The influence of wave direction on the morphological response of a double sandbar system

Nearshore sandbars are often characterized by three-dimensional bed patterns. To analyze the influence of wave direction on the morphological response of a double sandbar system, this paper uses the 2DH nonlinear surf zone model MORFO55. Depending on wave direction, different morphologies emerge as free instabilities. These morphological responses differ in terms of geometry (shape of the alongshore rhythmic patterns) and temporal evolution. Nearly shore-normal waves favor the emergence of crescentic patterns along both the inner and outer bars. These instabilities arise from “bedsurf” coupling, which is the positive feedback between the bed perturbations and the wave-breaking patterns resulting in rip-cell circulations. The system is stable for intermediate wave incidence angles because the longshore current shifts the circulation cells alongshore, inhibiting the growth of initial perturbations. For larger angles, the system is again unstable. Two types of oblique bars emerge. The first type forms between the crest of the inner bar and the shoreline and is oriented down-current. The second type is superimposed on the two bars and is oriented up-current. Although previous studies attribute the formation of oblique bars to the deflection of the longshore current (that is a cross-shore process), we show here that this mechanism contributes to the bar formation but the growth rate is mainly governed by a longshore process. Specifically, it is the positive feedback between the bathymetry and the longshore gradient of the sediment concentration. Finally, interactions between the patterns in the two shore-parallel bars are analyzed. In the observed configurations, the influence is always one way as the inner-bar dynamics never influence those of the outer bar. The outer-bar instabilities cause undulations in the inner bar when the outer-bar instabilities grow faster than those of the inner bar.