<p>The Vietnamese Mekong Delta (VMD) is home of 18 million people, provides enough food to cover 50% of the country’s nutritional needs and underpins the welfare of the rapidly growing population of the wider region. The longer-term future sustainability of this great delta, formed over millennia, is uncertain. The region is threatened by climate change induced eustatic sea-level rise (SLR), and by severe land loss. The latter is the result of a number of factors that are, in their majority, driven by human activities. They include dam impoundment that reduces the amount of sediment reaching and slowly building up the delta, sand mining which rapidly depletes the delta from its slowly accumulated sediment reserves and ground water extraction which enhances sediment compaction and accelerates delta subsidence.</p><p>In May 2018 we undertook a delta-scale survey to map the bathymetry of all of the main distributary channels of the VMD. Comparisons of these survey data with existing datasets from 1998 and 2018 reveal major increases of channel depth. They show that between 1998 and 2008 the VMD lost in excess of 370 million cubic meters of sediment, while the respective value for the period between 2008 and 2018 is 635 million cubic meters, suggesting an accelerating trend of sediment loss from the system.</p><p>We assume a ‘business as usual’ scenario for delta management practices and propagate delta degradation into the future, generating delta analogues for years 2028 and 2038. We combine these delta analogues with projections of SLR for the region for up to year 2098 and a number of boundary condition scenarios into a delta-scale hydraulic model. The fluvial-tidal interactions resolved in our numerical modelling simulations reveal that channel deepening is the key driver of tidal ingress into the delta plain for the next few decades. For the longer-term future (2098), the combined effects of predicted SLR and channel incision can lead to an increase of tidal ingress by 20%. This may destabilise delta bifurcations, is likely to increase bank erosion and flood risk into the future and can have sever implications for saline intrusion into the delta plains.</p>

<p>Deltas are home to 4.5% of the global population and support a range of ecosystem services that are vital to lives and livelihoods. As low-lying regions, deltas are also amongst the most vulnerable areas to the threat climate change and relative sea-level rise, which are being exacerbated by ongoing local resource exploitation. Anthropogenic activities such as riverine sand mining, construction of flood embankments, deforestation and changes of land use and hydropower dams are disrupting the natural evolution of deltaic systems, with many of the world’s large deltas now being sediment starved. This is important because changes of the sediment flux into large deltas can have implications for the evolution of the morphology of delta bifurcations and their function at routing water and sediment seaward. This can amplify flood hazard and risk for riparian communities and intensify processes such as bank erosion, presenting hazards to human lives and exacerbating land loss. The present study focuses on the Chaktomuk junction at the apex of the Mekong delta, connecting the Mekong with the Tonle Sap Lake and the downstream delta. The junction is important as it provides the connection between the Mekong and the largest freshwater lake in Southeast Asia and because of the proximity of the junction to the rapidly expanding urban centre of Phnom Penh. We present a combined 2D hydrodynamic and sediment transport model for the Chaktomuk junction, constructed and based on high-resolution bathymetric data obtained with multibeam echosounders. A series of established sediment transport equations are adopted and tested through a sensitivity analysis to identify the most appropriate sediment transport solver for the model, which is then validated against field observations. The model was forced with a series of scenario combinations including changes of water and sediment flux and rates of sand mining. Simulation runs are presented that project the future evolution of the apex of the Mekong delta, including changes in bifurcation morphology, water and sediment routing seaward through delta distributary channels and changes in water and sediment exchanges between the Mekong and the Tonle Sap. The implications of these future trajectories will be discussed in terms of the sustainability of the delta to future change.</p>