Mechanism of pollutant diffusion response to coupled hydrodynamics and sediment layer initiation in combined sewer...

Sediment re-suspension is a major source of overflow pollution in combined sewer systems. Based on the sediment stratification theory, this study constructed a pilot-scale sewage pipeline system and proposed a precise prediction method for sediment scour initiation and overflow pollution. Sediment stratification governs pollutant behavior throughout the "scouring-release-transport" cycle: loose surface layers are easily scoured, with fine particles and high-concentration pollutants rapidly released and widely dispersed by flow, whereas dense middle-to-bottom layers exhibit strong scour resistance and low pollutant background levels, causing released particles to readily settle with limited dispersal capacity. Under simulated heavy rainfall conditions (shear stress of 5.42 N/m²), the average erosion rate of surface sediments reached 3000 mg/s, accompanied by high chemical oxygen demand, total nitrogen, and total phosphorus release fluxes, constituting the primary risk source for overflow pollution. By contrast, the erosion rates in the middle and bottom layers were only 450 mg/s and 80 mg/s, respectively, indicating significantly reduced pollution contributions. Based on these mechanisms, a coupled kinetic model was developed for the stratified scouring and transport of sewer sediments. By integrating hydrodynamics, scouring, and transport modules, the model calculates the hydraulic conditions, stratified erosion, pollutant release, and pollutant migration processes. This study systematically reveals the mechanisms by which sediment transport affects water quality under different rainfall conditions, providing a reliable model and quantitative method for the accurate prediction of pollution from drainage pipeline overflows.