Effect of monsoonal rainfall and tides on salinity intrusion and mixing dynamics in a macrotidal estuary

Studies on the temporal and spatial variations of estuarine hydrodynamics, particularly those focusing on the combination of extreme and transition periods in the estuary with seasonal fluctuation of discharge, are rarely reported. Due to its importance, this study investigates the effect of rainfall and tide on the salinity intrusion and mixing conditions in the monsoon-affected macrotidal Tanintharyi River estuary (TRE), Myanmar, during the period of 2017–2019. The maximum salinity intrusion and partially mixed to well-mixed conditions were found during the neap-spring tidal cycles of dry seasons. The minimum salinity intrusion, along with partially mixed to stratified conditions, was found during spring to neap tidal cycles of wet seasons. Further, the transitional periods before and after the wet season have different salinity intrusion and mixing conditions based on the different discharge fluctuations in the past months. The salinity intrusion and mixing conditions in the TRE were largely influenced by the lack of rainfall during the dry season, whereas the combined effect of rainfall and the tidal range dominated during the wet season. Finally, the salinity interface gradient (SIG₁₀) index was found to be the convenient index to examine the mixing condition of a large area compared to the traditional indices because of the minimum data requirement with easiness of calculation by using the reference figures from the published articles.