Non-hydrostatic numerical modelling of nearshore wave transformation over shore-oblique sandbars

Abstract

Observations from high resolution bathymetric surveys on the coast of North Carolina indicate areas with shore-parallel depth contours and other areas with shore-oblique sandbars. These shore-oblique bars are large-scale morphological features that extend up to 1000 m from shore, and previous studies have shown that they are spatially correlated with higher shoreline change rates and induce erosional hotspots. To investigate the influence the oblique sandbar morphology has on nearshore wave transformation and energy distribution, a non-hydrostatic wave model is used with the bathymetry corresponding to two areas with shore-parallel depth contours and two areas with shore-oblique sandbars. The model is applied with a horizontal resolution of 5 m and a boundary condition representative of the directional spectrum of a typical southeast swell event. An idealized approach with relatively short alongshore model domains and cyclic lateral boundary conditions is adopted due to limited availability of high resolution bathymetry, however this approach elucidates the influence of shore-oblique sandbars on the nearshore hydrodynamics. The numerical results show that higher alongshore variability in depth due to the shore-oblique bars drive large alongshore changes in cross-shore wave energy flux and cause spatial variability in dissipation in the nearshore region. The simulations indicate that alongshore variations in wave energy caused by the shore oblique bars results in a 50–75% reduction in wave setup and a 25–40% decrease in alongshore velocity averaged across the surf zone for the conditions investigated. The results suggest that wave-driven hydrodynamics can influence nearshore sediment dynamics to create erosional zones compared to areas with shore-parallel bathymetric contours, since the morphology of shore-oblique sandbars are an important control on wave energy that impacts the beach.