The future of the Portuguese most vulnerable coastal areas under climate change – shoreline evolution and future extreme coastal flooding from downscaled bias corrected ensembles

Some of the most disruptive effects of climate change are projected to be felt along the coastlines. The combined effects of future changes in water levels and wave climate along the coastal areas constitute one of the most serious threats to their sustainable evolution, compromising critical infrastructures, resources, ecosystems, and communities. Understanding long-term changes in coastal areas remains challenging, however, due to their multivariate and multi-time-and-space-scale nature. In this study, we propose an innovative methodology for a complete vulnerability assessment of sandy low-lying coastal areas, based on dynamic, ensemble-based projections from the Coupled Model Intercomparison Project phase 5 (CMIP5). The effects of sea level rise (SLR) and nearshore wave climate changes on future shoreline evolution are firstly assessed at five key-locations along the Portuguese coastline. Longshore sediment transport (LST) projections are computed, and sedimentary imbalances are quantified. Robust shoreline retreat of up to 300 m is projected, especially along the Portuguese northern and central coastal areas, with continued erosion driven mainly by sediment imbalance and SLR. The projected decrease in future nearshore wave energy is responsible for a slight alleviation in erosion trends, up to 6.33%, whereas the increase of northerly incoming waves is expected to lead to northward beach rotations along western Mainland Portugal. The resulting shoreline evolution is responsible for the loss of up to 0.786 km2 of dry land by 2100 along the 14 kilometers of analyzed coastline. Based on the shoreline projections, new digital terrain models are built for the five key-locations, and future extreme total water levels are obtained through a probabilistic approach, defining wave events considering high wave energy thresholds in a changing climate. The results reveal that extreme coastal flooding is projected across several urbanized sections along the Portuguese coastline, especially in areas without artificial protection infrastructures. As dune erosion is expected along the sandy stretches, the natural protection against extreme coastal events is projected to be reduced by up to 13.3%, promoting widespread overtopping, leaving populations more exposed. Future projections reveal the episodic flooding of up to 1.47 km2 of land across the five key-locations, threatening households and commercial hubs, besides services and communication routes. Overall, as physical and human losses may increase substantially in the future, our results call for the implementation of adequate coastal management and adaptation plans, strategically defined to withstand changes until 2100 and beyond. This research has been funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).