On Three Gorges Reservoir control water level and operating conditions in flood season

As the cascade reservoirs in the upper reaches of the Yangtze River have been built and put into operation, the hydrological regime and functional requirements of the Three Gorges Reservoir (TGR) have changed significantly compared with that in the design phase. The reservoir operation of maintaining a fixed flood control limited water level (FCLWL) can no longer meet the needs of the new situation. In this paper, the feasibility of the dynamic control of the operating water level of the TGR during the flood season is demonstrated by analysing the setting conditions of the FCLWL during the design phase of the TGR, excavating the characteristics of the floods and the flood encounter rules in the basin. The results show that:① The applicable condition of the 145 m FCLWL in the design stage of TGR is to cope with major floods in the basin, and the occurrence probability of the basin-wide floods is small and has obvious characteristics, which can be predicted in advance by meteorological and hydrological forecast. ② According to the types of floods in the basin, division of flood stage and encounter rules of floods, when it is predicted that a regional flood will occur, FCLWL of the TGR will be set at 145 m from the beginning of June to the end of the Meiyu period, and then the water level will be gradually increased from the end of the Meiyu period to 155m by the 20^th of August. ③ With the cooperation of meteorological and hydrological forecasting and joint operation of upstream reservoir group, the operating water level of the TGR during the flood season can fluctuate around 155 m in normal years, and earlier reservoir impoundment would be feasible. ④ The dynamic control of the water level of the TGR during the flood season will not increase the risk of flood control and siltation in the reservoir area. Or rather, it is beneficial to the hydrological regime and the relationship between the rivers and lakes in the middle and lower reaches of the Yangtze River, which can significantly improve the comprehensive utilization benefits of power generation, shipping, ecological protection and water supply.