Effect of unsteady flow on hydrodynamic characteristics at confluences

Unsteady tributary inflow imposes time-varying lateral momentum forcing on river confluences, but its effects on transient confluence-flow adjustment remain less understood than those under steady inflow. In this study, two-dimensional (2D) planform simulations examined velocity redistribution, separation-zone adjustment, backflow occurrence, and modeled turbulent kinetic energy (TKE) under different peak tributary velocities and inflow durations. Three-dimensional (3D) simulations clarified vertical flow structure, secondary circulation, TKE redistribution, and water-surface response in representative cases. The results show that unsteady tributary inflow reorganized the planform velocity field and modified separation-zone adjustment, secondary circulation, modeled TKE, and water-surface elevation. Peak tributary velocity mainly affected the magnitude and spatial extent of the planform response. Stronger peak forcing enhanced mainstream deflection and enlarged the separation-related low-velocity region downstream of the confluence. It also intensified modeled TKE near the mixing interface and separation-related region. Localized opposite-bank near-wall backflow appeared under selected unsteady conditions, indicating a conditional transient response within the planform adjustment. Inflow duration mainly affected the timing, persistence, and recovery of the unsteady response. Longer-duration forcing slowed separation-zone development and shifted elevated TKE to a later and more persistent interval. The 3D simulations further showed that planform adjustment was linked to vertical flow restructuring. Secondary circulation was reorganized, streamwise velocity became vertically non-uniform, TKE redistribution was delayed, and local maxima in water-surface elevation migrated downstream. These findings suggest that both forcing amplitude and forcing timescale should be considered when interpreting confluence dynamics under rapidly varying tributary-inflow conditions.