Modelling the transport and fate of buoyant macroplastics in coastal waters

Abstract

As a result of the unabated growth in plastic usage worldwide, their abundance in the marine environment has steadily increased over the last few decades. Nowadays, plastic litter is observed across all oceans and shores. Due to their wide spread and adverse effects on ecology, economy and potentially human health, plastic pollution has been recognized as a worldwide environmental and ecological threat. For these reasons, it is important to reduce and mitigate the abundance of plastic litter in the marine environment. As marine plastic litter predominately originates from near the coast, it is critical to study the plastic behaviour in coastal waters. However, there is considerable uncertainty regarding what factors are influencing the trajectories, distribution and deposition sites of plastic litter. Along with complex physical processes and scarcity of empirical data, there is currently little knowledge and understanding on the transport and fate of plastics in coastal waters.

With the aim of obtaining more insight into plastic behaviour, the present study intends to examine the most important processes and quantify the effects of parameter uncertainty on modelling the transport and fate of buoyant macroplastic in coastal waters. The transport and fate of plastics is modelled by combining hydrodynamics with particle tracking concepts. For the simulations the Delft3D software Suite was used, where a Delft3D-FLOW model of the southern North Sea (ZUNO-DD) was coupled to Delft3d-PART. The model calculates how the position of plastic particles evolves in time from their release until the end of the simulation. In this study, model simulations are used as a numerical tool for exploring the relative influence of current uncertainties inherent in process parameters and data inputs on model results. A set of scenarios were defined by changing parameter values on at a time. By studying the changes in particle trajectories and shoreline deposition areas, a better understanding of their relative importance was obtained.

The modelling results imply that the effect of windage and release location are the most important parameters. Further, it is observed that dominating driving mechanisms may change with varying forcing conditions and object characteristics. Other factors such as small-scale processes and moment of release may impact particle trajectories and fate. However, the relative influence of these processes is less significant and therefore considered less critical. Adopting the findings of this thesis into decision making policy can support emergency response operations and monitoring strategies.

The research on plastic behaviour in the coastal environment is still in its early stage, and much has yet to be revealed. Therefore, further improve understanding of buoyant macroplastic behaviour is required. Validation of the results presented in this study is limited due to the scarcity of empirical data. Thus, further research should be directed towards collecting more field data. Further, it is recommended that effort is put into parametrizing accurately the effect of windage. Furthermore, expanding numerical simulations should be
expanded to a range of conditions and coastal environments so that trends can be compared and highlighted, but also allows for exploring new hypotheses.