Long-term drivers of shoreline change over two centuries on a headland-embayment beach

Shoreline evolution over the last two centuries was analysed for Dundrum Bay, Co. Down, Northern Ireland, using historical and recent shoreline datasets from 1833 to 2020. The area of interest comprises two sandy beaches and vegetated coastal dune fields, Newcastle-Murlough and Ballykinler, separated by an inlet channel which connects the inner with the outer bay. Twenty-four temporal shorelines were extracted, and a quantitative assessment of their positional uncertainty was performed. This was combined with analysis of foredune volume variations by applying the Structure-from-Motion-Multi-View-Stereo technique to 1963 aerial photography and comparing it with a 2014 Light Detection And Ranging (LiDAR) dataset to better inform on links between the sediment dynamics and the observed shoreline changes. Storm events were identified using recorded extreme water levels (EWLs) (1901-2020) and hindcasted wave data (1948-2020). In the first 87 years, the shoreline was largely stable, and change was focused at the inlet area. In the 20th century, localised retreat characterised the western (Newcastle-Murlough) sector, whereas the eastern sector (Ballykinler) experienced general shoreline advance. The rate and extent of shoreline retreat in the western sector increased post-1997 in concert with accelerated accretion at Ballykinler. The strongest erosional episodes were recorded in 1920-1951, 1997-2005, and 2012-2014. Although no direct link was established between single storms and shoreline retreat rates, the three major retreat periods coincided with several consecutive EWLs or EWLs with a return period greater than 100 years. These were generated by storm directions ranging from south to southwest. The long-term pattern of shoreline change points to a complex coastal system that is still evolving.