R&D highlights edition 2019

PROJECTS Water and subsoil resources 105°E 106°E 11°N 10°N 10°N 105°E 106°E 11°N 10°N 9°N 10 15 2.0 5 0.5 1.0 1.0 1.0 1.0 1.5 0.5 0.5 1.0 1.5 2.5 1.5 2.0 2.5 >4.0 0.5 Modelled subsidence rates for 2015 Modelled cumulative subsidence since 1991 0.5 20 25 30 25 30 20 15 10 20 25 15 10 15 10 35 >50 cm/yr 0 2 >4 centimeter 0 25 >50 34 T he Mekong river delta is the world’s third-largest delta, and around 18 million people call it home. The Mekong delta is very low-lying and, on average, less than 1 metre above sea level, making it very vulnerable to sea level rise and land subsidence. GROUNDWATER EXTRACTION CAUSES MEKONG DELTA TO SINK INTO THE SEA A 3D hydrogeological model of the entire Mekong delta was developed to calculate groundwater flows and land subsidence caused by groundwater extraction. We used the model to determine the acceleration of land subsidence rates over recent decades following the increase in groundwater In the late 1980s, Vietnam made the switch to a market economy. The transition led to increased agricultural production, population growth and urbanisation, all of which heightened the demand for groundwater. However, increased groundwater extraction can accelerate land subsidence and therefore the rate of relative sea level rise. Furthermore, because land subsidence is a slow process that is difficult to measure, it went largely unnoticed in the Mekong delta. The Rise and Fall research project, an alliance between Deltares and Utrecht University, TNO and Vietnamese partners, was established to study and develop solutions for subsidence and salinization in the Mekong delta. As part of this project, we developed a novel 3D hydrogeological model of the entire Mekong delta to calculate groundwater flows and land subsidence caused by groundwater extraction. We used the model to determine the acceleration of land subsidence rates over recent decades following the increase in groundwater extraction, to rates that are up to an order of magnitude larger than rates of sea level rise driven by climate change. Although groundwater extraction is not the only reason the Mekong delta is sinking fast (other reasons are the natural compaction of soft shallow sediments, physical loading by infrastructure and the artificial lowering of the surface water table), it is the largest anthropogenic driver. The 3D numerical model was then used for projections of future land subsidence based on different extraction pathways in order to test the potential effects of mitigation policies aimed at reducing groundwater extraction and therefore subsidence in the future. The results showed that reducing groundwater extraction could slow down subsidence in the future to a major degree. This research project resulted in several high-impact publications in scientific journals and it has been covered by national and international media (such as Trouw, VTV4 Global TV, Voice of America). Furthermore, in collaboration with the Dutch Embassy in Hanoi, the results were presented to the central Vietnamese government, raising awareness and contributing towards solutions to safeguard the sustainable future of the Mekong delta. This research was part of Philip Minderhoud’s PhD thesis: “The sinking mega-delta. Present and future subsidence of the Vietnamese Mekong delta”, which was defended in February 2019. The Rise and Fall research project is part of the Urbanising Deltas of the World programme funded by the Netherlands Organisation for Scientific Research (NWO), Deltares, and TNO- Geological Survey of the Netherlands. Contact: Philip Minderhoud, Philip.Minderhoud@deltares.nl , t +31 (0)6 1967 6737 Further reading : http://iopscience.iop.org/article/10.1088/1748-9326/aa7146/meta Modelled cumulative subsidence due to groundwater extraction from1991 to 2016. B) Modelled groundwater extraction-induced annual subsidence rates for 2015 (Minderhoud et al., 2017, ERL) Changes and consequences inmodern urbanising deltas (Minderhoud, 2019, PhD thesis) Evidence of subsidence in the Mekong delta. The surface level has dropped about 30 centimetres. © Philip Minderhoud

RkJQdWJsaXNoZXIy Mjc4NjU=