Processes controlling the dispersion and beaching of floating marine debris in the Barcelona coastal region

Coastal zones are considered one of the main sinks for floating marine litter (FML) in the marine environment although transport mechanisms in coastal zones are poorly understood. The Barcelona coastline is considered one of the main hot-spots in the Mediterranean Sea, with high flux rates reported between land and coastal waters and FML concentrations of the order of magnitude of subtropical oceanic garbage patches. A case study was conducted on the Barcelona coastline using observational data of debris output from two rivers within the metropolitan area (Llobregat River to the south and Besòs River to the north) and a Lagrangian numerical model specifically adapted for use in coastal areas (LOCATE) using high-resolution hydrodynamic data and a beaching module based on high-resolution shoreline data that calculated real-time particle distance to the shore to detect the land-water boundary. The coastline was divided into 16 zones based on geographical structures and high-interest areas allowing for the differentiation of the level of impact of different zones.  High levels of beaching in the area (>91.5%) with relatively short residence times (<6.4 days) were observed.

Coastline areas adjacent to the debris release points were observed to be more prone to receive particles. To separate the likelihood of a particle being deposited close to the point of origin from the effects of the transport mechanisms, a measurement of potential beaching likelihood was introduced based on the comparison of modelled sustained displacement of particles to an area with the minimum time and distance required for particles to be deposited in an area with the hydrodynamic conditions during the simulation. The Llobregat River mouth was overwhelmingly affected by debris flowing out of the river, confirming that proximity to the source is the main driver for particle beaching at smaller scales, followed by the influence of geometric structures on hydrodynamic conditions. Statistical analyses revealed that significant wave height and wave energy flux correlated positively with the amount of particle beaching with a strong association between the variables. Current velocity, however, had a weak negative correlation indicating that a strong prevailing Northern Current has an inverse effect on particle deposition. The relationship between wave direction and coastline orientation was also seen to have a strong influence on the levels of beaching, with vertically oriented sections of the coastline having a greater predisposition to receive particles.

Mass was calculated for the modelled particles based on their composition and compared to the amount of debris recorded in beach cleaning data, revealing that only 4.9% of debris reaching beaches in the Barcelona metropolitan area can be attributed to river outflow. Other sources of debris such as discharges from sewage and wastewater overflows, or land-based waste generated by beach use must be considered for more accurate representations as well as more precise beaching parameterisations to account for land-water fluxes.