A laboratory assessment of the influence of wind and sea currents on the dispersion of pellets from the Toconao incident

On December 8, 2023, the cargo ship Toconao was involved in a maritime incident off the coast of northern Portugal that resulted in the loss of 1,000 bags of buoyant plastic pellets, or nurdles. The pellets fell into the sea and reached the Bay of Biscay, posing a potential threat to the coasts of Spain and France. In the weeks following the incident, pellets were found washing up on beaches throughout northern Spain, particularly in Galicia, Asturias, Cantabria and the Basque Country. In response to this environmental emergency, several contingency plans were activated along the Spanish coast, requiring the use of scientific tools such as numerical modeling to develop effective responses. Nevertheless, a lack of detailed knowledge of the factors governing the transport and dispersion of these pellets made accurate predictions difficult, posing a significant challenge to effective management and mitigation of the spill.

For this reason, a set of laboratory experiments was conducted to study the dispersion of pellets under a range of hydrodynamic and wind conditions in two different physical settings. The behavior of the pellets collected from the Toconao spill, which showed a density of 900 kg/m³, was assessed in two different sections of a hydraulic flume (Z1 and Z2 zones) under different combinations of water level, current, and wind. Zone Z1, located within the flume itself (2 m x 0.35 m), exhibits a unidirectional flow pattern. In contrast, zone Z2, located in the expanded section of the flume (3 m x 1.5 m), shows three-dimensional asymmetric flow patterns. Hydrodynamic conditions were defined by combining one water level in each zone (30 and 40 cm in Z1 and Z2, respectively) and three flow rates (30, 40 and 50 l/s in both zones). In addition, the effect of four wind conditions was tested for the average flow rate of 40 l/s (in Z1: no wind, 0.5, 1.0, and 1.5 m/s; in Z2: no wind, 0.3, 0.7, and 1.1 m/s). The results provide valuable information on the effects of wind and ocean currents on the dispersion of a group of plastic pellets and demonstrate the importance of surface currents in this process. In addition, these findings provide a comprehensive database for validating numerical transport models, which will improve their predictive ability and usefulness in future emergencies.