The southern part of Thailand is heavily affected by tidal changes in the Gulf of Thailand and the Andaman Sea. This study investigates the long-term trends of diurnal and semidiurnal tides along the coasts of the Andaman Sea and the Gulf of Thailand. Daily observations of diurnal and semidiurnal tides along the coasts of the Andaman Sea and the Gulf of Thailand from 2004-2017 were obtained from the Marine Department of Thailand. Astronomical effects in the data were accounted for using a periodic regression model, while a linear regression model was applied to determine the long-term trends in highest high water (HHW), lower high water (LHW), lowest low water (LLW) and higher low water (HLW) levels for semidiurnal tides and diurnal tides. Results showed that diurnal tides at Tha Chaleap had positive trends for HHW and LLW. The results for semidiurnal tide at the Kantang and Krabi stations were positive for all water levels while trends at the Tammalang station were positive for LHW. The study revealed significantly increasing trends in water levels for diurnal and semidiurnal tides. The long-term effects could be catastrophic if proper management plans are not implemented to reduce these trends.
