade reservoir groups are closely coupled.Under the storage control of the cascade main and tributary reservoir groups,the flood evolution process of the upstream and downstream reservoirs is complex and changeable.The evolution of river flooding is elusive due to the comprehensive influence of tributary inflows and sinks,the outflow of upstream reservoirs,and the water level in front of the downstream reservoirs.Therefore,under the regulation and storage of tributary reservoirs,the impact of upstream reservoir operation and dispatch on downstream reservoirs and the comprehensive effect of joint dispatch of cascade reservoir groups have important theoretical research significance and engineering practical value.For this reason,the research work takes the cascade reservoirs in the upper reaches of the Yangtze River as the main research object,and starts with the construction of high-efficiency river runoff simulation models.Combining mathematical model deduction,hypothesis verification analysis,and engineering case studies,conduct in-depth research on river flow evolution,flood propagation between cascade reservoirs,rainfall runoff forecast in the basin,and joint operation and scheduling of cascade reservoirs.The research results have been successfully applied in the Three Gorges Group Basin Cascade Dispatch Operation Center,providing reference for other cascade reservoir groups or river basin dispatching decisions with similar conditions.The main research contents and achievements are as follows:(1)Aiming at the problem of river flood simulation between cascade reservoirs,combining the finite difference method and the finite volume method to discretize the control equations and divide the calculation grid,draw on the operator splitting method and the Euler-Lagrangian method to solve the model with high speed and accuracy.A one-dimensional hydrodynamic model(H1DM)of a branched river network with high simulation accuracy was established.In addition,on the basis of H1DM,the Open MP parallel technology is introduced to fully improve the calculation efficiency of the model.Under the premise of ensuring the calculation accuracy of the model,the solution speed of the model is further improved.The example results show that the proposed model has extremely high simulation accuracy for the flood process,and the solution cost is low.Parallelization technology can bring up to 41%efficiency improvement,which can provide strong basic technical support for the operation and dispatch of cascade reservoirs in the basin.(2)In order to prove the law of flood propagation between cascade reservoirs under the conditions of storage and control in the main and tributary basins.The research work uses H1DM as a tool to explore the evolution law of water flow between cascade reservoirs,in river channels and in the reservoir area.Combining with the actual operation and scheduling process,the concept of"the entrance of the reservoir"in the sense of non-traditional scheduling was put forward,and the calculation method of reservoir inflow flow was clarified.In addition,the influencing factors of the short-term optimal operation and dispatch of the reservoir were further analyzed,and the accurate prediction method of the reservoir tail water level was proposed.The examples used show that the well-trained BP neural network method has high simulation accuracy and strong stability,can achieve accurate and rapid prediction of the tail water level under reservoir operating conditions,and provide decision support for the operation and scheduling of cascade reservoirs.(3)Focusing on the problems that traditional hydrological models of river basins cannot describe the law of river flow movement and the lack of hydrological data,the accuracy of rainfall-runoff models is reduced.Analyze the characteristics of the hydrological model and the hydraulic model,combine the advantages of the two,and use the hydraulic model to replace the river evolution part in the hydrological model.Adopting different coupling strategies,considering topographical conditions for proper parameter displacement in areas with relatively lack of measured data,and developing two hydrological-hydrodynamic coupling models(XAJ-H1DM and H1DM-XAJ).Compared with the single model,the coupled model has improved simulation accuracy to varying degrees.XAJ-H1DM has a clear physical meaning and conforms to the rainfall runoff law.The H1DM-XAJ model draws on the solution ideas of the traditional Xin'anjiang model,which can provide simulation accuracy equivalent to the aforementioned coupled model and can better adapt to areas lacking hydrological data.The hydrology-hydrodynamic coupling model helps to improve the understanding of rainfall,runoff generation and confluence processes in the basin,and has strong engineering practical value.(4)Study and analyze the flow evolution method used in the optimal operation of cascade reservoirs,and demonstrate the advantages and disadvantages of the calculation method of flood evolution in the channel between cascade reservoirs.Discussed the law of the influence of the water level change in front of the dam on the flood entering the reservoir under the backwater condition of the downstream reservoir.On this basis,combined with the aforementioned example of flood propagation law analysis among cascade reservoirs and the research results of"the entrance of the reservoir",innovatively put forward the idea of using H1DM to replace the traditional delay method or Muskingum method,and a short-term optimal dispatch model for cascade reservoirs based on H1DM is established accordingly.The simulation results of the cascade dispatching system of the upper reaches of the Yangtze River show that the model proposed in this paper can effectively perform short-term optimal dispatch of cascade reservoirs and accurately obtain the inflow of downstream reservoirs,which is of great significance for improving the comprehensive benefits of hydropower stations and the safe operation of reservoirs. |
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