Comprehensive river cleaning: PTV analysis of a novel plastic debris removal concept in a large-scale physical model.

Plastic pollution in rivers represents a significant environmental threat, particularly in small- and medium-sized urban rivers, which account for major global riverine plastic emissions. This necessitates developing and optimizing effective countermeasures. However, limited research on existing clean-up technologies, precisely their plastic removal capabilities and hydraulic impacts on river systems, makes selecting and properly designing effective plastic removal measures challenging. This study evaluates a patented technology for river-wide plastic debris removal, which uses rotating screen drums mounted on groyne-like structures along the riverbanks. We tested the technology in a hydraulic model under 54 different flow and operational conditions representing tidal-influenced, low-gradient rivers with discharges of 17 m³/s - 115 m³/s. Particle Tracking Velocimetry (PTV) was used to analyze the 2D-horizontal flow field around the cleaning modules and to identify distinct flow regions and shear layers around the system. The resulting flow fields and vectors provide insights into the system's hydraulic impacts on the flow dynamics within the river stretch. Streamlines were calculated to assess the system's cleaning performance and macroplastic removal ability across the entire river width for different tested configurations. Hydraulic model tests demonstrated that the system's width-related barrier efficiency varies from 85 % to 100 %, depending on the chosen design parameters. The tested modular system achieves full efficiency while maintaining ecological corridors (fish/sediment passage) and ship passage in the reproduced medium-sized prototype river of 50 m width. Our study developed a robust methodology based on PTV measurements for evaluating both cleaning performance and hydraulic impact, which can be applied to a wide range of river clean-up technologies and applications. This study enhances understanding of flow-interaction in macroplastic-polluted rivers and may support effective river management and restoration by optimizing clean-up measures.