Pioneering large-scale study of land subsidence in Vietnam by Utrecht researcher

Published: 14 February 2019

Sea level rise is not the only threat facing river deltas. Physical geographer Philip Minderhoud of Utrecht University and research institute Deltares investigated land subsidence in the Mekong delta and was successful in bringing the issue to the attention of the Vietnamese government. He will defend his doctorate in the Academy Building of Utrecht University on 15 February.

Vietnam made the transition to a market economy in the late 1980s. That led to higher agricultural production, an increasing population and more urbanisation. And more demand for groundwater as a result. The extraction of groundwater contributes to land subsidence, writes Minderhoud in his thesis. ‘And the subsurface is soft here, as well. Economic development and the associated infrastructure generate additional pressure that leads to land subsidence. So the difference between the sea level and the land level increases even faster. The delta starts to sink below the waves, as it were. Salt water penetrates further and further inland, causing the salinisation of the delta.’

Combination of hard and soft interventions

This is the world’s third-largest delta. It has a population of about 18 million. Vietnam is the world’s second-largest exporter of rice, most of which comes from the Mekong Delta. In his thesis, Minderhoud shows that, if policy remains unchanged, land subsidence could have far-reaching humanitarian and economic consequences. He argues for a combination of ‘hard’ and ‘soft’ interventions. Hard, for example, means building dikes and polders around economically and socially important locations such as cities. Soft measures include things like encouraging natural siltation, which can limit the need for dike construction. Other possible measures include pumping surface water back into the subsurface to reduce the rate of subsidence. Minderhoud emphasises that it is not economically feasible to build dikes everywhere.

Land subsidence is on the agenda

Researchers at Stanford University (U.S.) presented an initial exploratory analysis of subsidence in the Mekong Delta in 2014 but it did not result in any concrete measures. Inspired by the American study, Minderhoud mapped out the subsurface and built a 3D groundwater model that can calculate land subsidence for the entire Mekong Delta. The model is based on 25 years of geological and geotechnical data, and groundwater levels, and it has been calibrated using satellite data. Never before has such a large delta been investigated in this way. The model shows that groundwater extraction is a major driver of subsidence in the Mekong Delta. An article by Minderhoud on this subject was published in 2017 and, in part because of his research, subsidence is now firmly on the agenda of the Vietnamese government. That same year, the government adopted a resolution to combat the overexploitation of groundwater.

Model for other sinking deltas

Land subsidence is a slow process and it often goes unnoticed until the damage becomes clearly apparent. Because everything subsides together, there is almost no visible impact on the landscape. The Mekong Delta is one of the many deltas to suffer from land subsidence. Minderhoud’s research can serve as a model for other deltas.

His thesis was written as part of the ‘Rise and Fall‘ research project which receives financial support from the Netherlands Organisation for Scientific Research (NWO) and which is co-financed by Deltares and TNO. The research contributes to the Utrecht Pathways to Sustainability programme ‘Water, Climate & Future Deltas’.