Impact of a high-energy storm event (Storm Barra) on a multiple intertidal barred system.

Multiple intertidal barred systems (MITB) are complex coastal features observed under specific conditions including low to moderate energy waves and macrotidal forcing. Although primarily tide-dominated, MITBs can undergo significant morphological changes under storms and extreme events. In December 2021, storm Barra crossed the U.K. and was the 2^nd most energetic event in the past 25 years, to occur on Northern Ireland’s east coast. At the MITB site in Dundrum Bay, offshore waves reached a maximum significant wave height of 5.5 m on December 7^th, 2021 and were associated with a peak period and wave direction of 10s and 179°N-oriented respectively. Pre- and post-Barra intertidal DGPS surveys were conducted at Dundrum Bay on the 6^th and 9^th of December to identify beach morphological changes on MITBs. Despite direct onshore waves reaching the bay, a strong alongshore variability was recorded in the response to Murlough and Ballykinler beaches to Storm Barra. Indeed, according to preliminary results, the western end of the bay shows an elevation of the beach profile, the central area presents onshore migrations of the bars to no significant changes, while the eastern side of the bay (Ballykinler) displays bar crests flattening and linear post-storm profiles.

Although storm Barra was the most energetic event recorded during the winter 2021/2022, smaller storm events had modified the morphology of Murlough and Ballykinler beaches throughout the preceding season, leading to some already low pre-Barra beach profiles. Secondly, at the peak of Barra’s energy, the waves were southeast oriented. But the direction of the waves rapidly shifted from onshore to offshore, possibly modifying the impact of Barra on the system. Finally, the shape of the bay, the location of the different profiles and the complex nearshore bathymetry and local geology must have also played a key role dictating the alongshore pattern shown. Previous studies of Dundrum Bay have shown that physical processes driven by waves/tide and geomorphology interactions can undergo significant local modification, leading to a strong alongshore variability in the profiles’ response to seasonal events. Nearshore SWAN simulations will help highlight the role of nearshore hydrodynamics including wave dissipation and re-orientation, wave-driven sediment transport and the impact of storm surge on MITB during an extreme event.