Analysis of extreme sea states under the impact of typhoon in different periods: A nested stochastic compound distribution applied in the South China Sea

Abstract

• A nested stochastic compound distribution (NSCD) is introduced to analyze the extreme sea states in typhoon-prone areas. • The applicability of the NSCD model in the South China Sea is discussed. • The NSCD model is found to be a better model to consider the impact of typhoon in different time periods. • It is shown that the NSCD model has a broader scope of application than existing models. Extreme sea conditions are often caused by typhoons that occur obviously in season. How to calculate the design values of marine environmental elements based on statistical characteristic of typhoon in different periods is important. A nested stochastic compound distribution (NSCD) model is proposed based on the stochastic process theory. During a typhoon occurrence, the distributions of the marine environmental elements such as wind velocity (WV) and storm surge (SS) at any moment are described in continuous stochastic processes, and the distribution of typhoon occurrence frequency (TOF) during any period is described by a discrete stochastic process. Based on the measured data in the South China Sea (SCS), the applicability of different marginal distribution functions and Copula functions in the SCS are further discussed, and the specific form of the NSCD model suitable for the SCS is obtained. The Design values of WV and SS for different joint return periods and coincidence return periods are calculated according to the NSCD and the compound extreme value distribution (CEVD). The effects of treating TOF as a stochastic process and a random variable on the design values of the two marine environmental elements are discussed. Results show that the NSCD model contains the traditional CEVD model, which is a form of the NSCD model in a specific time situation. The design values of WV and SS obtained by the NSCD model can reflect the impact of TOF on marine environmental elements. Moreover, the probability of simultaneous occurrence of extreme WV and extreme SS is higher over longer return periods under the influence of typhoons. The NSCD model can therefore be used to make joint probability predictions for a wide range of extreme sea conditions in different time periods according to engineering needs.