Impact of an extreme storm event on a multiple intertidal barred coastal system

In December 2021, the British Isles experienced Storm Barra, an exceptionally intense storm event, categorised as the second most energetic event on the east coast of Northern Ireland in the past 25 years. Dundrum Bay (Co. Down) on the Irish Sea coast features a Multiple Intertidal Bar (MITB) system, with an additional subtidal sandbar. Here, we examine the direct coastal impact of this event on the MITB system, focusing on (1) the morphological response of beaches, and (2) the role of the multiple sandbars in dissipating wave energy at different tidal stages. Topographic profiles showed high variability in morphological response. Distinct erosional, accretional and transitional zones were evident that reflect complex interactions between the local geology, pre-storm profile shape and the impact of previous events. Accretion occurred on profiles that were depleted by previous storms. Erosion was prevalent on profiles where the MITB features were poorly developed. Cross-shore sediment transport was dominant in the NE (Ballykinler), while sediment was transported alongshore in the SE sector (Newcastle to Murlough). Analyses of wave height and wave energy dissipation illustrate the key role of sandbars in dissipating extreme storm energy at different water levels. The storm peak occurred on a falling tide stage, and most of its energy was dissipated by the subtidal bar. Following the peak, at high tide and under moderate energy conditions, part of the wave energy bypassed the subtidal and seaward intertidal bars but was fully dissipated on the landward intertidal bars. Consequently, even though Storm Barra generated high-energy waves, the effectiveness of the sandbars in dissipating the wave energy at different tidal stages prevented severe impacts on the shoreline. This work demonstrates the important role of sedimentary bedforms in buffering storm energy before it reaches the shoreline. Additionally, the work shows the crucial role of coincidence or otherwise between storm peak and tidal stage on energy dissipation across the nearshore.