Bathymetric control on the spatial distribution of wave breaking in the surf zone of a natural beach

Abstract

A non-hydrostatic wave model (SWASH) that phase-resolves the free surface and fluid motions in the water column is applied to investigate wave transformation and the spatial distribution of wave breaking over different morphological features. The model is forced using observed directional energy spectra and results are compared to wave observations collected outside the surf zone using acoustic wave sensors, and over a 100m nearshore transect using high-frequency measurements of the sea surface from a LIDAR sensor mounted on the beach dune at the Field Research Facility in Duck, NC. The model is applied to four cases with different wave conditions and bathymetry, tested for sensitivity of model parameters to these different natural conditions, and used to predict the spatial variability in wave breaking and correlation between energy dissipation and morphologic features. Model results compare very well with observations of spectral evolution outside the surf zone, and generally well with the remotely sensed observations of wave transformation inside the surf zone with R = 0.85-0.93 for Hs along the cross-shore transect. In particular the model is able to spatially resolve wave shoaling and dissipation at the shore break at the same location as observed in the LIDAR data. The results indicate that nearshore morphology has a significant effect on the spatial distribution of wave energy dissipation. Alongshore variability in bathymetry due to bars, rip channels, and larger morphological features such as the scour depression under the pier, causes large alongshore changes in cross-shore wave energy flux that influence the location and intensity of wave breaking.