Seasonal Variability of Surface Currents in the Kawal Estuary, Bintan Island, Indonesia: A Hydrodynamic Modelling Approach

Background: From a social point of view, the water area around the mouth of the Kawal river, Bintan regency, is a tourism area, settlement, fisherman harbour, and seagrass conservation area that affects the sustainability of coastal activities and conditions such as abrasion, sedimentation and changes in coastline. Current patterns and wind speeds around the region are strongly influenced by winds blowing from the dominant monsoon system in Southeast Asian waters. Aims: This study analyses the seasonal hydrodynamic characteristics, particularly surface current patterns, in the Kawal Estuary, eastern Bintan Island, Indonesia, a monsoon-dominated area intensively utilised for tourism, settlements, seagrass conservation, and fisheries activities. Study Design: A numerical hydrodynamic modelling approach was employed to simulate and evaluate seasonal variations in the magnitude and direction of surface currents over a one-year period. Place and Duration of Study: The study was conducted in the Kawal estuary, Bintan Island, Indonesia, covering a one-year simulation period from December 2020 to November 2021. Methodology: Hydrodynamic simulations were performed using the MIKE 21/3 Flexible Mesh integrated flow model. Seasonal surface current characteristics were examined across representative tidal conditions, including flood and ebb phases under spring and neap tides. Model validation was conducted using tidal data and in situ current measurements obtained from a Recording Current Meter deployed in March 2021. Surface current magnitude and direction were analysed at four representative locations within and around the estuary. Results: The modelling results show good agreement with observations, achieving validation accuracies of 81.69% for tidal data and 85.04% for field current measurements. Surface current patterns exhibit pronounced spatial and seasonal variability, with predominantly northeastward flow at the northern and southern locations, while the frontal area displays greater directional variability. Two dominant flow patterns were identified, characterised by southwest–south and northeastward directions associated with tidal reversals. Peak surface current velocities consistently occur during the flood tide phase. Conclusion: The Kawal Estuary exhibits complex seasonal surface current dynamics controlled by monsoon forcing, tidal variability, and estuarine geometry. These findings provide essential baseline information for understanding sediment transport pathways, supporting coastal management and navigation planning, and enhancing the sustainability of estuarine ecosystems in monsoon-influenced tropical regions.