SWAN predictions of waves observed in shallow water onshore of complex bathymetry

a r t i c l e i n f o SWAN model predictions, initialized with directional wave buoy observations in 550-m water depth offshore of a steep, submarine canyon, are compared with wave observations in 5.0-, 2.5-, and 1.0-m water depths. Although the model assumptions include small bottom slopes, the alongshore variations of the nearshore wave field caused by refraction over the steep canyon are predicted well over the 50 days of observations. For example, in 2.5-m water depth, the observed and predicted wave heights vary by up to a factor of 4 over about 1000 m alongshore, and wave directions vary by up to about 10°, sometimes changing from south to north of shore normal. Root-mean-square errors of the predicted wave heights, mean directions, periods, and radiation stresses (less than 0.13 m, 5°, 1 s, and 0.05 m 3 /s 2 respectively) are similar near and far from the canyon. Squared correlations between the observed and predicted wave heights usually are greater than 0.8 in all water depths. However, the correlations for mean directions and radiation stresses decrease with decreasing water depth as waves refract and become normally incident. Although mean wave properties observed in shallow water are predicted accurately, nonlinear energy transfers from near-resonant triads are not modeled well, and the observed and predicted wave energy spectra can differ significantly at frequencies greater than the spectral peak, especially for narrow-band swell. Surface gravity waves propagating across shallow coastal areas are affected by topography (shoaling, refraction, trapping, diffraction, and reflection), nonlinear wave–wave interactions (triad and quartet resonances), and dissipation (wave breaking and bottom friction). The observed evolution of non-breaking waves on narrow, bathymetrically simple shelves is predicted accurately in intermediate water depths by linear wave propagation models (In shallower nearshore waters, near-resonant spectral energy transfers (described accurately by Boussinesq models) change the shape of the wave energy (frequency) spectrum prior to Breaking wave evolution on alongshore-homogeneous bathymetry has been simulated with linear and nonlinear models by incorporating dissipation terms that are tuned with observations Apotsos et al., 2008a). Wave propagation on beaches with alongshore-inhomogeneous bathymetry is more complicated. For example, curved sand bars (Lippmann and Holman, 1990) and rip channels in the surf zone (MacMahan et al., 2006), both with alongshore length scales of order 100 m, cause alongshore variations in wave heights and directions. Large alongshore gradients in surfzone waves, setup, and circulation also can be caused …