Oil spill hazard and risk assessment for the shorelines of a Mediterranean coastal archipelago

Abstract

Oil spill is a serious threat for all marine and coastal environments. This is even more true for areas having high environmental, social and/or touristic value. A serious and scientifically rigorous identification of the hazard and risk related to oil slicks is becoming mandatory, in order to reduce or mitigate the impact of oil dispersal at sea and its stranding. A new model based method for evaluating hazard of oil slicks contact with shorelines of the Archipelago of La Maddalena (Strait of Bonifacio, Sardinia, Italy) has been developed and applied. The core of the methodology is a coastal 3D finite elements model, able to simulate hydrodynamics and waves of the strait of Bonifacio and, through a Lagrangian module, the physical/chemical fate of the oil at sea. In order to estimate the hazard due to oil slicks for the Archipelago and Northern Sardinia shorelines, a two-years interannual experiment has been conducted. An hazard index, given by the ratio between the oil concentration reaching each predefined coastal cell and the maximum stranded concentration, has been computed and mapped by using geostatistic tools in GIS environment. Temporal and spatial variability as well as the climatological distribution of the hazard index were therefore described: this can be an useful information for local authorities in order to efficiently manage oil slick emergencies. A significant temporal and spatial variability has been observed in the distribution of the hazard index, showing highest values for winter months, in agreement with stronger wind-induced currents. Large hazard values were found mainly along westerly exposed shorelines, as expected considering the prevalence of westerly winds blowing through the Strait. In order to assess the risk, such a hazard index can be easily combined with quali-quantitative factors of vulnerability of the coastal environment, assuming the risk is the product of hazard and vulnerability. Two of the most important factors of vulnerability have been combined with the hazard index: the shores geomorphology and the level of environmental protection (proxy for the environmental value). The southern side of Spargi Island shows the highest risk values, because of coincident presence of large hazard index values, beaches presenting last classes of geomorphological vulnerability and a moderate/high level of protection.