One of the preferred features of plastic is its high durability. This merit is its main disadvantage in the natural environment. When poorly disposed of, plastic waste can enter rivers from surface waters and subsequently end up in the ocean. Here, plastic spreads out over the entire ocean and ultimately ends in ocean gyres. Plastic material in the ocean causes environmental damage. Marine animals can get entangled in plastic debris, plastic accumulation occurs when consuming seafood and the material spreads toxins in the seawater. It has also caused economic damage: marine plastic accumulation led to €5 to 18 billion to key economic sectors for 87 coastal countries in 2020 (Deloitte, 2021). Attempts have been made to capture buoyant plastics from the ocean with interception devices, with mixed success.

The placement of an interception device in a river is thought to be a better approach to the problem. A standard interception design is that of a floating boom. Plastic debris is transported by the river flow and these objects guide buoyant plastics into a collection tray. Currently, these interception designs are still in their infancy. A difficulty in the optimisation of these interception devices lies in the limited knowledge of the trajectories of plastic waste. Computational fluid dynamics (CFD) modelling can help to create a better understanding of these aspects. This tool can provide a rapid assessment of different flow conditions with different types of plastic debris modelled. Further knowledge into this field can ultimately help into better retention of plastics in rivers.

Plastic modelling for buoyant particles has been performed in the marine environment (Van Utenhove, 2019) and in the river (Van Welsenes, 2019) with a Lagrangian study. However, both works did not perform a validation study. The combination of a CFD model and the results of physical model tests can be valuable in removing uncertainties related to plastic trajectories and the modelling of these.  One physical model test performed on plastics is the release of plastic particles in a flume (Zaat, 2020) Small films were released in a 2DV physical model, representing plastic bags. It was found that these particles followed an inverted Rouse profile for spherical particles times a shape factor.

The study by Zaat (2020) was replicated in the CFD modelling environment of OpenFOAM in this thesis. Subsequently, the retention of plastic particles was investigated. Three hydrodynamic conditions were investigated, with uniform inlet flow velocities of 0.10, 0.55 and 0.90 m/s. The $k-\epsilon$ turbulence model was used. An Euler-Euler approach was applied with a discrete and continuous phase. This modelling approach is computationally less expensive compared to a Euler-Lagrangian framework and can be easily established for different cases. The drag models Gibilaro, Wen-Yu and Syamlal-O'Brien were tested to investigate which concentration profile best fits an inverted Rouse profile for rising particles. The interception device was modelled as a square obstruction of 10\% of the water depth and the model was assumed as a rigid-lid. The retention of particles by the system was explored for neutrally buoyant particles, light material and high-density polyethylene (HDPE) and polypropylene (PP) films.

This study found relative differences in near-surface flow and the analytical approach of 3 to 4%. The Gibilaro model was best applicable and model coefficients for the virtual mass, lift and turbulent dispersion of 0.5, 1.6 and 1.0 were found. The concentration profiles followed the inverted Rouse profile and that for films closely for the medium and high flow cases, but not for the low flow case. A lower retention of plastics was observed for increasing flow velocities. In the extreme case of low flow and light particles a build-up of particles near-surface can be observed and runs against the streamwise direction.

Several limitations were present in this study. Only two parameters of the plastic particles were adjustable, which were the particle diameter and density. In reality, plastic debris has different shapes will experience drag differently, especially the friction drag. The rigid-lid model is valid only if the velocity head is smaller than that of 10% of the water depth, which is not the case for the high flow velocity case.

The results show that on the basis of a rise velocity and a density a representative diameter can be determined from Stokes’ law. The Euler-Euler modelling approach provides an accurate assessment and gives a proper representation of the particles for the Rouse distributions. This is done without handling the particles individually and thus results can be found more quickly. Both for HDPE and PP inverted Rouse profiles were found that followed the theoretical profile with an underestimation of 10\%-20\% near-surface. The retention was influenced mainly by the size of the wake. It is evident that the interception system can work for buoyant particles. The object can be placed during low flow conditions for the highest efficiency. For high discharge in rivers, more particles will be in suspension and thus further away from the water surface, which makes this design less preferred during high flow conditions.

Recommendations for further research are to perform more physical model tests, implement a non-spherical drag model applicable for plastic material and to apply a three-phase model. This removes uncertainties related to the model and to the current knowledge in particle dynamics. The addition of physical model tests would benefit the current knowledge the greatest. It is only possible to expand the complexity of the CFD model with a larger availability of these physical model tests.