Modeling dispersal and beaching of urban plastic waste along the coastline of Côte d'Ivoire

Plastic pollution is a growing concern in West Africa's marine environment, yet the processes governing its fate remain poorly understood. The coastline of Côte d'Ivoire is particularly vulnerable due to dense coastal populations and significant plastic inputs from rivers and lagoons. This study investigates the dispersal and coastal retention of urban plastic debris using a Lagrangian particle-tracking model (Ichthyop) forced by surface currents from a high-resolution ocean circulation model (CROCO). We provide the first regional-scale assessment of seasonal beaching dynamics along the Ivorian coast using a high-resolution coupled hydrodynamic-Lagrangian framework. Simulations show that while a large fraction of particles is transported offshore (~52%), a substantial proportion (~48%) remains nearshore and beaches at least once. Persistent accumulation hotspots are identified around Abidjan, regardless of the release location. Seasonal variability strongly controls debris pathways. During the minor upwelling season (January-April), beaching occurs predominantly west of Abidjan, associated with weaker currents and enhanced coastal retention. In contrast, during the major upwelling season (June-September), intensified currents and mesoscale activity promote wider dispersal and shift beaching eastward. From October to December, beaching intensifies along the eastern coastline. These results highlight the dominant role of regional hydrodynamics in shaping marine debris distribution and demonstrate the value of high-resolution Lagrangian modeling for identifying pollution hotspots and supporting mitigation strategies in data-scarce coastal regions.