Verification of Quasi-Determinism theory against Baltic Sea Data

The second formulation of Boccotti’s quasi-determinism theory is investigated. Deep water wave records collected in the Baltic Sea with AWAC (Nortek Acoustic Wave and Current profiler) are used in this study. The adequacy of Boccotti’s theory for Baltic Sea conditions is evaluated on the basis of some simulations of wave groups with a very large maximum wave height. The large waves are chosen using an abnormality index (ratio of maximum wave height to significant wave height) with a value greater than 2. Such values of this index are also used by different authors for the definition of freak waves. In order to obtain better results, JONSWAP formula is fitted to the measured spectra instead of using default JONSWAP parameters for the simulations. Verification of values of spectral parameters obtained from the autocovariance function against the same parameters calculated from smoothed and not smoothed spectra is presented. Much attention is given to spectral peak period value. The spectral width parameter, in QD theory known as narrow bandedness parameter, requires further investigation as the results obtained here did not allow the formulating of any functional relation with another – more often used – spectral width parameter. The low frequency of sampling of the data allowed only the most important aspects of the theory to be checked. A short Matlab function used in QD simulations is presented.