Turbulent flow at confluences under different tributary floodplain topographies

River confluences are key nodes in river network systems and exhibit complex three-dimensional hydrodynamics strongly influenced by tributary topography. In this study, the effects of tributary floodplain side slope angles [(90° (C1), 45° (C2), 30° (C3), and 0° (C4, bed discordance)] on confluence flow structures and turbulence characteristics were investigated using large-eddy simulation. The numerical model was validated against laboratory measurements, showing good agreement in three-dimensional mean velocity and turbulent kinetic energy, as well as in the position and size of the separation zone and secondary circulation. As the tributary side slope angle decreases, the secondary circulation shifts toward the inner bank, and the near-bed separation zone is progressively disrupted by the secondary flow and eventually disappears in C4. The secondary circulation intensity near the tributary floodplain step remains similar among cases due to comparable tributary inflow momentum; however, in C4 it becomes significantly stronger downstream and then rapidly weakens after impinging on the inner bank wall. Elevated turbulent kinetic energy and coherent vortical structures are mainly concentrated in the shear layer, tributary floodplain step, and boundary of the separation zone. Q-criterion analysis shows that these structures remain stable in C1 and C2, are partially disrupted by secondary circulation in C3, and are substantially reduced in C4 due to the disappearance of the separation zone. These findings advance the understanding of confluence hydrodynamics under different tributary topographies and provide potential support for river network systems management.