Numerical simulation and analysis of floating marine litter dispersion and accumulation in coastal regions around Barcelona city

Transport mechanisms of floating marine litter (FML) in coastal zones are poorly understood. Tracking FML dispersion with numerical models is complex due to the geometry, hydrodynamic processes and the influence of coastal processes, the latter being especially challenging to incorporate. Within the TRACE (Tools for a better management in coastal environments to accelerate tRAansition to Circular plastic Economy) project, however, the LOCATE tool was developed to simulate the motion and accumulation of plastic particles in coastal areas, using nested grids of varying spatial scales and resolutions (2.5 km, 350 m and 70 m) to account for coastal processes. LOCATE couples Eulerian hydrodynamic data with a Lagrangian particle solver, thus requiring configuration and optimization. Regional Eulerian hydrodynamic data are obtained from Copernicus Marine Environment Monitoring Service (CMEMS) products whereas coastal hydrodynamic simulations use the open-source Coupled Regional Modeling System (COAWST) system. The Lagrangian solver uses the open-source OceanParcels (Probably a Really Efficient Lagrangian Simulator) model (van Sebille, et al., 2020). As proof of concept, the model was applied to the Barcelona coastline where breakwaters can behave as marine litter traps and concentrations are comparable to some other heavily polluted areas such as the Atlantic and Pacific gyres (Sánchez-Vidal, et al, 2021).

Observational data from 2017 from the Llobregat and Besòs rivers, two known sources of FML around Barcelona, were used to run simulations to determine how LOCATE can predict litter accumulation zones when compared to beach cleanup data. Both rivers are the major rivers around the Barcelona coastline and have been hypothesised to be an important source of plastic in the region. A beaching module that detects, quantifies, stores location and time, and removes particles that have crossed the land-water boundary was developed that uses a vector-resolution coastline, and as such is independent from the limiting spatial resolution of the nested grids at coastal scales. The coastline was divided into 16 zones, as well as five water boundary zones on the perimeter of the study area (40.88°N to 41.81°N, 1.38°E to 3°E). 

Simulations of particles released at hourly intervals were carried out between February and October 2017, with 552,400 released in total. Results show overall beaching rates of 91.5%, higher than other larger scale studies. The most impacted areas were around the release sites and adjacent beaches to the south, showing a NE to SW transport; the Llobregat river mouth being the most affected with over 200 particles km^-1 day^-1. A non-dimensional beaching bias index was used to identify areas that have a greater partiality to receive particles from a release point, such as areas south of the Llobregat river. Residence time at sea was highly variable with very short times recorded for the Besòs river mouth and adjacent areas (< 1 hour). Preliminary results will be presented to determine possible factors contributing towards high beaching rates and litter accumulation in zones. The ultimate aim of the present system and model is to demonstrate its exportability and adaptability to other coastal regions.