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A new parameter is proposed to characterize the tail spreading performance of extremal distribution functions as defined by the ratio of the 50-year return wave height to the 10-year height, and it is called the spread parameter. Extreme data sets of annual maximum wind speed and peaks-over-threshold significant wave heights have been analyzed for best-fitting distribution functions, for which the spread parameter is evaluated. The observed values of the spread parameter are in good agreement with the predicted values based on the coefficient of variation of an extreme data sample. Formulas are given to derive the scale and location parameters from a given value of the spread parameter. Some typical values of the spread parameter are presented for promotion of performance-based design of maritime structures in the world. DOI: 10.1061/(ASCE)0733-950X(2004)130:1(29) CE Database subject headings: Statistical analysis; Distribution functions; Parameters; Standard deviation; Design waves; Wave height; Offshore structures.

A procedure for predicting the significant height and period of wind waves generated in a uniform fetch in deep water is presented in the closed-form expressions based on Wilson's formulas, together with relevant design diagrams. Although Wilson's formulas are relatively unknown in the United States, they are quite reliable. The relationship between the nondimensional minimum duration and the nondimensional fetch length is empirically expressed in a power law, which enables the analyst to judge whether the wave growth is limited by the wind duration or the fetch length. The correlation between the significant wave height and period also indicates the period approximately proportional to the 2/3 power of the wave height. CE Database keywords: Wind waves; Wave height; Prediction.