The impact of outer-bar alongshore variability on inner-bar rip dynamics

A field-calibrated morphodynamic model (MIKE21) is used to investigate the importance of the dimensions of a rip channel across an outer sand bar to the hydrodynamics and morphological evolution of an inner sand bar and rip channel for a range of initial bathymetries and wave conditions. The model was driven with offshore wave conditions and idealized bathymetry representative of field conditions near Duck, NC, USA during which strong rips and associated channel erosion were observed to occur over an inner bar. Consistent with prior results, the strength of the hydro-morphological coupling between the two bars depends on the dimensions of the outer-bar perturbation, as well as the wave forcing. The results suggest that in double-barred systems, a single moderate-scale perturbation (O(0.1 m deep, 10 m wide)) in the outer-bar elevation can lead to the generation of a rip current and associated erosion of a rip channel across the inner bar. The simulations suggest that the magnitude of the inner-bar rip flow, the depth to which the inner-bar channel is eroded, and the alongshore position of the inner-bar rip relative to the outer-bar perturbation depend on the non-dimensional outer-bar channel depth, the transverse rip-channel slope, and the wave height, period and directional spreading. For deep and narrow outer-bar channels, the outer-inner bar coupling is strong. In contrast, for shallow and wide outer-bar channels, the system may alternate between being coupled and uncoupled with unstable locations of the inner-bar rip.