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We describe and partly explain the change of the form of bichromatic surface waves with large amplitudes. This phenomenon was recently observed in laboratory experiments and reported by C. T. Stansberg at the Third International Symposium on Ocean Measurement and Analysis (WAVES 97) in 1997. We motivate the use of a Korteweg-de Vries-type equation; improved dispersive properties are necessary in view of the relatively short wavelengths in the experiments. A second-order expansion is shown to be quite capable of describing waves of small and moderate amplitudes. However, waves of extreme amplitude require a more sophisticated attack. Based on phase-amplitude equations of a nonlinear Schrodinger model, we are able to give an analytical description of the phenomenon which provides additional insight into the ingenious pseudo-empirical explanation of Stansberg.

In this contribution the performance is shown of a hybrid spectral-spatial implementation of the AB model for uni-directional waves above varying bottom. For irregular waves of JONSWAP-type, with peak periods of 9 and 12[s], significant wave height of 3[m], running from 30 to 15[m] depth over a 1:20 slope, comparison with scaled experimental data show reasonable (for the 9[s] wave) to good (for the 12[s] wave) results in calculation times less than 15% of the physical time. Especially, the most extreme, freaklike, waves are well simulated for both cases.