The mechanism of dissolved oxygen mixing and atmospheric reoxygenation at the confluence with different flow ratios and junction angles

Abstract

As the control cell of river networks, the complex hydrodynamic conditions at river junctions pose significant challenges to pollution control and water ecology protection. The purpose of this study is to construct a 3D hydrodynamic model for dissolved oxygen (DO) transport at the confluence, studying its unsteady mechanism under different flow ratios and junction angles. The results of the study show that: i) The atmospheric reoxygenation distribution at the confluence exhibits a typical layered 3D feature, and the strong flow turbulence therein exerts a significant promoting effect on such reoxygenation. ii) The DO transport pattern at the confluence is characterized by the formation of an arc-shaped concentration distribution zone, followed by a contraction towards the opposite bank of the tributary and downstream transportation. A delay in DO concentration increase occurs within the separation area, while lateral oscillations are observed during downstream transport. iii) The flow ratio primarily governs the lateral migration extent of DO, while the junction angle serves as the main driver for downstream transport rate of DO. Increasing both flow ratio and junction angle can expedite mixing efficiency of DO at the confluence. The findings of this study can further enhance the hydrodynamic mechanism for water quality control at the confluent flume, and serve as a valuable reference for improving water environments and protecting water ecology in natural river confluence.