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 con-ducted 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 morpho-logical 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.

Automated summary

Although Multiple Intertidal Bar Systems (MITBs) exhibit long-term morphological persistence, they display short-term, especially seasonal, morphodynamic behavior. The analysis of monthly surveys reveals a link between strong seasonality in wave climate and MITB beach behavior. Summer conditions limit sediment exchange and stabilize MITB beach morphology, while high-energy winter conditions drive sediment transport and promote bar recovery. Alongshore dynamics are also influenced by seasonality, with winter high-energy forcing enhancing channel migration rates and sediment transport. Differences in local geology and beach morphology contribute to variations in dynamics between study sites, but the primary role of the inlet in sediment circulation is highlighted.