Curvature-controlled three-dimensional flow structures in a natural sharp-bend confluence with tributary inflow: The Baima reach of the Wujiang River

Natural sharp-bend confluences involve simultaneous tributary momentum injection and curvature-induced pressure adjustment, but their coupled control on three-dimensional flow organization remains poorly understood. Here, a validated three-dimensional Reynolds-averaged Navier–Stokes-based model was applied to the Baima Reach of the Wujiang River to examine flow responses to different tributary-to-mainstream discharge ratios (RQ) at a tributary confluence near a 90° bend. The results show that the free-surface response was primarily organized by bend curvature. The longitudinal rise–fall pattern in water level persisted even without tributary inflow, whereas increasing RQ mainly changed its amplitude. The transverse water-surface gradient strengthened near the upstream bend apex, weakened immediately downstream of the confluence, and recovered in the middle–lower bend reach; its dependence on RQ was spatially variable, decreasing with RQ immediately downstream of the confluence but increasing farther downstream. Velocity distributions showed a curvature-controlled migration of the high-velocity zone from the upstream convex-bank side to the downstream concave-bank side, with vertically asynchronous adjustment over depth. Tributary inflow modified local velocity magnitude, lateral contrast, and positions of the circulation cells, but did not replace the curvature-controlled migration pathway. Under high tributary discharge, turbulence intensity increased markedly in the inflow-side surface layer and was redistributed toward the downstream near-bed region. These findings demonstrate that tributary momentum acts mainly as a local modulation imposed on preexisting curvature-controlled pressure and secondary-flow fields, providing a physical basis for understanding three-dimensional flow structures in natural sharp-bend confluences.