A process-based physical climate storyline for the Hercules storm in Portugal: extreme coastal flooding under climate change

In the beginning of 2014, exceptionally energetic swells associated with the Hercules storm (also known as “Christina”) produced one of the most devastating coastal events ever recorded in Portugal. Between January 6^th and 7^th, coastal flooding affected more than 30 municipalities along the Portuguese coastline, with offshore buoys registering maximum individual wave heights and periods of 14.91 m and 28.10 s, respectively. The storm resulted in more than 16 million euros in direct damages due to overtopping and coastal flooding, while indirect losses (considering affected businesses and populations) are estimated to have reached hundreds of millions of euros. In this study, two physical climate storylines are developed to assess the impacts of a “Hercules”-like storm, at five key-locations along the Portuguese coastline, occurring by the end of the 21^st century, under the combined influence of sea-level rise (SLR), projected changes in wave climate, and altered coastal morphology, while retaining the same statistical representativeness observed in 2014. The storyline approach enables a clear linkage to the original event and facilitates the assessment of future extreme events such as Hercules within the context of a changing climate, supporting decision-making by working backwards from specific vulnerabilities or decision points. Results indicate that the impacts of a future Hercules-like storm are projected to intensify, considering SLR and increases in high-percentile wave energy. Extreme coastal flooding is expected to affect 1.9 to 2.4 times more area than in 2014, resulting in 3.2 to 6.5 times more physically impacted buildings, particularly in densely urbanized coastal sectors. As coastal erosion is expected to reduce the natural protection of Portuguese sandy coastlines, the currently employed protection mechanisms will require robust adaptation measures, strategically defined to withstand long-return-period extreme events.

 

This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020 - https://doi.org/10.54499/LA/P/0068/2020, UID/50019/2025, https://doi.org /10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025. The authors would like also to acknowledge the project “Elaboração do Plano Municipal de Ação Climática de Barcelos (PMACB).