Statistical Modeling for Sea Level Change in Thailand

Abstract

Increasing sea levels can change the shape of coastlines, contribute to coastal erosion, and lead to flooding and increased underground salt-water intrusion. The water levels of coastal areas are important because the effects of sea level change can be devastating to vulnerable coastal and marine areas and can impact the function and structure of their ecosystems. This study aimed to investigate variations and longterm changes in the frequency of distribution of water levels along the Gulf of Thailand and the Andaman Sea, and to examine the secular trends in tidal levels, tidal ranges, mean sea level (MSL) and four main tidal constituents. The water level data were obtained from the Marine Department and include 14 stations, namely LaemNgop, ThaChalaep, Rayong, BangPakong, SamutSakhon, SamutSongkram, BanLaem, LangSuan, Sichon, PakPanang, Narathiwat, Krabi, Kantang and Tammalang. The data span for each station, as well as the length of available water level records, was different. This research was divided into two studies. In the first study, six stations were included in the analysis, namely LaemNgop, ThaChalaep, Rayong, Krabi, Kantang and Tammalang, which had tidal characteristics, diurnal and semi-diurnal tides along the coast of Thailand. The distinct water levels for these six stations spaned 14 years. The secular trends in tidal levels were analysed using periodic and linear regression. The results showed that all water levels at ThaChalaep, Kantrang and Krabi stations had increasing trends, contrasting to water levels in LaemNgop and Rayong stations, which had decreasing trends. In the second study, hourly data from 14 tidal gauge stations included three types of tides, namely diurnal, semi-diurnal and mixed-diurnal tides along the coast of Thailand over different periods were analysed. The variations, long-term changes in the frequency of distribution of water levels, secular trends in tidal level, tidal range and MSL, as well as the four main tidal constituents (lunisolar declination diurnal tide (K1), principal lunar declination diurnal tide (O1), principal lunar semidiurnal tide (M2) and principal solar diurnal tide (S2)), were analysed using percentile, harmonic and linear regression methods. The result from percentile analysis revealed that the height of water levels in the upper Gulf of Thailand was higher and varied more than in the lower Gulf of Thailand. In contrast, the Andaman Sea had a more stable water level than in the Gulf of Thailand. Most water levels in the upper Gulf of Thailand showed significant long-term changes, which occurred due to MSL rise and long-term trends in the tidal component and non-tidal residuals. The findings from the harmonic analysis confirmed the change in tidal components and secular trends in all tidal levels. These trends were caused by changes in the four main tidal constituents (M2, S2, O1, and K1), with the exception of Sichon, Tammalang, and Kabi stations, which showed no significant trend in both amplitude and phase. The overall finding indicated that water level change along the coast of Thailand occurred due to MSL, astronomical tides and non-tide residual. The changes were more prominent in the upper Gulf of Thailand compared to the lower Gulf and the Andaman Sea.