Extreme Wind Gusts Calculated by Combining Short and Long Term Distributions

Knowledge of the maximum gust expected over a period of years is essential for off-shore structures design. Because long records of gust speed are not normally availa-ble, maximum gusts have traditionally been estimated by multiplying the maximum expected hourly or 10-minute wind speed by a gust factor. That calculation ignores the possibility that the highest gust might not occur in the hour with the highest mean wind speed. A similar problem arises in the estimation of the maximum expected in-dividual wave height. By analogy with the accepted method of calculating maximum wave heights, we demonstrate how maximum gusts can be calculated from time series of average wind speed and wind gust distributions. We used measurements from the IJmuiden meteorological mast offshore The Netherlands to find wind gust distribu-tions. The IJmuiden data is particularly useful for studying gusts because four years of measurements were made at a sampling frequency of 4 Hz. Those distributions were used to predict extreme values of gusts in a storm using methods similar to those used in wave height calculations. The resulting extreme values closely matched ex-treme values calculated directly from the measured maximum gusts in each storm. The methods described here can calculate extreme gust speeds more accurately than the methods currently in use.