Due to the unprecedentedly increasing demands of waterway transportation on the Yangtze River during the latest years, traffic congestion at the Three Gorges Dam (TGD) has becoming a serious problem. In busiest seasons, a vessel may need to wait up to 3 days for passing the TGD via the ship lift or the five-stage ship lock. In order to improve the navigation efficiency and the utilization of passing facilities, a co-scheduling problem of ship lift and ship lock at the TGD is modeled in this paper. Besides, the improvement on effectiveness, safety and fairness of the navigation scheduling by introducing the ship lift is taken into account. The mathematical model proposed is, by nature, a two-stage problem, where the firststage variables determine the assignments of passing facilities and the second-stage variables schedule the vessels. In order to solve the proposed mathematical model, a hybrid metaheuristic algorithm is used at the first stage to determine the allocation of vessels to different passing facilities and to different lockage through the maximization of facility utilization rate. The second-stage problem is then solved by CPLEX. The proposed mathematical model and algorithm are validated through a set of numerical experiments and a case study. The results show both the navigation efficiency and the utilization of passing facilities can be improved by using the proposed methods. INDEX TERMS Three Gorges Dam, scheduling, co-scheduling, metaheuristics, optimization.

In recent years, the environmental pollutions at the Three Gorges Dam have become an increasingly concerning issue of the Chinese government. One of the most significant environmental problems is the carbon emissions from the lockage operations at the two ship locks of the Three Gorges Dam. Currently, due to the large amount of vessels passing through the dam, there is always a long queue of vessels on both sides and the average waiting time is long. This has further lead to an increased amount of fuel consumption and carbon emissions. Therefore, it is of great importance to develop a decision-support model for a better navigation scheduling and planning of the lockage operations at the Three Gorges Dam. This paper proposed an improved mixed integer non-linear programming model for the green lock scheduling problem at the Three Gorges Dam. The model aims at minimizing the carbon emissions and the waiting time in the lockage process through scheduling the vessels in a fairer and more efficient manner. Moreover, a greedy particle swarm optimization (G-PSO) algorithm is developed to solve the complex optimization problem. The proposed mathematical model and algorithm are validated through a numerical experiment. The result shows that it may lead to a significant reduction on carbon emissions by giving a specified speed to each vessel with a pre-optimized sequence. Meanwhile, the fairness and efficiency of the lockage process may also be improved.