Coastal flood pathways driven by extreme storm surge and dike failure in the Northern Adriatic

Abstract

<p>Extreme events can cause damage to coastal defences resulting in overtopping, breaching, or their total destruction. The resulting flood can impact the population and assets, causing short- to long-term losses to the economy. Two examples of extreme coastal storms are the Katrina Hurricane (2005) in the USA, and the Xynthia storm (2010), in France. The Northern Adriatic Sea is often impacted by coastal storms, impacting residential and commercial areas. The region of Emilia-Romagna (Italy) is so often affected that it is common practice to create an artificial dune on the beach during winter to protect the properties.<br />On 22 November 2022, a coastal storm developed in the Northern Adriatic Sea, impacting the coasts of Veneto and Emilia-Romagna Italian regions. This storm, coming from ESE, was characterised by non-extreme waves, but coincided with spring tides, producing an extreme surge, reaching a total water level of 1.48 m above MSL, recorded for the first time by tide gauge at Porto Garibaldi (Comacchio), which corresponds to a return period greater than 100 years. The Saint Agatha storm that hit the same areas in 2015, reached a TWL of 1.2 m at the same location. The event of November 2022, caused damages on the coast, erosion of beaches, artificial and natural dunes, damage to coastal infrastructures, and flooding of residential buildings and local business activities. <br />While this storm represented a success for the MOSE, which succeeded in protecting the lagoon of Venice against one of the most hazardous events of the last decades, its effects on the Ferrara coasts were, in some cases, devastating. The most affected area was the Lido di Volano (Comacchio), where the extreme event caused a dike breach in the inner part of the Po di Volano mouth, leading to the recurring flooding of the town, due to the high spring tides that followed the storm. Indeed, the breach remained open for several days after the event. The event required the heavy involvement of first responders.<br />The site was surveyed on 23 and 25 November 2022. The research team measured flood extension and flood markers with the use of DGPS and aerial images, while UAV aerial surveys were implemented on the emerged beach to assess the morphologic impacts.<br />The flood associated with the extreme event was simulated using a hydrodynamic model (LISFLOOD-FP) to verify the causes and evolution of the flood event. The model was set up using topo-bathymetric data from 2019, water levels from the tide gauge of Porto Garibaldi.  A thorough calibration was implemented using the fieldwork data. The model was used to simulate scenarios considering different dike-breach configurations and/or forcing it with other recent events (e.g. Saint Agatha, 2015). <br />The model was able to properly simulate the flooding event of November 2022. The tested scenarios highlighted the role of the magnitude of the event and the effects that a breaching dike can have on the associated areas. This is a contribution to the ECFAS project (EU H2020 GA 101004211).</p>