Inter-annual morphological evolution and volume changes of a meso- to macrotidal beach exhibiting multiple intertidal bars (MITB).

Although morphologically persistent in the long term, Multiple Intertidal Bar Systems (MITBs) display short-term, especially seasonal, morphodynamic behaviour. Analysis of high-density, monthly DGPS surveys conducted at Murlough and Ballykinler beaches, inter- and supratidal sediment volumes and hydrodynamic forcing (wave conditions and water levels), demonstrates a link between strong seasonality in wave climate and MITB beach behaviour. Summer, low-energy conditions limit cross-shore sediment exchange during which MITB beach morphology tends to stabilise throughout the season. With the onset of high-energy winter conditions cross-shore sediment exchanges occur between inter- and supratidal areas. Sediment transport is then enhanced during storm conditions that are coincident with spring tides, leading to high total water levels (TWL). Ultimately the storm sequencing, (frequency, magnitude and inter-storm interval), is the key parameter driving the beach morphological response. Major erosional patterns occur when the most energetic event, combined with spring high tide, occurs at the beginning of the winter season. Subsequent, less energetic storms then promote bar recovery until another extreme event occurs. Seasonality is also evident in alongshore dynamics. Cross-shore drainage channels that dissect the intertidal bars migrate alongshore, driving alongshore sediment transport and MITB longshore migration patterns. In summer migration of drainage channels is limited, whereas the winter high-energy forcing enhances channel migration rates and resulting sediment transport. Differences in dynamics between the two study sites are attributed to differences in local geology, beach morphology and sediment size, but in both locations, drainage channels are in fine migrating toward the inlet and associated ebb delta that divides the bay. Subsequent transport vectors are unknown, but the observations highlight the primary role of the inlet in the sediment circulation dynamics in the system as a whole. • Long-term Multiple Intertidal Bar systems stability is based on a dynamic equilibrium driven by short-term seasonal morphodynamics • Low-energy conditions limit cross-shore sediment transport and longshore drainage channels migration, supporting MITB system stability • High-energy forcing drive sediment exchanges between upper and lower beach; patterns are enhanced by storm events and high-water levels • Winter forcing increases alongshore drainage channels migration rates and subsequent sediment transport leading to alongshore bar migrations • MITB beach dynamics are driven by wave forcing, water levels, storm sequencing, local geomorphology and the presence of hard features and an inlet [ABSTRACT FROM AUTHOR]