The quantitative response of pollutant spatial distribution and mixing dynamics to confluent conditions in an asymmetric open-channel confluence

Abstract

River confluences are key nodes of pollutant transport in fluvial ecosystems and they are accompanied by complex hydrodynamic and mixing processes. The generalization and quantitative response of mixing to highly variable confluent conditions remain unclear. This studyinvestigates the spatial distribution and mixing process of pollutants through a series of meticulous experiments in an asymmetric open-channel confluence. Furthermore, we propose quantitative responses and characterization equations for the mixing rate and mixing metric for various confluent conditions, namely junction angle, momentum flux ratio, and width-depth ratio. The spatial distribution of pollutants have a strong three-dimensional structure, with a narrow-long pollution belt near the tributary and an almost vertical mixing interface adjacent to the downstream junction corner; this interface exhibits a logarithmic growth trend downstream and is affected by shear layer distortion. The standard deviation of the cross-sectional pollutant concentration at the confluence decreases gradually along with the downstream mixing of convergent flows, and the fitting curve slope increases as the junction angle, momentum ratio, and width-depth ratio increase. The mixing rate is promoted by the enhancements of helical motion, lateral advection, and turbulent diffusion caused by the strengthening of mutual deflection and lateral momentum flux under a large junction angle and momentum ratio, as well as the reductions of vertical velocity gradient and cross-sectional area under large width-depth ratio conditions. Sufficient and complete mixing occurs under a larger momentum ratio condition of >1 within a short distance of 4–5 widths of the main channel. In addition, the mixing metric shows a strong linear relationship with each of the three confluent conditions. In summary, the significant (p < 0.001) quantitative responses of the mixing rate and mixing metric can be characterized by a polynomial equation. These results have major implications for the systematic understanding of the three-dimensional distribution characteristics, mixing patterns, and mixing dynamics of pollutants at asymmetric confluences, as well as for the prediction or calculation of the downstream mixing metric.