Sediment starvation is the primary factor of tidal ingress in the Mekong delta

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.

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.

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.