NONLINEAR TRANSIENT WATER-WAVES - PART I - A NUMERICAL-METHOD OF COMPUTATION WITH COMPARISONS TO 2-D LABORATORY DATA

This paper concerns the description of extreme two-dimensional water w aves in an unsteady or irregular sea state. An existing wave propagati on model (Fenton, J.D. and Rienecker, M.M., 7th Int. Conf. Coastal Eng ng, Sydney, Australia, Vol. I, pp.50-60, 1980) is re-considered and a new methodology proposed so that direct comparisons can be made with r ecent laboratory measurements (Baldock, T.E., Swan, C. and Taylor, P.H ., Phil. Trans. Key. Sec. Lend., Ser. A, 1996, 354, 649-676). These co mparisons confirm that, in respect of both the water surface elevation and the underlying water particle kinematics, the numerical calculati ons are in excellent agreement with the laboratory data. Furthermore, since the numerical calculations are merely based upon an appropriate description of the underlying wave spectra, no prior knowledge of the extreme water surface elevations is required. Calculations confirm tha t the time-stepping procedure is able to describe the evolution of an extreme wave event and that the transfers of energy within the frequen cy/wave-number domain can be effectively modelled. This is shown to be essential if the maximum water particle velocities (appropriate to dr ag force calculations) are to be adequately predicted. Finally, compar isons with existing design solutions highlight important discrepancies and confirm that an accurate description of an extreme wave event mus t incorporate both the nonlinearity and the irregularity of the sea st ate. Although the proposed model is, at present, only compared to labo ratory data, it has clear design applications. (C) 1998 Elsevier Scien ce Limited. All rights reserved.