Vulnerability assessment of two Adriatic mixed beaches for potential oil spill events

Oil still represents the most adopted source of energy for several human activities and goods production. Its extraction, transport and storage frequently occur across the sea and affect the largest ports all over the world. These actions have an environmental cost which can affect the marine environment at several levels among which the coastline is one of the most vulnerable. The NOAA (National Oceanic and Atmospheric Administration) in 2002 established the ESI guidelines (Environmental Sensitivity Index) in order to create vulnerability maps of the coastline for oil spill emergencies. Here it is presented an assessment of the vulnerability index for two Adriatic beaches (Portonovo and Sirolo) located in the Conero Headland which is one of the most congested areas for oil transportation due to the close port of Ancona. The aim of the work is to understand the specific role of coastal morphodynamics on the vulnerability assessment for oil spill events on mixed beaches. Portonovo and Sirolo are two mixed sand and gravel beaches (MSG) where sediment ranges from medium sand to cobble and boulders with a prevalent pebble fraction especially in Portonovo. Sediments are made of limestone and marl originated by cliff erosion which is the only sediment source of the beaches. The average tidal range at spring tide is 0.5 m. The dominant winds come from NE and SE, which correspond also to the main directions of storms. The wave heights are between 0.25 m and 2 m for most of the time. Topographic surveys of the beach surface and sediment sampling were undertaken in April 2015 in order to characterize the two beaches from a geomorphic and sedimentological point of view. Portonovo can be considered a borderline case between levels 5 and 6 of oil spill vulnerability given the larger quantity of gravel which increase the potential permeability of oil within the sediment wedge. Sirolo can be classified as lower vulnerability (5) because is a better defined mixed sand and gravel beach due to the larger sand fraction. The northern part of Sirolo, which is mainly comprised of cobbles and where the shoreline is almost in touch with the rocky cliff, is classifiable as the lowest vulnerability (1C). The burial entity of sediments can reach high depths (1 to 1.5 m), with several orders of storm berms on top. In normal conditions of wave motion the entity of burial is 25-30 cm in Portonovo (field tested) and up to 16 cm in Sirolo (formula derived). The temporal sequence of storms is also important: given the bimodal direction of storms in the Adriatic (NE and SE) consistent shoreline rotation of beaches has been already demonstrated [15; 17]. The high dynamism of beach topography and surface sediments is highlighted by obstructing structures that occupy and limit the beach in Portonovo where the entity of burial and sediment transport is higher. Despite the NOAA (2002) classification for oil spill vulnerability covers a wide range of environments and consider several aspects [23], an improvement is still needed focused on the geomorphic changes that mixed beach can experience in short periods of time.