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On the anatomy of nearshore sandbars: A systematic exposition of inter-annual sandbar dynamics

Nearshore sandbars have a lifetime of many years, during which they exhibit cyclic, offshore directed behaviour with strong alongshore coherence. A bar is generated near the shoreline and grows in height and width while migrating offshore, before finally decaying at the seaward limit of the surf zone. It may take 10 to 15 years for a bar to exhibit this cycle. Four to five bars may occur simultaneously within a cross-shore bed profile. Alongshore variations in cross-shore bar position and bar amplitude are commonly observed. A strong or abrupt alongshore variability is referred to as a bar switch. At large spatial scales, the inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the bar cycle return period (T r , i.e. the duration between two successive bar decay events) may differ by a factor of three to four. This type of change in T r appears to be always present in time and is characterized as a persistent bar switch. At smaller (kilometer) scales, bar switches typically occur in areas with similar T r -values on both sides of a bar switch and occasionally disappear when the bars re-attach. These are characterized as non-persistent bar switches. The assimilation of shoreface nourishments into the coastal system involves a strong interaction with the pre-existing sandbar system. Typically the placement of a shoreface nourishment just seaward of an outer bar reverses the bar cycle temporalily, inducing a landward migration of the bar system. The shoreface nourishment becomes absorbed in the coastal system as the new outer bar. At the distal ends of the shoreface nourishment bar switches often manifest, owing to a distinct difference in the bar migration cycle phase that is induced. Given the importance of the bar-nourishment interaction, an improved understanding of the nearshore bar dynamics is expected to improve the efficacy of shoreface nourishments. Furthermore, the long-term evolution of the nearshore barred profiles is generally considered indicative of the quality of the modelling for the response of the entire nearshore coastal system. Therefore, the ability to perform reliable and robust a-priori, long-term predictions has broad societal relevance in view of anticipated adverse impacts of climate change and sea level rise on the stability of coasts worldwide. Until now the anatomy of the nearshore sandbars has primarily been studied using field data. Although these studies have provided insight into how the geometric bar parameters respond to the external forcings, no comprehensive conceptual framework is available that explains the full life cycle of a sandbar and its associated characteristics. The overarching objective of this study is to elucidate the anatomy of the inter-annual bar morphology using a combined data and model approach. This overarching objective is in turn devolved into three objectives aiming to understand key features of bar morphology and a further objective to enable a comprehensive…