The sinking mega-delta: Present and future subsidence of the Vietnamese Mekong delta

Abstract

The Vietnamese Mekong delta has world’s third largest delta plain, inhabited by nearly 18 million people and responsible for 50% of Vietnam’s total food production. The delta is impacted by decreased fluvial sediment supply, salinization, coastal erosion and global sea-level rise. On top of that, annual subsidence rates up to several centimeters per year cause the Mekong delta to rapidly lose elevation. The main objective of the research of this thesis is to understand and quantify the main natural and human-induced drivers and processes causing subsidence, and its impact on current and future surface elevation of the Vietnamese Mekong delta. A newly created elevation model of the Mekong delta reveals that the delta is lower elevated above sea level than previously concluded based on spaceborne DEMs and shows that it is more prone to relative sea-level rise than previously anticipated. Using a novel numerical model capable of simulating sediment accretion and natural consolidation the formation and evolution of the Mekong delta over the past 4000 years was simulated. This showed that present-day rates of natural compaction of the shallow Holocene deposits can reach annual rates up to several centimeters and identified natural compaction as a major subsidence driver in the coastal zone of the Mekong delta. During past decades, the Mekong delta experienced large-scale anthropogenic land-use changes as a result of increased agricultural production, population growth and urbanization. Using remote sensing analyses, land-use specific subsidence rates, were quantified for the Mekong delta. This showed that the spatial patterns of the InSAR-derived subsidence rates can largely be explained by land use-related drivers and revealed the role of human activities on subsidence in the Mekong delta. Since the nineties, groundwater exploitation has increased drastically. By creating a new delta-wide numerical model, a quantitative spatially-explicit assessment was made of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation until present. This revealed that extraction-induced subsidence in the Mekong delta has steadily increased over the past decades towards the highest sinking rates at present. Using six scenarios in which the delta follows different groundwater extraction pathways until 2100, future groundwater dynamics and consequent delta subsidence were modeled. The results revealed that when groundwater extraction is allowed to increase continuously, extraction-induced subsidence could potentially drown almost the entire Mekong delta. The present accelerating rates of subsidence in the Mekong delta, in combination with absolute sea-level rise cause the delta to rapidly lose elevation to local sea level. The delta is in a critical position due to its low elevation and it is almost inevitable that large parts of the Mekong delta will fall below sea level during this century. The best chance for the Mekong delta to sustain itself in the future is through mitigation measures to reduce groundwater use and stimulate groundwater recharge, re-enable sedimentation on the delta plain, and by implementing a smart combination of both hard and soft adaptive measures.