Coupled sandbar patterns and obliquely incident waves

Abstract

In double sandbar systems, the alongshore variability in the inner bar oftenresembles that of the outer bar, suggesting that the outer bar acts as a morphologicaltemplate for the inner bar. Earlier observations have indicated that this resemblance, alsotermed “coupling,” may take several forms. Here we apply a nonlinear 2DHmorphodynamic model with time-invariant forcing to show that the angle of waveincidence (Â) is crucial for the alongshore-variable morphodynamic evolution of the innerbar, for a given crescentic outer bar. In contrast to previous modeling efforts ofdouble-barred systems, which mostly used highly idealized boundary conditions, weforce our model with realistic hydrodynamics and bathymetrical data derived from videoobservations at the double-barred Gold Coast, Australia. The results show that for smallangles of wave incidence (Â <10ı) over a crescentic outer-bar, cell-circulation patternsgovern the flow at the inner bar, giving rise to rip channels that incise the inner bar at thelocations of the landward perturbations in the outer bar (horns). On the other hand, forobliquely incident waves (Â =10ı–20ı) over a crescentic outer bar, the circulatory natureof the flow disappears and gives way to a meandering alongshore current. Theoffshore-directed sections of this meandering alongshore current erode the inner-bardowndrift of the outer-bar horns, leading to landward perturbations of the inner bar thatare coupled to the outer-bar horns; an observed coupling type that had not beenreproduced as yet. Oblique wave incidence thus proves to be crucial to the developmentof this type of sandbar coupling, as previously hypothesized from sandbar-couplingobservations at the Gold Coast. Additional simulations including tidal water levelvariations and bar depth variations demonstrate the robustness of our findings.