Hydrodynamic characteristics and channel morphodynamics at a large asymmetrical confluence with a high sediment-load main channel

Abstract

Previous studies of confluences have concentrated mainly on confluences of small streams, confluences with simple geometric configurations, and confluences of curving rivers. However, the confluences of natural rivers, especially large confluences of high sediment-load rivers, generally have complex geometric confluent boundaries and various incoming flow and sedimentation conditions. In this study, several field measurements of the three-dimensional velocity components, water surface elevation, bed topography, and suspended sediment particle sizes reveal the complex hydrodynamic characteristics and bed morphology of the asymmetrical confluence of the high sediment-load Yellow River and its clear tributary, the Fen River. The results show that the water surface morphology of this confluence is elevated in the stagnation zone, mixing interface, and the opposite side of the tributary, whereas the converse is true at the downstream junction corner. The geomorphologic features of the bed show a small scour hole and a long narrow scour belt under conventional hydrological conditions, or a larger scour hole under hydraulic conditions when the value of the momentum flux ratio is much greater than one. In contrast to the confluences of small streams and curving rivers, this confluence does not exhibit continuous flow separation or helical motion, but has pronounced regions such as a stagnation zone, flow acceleration and deflection areas, a shear layer/mixing interface, and a flow recovery zone. The position and size of these regions vary with the momentum flux ratio. The particle size of suspended sediment at the confluence is normally distributed, and the changing sedimentation process corresponds to the scour and siltation state of the bed. In general, the particle size of the suspended sediment is small in the stagnation zone and the downstream channel near the tributary, but large in the acceleration zone and shear layer. These results are helpful to better understand the hydrodynamic and morphodynamics characteristics of confluences with complex geometric configurations and junction conditions.