Optimize multi-objective transformation rules of water-sediment regulation for cascade reservoirs in the Upper Yellow River of China

Frequent occurrence of flood and ice disasters caused by river channel sedimentation in the Upper Yellow River of China has seriously threatened the lives and property of downstream residents, which has become the most challenging issues in harnessing the Yellow River. In this paper, we aim at optimizing water-sediment regulation that considers two cascade reservoirs (Longyangxia and Liujiaxia) as the regulatory bodies using one multi-objective model (sediment transport and hydropower generation) and two single-objective models (sediment transport only, and hydropower generation only). We propose an innovative approach (FSS-MOPSO) that hybrids the Feasible Search Space (FSS) with the Multi-Objective Particle Swarm Optimization (MOPSO) algorithm to search the optimal solutions for the multi-objective joint operation of cascade reservoirs. We analyze the transformation rules of water-sediment regulation among five objectives (hydropower generation, sediment transport, water supply, flood control and ice control) under various optimization schemes. The results indicate that the conflict between the hydropower generation objective and the sediment transport objective is prominent. An extreme case indicates that an increase in hydropower output by 2.31 billion kWh (17.6% increase) would greatly reduce the amount of sediment transport (73.5% decrease) while only makes little effects on the other three objectives. The results demonstrate that the optimal water-sediment regulation not only can ensure to meet water demands in the future (2030) but can provide an important guideline to safely operate cascade reservoirs during ice and flood periods. The research findings contribute to the identification of the relationship among objectives and strategy recommendations on water-sediment regulation for efficient cascade reservoirs operation in the Upper Yellow River.