Dispersion of floating marine litter: Lagrangian numerical simulations at coastal scales

Coastal zones are recognised sinks for plastic debris, although studying the transport and dispersion of plastic debris in nearshore waters remains challenging. The understanding of coastal processes and how these can be applied to numerical modelling studies is not yet fully mature. Most Lagrangian numerical modelling studies of plastic debris transport to date have been conducted at the basin or sub-basin level, typically using low-resolution hydrodynamic data (>2.5 km). These resolutions are generally sufficient at larger scales, but at smaller scales they create uncertainties regarding mainly the beaching of particles on the coastline and poor resolution of coastal structures or complex geometries.

The LOCATE numerical model addresses these constraints by incorporating high-resolution hydrodynamic data in nested grid configurations, focusing on areas of high interest. Specifically, this has been applied to the Barcelona coastline, where the LOCATE model used a coupled current-wave dispersion module with a three-way nested grid for current data, of 2.5 km resolution from CMEMS within the study domain, a 350 m high-resolution grid from the Spanish Port Authority (Puertos del Estado) covering the Barcelona metropolitan area, and a 70 m resolution grid covering the Port and the urban coastline. The use of high-resolution hydrodynamic data with the LOCATE model was validated using drifter data. Complex geometric structures were resolved, demonstrating that particles had residence times over 18 times longer within the Port when using high-resolution data compared to only using low-resolution data. Furthermore, a beaching module that calculated the real-time distance of a particle to the shoreline was developed using high-resolution shoreline data. This provided much more realistic beaching patterns, compared to determining particle beaching using its advected velocity, as well as shoreline continuity between the different resolutions in the hydrodynamic data.

These adaptations alone, however, do not address coastline or beach processes not included even in the highest-resolution hydrodynamic data. For this, a probabilistic framework was assumed, where a beaching timescale of 26.35 h was determined using a sensitivity analysis for backtracking simulations initiated in nearshore environments where particles could only cross the land-water boundary at known discharge sources. A further condition based on a minimum particle trajectory distance was also introduced to avoid artefacts. This final probabilistic configuration allowed for particle advection near the shoreline with more realistic trajectories within complex shorelines, considering stochastic elements of particle transport at localised scales. These parameterisations highlight the adaptations required to coastal and nearshore Lagrangian numerical modelling and serve as an area of future research which may be transposed to other locations.

Acknowledgements

This study was carried out within the TRAP project (Participatory Strategies for the Management of Plastic Pollution on the Transboundary Coast), which was 65% co-financed by the European Union through the Interreg VI-A Spain-France-Andorra Programme (POCTEFA 2021-2027). The objective of POCTEFA is to strengthen the economic and social integration of the Spain-France-Andorra border region