Hydrodynamic Impacts of Tributary Sluice Gate Operations on Salinity Intrusion in the Tien River, Vietnamese Mekong Delta

Salinity intrusion and drought pose an escalating threat to freshwater resources in the Tien River, a primary distributary of the Vietnamese Mekong Delta. Sluice gates on its branch channels are widely implemented to counteract this threat by preventing saltwater intrusion and storing freshwater in inland areas; however, their comprehensive impacts on main-river salinity remain inadequately understood. This research utilized a calibrated MIKE 11 hydrodynamic-salinity model to assess how different sluice gate operational scenarios influence the salinity intrusion pattern. Findings demonstrate that closing all tributary sluices (a maximum intervention scenario) results in a landward extension of the 4 g/L salinity isohaline by up to 2.09 km, with localized salinity increases of up to 12.6% at 40 km from the river mouth, diminishing freshwater availability. Scenario analyses revealed that closing sluices near the estuary, especially on high-discharge tributaries, precipitates the most significant adverse effects. In contrast, operating sluices in upper or middle reaches, or on low-flow tributaries, exerted minimal influence. These results highlight that uncoordinated sluice operations can unintentionally worsen salinity intrusion, compounding pressures from sea-level rise, upstream flow decline, and riverbed degradation. Consequently, strategic, spatially aware planning and adaptive management of sluice infrastructure are imperative for ensuring freshwater security in the Vietnamese Mekong Delta.