Unsteady Flow Characteristics and Navigation Safety Downstream of Hydropower Stations

Unsteady flow induced by peak-regulation of large hydropower stations can cause rapid variations in downstream hydraulic conditions, posing significant challenges to navigation safety in restricted navigation reaches. Previous studies have provided valuable insights into the hydraulic response of downstream reaches to hydropower regulation, particularly in terms of water-level and flow-velocity variations, while the combined analysis of field-validated unsteady-flow propagation, local wharf response, and vessel maneuvering capability remains limited. Focusing on the 30 km restricted navigation reach downstream of Xiluodu Hydropower Station, the paper integrates prototype observations, hydrodynamic simulation, and ship maneuvering simulation to investigate the propagation characteristics of unsteady flow. The results show that hydropower-induced unsteady flow undergoes pronounced attenuation during downstream propagation, with stronger water-level and flow-velocity. Discharge variation amplitude is identified as the dominant factor, whereas base flow and ramping duration exert comparatively limited effects. An increase in the operating level of Xiangjiaba Reservoir strengthens the backwater effect. Based on these characteristics, an empirical model incorporating hydropower-station discharge characteristics, local channel conditions, and longitudinal attenuation is developed to predict water-level variation at downstream wharves, and the predicted results show good agreement with prototype observations. Validation against prototype observations gives a mean relative error of 6%, a maximum relative error of 8%, and R2 = 0.988. Ship-handling simulations further demonstrate that navigation feasibility is jointly governed by discharge, vessel class, and propulsion power, with the allowable discharge for navigation increasing as propulsion power increases. On this basis, two complementary navigation-safety criteria are proposed: an allowable discharge ramp rate constrained by hourly water-level variation and navigation discharge criteria determined by vessel navigation feasibility. The results provide a quantitative basis for coordinating peak-regulation operation of large hydropower stations with navigation management in downstream restricted navigation reaches.