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A third-generation numerical wave model to compute random, short-crested waves in coastal regions with shallow water and ambient currents (Simulating Waves Nearshore (SWAN)) has been developed, implemented, and validated. The model is based on a Eulerian formulation of the discrete spectral balance of action density that accounts for refractive propagation over arbitrary bathymetry and current fields. It is driven by boundary conditions and local winds. As in other third-generation wave models, the processes of wind generation, whitecapping, quadruplet wave-wave interactions, and bottom dissipation are represented explicitly. In SWAN, triad wave-wave interactions and depth-induced wave breaking are added. In contrast to other third-generation wave models, the numerical propagation scheme is implicit, which implies that the computations are more economic in shallow water. The model results agree well with analytical solutions, laboratory observations, and (generalized) field observations.

A third-generation spectral wave model (Simulating Waves Nearshore (SWAN)) for small-scale, coastal regions with shallow water, (barrier) islands, tidal flats, local wind, and ambient currents is verified in stationary mode with measurements in five real field cases. These verification cases represent an increasing complexity in two- dimensional bathymetry and added presence of currents. In the most complex of these cases, the waves propagate through a tidal gap between two barrier islands into a bathymetry of channels and shoals with tidal currents where the waves are regenerated by a local wind. The wave fields were highly variable with up to 3 orders of magnitude difference in energy scale in individual cases. The model accounts for shoaling, refraction, generation by wind, whitecapping, triad and quadruplet wave-wave interactions, and bottom and depth-induced wave breaking. The effect of alternative formulations of these processes is shown. In all cases a relatively large number of wave observations is available, including observations of wave directions. The average rms error in the computed significant wave height and mean wave period is 0.30 m and 0.7 s, respectively, which is 10% of the incident values for both.

Swell enhances wind-induced wave growth in the third-generation wave models of the WAM family (e.g., WAM, SWAN, TOMAWAC). This is contrary to observations in laboratory conditions where swell reduces wind-induced wave growth. To improve the performance of these models, it is hypothesized here that high-frequency whitecapping is increased by surface-straining near the crests of low-frequency waves. The pulse-based parameterization of whitecapping in these models is accordingly scaled at every frequency with the ratio of high-frequency steepness over total wave steepness (in an experimental version of SWAN). A generic laboratory experiment which illustrates the phenomenon can now be reproduced. The growth of young wind sea in idealized conditions is not affected. Moreover, the spectral shape (both in frequency and direction) improves. However, in the one field case that is considered here, the effects are marginal.