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?”

Plastic pollution threatens ecosystems and human health, making it a critical global issue. Since rivers transport large amounts of plastic to the ocean, intercepting debris within river systems is a promising mitigation strategy. However, effective waste-collection design requires understanding how plastic interacts with flow conditions and hydraulic structures. Debris accumulation can reduce water conveyance, increase flood risk, and impose excessive structural loads.

This research investigates the stability of floating debris accumulations (“carpets”) using experiments and numerical simulations. Two instability mechanisms are identified: squeezing, driven by cumulative compressive forces within the debris, and erosion, caused by flow and pressure fluctuations at the leading edge. Results show that flow transitions—from open to closed channel conditions due to debris buildup—play a key role in these processes by altering boundary layers, velocity profiles, and shear stresses.

The findings reveal that rough debris layers intensify turbulence and increase instability risks, while particle position influences forces such as drag and lift. Overall, the study highlights how debris affects flow dynamics and structural safety, offering insights for improved hydraulic design, flood modelling, and debris interception strategies.

This research investigates the hydrodynamics of a physical boundary transition from free slip to no slip, which usually occurs in ice-jams, large wood and debris accumulation in free-surface flows. Using direct numerical simulation coupled with a volume penalisation method, a series of numerical simulations is performed for an open-channel flow covered with a layer of floating spherical particles, replicating the laboratory set-up of Yan Toe et al. (2025 J. Hydraul. Eng., vol. 151, 04025010). Flow transition from the open channel to the closed channel induces a new boundary-layer development at the top surface, accompanied by a flow separation and an increased bottom shear stress that enhances particle mobility at the bottom. Analysis of a fully developed flow in an asymmetric roughness channel (rough surface at the top boundary and smooth surface at the bottom boundary) also shows that the vertical position of maximum velocity is higher than the position of zero Reynolds shear stress, which supports the experimental observation of Hanjalić & Launder (J. Fluid Mech., vol. 51, 1972, pp. 301–335), demonstrating the shortcoming of traditional turbulence closure models such as the k−ε model. Finally, the stagnation force acting on a particle at the leading edge of the accumulation layer is compared with the analytical prediction of Yan Toe et al. Understanding the flow transition improves the prediction of the stability threshold of the accumulation layer and design criteria for debris-collection devices.

Plastic debris can accumulate at hydraulic structures and waste-collection devices, leading to a so-called floating carpet formation. Understanding the accumulation of plastic debris at structures is pivotal in the prediction of increased flood risk and design of waste-collection devices. In this research, we studied the stability of plastic carpets under different flow conditions using laboratory experiments, and we developed analytical models to predict critical velocities that led to two instabilities: (1) squeezing—particles inside the carpet are pushed downward due to cumulative compressive force, and (2) erosion—particles at the upstream edge of the carpet mobilize completely. Velocities of the fully developed flow were measured under a stable carpet to estimate boundary shear stress, which was applied to calculate the compressive force of the particles. Using measured flow velocity data and particle’s properties, the critical flow velocities that led to instabilities were calculated. Overall, this research supports a better understanding of physical processes associated with plastic accumulation, supporting the development of optimized plastic removal strategies.

Knowledge of plastic debris transport mechanism in open waters and its interaction with hydraulic structures (i.e. accumulation and clogging) is of paramount importance for effective waste-removal strategies and sustainable management of plastic debris. To the author’s best knowledge, current models for prediction of plastic debris transport assume a highly simplified geometry, while making use of parameterization of the physical processes, therefore pointing out the need for further research. Herein, the effect of shape and buoyancy on the motion of a single particle was studied employing point-particle approach while the background flow is solved using RANS approach. It is observed that the particles with the same amount of plastic mass but different shape and density showed substantially different behaviors, resulting in different trajectories. Since parametrization and point-particle approach were used, even if the particle size is larger than the mesh size, these preliminary results showed that further validation is required for prediction of accurate trajectory by means of resolved-particle approach.

"Plastic pollution is a threat for all ecosystems due to its effects on people, animals, and environment. Rivers are estimated to transport around 0.5 millions tons of plastic per year. When plastic enters a river system, it is transported downstream towards the sea but it is also likely to accumulate at specific cross sections and locations, including hydraulic structures, eventually increasing the risk of floods. Gates, locks, weirs, and bridges are commonly present in rivers and canals and have several functions, including water level regulation, flood safety, and inland water shipping. These can also be found in water treatment plants, hydropower stations as well as debris/plastic collection systems. Riverine plastic accumulation is also known to cause geomorphic changes. In-depth knowledge on how plastic particles accumulate upstream of hydraulic structures is therefore crucial to understand the processes that affect plastic transport, its influence on the safety and functionality of hydraulic structures and their effects on the hydro- and morphodynamic conditions of the flow. In this research experiments were performed using simplified plastic particles to analyse the processes that lead to the instability of accumulated particles upstream of a simple gate."