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CURRENTS
The hydrodynamics of the Mekong Delta is controlled by a complex interaction among upstream discharge from the Mekong River, tidal forcing from the East Sea and the Gulf of Thailand, and the dense network of distributaries and canals. The system exhibits a strongly seasonal, bidirectional flow pattern. During the flood season, fluvial discharge dominates, generating a net seaward flow and enhanced freshwater outflux. In contrast, during the dry season, reduced river discharge allows tidal forcing to penetrate further inland, leading to pronounced tidal asymmetry and widespread flow reversal within channels and floodplains. Flow velocities are generally low (typically <1 m/s in natural channels), but exhibit high temporal variability driven by tidal cycles, river discharge, and wind forcing.
Hydrodynamically, the flow regime plays a central role in sediment transport and redistribution, thereby governing the balance between accretion and erosion. Under pre-disturbance conditions, high sediment loads from upstream sustained efficient downstream transport, promoting deposition at river mouths and along the coast, which supported delta progradation. However, the substantial decline in sediment supply in recent decades has fundamentally altered this balance. The present flow regime lacks sufficient sediment load to counteract removal processes driven by waves and alongshore currents, resulting in a net sediment deficit and widespread coastal erosion.
In estuarine and nearshore zones, river flow interacts nonlinearly with tides and waves, generating complex current systems, including tidal jets, alongshore currents, and rip currents. These processes enhance both cross-shore and longshore sediment transport. During periods of strong monsoonal winds, particularly the Northeast monsoon, wind-driven currents reinforce alongshore transport, redistributing sediments over large spatial scales and contributing to the observed heterogeneity in shoreline change, with alternating zones of erosion and accretion.
Anthropogenic modifications have further altered the natural flow regime. The expansion of hydraulic infrastructure—including dike systems, sluice gates, and canal networks—has disrupted natural flow connectivity and sediment pathways. Water regulation for agriculture and aquaculture modifies discharge distribution, reduces overbank sediment deposition, and can locally intensify flow velocities, thereby increasing the susceptibility of riverbanks and canal margins to erosion.
Moreover, the combined effects of relative sea-level rise (driven by both eustatic rise and land subsidence) have increased water depths across the delta plain, altering hydrodynamic conditions. Greater depths reduce bottom friction and allow tidal energy to propagate further inland, enhancing tidal prism and salinity intrusion. These changes modify sediment transport pathways and deposition patterns, generally reducing sediment retention within the delta. As a result, the Mekong Delta is increasingly transitioning from a sediment-accumulating to a sediment-deficient system, with significant implications for long-term geomorphological stability and coastal resilience.