Coastal flooding risk scenario for the city of Grado, northern Adriatic Sea

Sea level rise, along with fast urbanization, has significantly raised flood hazards in coastal areas due to both increased population and the frequent urgent situations encountered in the urban context (Neumann, 2015). Rigid defensive systems along shorelines, backshore, and perimeter embankments at the lagoon border must therefore undergo structural verification and accurate elevation control, adapted to the expected sea level rise over which extreme events emerge. In this regard, a thorough investigation was conducted on the city of Grado, located in the North Adriatic Sea and representing one of the most important coastline and heritage sites in Friuli Venezia Giulia. The city is located on a barrier island and is prone to flooding from both the sea and from the back lagoon. The examination of the historical dataset of mean sea level and maximum levels recorded between 1991 and 2022 allowed us to first obtain local elevation estimates while also including subsidence. The study then looked at the duration of annual exceedance of the minimal threshold level of 110 cm, which marks Grado’s first significant marine ingression. Annual extremes were then processed using the Gumbel-type distribution law (Gumbel, 1958; Coles, 2001), which modulated the level thresholds with their respective return times for the location. Finally, the reconnaissance includes a downscaling examination of the impacts on the urban fabric, which is divided into regions of different vulnerability. Inundated areas were compared from high resolution DEMs using a GIS-based technique and a modeling approach utilizing MIKE software, analyzing both the magnitude and exposure of the urban setting to flood risk estimated for 2050 and 2100. As of today, approximately 56% of Grado’s inhabited area is inundated with a sea level threshold value of 151 cm, which occurs during surge episodes with a 30-year return period. By 2100, with an optimistic forecast (SSP1-2.6) of local sea level rise of around +50 cm, the same threshold will be met with events with a 30 times shorter return time. As a result, by 2100, 56% of the land area will be submerged at least once a year due to 101 cm surges. Extreme levels linked with more catastrophic events with secular recurrence times will be achieved on a multiyear basis, flooding more than 70% of the urbanized area. This makes it more important than ever to develop proper adaptation measures from the outset. The simulations developed are thus essential for medium to longterm urban planning, with the goal of securing the city, which, like Venice, is indicative of the processes that may affect other coastal realities and historical cities of great national worth.