Numerical Investigation of Dissolved Oxygen Transportation through a Coupled SWE and Streeter–Phelps Model

Dissolved oxygen (DO) reflects the self-purification ability of a water body and is also an important indicator for quantifying the water quality. The morphological changes in the cross sections of river channels will affect the hydraulic conditions, and the distribution of pollutants and DO may also be affected, possibly resulting in local oxygen deficits and pollution. To effectively predict the water quality, a coupled model is introduced in this study. The shallow water equation (SWE) is adopted to calculate the hydrodynamic processes, and the modified Streeter–Phelps model is further coupled with the SWE model to evaluate the reaeration. By applying this model, mass transportation and reaeration in rivers are analyzed. The influences of the sudden cross-sectional changes in the river channel on the DO distribution and the reaeration ability are identified. The results reveal that a certain degree of expansion in the river is conducive to reaeration and can also accelerate the consumption of pollutants through the water body’s self-purification. DO transport in two real terrains, including a mountain basin and plain river, is extensively investigated, and the results indicate that the morphological characteristics in the mountain basin will cause the concentration distribution to form inside dead zones, while in the plain, the distribution will form a fan-shaped downstream zone.