Density Differences Affect the Mixing Process of Pollutants in The River Confluence Area

The influence of density differences on the mixing process of pollutants in the confluence area of the Yellow River (YR) and Fen River (FR) is unclear. This study conducts several field experiments on the hydrodynamic and pollution concentrations in a large asymmetric confluence. The mixing rate and deviation from complete mixing (δ) for the pollutant concentrations are used to describe the mixing process and intensity of contaminants. The results show that the density differences caused by the sediment concentration are an essential factor affecting the pollutant mixing process in the confluence area. After ≈14.7 dimensionless river lengths, the δ of the pollutants in the confluence area, under the strong density difference, drops below 10%. In comparison, the maximum δ under the other density differences still exceeds 10% after 27.5 dimensionless river lengths. The strong density difference in the confluence area enhances the pollutant mixing process and shortens the mixing distance. The flow velocity distribution and shear layer changes under different density differences are important driving forces that affect the mixing process of pollutants. This study is of great scientific significance, as it helps gain insight into the mixing pattern of pollutants in river confluence areas. A preliminary investigation is performed concerning hydrodynamic and water quality of the Yellow and Fen River confluence area. The results show that the density differences caused by the sediment concentration is an important factor affecting the mixing of pollutants in this area. The flow velocity distribution and shear layer changes under varying density differences are dynamic mechanisms that affect pollutant mixing.