Influences of wave forcing and morphological variability on the evolution of a double-sandbar system

Double-sandbar systems are common along sandy, wave-dominated coastlines. The evolution of a double-sandbar system is a complex process and is affected not only by the time-varying wave forcing but also by the local seabed morphology itself. The influences and relative importance of both wave condition and morphological variability on the evolution of a double-sandbar system still remain unclear and are investigated in this study using a well-established operational quasi-three-dimensional coastal evolution model. Results show that these two factors contribute to the final double-sandbar morphology in different ways. For energetic waves with high angles of wave incidence, waves determine the final sandbar morphology, regardless of the antecedent bathymetry. For moderate waves with small angles of wave incidence, however, the pronounced morphological variability (crescentic pattern of sandbars) will dominate the evolution, which remarkably enhances the existing morphological pattern regardless of the changed wave condition. A nondimensional index is proposed to quantitatively evaluate the relative importance of these two mechanisms. Further quantitative analysis reveals that the relative importance of wave condition and morphological variability depends on the alongshore current velocities over the bar crests. There also appears a critical value for these alongshore current velocities, below which morphological variability tends to be dominant and wave forcing is of less importance.