Abstract:

[Objective] Optimizing the scheduling of flood control water levels in reservoirs has become a key technical approach to cope with extreme flood and drought disasters. Focusing on the challenges of unclear allocation efficiency of reservoir capacity and hierarchical scheduling of water levels for flood control in the cascade reservoirs of the Yalong River Basin, we established a multi-objective scheduling model for flood control water levels of cascade reservoirs, aiming to achieve coordinated scheduling of both flood control and beneficial utilization goals. [Methods] The study focused on three key control reservoirs—Lianghekou, Jinping I, and Ertan—in the Yalong River Basin, and used flood hydrographs derived from typical flood events in 1965 and 2000 under different design frequencies (P=1%, 0.5%, 0.2%, and 0.1%) as input data. The objective functions of the proposed model include: minimizing the peak discharge at the outlet cross-section, minimizing the water level at the end of the scheduling period, minimizing the highest water level during the scheduling period, and minimizing the total utilized flood control reservoir capacity, while considering constraints such as water balance, discharge capacity, and upper and lower water level limits. The model was solved using the Non-dominated Sorting Genetic Algorithm II (NSGA-II) to generate the Pareto solution set, and the entropy-weighted VIKOR method (EMW-VIKOR) was used to rank and select the optimal scheduling schemes. [Results] (1) Significant trade-off among the objectives existed. The peak flood discharge at the outlet cross-section of the river basin showed a clear competitive relationship with the water level recovery rate of upstream reservoirs at the end of the scheduling period, the maximum water level during the scheduling period, and the total utilized flood control reservoir capacity. Analysis of flood control reservoir capacity allocation indicated that as the flood magnitude increased, the allocated flood control capacity of Jinping I increased, the total utilized flood control capacity of cascade reservoirs decreased, and the peak discharge affecting downstream protected areas decreased. (2) The optimal flood control water level schemes were obtained after comparison and selection. For Lianghekou Reservoir, the flood control water level could be raised from 2 845.9 m to 2 848.44-2 850.88 m, an increase of 2.5-5 m. For Jinping I Reservoir, from 1 859.06 m to 1 859.76-1 860.83 m, an increase of 0.7-1.8 m. The flood control water level of Ertan Reservoir could be raised from 1 190 m to 1 192.13-1 192.96 m, an increase of 2-3 m. (3) The optimized scheduling schemes demonstrated significant comprehensive benefits. Compared with conventional scheduling schemes, the error in reservoir water level recovery rate at the end of the scheduling period was less than 0.5% under different design frequencies. The highest water level indicator approached 1 (e.g., 0.997 9 at P=0.1%) without exceeding flood control upper limits. The utilized flood control reservoir capacity was reduced by 300 million m³ or more, including a reduction of 566 million m³ under the design frequency of P=1%. Taking the flood event on July 29, 2024 as an example, the optimal scheme increased power generation by 255 million kW·h (an improvement rate of 9.7%) compared with the actual scheduling, while achieving a lower peak flood control water level and a water level recovery rate of 100% at the end of the flood season, demonstrating significant improvements in both flood control and beneficial utilization. [Conclusion] This study proposes a multi-objective optimization model for flood control water level scheduling in cascade reservoirs, reveals the allocation efficiency of flood control reservoir capacity, and provides a multi-objective coordinated scheduling scheme for flood control water levels of cascade reservoirs. The findings provide technical support for floodwater resource scheduling of cascade reservoirs in the Yalong River Basin and enhance the utilization level of water resources in the river basin.

Key words: cascade reservoirs, flood control scheduling, flood control water level, multi-objective optimization, Yalong River Basin