An improved ‘Battjes’ method for predicting the probability of extreme waves

The Battjes method for predicting the 50 or 100-year design wave was developed to allow for the possibility that the highest wave in a 50 or 100 year period may occur during the second highest storm or even in lower storms. It uses the probability distribution of individual waves. It is first shown that a slightly different logical approach removes some of the problems encountered with the use of the method. It is then shown that it actually uses a different definition of return period to that used by the classic method because if two or more waves in a severe storm exceed H₅₀, then these are counted as separate events. A formula is developed which considers each storm as one event, but still takes account of the possibility of the highest wave in 50 years not coming from the most severe storm. Computation using this formula shows that it reduces H₅₀ by about 3% relative to the Battjes method.