Impact of bed morphology on microplastic transport at a confluence between a channel and a pipeline

Urban drainage systems are crucial pathways for the transport of terrestrial microplastics (MPs) to urban rivers. This study investigates the impact of the bed morphology on the transport of MPs at a laboratory-scale 90° confluence between an open channel and a pipeline. A series of flume experiments were conducted to identify the bed morphology and MPs distribution downstream the confluence junction for different confluence discharge ratios (Q_p/Q_m) and pipe heights (h_p). This study considered three distinct categories of MPs: polyethylene terephthalate (PET), polyamide (PA), and polyvinyl chloride (PVC), which were subjected to accelerated aging via the Fenton treatment. The results highlighted the role of confluence discharge ratios and pipe heights in shaping the bed morphology. This study revealed that the undulating bed increased the retention rate of MPs by 30 %-40 %, and even the flat bed exhibited a retention rate of nearly 50 %. Compared to the inherent properties of MPs, bed morphology plays a decisive role in the transport of MPs. The retarding effect of the riverbed on MP transport may have been underestimated. MPs migrate from the stoss side to the lee side of sandbars, where they are progressively buried by accumulated sediments. It is necessary to conduct key monitoring of these MP concentrated areas, promptly remove MPs, and prevent further pollution of downstream water bodies.