Variability of sandbar morphometrics over three seasonal cycles on a single-barred beach

Abstract

Understanding the morphodynamics of surfzone sandbars is very relevant from a sediment transport perspective. Three complete annual sandbar migration cycles were studied on a single-barred beach. The study period included the 2015–2016 El Niño anomaly and the incidence of a highly energetic swell-storm. The sandbar responded directly to the seasonal wave forcing, thus, it migrated offshore during the energetic winter and onshore during the milder summer conditions. Its seasonal migration cycle comprised four stages: (1) generation in autumn (November) at lower than 1 m depth; (2) offshore migration over the winter (until January/February) while H¯s > 1.3 m; (3) onshore migration over the early spring (March and April) while H¯s < 1.0 m; and during May–June: (4a) subaerial beach welding (unbarred beach) when a dynamic equilibrium reached; or (4b) terrace-bar formation (sandbar flattening) as a consequence of a lack of a dynamic equilibrium. The energetic El Niño winter conditions induced the same amount of offshore sandbar displacement as the individual extreme swell-storm, placing the sandbar at a maximum cross-shore distance of ~190 m, beyond the dynamic equilibrium. Contrary to the expected, a period of mild wave conditions during the 2017 winter favored the landward migration of the sandbar, locating it near its generation point. The duration of mild wave energy conditions and the offshore sandbar location and volume are considered relevant factors that limit the capabilities of the sandbar to reach shallow waters and weld to the subaerial beach before the summer.