Research On Flood Control Strategy Of The Three Gorges Reservoir And Upstream Cascade Reservoirs In A Catastrophic Flood

The areas with the most flooding in the country are the middle and lower reaches of the Yangtze River,which has a great influence on the entire basin and the national economy.Among these,the floods in the Jingjiang area(middle reach)are the most serious,and it is an urgent and most important task to ensure flood control security along the Jingjiang reach,to prevent catastrophic flood disasters.The Three Gorges Project can directly control more than 95%of the flood volume of the Jingjiang River,and plays an important role in ensuring the safety of flood control in the middle and lower reaches of the Yangtze River.Since the completion of the Three Gorges Reservoir(TGR),a large number of large reservoirs with large flood control capacity and strong regulation ability have been built in the upper reaches of the Yangtze River,such as Xiluodu,Xiangjiaba,Wudongde,and Baihetan.The process of water storage and discharge since the completion of the upstream cascade reservoirs has greatly changed the original form of the TGR inflow flood,which has had considerable impact on the flood control risk in the downstream Jingjiang area.Therefore,in the context of the successive completion of multiple large reservoirs in the upper reaches of the TGR and the approval of the new version of the regulation rules,research on the flood control ability of the TGR and upstream cascade reservoirs is of great significance for enhancing the flood control safety of the reservoirs and downstream rivers and maximizing the flood control benefits of the reservoirs.This paper is based on the theories of hydrological stochastic simulation,one-dimensional hydrodynamic calculation,dynamic capacity flood regulation calculation,and joint operation of cascade reservoirs,and comprehensively considered the influence of multiple factors such as different design frequencies,different inflow flood shapes,different flood regulation rules,and joint dispatching of different cascade reservoir groups.The maximum water level in front of the dam,the maximum discharge of the TGR,the flood control protection time for the Jingjiang and Chenglingji areas,the flood diversion volume of the Jingjiang area,and the minimum capacity that needs to be reserved for the upstream cascade reservoirs are calculated systematically during the joint operation of the TGR and the upstream cascade reservoirs.The main research contents and results include:(1)Inflow flood generation of the TGR.Based on the long series of daily runoff data of the main inflow stations,a stochastic model of inflow floods for the TGR was established using the periodic stationary autoregressive method.The quantitative test results show that the simulation accuracy of the model meet the requirements,and can effectively generate the stochastic flood of the TGR.The historical typical flood and stochastic inflow floods are amplified into the design flood at four frequencies:1%,0.1%,0.01%,and 0.01%×1.1 by using the same frequency flood volume composition method of Cuntan and Yichang(or Zhicheng).(2)The traditional one-dimensional hydrodynamic model was improved to effectively realize the accurate simulation of flood evolution in the TGR.Based on the high-precision DEM data of the TGR,considering the effects of the tributaries and river bends on the flood evolution,the problem that the traditional one-dimensional hydrodynamic model cannot consider the real topography of the river was corrected,and the dynamic capacity flood regulation model of the TGR was constructed.The model is calibrated and verified with measured flood data during the normal storage period of the TGR,ensuring the accuracy of the model for the high water level flood evolution in the reservoir area.Sensitivity analysis of the model shows that when calculating the flood evolution of a river-type reservoir,considering the storage effect of tributaries and river bends can greatly improve the calculation accuracy of the traditional one-dimensional hydrodynamic model.(3)The flood control ability of the TGR at three frequencies:0.1%,0.01%,and 0.01%×1.1 was systematically explored when it was operated independently.When encountering the design floods with frequency of 0.1%,only relying on the TGR to hold the flood independently and the water level in front of the dam does not exceed 175m,the Jingjiang flood diversion area must be used to ensure the safety of flood control in the Jingjiang area.When encountering the design floods with frequency of 0.01%and 0.01%×1.1,even if the Jingjiang flood diversion area is opened,the flood cannot be effectively blocked.The upstream cascade reservoirs must be jointly dispatched to prevent floods to ensure the safety of flood control in the Jingjiang area.(4)The flood control strategy for the joint operation of the TGR and the upstream cascade reservoirs is systematically calculated.When encountering the design floods with frequency of 0.1%,the Wudongde,Baihetan,Xiluodu,and Xiangjiaba cascade reservoirs need to reserve a minimum flood control capacity of 7.211?9.474 billion m3 to avoid the use of the Jingjiang flood diversion area,accounting for 46.5%?61.1%of the total flood control capacity.When encountering the design floods with frequency of 0.01%,the minimum flood control capacity required to reserve for 14 large reservoirs(e.g.,the Wudongde,Baihetan,Xiluodu,Xiangjiaba Reservoir,Liyuan,and Ahai reservoirs)is 14.957?17.771 billion m3,accounting for 71.1%?84.5%of the total flood control capacity.When encountering the design floods with frequency of 0.01%×1.1,even if all the reservoirs built and under construction in the upper reaches of the TGR are put into use,the downstream Jingjiang flood diversion area still needs to be used to ensure the safety of flood control in the Jingjiang area.(5)The influence of the inflow flood shapes and flood control regulation rules on the flood control strategy of the TGR and the flood control safety of the Jingjiang area has been demonstrated.Calculation results shows that the shape of the inflow floods with the same design frequency has a huge impact on the maximum discharge that can be controlled by the TGR.In the flood control design,it is necessary to explore the flood control ability of the TGR by using the stochastic flood simulation method to find the most dangerous inflow flood of the TGR at a certain design frequency.Regarding the regulation rules of the TGR,while ensuring that the Chenglingji reach does not exceed the guaranteed water level,the compensation discharge of the TGR should be increased as much as possible,and more flood prevention capacity should be reserved for the subsequent flood arrest.This will not only help increase the protection time for the Chenglingji area as a whole,but also greatly reduce the flood diversion capacity of the Jingjiang area when encountering catastrophic floods.