High-resolution numerical simulation of urban water environment under the influence of river sediment.

To investigate the potential impacts of adsorption-desorption processes on the pollutant transport and transformation in urban water channels by river sediments, this study employed the hydrodynamic and water quality dynamic bidirectional coupling model (GAST-SWMM), validated under static idealised conditions. The research simulated urban river sediments functioning as both a 'sink' and a 'source' of pollutants, clarifying their role in influencing river water quality mechanisms during combined sewer overflow events. Results showed that sediments effectively mitigated water quality deterioration when acting as a 'sink' for pollutants. After four and a half hours, the pollutant concentration at the outlet was reduced to 0.56 mg/L due to sediment adsorption, compared to 23.573 mg/L without sediment. When acting as a 'source,' sediments released pollutants into the water, resulting in elevated concentrations in shallower river sections that subsequently migrate downstream, leading to pollution levels exceeding Class IV water standards. During combined sewer overflow incidents, sediments shortened retention times of elevated pollutant concentrations and reduced overall levels. At selected cross-sections, the durations during which water quality exceeded standards were reduced by 5.0%, 23.4%, and 27.5%, respectively. The highest pollutant reduction rate at the river outlet reached 55.95%. Although pollutants were released gradually from sediments after rainfall, this process was slow and prolonged. This study elucidates and quantitatively assesses the specific impacts of river sediments on combined sewer overflow pollution processes, offering a theoretical basis for the management of urban black-odor water bodies and control of combined sewer overflow pollution.