Seasonal morphodynamics of multiple intertidal bars (MITBs) on a meso‐ to macrotidal beach

Multiple intertidal bar (MITB) beach systems comprise a succession of subdued, shore-parallel sandbars, developed under low-energy conditions in meso- to macrotidal settings. Their relatively stable morphologies over long timescales are commonly attributed to a dynamic equilibrium, driven primarily by seasonal morphodynamics. The seasonal behaviour is, however, poorly understood. The relationship between temporal and spatial hydrodynamic forcing and morphological changes was investigated through monthly differential GPS surveys in Dundrum Bay (Northern Ireland) from April 2019 to March 2020, associated with variations in nearshore wave conditions, simulated using the SWAN wave model. During low-energy summer conditions, southeast waves helped promote MITB stability and a slight increase in bar crest elevation, with the seaward-most bar buffering and preserving the inner bar system through wave dissipation. In the winter, a progressive increase in wave energy, and a switch in wave direction (southwest to south), initiated the highest rates of cross-shore bar migration and a lowering of bar crests. While the seaward-most bar remained largely submerged, the shoreward-most bar played a key role in protecting the upper-beach and/or foredune. Winter conditions also forced a newly observed mesoscale longshore drift involving offshore sediment transport in the southwest and onshore transport in the northeast. Cross-shore variability in MITB behaviour at seasonal timescales was, however, primarily driven by local hydrodynamic conditions, including variations in wave energy and direction. Conversely, alongshore variability was largely influenced by the complex nearshore bathymetry, headlands and an active ebb delta, all interacting with changing hydrodynamic forcing. These results challenge the seasonal equilibrium model for MITB systems and highlight longer-term patterns of sediment movement.