A physical climate storyline for the Hercules storm in Portugal: Extreme coastal flooding in southwestern Europe under a changing climate

In the first days of 2014, exceptionally energetic swells associated to the Hercules storm (also known as "Christina") produced one of the most devastating coastal events in Portugal, establishing new records in long-term wave observations for the area. Between January 6th and 7th, extreme coastal flooding affected >30 municipalities along the Portuguese coastlines, with buoys measuring maximum individual wave heights and periods of 14.91 m and 28.10 s, respectively. The storm caused >16 million euros in direct costs, due to overtopping and coastal flooding, and hundreds of millions in indirect ones, including affected businesses and populations. Here, two physical climate storylines are built to assess the impacts of a "Hercules" storm, at five key-locations along the Portuguese coastline, happening by the end of the 21st century, under the influence of SLR, changes in wave climate, different shorelines and overall coastal morphology, yet, keeping the same statistical representativeness as observed in 2014. The storyline approach allows the establishment of a clear connection with the 2014 event and conceive the impacts of future extreme phenomena like Hercules, in the context of a changing climate, fostering decision-making by working backwards from specific vulnerabilities or decision points, integrating climate change data with other factors to address compound risks. Results reveal that the impacts of a future Hercules-like storm are projected to become more severe, considering the contribution of SLR and projected increases in the associated wave energy. Extreme coastal flooding is expected to impact 2.21-2.92 km² along 14 km of analyzed coastline, 1.9 to 2.4× more than in 2014, resulting in 3.2 to 6.5× the number of physically impacted buildings, in densely urbanized stretches. As continuous coastal erosion towards the end of the 21st century is expected to reduce the natural protection of the coastline, leaving populations more exposed, the currently employed coastal protection mechanisms will require robust adaptation measures, strategically defined to withstand extreme events with long return periods.