Channel confluence hydrodynamics over a gravel bed

Bed roughness plays a critical role in shaping flow structures, especially in large width-to-depth ratio channels. River confluences, characterized by complex flow dynamics, are further modulated by variations in bed roughness. However, the effect of roughness on confluence hydrodynamics remains unclear. Through flume experiments with three width-to-depth ratios, this study compared hydrodynamic responses between smooth and gravel beds, analyzing velocity fields, secondary current intensity, water depth, streamline curvature, and Reynolds stress. Unexpectedly, the introduction of bed roughness was found to enhance, rather than weaken, the secondary current. This is primarily attributed to the decreased separation zone (especially near the bed), which led to an increase in the curvature of the secondary current. The rise in local water levels further limited the effect of roughness in reducing the secondary current. The reduction of the separation zone caused a decrease in the primary velocity of the nearby tributary flow and also resulted in a more immediate recovery of downstream flow velocity.