Hydrodynamic Characteristics and Navigation Risk Zonation in the Lower Min River Estuary Under Mainstem Backwater Effects

Daily hydropower regulation and mainstem backwater generate complex hydrodynamic conditions in the Min River estuary, posing significant challenges to navigation safety. To analyze the impact of mainstem backwater on tributary navigation safety, this study focuses on the lower Min River reach affected by backwater from the Jinsha River. A depth-averaged 2D hydrodynamic model is established, and a water level difference parameter is used to construct the stage–discharge relationship at the estuary based on long-term measured water level and discharge data. Indicators including backwater distance, water surface slope, hydrodynamic axis migration, flow velocity, and cross-flow are used to delineate navigation risk zones. The results indicate the following: (1) The backwater intensity and extent are primarily governed by the mainstem and tributary discharges and by the distance from the estuary. High discharge and water levels produce significant backwater effects and reduced flow velocity. Empirical formulas for backwater length under various discharge conditions are established to support navigation decision-making, with RMSE values ranging from 0.42 km to 0.92 km. (2) Variations in estuarine water level induce oscillations in the hydrodynamic axis. When the upstream discharge is 900 m 3 /s and the estuarine water level is 258.4 m, the maximum oscillation amplitude reaches 20.33 m. (3) During periods of medium and low water, the reach exhibits significant navigation-obstructing behavior, with high-risk zones concentrated in Tongluowan, Yangjiaoshi, and other shoals 5–8 km being found from the estuary. (4) Under the design discharge condition, the minimum estuarine water level required to ensure adequate channel depth, appropriate flow velocity, and manageable ship resistance for safe navigation is 267.96 m. This study provides a scientific basis for navigation safety and channel regulation in the Min River estuary and similar reaches affected by mainstem backwater, thereby supporting sustainable waterborne transport.