Flow transition from open-to-closed channels in rivers

Abstract

Plastic waste results from human innovation, emerging from our development and use of synthetic materials called plastics. However, because of their long-lasting durability, plastics do not break down quickly and safely in the environment after the duration of intended use. This causes plastic waste to accumulate in the environment, threatening valuable ecosystems and human health (Thompson et al., 2024). Therefore, reducing and mitigating plastic pollution is an urgent priority. Rivers are considered the main conveyor of plastic debris toward the ocean (Thompson et al., 2024). Once the plastic debris enters the ocean, it becomes more challenging to track and collect them due to a highly dynamic nature of the oceanic processes and wind transport (DiBenedetto, 2025). Hence, collecting macroplastic debris from the river will facilitate clean-up procedure before they reach the ocean. On the other hand, mismanaged macroplastic debris also harms urban drainage system due to the accumulation at hydraulic structures. These debris accumulations affect the performance of hydraulic structures and result in additional flooding. Hence, understanding the accumulation process of plastic debris is crucial to better design hydraulic structures. It should be noted that ice-jams in rivers can cause similar problems to hydraulic structures. To analyse the research problem, we subdivided the accumulation process into three subprocesses as shown in Fig. 1: (1) the overall stability of the accumulation layer, also called the carpet, (2) the flow response (e.g. mean flow velocity profile, boundary shear stresses) to the presence of the carpet, and (3) detailed analysis of hydrodynamic forces (lift and drag) acting on the individual particles located in different horizontal and vertical positions with respect to the carpet. In this study, we are interested in how the particles are influenced by the flow transition induced by the debris carpet or ice jams within a turbulent flow, leading to the following research question: “How does the accumulation affect the hydrodynamic forces, drag and lift forces, acting on the individual particles in the vicinity of the transition point and underneath the layer?”