Mapping oil spill hazards in the coastal Mediterranean

This study presents a general framework for quantifying coastal hazard along Mediterranean national coastlines resulting from recurrent and small operational oil spills, which have been shown in recent years to have a greater cumulative impact than large accidental spills in the Mediterranean Sea. It addresses uncertainty in coastal hazard due to the variability in met-ocean conditions by simulating spills every 5 days during a five-year ensemble simulation, and uncertainty in spill locations by placing different releases on a uniform grid located between 10 and 50 km from the coast. The spatial heterogeneity in spill probability, usually estimated based on proximity to ship traffic routes, was not considered due to the high uncertainty associated with its estimation. This setup thereby allows for the isolation of the effect of ocean currents and the quantification of the susceptibility of Mediterranean coastal states to oil pollution driven by shelf and slope transport. The oil concentration distributions arriving at each national coastline were found to be heavy-tailed, meaning that extreme pollution events were more likely to occur and at considerably higher concentrations than a Gaussian distribution would predict. These distributions were therefore fitted to two-parameter Weibull probability density functions. Results reveal six distinct hazard clusters across countries, characterized by distributions with different scales and kurtosis, the latter being a proxy for the frequency and magnitude of extreme coastal pollution events. Coastal hazard was defined by the Coastal Hazard Index (CHI), which computes the percentage of events in the theoretical Weibull distribution exceeding a user-defined threshold. To define the probability of extreme events, the threshold considered was 0.1 tons/km, as oil concentrations exceeding this threshold have been reported to cause significant wildlife mortality. This threshold was exceeded in up to one quarter of the events arriving at the coasts of Libya and Egypt (CHI = 0.25), while the Southern Adriatic (Albania and Montenegro) presented the lowest hazard (CHI = 0.04). Seasonal variability was significant, with hazards consistently peaking in spring and reaching their minimum in autumn, particularly along the southern coasts of the eastern and central Mediterranean. Furthermore, the established methodology identifies a positive association (r = 0.720) between coastal complexity, measured by the fractal dimension of the coast, and the kurtosis of the oil distributions, suggesting that highly indented shorelines may reflect greater oil entrapment and convergence and create extreme pollution events.