[1] The performance of the spectral wind wave model SWAN in tidal inlet seas was assessed on the basis of extensive wave measurements conducted in the Amelander Zeegat tidal inlet and the Dutch Eastern Wadden Sea, as well as relevant data from other inlets, lakes, estuaries and beaches. We found that the 2006 default SWAN model (version 40.51), the starting point of the investigation, performed reasonably well for measured storm conditions, but three aspects required further attention. First, over the near-horizontal tidal flats, the computed ratio of integral wave height over water depth showed an apparent upper limit using the default depth-limited wave breaking formulation and breaker parameter, resulting in an underprediction of wave heights. This problem has been largely solved using a new breaker formulation. The second aspect concerns wave-current interaction, specifically the wave age effect on waves generated in ambient current, and a proposed enhanced dissipation in negative current gradients. Third, the variance density of lower-frequency wind waves from the North Sea penetrating through the inlets into the Wadden Sea was underpredicted. This was improved by reducing the bottom friction dissipation relative to that of the default model. After a combined calibration, these improvements have resulted in a relative bias reduction in H m0 from À3% to À1%, in T mÀ1,0 from À7% to À3%, and in T m01 from À6% to À2%, and consistent reductions in scatter, compared to the 2006 default model.

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Acoustic Doppler current profilers (ADCPs) can measure orbital velocities induced by surface gravity waves, yet the ADCP estimates of these velocities are subject to a relatively high noise level. The present paper introduces a linear filtration technique to significantly reduce the influence of noise and turbulence from energy spectra of combined orbital velocity measurements. Data were collected in 13-m-deep water with a 1.2-MHz ADCP sampling in mode 12, where a collocated wave buoy was used for verification. The surface elevation spectra derived from the filtrated and nonfiltrated measurements were compared with corresponding wave buoy spectra. In the frequency range between 0.12 and 0.5 Hz, ADCP-and wavebuoy-derived spectral estimates matched very well, even without applying the filtration technique. At frequencies below 0.12 Hz, the ADCP-derived surface elevation spectra are biased, caused by a depthvarying excess of spectral energy density in the measured orbital velocities, peaking at middepth. Internal waves may provide an explanation for the energy excess, as the experiment was conducted in the region of influence of the Rhine freshwater plume. Alternatively, infragravity waves may be the cause of the depth variation of low-frequency spectral energy density.