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The area of the Central North Sea is notorious for the occurrence of very high waves in certain wave trains. The short-term distribution of these wave trains includes waves which are far steeper than predicted by the Rayleigh distribution. Such waves are often termed “extreme waves” or “freak waves.” An analysis of the extreme statistical properties of these waves has been made. The analysis is based on more than 12 yr of wave records from the Mærsk Olie og Gas AS operated Gorm Field which is located in the Danish sector of the Central North Sea. From the wave recordings more than 400 freak wave candidates were found. The ratio between the extreme crest height and the significant wave height (20-min value) has been found to be about 1.8, and the ratio between extreme crest height and extreme wave height has been found to be 0.69. The latter ratio is clearly outside the range of Gaussian waves, and it is higher than the maximum value for steep nonlinear long-crested waves, thus indicating that freak waves are not of a permanent form, and probably of short-crested nature. The extreme statistical distribution is represented by a Weibull distribution with an upper bound, where the upper bound is the value for a depth-limited breaking wave. Based on the measured data, a procedure for determining the freak wave crest height with a given return period is proposed. A sensitivity analysis of the extreme value of the crest height is also made.

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A mathematical model driven by meteorological forcing and sea level differences has been established in order to describe the water exchange and vertical mixing in the Kattegat and Samsø Belt. The model has been run with a time-step of 1 day for the period April - October from 1961 to 1981. The mixing has been represented by equations from the theory of turbulence with the normally used coefficients, i.e. no calibration has been necessary. Results from earlier investigations have been used to describe the flows in the Belts and the Sound. The vertical exchange between the upper, brackish and the lower, saline water is strongly dependent on the meteorology. A small meteorological activity results in small exchange flows which implies that the oxygen supply to the lower layer becomes smaller and unfavourable biological conditions may occur. A simple calculation has been carried out on the assumption that the chemical and biological oxygen demand is a first order process with a time scale for the oxygen decay, which is halfed for each 10° increase of temperature. Thus the model describes the effect of the meteorological activity on the oxygen concentration, all other parameters kept constant.