Swell Transformation across the Continental Shelf. Part II: Validation of a Spectral Energy Balance Equation

Abstract

State-of-the-art parameterizations of the interactions of waves with a sandy bottom are evaluated using extensive field observations of swell evolution across the North Carolina continental shelf and hindcasts performed with the spectral wave prediction model CREST. The spectral energy balance equation, including bottom friction and wave?bottom scattering source terms, was integrated numerically for selected time periods with swell-dominated conditions. Incident wave spectra at the model boundary were estimated from buoy measurements near the shelf break, assuming weak spatial variations in the offshore wave field. The observed strong and variable decay of the significant wave height across the shelf is predicted accurately with an overall scatter index of 0.15. Predicted wave directional properties at the peak frequency also agree well with observations, with a 5° root-mean-square error on the mean direction at the peak frequency and a 0.22 scatter index for the directional spread. Slight modifications are proposed for the laboratory-based empirical constants in the movable bed bottom friction source term, reducing the wave height scatter index to 0.13. A significant negative bias in the predicted directional spread (about ?20%) suggests that other wave scattering processes not included in the energy balance equation broaden the wave field near the shore. Other residual errors may be largely the result of neglected spatial variations in the offshore wave conditions and, to a lesser extent, insufficient knowledge of the sediment properties.