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.@en

"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."@en

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.

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