Land subsidence is leading to the loss of land and fresh water resources in the southern Mekong Delta. This development poses enormous challenges for the local population. While the salinisation of surface waters and groundwater is increasingly limiting the usable water resources, the southern Mekong Delta is also confronted with coastal erosion and land subsidence, which, intensified by rising sea levels, is seriously threatening the livelihoods of more than 20 million people.

The first measurement data from the land subsidence observatory of the CLIENT II project ViWaT-Engineering now offer a unique insight into the process dynamics of land subsidence and can thus make an important contribution to clearly defining the causes of land subsidence and, based on this, to developing possible countermeasures.

Heavy groundwater extraction was considered one of the main causes of land subsidence – this assumption has now been clearly proven with the help of the ViWaT-Engineering research project’s measuring system installed in the Mekong Delta. For a depth-differentiated measurement, a 164 m deep borehole was drilled at the pilot plant and a measuring chain with eight digital extensometers (expansion measuring devices) was installed. The measuring chain hangs freely in a measuring tube, which is divided into separately movable sections by telescope segments. In various depths magnetic rings are attached to the measuring tube. The smallest displacements underground in the range of just 0.01 mm are transferred to the movable measuring tube sections and their magnetic rings. The relative movement between the extensometers and the magnet rings is digitally recorded in 10-minute intervals and, like the data of three groundwater measuring points, a weather station and various soil moisture sensors, automatically transmitted to the servers of KIT and the Vietnamese partner authorities.

The data collected since December 2022 shows a clear correlation between land subsidence and sinking groundwater levels as well as temporary stagnation of land subsidence during the rainy season, when groundwater pressure levels temporarily rise. Further investigations are planned for the coming years.

The initial findings from the measurements contribute significantly to the development of possible countermeasures, which must focus in particular on avoiding further groundwater lowering. The development of alternative water resources plays a particularly important role in this undertaking. Furthermore, the development of coastal protection measures, the implementation of geochemical and hydro chemical investigations and the transfer of knowledge to local project partners have been a key focus of ViWaT-Engineering since the start. Although the project ended in December 2022, four PhD students at KIT are currently continuing their research on the issues arising from the project.

Further information on the ViWaT-Engineering research project can be found at the project’s website and on the project page.