Water Level Lowering and Increased Tidal Influence in the Mekong Delta driven by Human-Induced Riverbed Incision

River deltas provide ecosystem services that are vital to the world's population, supporting both lives and livelihoods. However, these low-lying areas face heightened vulnerability to the effects of climate change and increasing sea levels; a vulnerability further intensified by local resource exploitation. In recent decades, population growth, urbanization, and economic development have cause a surge in the demand for natural sand and hydropower. Sand mining across lowland rivers and deltas alongside river impoundment in upstream catchments is resulting in the rapid incision of riverbeds. These cumulative impacts, coupled with alterations in input hydrological conditions and rising sea levels at the delta front, have the potential to cause considerable disruptions in the flow hydraulics at the delta scale and alter related water-level dynamics many kilometres from the coastal zone. Despite numerous studies into anthropogenic influence in delta evolution, a significant knowledge gap persists regarding how the combination of stressors that drive river bed lowering influences alterations in water level across lowland rivers and deltas.

In this paper, we utilize long-term observation data to examine the relationships between water level and water discharge in the Vietnam Mekong Delta. Assessing these relationships across both spatial and temporal dimensions allows us to determine the effects of riverbed lowering from 1998 to 2018 while identifying the main hydrological and morphological drivers and impacts of these changes. In addition, we employ a 1D hydraulic modelling routine to assess the projected progression of riverbed degradation in the future, and assess the likely impacts of the water level regimes in the entire lower Mekong River and Delta, extending from Kratie to coastal Vietnam. Our results suggest that the delta's historical river bed lowering of approximately 3.06 m from 1998 to 2018 has led to simultaneous declines in mean water levels of approximately 0.65 m and an increase in the mean tidal range by approximately 0.19 m. The reduction in water level is more pronounced in the landward direction, whereas the increased tidal range is more prominent in the seaward direction. Under anticipated future scenarios, where the riverbed lowering is projected to average around 5.92 m compared to the conditions in 1998, there could be declines in mean water level of approximately 1.27 m, while, the maximum water level reduction upstream may reach 3.73 m. Simultaneously, the mean tidal range is expected to increase by approximately 0.46 m, with the maximum rise potentially reaching more than 1 m in the downstream delta region.

There are very significant implications of these trends which include a potential reduction in the level of flooding in landward parts of the delta but very significant consequences associated with tidal flood hazard seaward, as well as associated impacts such as the disconnection of channels from floodplains, decreased efficiency of infrastructure and irrigation works, an elevated risk of storm surge hazards, as well as the increased likelihood of water salinization.