Observations and conceptual modelling of morphological coupling in a double sandbar system

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Abstract

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.

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