Spread parameter of extreme wave height distribution for performance-based design of maritime structures

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.