NEAP-SPRING TIDAL AND SEASONAL VARIATIONS IN SALINITY INTRUSION AND MIXING CONDITION AT THE TROPICAL MACROTIDAL TANINTHARYI RIVER ESTUARY

Estuaries in developing countries have a strong impact on the livelihood of local people and the economic growth of the country through small-scale fisheries and farming. However, these estuaries are currently facing with a major problem of decline in fish stock due to the over-exploitation of fish resources. Additionally, the saltwater intrusion in the estuary will affect the freshwater supply and agriculture. A large amount of saltwater intrusion also causes tidal flooding and resulting in the coastal population homeless. The recent trend of changes in rainfall patterns associated with climate change can exacerbate the above issues. A deep understanding of the hydrodynamic processes in these estuaries is required for the better planning and management of their aquatic environment. Hence, the seasonal and tidal (neap-spring) variation of salinity intrusion and mixing conditions in the Tanintharyi River estuary (TRE), Myanmar was studied from 2017-2019. The study area was influenced by the monsoon-generated high river discharge from the Tanintharyi river (90% during the wet season) and high tidal ranges from the Andaman Sea. The results reveal that the salinity intrusion in the TRE was maximum during dry periods (>35km) and minimum (0.6-16km) during peak monsoon. The salinity intrusion during pre-monsoon and post-monsoon exhibits 17.4 km and 28.4 km, respectively. From a neap-spring tidal perspective, the mixing conditions in the TRE varied from partially mixed to well-mixed conditions in dry periods and from stratified to partially mixed conditions in peak monsoon. Therefore, the seasonal changes in rainfall patterns are a major influencing factor of salinity intrusion, and the combined effects of rainfall and tidal forces were responsible for mixing in the TRE. It was concluded that the narrower duration of monsoon periods with increased rainfall intensity patterns and strong tidal forces are responsible for the hydrodynamic changes in estuaries of Southeast Asian regions.