Effects of stratification on hydrodynamics and mixing process at a large asymmetric confluence in northern China

The hydrodynamics in the confluence area of rivers are typically governed by the geometrical configuration of the surrounding landforms, the momentum and discharge ratios of the converging watercourses, and the morphological features of the riverbed. Recently, it has been discovered that stratification effects resulting from disparities in temperature, salinity, sediment concentration can also have significant impact on the downstream hydrodynamics of the confluence area. In this study, the impact of stratification induced by sediment concentration on fluid dynamics and pollutant mixing in the Yellow - Fen River confluence area is studied by using a 3D numerical model. The distribution of the depth-averaged field and secondary flow are analyzed. The findings revealed that the alterations in density caused by sediment concentration had effect on the flow dynamics across both horizontal and vertical dimensions of the confluence area. These alterations encompassed flow magnitude, shear layer, low velocity zone, cross-section vortex, and secondary flow. When the sediment concentration in the main stream was high, a noticeable stratification phenomenon emerged, whereby the tributary pollutants rode above the main stream during transport and mixing. Under these conditions, the pollutant mixing process accelerated, resulting in a shortened mixing distance. This acceleration can be attributed to the intensified secondary flow in the cross-section, brought about by the stratification effect. The findings from this study enhance our understanding of pollutant mixing patterns in river confluence areas.