Morphological coupling in a double sandbar system

Abstract

Abstract. Subtidal sandbars often exhibit alongshore variable patterns, such as crescentic planshapes and rip channels. While the initial formation of these patterns is nowadays reasonably well understood, the morphodynamic mechanisms underlying their subsequent finite-amplitude behaviour have been examined far less extensively. This behaviour concerns, among other aspects, the coupling of alongshore-variable patterns in an inner bar to similar patterns in a more seaward bar, and the destruction of crescentic patterns. This paper aims to present our recent findings on the understanding of finite-amplitude behaviour of crescentic sandbars, with a focus on morphological coupling in double sandbar systems. Our results, based on a combination of remote-sensing observations, numerical modelling and data-model integration, illustrate that morphological coupling can be common in multiple sandbar systems. It is governed by water depth variability along the outer-bar crest and by various wave characteristics, including the offshore wave height and angle of incidence. The angle of wave incidence is crucial to the flow pattern, sediment transport, and thus the emerging morphology of the coupled inner bar. In addition, our results demonstrate that crescentic patterns predominantly vanish under high-angle wave conditions, highlighting the role of alongshore currents in straightening sandbars and challenging the traditional conception that crescentic patterns vanish under high-energy, erosive wave conditions only.