Morphological coupling in multiple sandbar systems – a review

摘要

Subtidal sandbars often exhibit alongshore variable patterns, such ascrescentic plan shapes and rip channels. While the initial formation of thesepatterns is reasonably well understood, the morphodynamic mechanismsunderlying their subsequent finite-amplitude behaviour have been examined farless extensively. This behaviour concerns, among other aspects, the couplingof alongshore variable patterns in an inner bar to similar patterns in a moreseaward bar, and the destruction of crescentic patterns. This review aims topresent the current state of knowledge on the finite-amplitude behaviour ofcrescentic sandbars, with a focus on morphological coupling in double sandbarsystems. In this context we include results from our recent study, based on acombination of remote-sensing observations, numerical modelling anddata–model integration. Morphological coupling is an inherent property ofdouble sandbar systems, where the inner bar may attain a type of morphologynot found in single bar systems. Coupling is governed by water depthvariability along the outer-bar crest and by various wave characteristics,including the offshore wave height and angle of incidence. In recentresearch, the role of the angle of wave incidence for sandbar morphodynamicshas received more attention. Numerical modelling results have demonstratedthat the angle of wave incidence is crucial to the flow pattern, sedimenttransport, and thus the emerging morphology of the coupled inner bar.Moreover, crescentic patterns predominantly vanish under high-angle waveconditions, highlighting the role of alongshore currents in straighteningsandbars and challenging the traditional conception that crescentic patternsvanish under high-energy, erosive wave conditions only.