Non-hydrostatic modelling of nearshore wave transformation and breaking over varying beach morphology

Abstract

A non-hydrostatic phase-resolving wave model is used to investigate wave transformation and energy dissipation due to wave breaking over different small and large scale morphological features. Simulations of waves in the nearshore region of a sandy beach at the Field Research Facility (FRF) in Duck, North Carolina (NC) were conducted to evaluate the Simulating WAves til SHore (SWASH) model ability to predict wave transformation and breaking in a high resolution 3-dimensional configuration for four moderate wave events. The model is forced with directional wave spectra from an acoustic wave sensor at the boundary of the computational domain, and validated with three acoustic sensors and water surface elevation observations from a LIDAR scanner mounted on the beach dune. Results indicate the model is able to accurately predict significant wave heights, spectral evolution, and energy dissipation due to wave breaking in the nearshore. This research shows that small-scale morphological features including shore-parallel bars, rip channels, and a large-scale feature corresponding to scour depression under the pier have a strong impact on the rate of wave energy dissipation and location of breaking in the surf zone. The validated model is applied to four additional field sites in NC to determine the influence shore-oblique bars have on wave transformation and the alongshore variability in wave energy. Two sites with shore-oblique sandbars, and two beaches with straight contours were simulated with SWASH. The results indicate that higher alongshore variability in depth due to the shore-oblique bars causes large alongshore changes in cross-shore wave energy flux. This work shows that shore-oblique bars, known to be spatially correlated with areas of high shoreline change rates, introduce high alongshore variability in nearshore wave energy that could be the primary driver of changes in sediment transport rates and induce localized erosional hotspots on the shoreline.