EXTREME WAVE HEIGHT ANALYSIS IN NATUNA SEA USING PEAK-OVER THRESHOLD METHOD
Ismail Abdul Jabbar (a), Nining Sari Ningsih (b*)
a) Study Program of Oceanography, Faculty of Earth Sciences and Technology, Bandung Institute of Technology
b) Research Group of Oceanography, Faculty of Earth Sciences and Technology, Bandung Institute of Technology
*) Jl. Ganesha 10, Bandung 40132, Indonesia
Abstract
Extreme wave height analysis has been conducted in Natuna Sea, Indonesia, using 25 years (1991-2015) Significant Wave Heights (SWHs) data from WAVEWATCH 3 (WW3) with spatial resolution of 1/8o. The Natuna Sea is geographically connected to the South China Sea (SCS) and is often crossed by massive storms (cyclones) with wind speeds greater than 30 m/s. These storms may have contributed to the existences of high waves in the SCS, which propagate as swell waves to the Natuna sea and could led to extreme waves in this region. Extreme value analysis has been done by classifying the extreme events of SWHs using Peak-Over Threshold (POT) method with a fixed threshold level at quantile 0.93 and a minimum time separation of 48 hours between two successive extreme events. Furthermore, the Generalized Pareto Distribution (GPD) has been applied to estimate return values of the extreme SWHs for several return periods. The shape and scale parameters of the GPD have been estimated by Maximum Likelihood Estimation (MLE) method. The characteristics of extreme SWHs in the Natuna Sea have been explained by maximum and seasonal distribution plots. Maximum value of extreme SWHs in the SCS can reach 13 m and around 3-5 m in the Natuna Sea. The seasonal distributions of extreme waves indicate that the occurrences of extreme waves in the SCS during Northeasterly Winter Monsoon (NWM) are higher than those during Southwesterly Summer Monsoon (SSM) although the tropical cyclones in the SCS are more likely to happen during SSM. The seasonal mean and maxima of extreme SWHs in the Natuna Sea are also high during the NWM and low during the SSM. To examine effects of swell waves from the SCS and also the future extreme waves in the Natuna Sea, we have analysed the characteristics and calculated the return values of extreme waves in front and behind of Natuna Island, which the former faces directly to the SCS. There are 172 (331) extreme waves happen from 1991-2015 in the front (behind) of Natuna Island and mainly from northeast (southwest). Most of them are around 2-3.5 (0.5-1.5) m with mean period of 7-9 (3-5) s. Moreover, the return values in the front (behind) of the Natuna Island show that there are possibilities of extreme waves with values 4.70, 4.87, and 4.96 (2.08, 2.20, and 2.27) m for return periods of 25, 50, and 75-year, successively.
Keywords: Natuna Sea; Peak-Over Threshold; Generalized Pareto Distribution; extreme waves; return values
Topic: Coastal Degradation and Coastal Hazard