Erosive effects of sea-storms, human pressure, and sea level rise on Volturno coastal plain (mid-Tyrrhenian area): present to future coastal hazard

Climate change and sea-level rise are crucial global effects in the coming decades. This study aims to estimate the future effects of sea storms on low-lying coastal areas, by using an integrated approach using advanced modeling tools and technologies that include in the analysis the anthropic impacts and coastal subsidence. The approach wants to analyze past storm surges, sea level changes, and shoreline displacements to model future coastal variations due to the increasing frequency and magnitude of extreme events related to climate change.

We focus on the Volturno coastal plain, which is one of the largest and most populated coastal areas along the mid-Tyrrhenian Sea in Italy. The area is mostly exposed to meteo-marine forcings in the sector between 180° and 280° N, therefore major storm surges registered by the nearby National Network’s wave buoy were analyzed for this sector, to evaluate the coastal effect of the main flooding that occurred in the last 35 years. Peak Over Threshold methodology was used for this evaluation by considering events with a minimum significant wave height greater than 1,50 meters and a minimum duration of 12 hours. The selected storms were then divided into 5 categories, based on their power index (Dolan R., Davis R.E., 1992). Numerical simulations of two storms for each category were carried out on a coastal sector with high social, touristic, and naturalistic values, between the Volturno and Regi Lagni rivers. The topographic base used for hydrodynamic modeling has been extracted by analyzing a set of LiDAR surveys collected by the Italian Ministry of the Environment at 2 m of resolution. The elevation data have been converted into the RDN2008/UTM 32-33 coordinate system above sea level elevation (geoid-related height) to create a revised DEM of the coastal zone, validated through multiple GPS surveys carried out in 2024. The DEM of the underwater coastal sector was created by interpolating data from a single beam survey.

The calculation of the shoreline displacement rates shows that during 1954-2003, the coastal sectors of the Volturno River mouth retreated at a velocity >10 m/y. After 2004, thanks to the construction of two submerged breakwaters, this trend inverted with a total accretion of the beach of about 100 meters in 20 years. Nevertheless, the hydrodynamic simulation results allowed the estimation of flooded areas, also considering the three more probable IPCC–AR6 predictive scenarios of sea level rise until 2150. Storms with a return time of 10–25 years reach the dune toe for 80% of the total length. In the sector protected by the breakwater, such events tend to overtop the dune flooding the back dune system area which is part of the WWF natural reserve “Oasi dei Variconi”. According to our procedure, in the future, with a sea-level rise to 1.1 m, not only will such effects increase, but also their return period will be reduced to 5-20 years. Finally, the extreme event that occurred in 1999 will shorten its return period from 50 to 25-30 years.