Assessing Coastal Flood Hazard under Climate Change in the northern Adriatic Sea: Extreme Sea Level Analysis and Implications for Urban Adaptation

Coastal flooding represents one of the most critical and rapidly evolving hazards affecting low-lying and urbanized coastal areas under climate change, particularly in semi-enclosed basins such as the Mediterranean Sea. The ongoing rise in mean sea level, combined with the intensification of meteomarine extreme events, is increasing both the frequency and severity of coastal inundation events, with significant socio-economic and infrastructural impacts. Within this context, the northern Adriatic Sea represent a hotspot of vulnerability, owing to its shallow bathymetry, elongated basin geometry, and strong sensitivity to wind-driven storm surges and basin-scale oscillations. This doctoral research investigates extreme sea level dynamics and coastal flood hazard along the Friuli Venezia Giulia coastline (NE Italy), focusing on the urban areas of Grado Lignano Sabbiadoro, Muggia and Trieste. The study integrates long-term tide-gauge observations, advanced extreme value statistics, physical process decomposition, and hydrodynamic modelling to characterize present-day and future coastal flood hazard at the urban scale. For Trieste, one of the longest and most reliable tide-gauge records in the Mediterranean (1939-2024) is analysed. Extreme sea levels are estimated using both the block maxima approach (GEV distribution) and the Peaks Over Threshold method (GPD), following detrending procedures based on annual mean sea level and a 19-year running mean to isolate meteomarine extremes from long-term sea-level rise oscillations. Statistical robustness is ensured through diagnostic tests and non-parametric trend analysis. A multivariate decomposition of total water levels allows the relative contribution of astronomical tide, storm surge, seiches, and low-frequency components to be quantified. Results show that extreme sea levels in Trieste are predominantly governed by barotropic processes, with the non-tidal residual becoming the dominant contributor during the most severe events. Seiches play a key amplifying role under persistent Scirocco conditions, leading to resonance effects that substantially enhance coastal water levels. Despite the observed increase in mean sea level, no statistically significant trends in storm intensity or frequency are detected over the analysed period, indicating that rising baseline sea level remains the primary driver of increasing flood hazard. For Grado and Lignano Sabbiadoro, characterized by low-lying sandy and lagoonal morphologies, extreme levels are assessed using annual maxima calibrated against the Trieste reference record. High-resolution digital terrain models are employed within a GIS-based framework to map inundation scenarios through a static “bathtub” approach, highlighting the critical role of lagoon connectivity and urban elevation in controlling flood propagation. In Muggia, a semi-enclosed bay setting promotes surge accumulation and prolonged inundation despite limited wave exposure. Hydrodynamic simulations performed with the MIKE 21 modelling suite further elucidate the interaction between sea level, waves, and coastal morphology, confirming that flooding mechanisms differ substantially among the analysed sites. Overall, the research provides an integrated and operational framework for coastal flood hazard assessment, supporting urban adaptation strategies and the implementation of the European Floods Directive. The findings underscore the urgency of targeted adaptation measures, particularly in lagoon-connected urban systems, where even modest sea-level rise may transform currently infrequent events into chronic flooding conditions.