Morphological and economic impacts of rising sea levels on cliff-backed platform beaches in southern Portugal

Projections from the Intergovernmental Panel on Climate Changes (IPCC) point to a global mean sea level rise (SLR) of close to 1 m by 2100 for a worst-case scenario. This will have a significant impact on coastal areas worldwide, primarily by modifying the shoreline position and coastal morphology, but also by influencing the coastal economy and livelihoods. Generally, it is assumed that sandy barriers will adapt to SLR through shoreline retreat and barrier inland migration. However, for embayed beaches backed by cliffs and/or underlined by shore platforms, constraints to inland migration will compromise such morphological response, with SLR-induced shoreline retreat leading to reductions in beach width and area. This will have impacts on beach use and carrying capacity. Aiming to analyse the morphological changes induced by SLR at cliff-backed platform beaches, this study explores simple mathematical models to quantify beach morphological change. 2D cross-shore profiles, representing the morphology of the beach and the underlying shore platform, were analysed using two geometric models of beach profile response. The model of Taborda and Ribeiro (2015) was applied for profiles with berm, while a new model is proposed for profiles without berm. The models assume that for profiles with berm there is both retreat and rise of the berm, while for profiles without berm the beach face becomes steeper and the sub-aerial beach narrower in response to SLR. Using a high-resolution topo-bathymetric LiDAR dataset, 94 cross-shore profiles from 32 beaches in southern Portugal were analysed. Their evolution was modelled considering the IPCC RCP8.5 scenario, which projects a SLR between 0.5 m and 1 m by 2100. From the 48 profiles with berm, 15 will experience complete berm erosion by 2100 for a 1 m SLR worst case scenario. The modelled average berm/beach width reduction is 7.9/5.8 m and 9.5/9.6 m for a SLR of 0.5 m and 1 m, respectively. A total of 26 beaches will become steeper and may be submerged if a threshold equilibrium beach slope is exceeded. Changes to the beach carrying capacity due to reduction in beach area will impact the local and regional economy, since the southern coast of Portugal is strongly influenced by beach tourism. The modelled changes to beach area result in a maximum potential economic loss ranging between EUR 215,000 and EUR 561,000 per day during peak summer months if no mitigation measures are considered. Beach nourishment was found to be a cost-effective measure to prevent the modelled reduction in beach area and mitigate the associated economic impacts