Coupled and noncoupled behavior of three‐dimensional morphological patterns in a double sandbar system

Abstract

Eight weeks of daily (low tide) time exposure images of the Gold Coast (Australia) were used to analyze the development of three‐dimensional morphological patterns in a double sandbar system. Under continued low‐energy wave conditions following a morphological reset (storm) event, the patterns in both the inner and outer bar evolved, albeit at different spatial and temporal scales, through the accretionary sequence commonly observed at single barred beaches. Continuous wavelet transforms (CWT) of the inner bar and outer bar breaker lines show that after a storm the patterns developed at a wide range of alongshore wavelengths and that variations in wavelength over space and time were common. With the bivariate extension of CWT, continuous cross‐wavelet transforms, it is found that the initial inner bar and outer bar three‐dimensional patterns were spatially noncoupled. However, over time the inner‐bar morphology appeared to couple with that of the outer bar, with the inner bar patterns developing in response to the increasingly three‐dimensional, onshore propagating outer bar. The wavelet analysis suggests that the outer bar geometry and the distance between the inner and outer bars are critical parameters governing the morphological evolution of the composite double sandbar system.