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.

The United States Army Corps of Engineers (USACE) is scheduled to present their solution for a storm surge barrier on Galveston Island in 2021 to congress for approval. A solution for an engineered dune system on the Galveston Island West End has been proposed, but storm surge models have shown that protection from this engineered dune only goes so far, moreover the search for a proper alternative that fulfills technical requirements and social political influences have proven to be challenging. This study aimsto assess different dune alternatives, proposed in different reports, with a range of multi disciplinary criteria. The assessment of dune alternatives will also result in guidelines that should be considered for design, maintenance and governance aspects for an engineered dune barrier on Galveston Island, TX. Using a multi disciplinary approach for the evaluation of the different dune alternatives, the following research question was formulated: To what extent do the various dune alternatives fit the requirements for a land barrier at the West End of Galveston Island, looking at both technical and sociopolitical aspects? In this context, technical requirements are defined as the storm surge-and rainfall coping capacities of the dune, i.e. against what kind of storm is the dune resistant. Social political influences are a combination of the perception by local residents that are directly influenced by the construction of a dune system, governmental forms of collaboration, and in provide an analysis of the maintainability of the dune alternatives using the storm surge capacities. The different dune alternatives that have been assessed consist of the dune system proposed by the USACE and GLO (2018), the big dune system proposed by Galvez (2019) and the hybrid dune system as proposed by Muller (2017) and will hereafter be called alternative 1, 2 and 3 respectively. In this report a fourth alternative was introduced which is based on the hybrid dune system by Muller (2017) and consists of a clay core instead of a concrete core. Alternative 4 was chosen in order to simulate the difference between a concrete core and a clay core. Based on XBeach calculations, the storm surge coping capability of each dune was determined by projecting 10 year-, 50 year- and 100 year storms onto the dune alternatives. ArcGIS maps from the Galveston Island allowed for projection of flow patterns on the island in order to determine the rainfall coping capacity. An evaluation of sociopolitical aspects was based on a review of the literature on dune systems, forms of collaboration between governmental and private entities, and interviews with various respondents consisting of private individuals and companies, as well as governmental agencies involved in the process. Analysis of the various dune alternatives, based on multi disciplinary criteria, demonstrated that alternative 1 is completely flattened in 50 year storm events, whereas alternatives 2, 3 and 4 show a good storm surge capacity. All alternatives aggravate the current rainfall capacity at Galveston Island West End, because each dune system poses an obstruction that is not there currently. Alternatives 2 through 4 show a good enough storm resilience, requiring post-storm recovery maintenance while still providing a reduced but fair storm surge capacity. The sociopolitical results indicate that Galveston Island West End residents wishes are only safeguarded for alternative 1. On this basis, the main recommendations are to perform tests upon the dune system alternatives regarding storm events occurring in succession, which is not unusual in the Gulf of Mexico. A combination of alongshore erosion rates from the Galveston Island and the effect of dune vegetation should be determined for the dune alternatives, since these aspects were not considered in this research. Further research is needed to identify the combined effects of rainfall and storm surge in order to get insights into the performances of a certain alternatives. Furthermore, the exact role including the desired storm surge capability should be well defined in order to determine which stakeholder wishes and influences are to be fully considered for the dune system design.