Crescentic sandbars and rip channels along wave-dominated sandy beaches are relevant to understand localized beach and dune erosion during storms. In recent years, a paradigm shift from hydrodynamic template models to self-organization mechanisms occurred to explain the formation of these rhythmic features. In double sandbar systems, both the inner-and outerbar rip channels and crescentic planshapes are now believed to be free instabilities of the nearshore system arising through selforganization mechanisms alone. However, the occasional occurrence of one or two inner-bar rip channels within one outer-bar crescent suggests a forced, morphologically coupled origin. Here we use a nonlinear morphodynamic model to show that alongshore variability in outer-bar depth, and the relative importance of wave breaking versus wave focussing by refraction across the outer bar, is crucial to the inner-bar rip channel development. The coupling patterns simulated by our model are similar to those observed in the fi eld. Morphological coupling requires a template in the morphology (outer-bar geometry) which, through the positive feedback between fl ow, sediment transport and the evolving morphology (that is, self-organization) enforces the development of coupling patterns. We therefore introduce a novel mechanism that blurs the distinction between self-organization and template mechanisms. This mechanism may also be extended to explain the dynamics of other nearshore patterns, such as beach cusps. The impact of this novel mechanism on the alongshore variability of inner-bar rip channels is investigated in the companion paper.

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Nearshore sandbars, located in <10 m water depth, can contain remarkably periodic alongshore undulations in both cross‐shore position and depth. In a double sandbar system, the alongshore spacing of these morphological patterns in the inner sandbar may be identical to those in the outer sandbar. Although this morphological coupling has been observed previously, its frequency and predominance remain unclear. In this paper, we use a 9.3‐year dataset of daily low‐tide time exposure images from the double‐barred beach at Surfers Paradise (Gold Coast, Australia) to analyse the temporal and spatial characteristics of morphological coupling within a double sandbar system. We distinguish five types of morphological coupling between the inner and outer sandbars, of which four coincide with a downstate progression of the outer bar. Coupling is either in‐phase (with a landward perturbation of the inner bar facing an outer‐bar horn) or out‐of‐phase (with a seaward perturbation of the inner bar facing an outer‐bar horn), where the coupled inner‐bar features either consist of rip channels or, predominantly, perturbations of the low‐tide terrace. Cross‐correlation of the image‐derived inner‐ and outer‐bar patterns shows coupling to be a common phenomenon in the double sandbar system studied here, with coupling in 40% of the observations. In contrast to previous observations of sandbar–shoreline coupling at single‐barred beaches, in‐phase coupling (85% of all coupled bar patterns) predominates over out‐of‐phase coupling (15%). Based on our observations and bathymetries assimilated from the images for a restricted set of coupling events, we hypothesize that the angle of offshore wave incidence, wave height and depth variations along the outer sandbar determine the type of flow pattern (cell circulations versus meandering currents) above the inner bar and hence steer the type of coupling. Copyright © 2012 John Wiley & Sons, Ltd.

Sandbars, submerged ridges of sand parallel to the shoreline, affect surfzone circulation, beach topography and beach width. Under time-varying wave forcing, sandbars may migrate onshore and offshore, referred to as two-dimensional (2D) behaviour, and vary in planshape from alongshore uniform ridges to alongshore non-uniform ridges through the growth and decay of three-dimensional (3D) patterns, referred to as 3D behaviour. Although 2D and 3D sandbar behaviour is reasonably well understood along straight coasts, this is not the case for curved coasts, where the curvature can invoke spatial variability in wave forcing. Here, we analyse sandbar behaviour along the 3000 m man-made curved coastline of the Sand Engine, Netherlands, and determine the wave conditions governing this behaviour. 2D and 3D behaviour was quantified within a box north and west of the Sand Engine's tip, respectively, using a 2.4-year dataset of daily low-tide video images and a sparser bathymetric dataset. The northern and western sides behaved similarly in terms of 2D behaviour, with seasonal onshore and offshore migration, resulting in a stable position on inter-annual timescales. However, both sandbar geometry and 3D behaviour differed substantially between both sides. The geometric differences (bar shape, bar crest depth and wavelength of 3D patterns) are consistent with computed alongshore differences in breaker height due to refraction. The differences in the timing in growth, decay and morphological coupling of 3D patterns in the sandbar and shoreline are likely related to differences in the local wave angle, imposed by the curved coast. Similar dependency of bar behaviour on local wave height and angle may be expected elsewhere along curved coasts, e.g. shoreline sandwaves, cuspate forelands or embayed beaches.

Abstract. Subtidal sandbars often exhibit alongshore variable patterns, such as crescentic plan shapes and rip channels. While the initial formation of these patterns is 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 review aims to present the current state of knowledge on the finite-amplitude behaviour of crescentic sandbars, with a focus on morphological coupling in double sandbar systems. In this context we include results from our recent study, based on a combination of remote-sensing observations, numerical modelling and data-model integration. Morphological coupling is an inherent property of double sandbar systems, where the inner bar may attain a type of morphology not found in single bar systems. Coupling 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. In recent research, the role of the angle of wave incidence for sandbar morphodynamics has received more attention. Numerical modelling results have demonstrated that the angle of wave incidence is crucial to the flow pattern, sediment transport, and thus the emerging morphology of the coupled inner bar. Moreover, crescentic patterns predominantly vanish under highangle 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.