The role of a multiple intertidal barred (MITB) system in dissipating extreme storm wave energy

First described by King and Williams in 1949, multiple intertidal barred (MITB) features are characterised by a succession of intertidal sandbars, comprising a complex system subject to variable hydrodynamics. Relatively stable under macrotidal conditions and low to moderate wave energy, MITB systems can however, display cross-shore migrations and morphological changes under energetic and extreme events. Storm Barra occurred in December 2021, and crossed the east coast of Northern Ireland. It was the second highest energetic storm to have occurred there in the last 25 years, with waves reaching a maximum significant wave height (H_s) of 5.5m for a peak period (T_p) of 10s and, from a southeasterly direction on the 7^th December. To examine the morphological impact on Dundrum Bay and its MITB system, DGPS surveys were conducted, before and after storm Barre, at two adjacent sites, Murlough and Ballykinler beaches, on the 6^th and 9^th of December respectively. Topographic beach surveys showed distinct alongshore variability at both sites. Despite onshore waves, the Ballykinler site (eastern side of the bay) presented linear post-storm Barra profiles due to a complete flattening of the bar crests and sediment in-filling within the runnels. In contrast, the western end of the bay displayed an elevation of the beach profiles, with the central zone a more transitional area associated with onshore bars’ migrations to no significant changes, eastward. Additionally, nearshore wave modelling (SWAN), including adjustments for surge, was conducted to better understand the wave dissipation patterns and local interactions with sandbars’ morphology. Preliminary wave model results show a significant role of the bars during the event. At the peak of the storm, which corresponded to a falling tide period, the maximum wave dissipation was focussed on the offshore-most bar, just outside the intertidal area and thus limiting impacts on the beach and nearshore MITB system. The following rising tide period however, coupled with decreasing wave height and energy, corresponded with much less intense energy dissipation at the site. During the final phases of the storm, maximum values of wave energy dissipation concentrated closer to the shore and were primarily induced by the MITB system in the intertidal beach area. It therefore appears that the offshore-most bar of the MITB features plays a significant role dissipating extreme events energy and limiting morphological changes, while the other bars are more effective during less energetic conditions and at low tide. SWAN simulations help explain the relatively low coastal impact detected at both Ballykinler and Murlough beaches from storm Barra, although further investigation of sediment transport patterns and (antecedent) storm chronologies throughout the winter 2020/2021 season are still required to fully understand the alongshore variability observed.