|Paper title||Subsidence in Hanoi, Vietnam; is it all due to groundwater abstraction?|
|Form of presentation||Poster|
Hanoi Province is located in the northern part of Vietnam, within the Red River delta plain, the city sits on unconsolidated Quaternary sediments of fluvial and marine origin which are 50-90m thick, these in turn rest on older Neogene deposits. Hanoi city is the capital and second largest city of Vietnam with 7.4 million inhabitants, however the population is projected to reach 9–9.2 million by 2030 and approximately 10.8 million by 2050 (Kubota et al., 2017). A recent study on land cover changes in Hanoi highlighted that between 1975 and 2020 artificial surfaces have increased by 15.5% while forests have decreased by 26.7%. As a result of this rapid urbanisation causing massive pressures on resources and the environment, the government of Vietnam officially presented the Hanoi Master Plan 2030 in July 2011. The target of the master plan is to develop Hanoi as a sustainable and resilient city and as such identified sites for urban expansion in satellite cities outside of the current city limits.
Groundwater extraction in Hanoi has long been recognised as the principal water source for the city and the negative effects of its rapid urban growth on the groundwater system have been identified early (Trafford et al, 1996). In more recent years, several studies of ground motion using Interferometric Synthetic Aperture Radar (InSAR) have measured rates of subsidence in Hanoi and, via the use of successive satellite sensors have documented the evolution of the subsiding areas. These studies have mainly attributed the high rates of subsidence to the increased extraction of groundwater.
In this study we use Sentinel 1 InSAR data for the last six years to examine subsidence patterns and link them to urban development. We find that although groundwater extraction undoubtedly plays a significant role, there is a clear spatial and temporal link to new development for all the observed subsiding areas close to Hanoi city itself. The use of historical optical satellite imagery allows the evolution of the development to be linked to the ground motion time series. We observe a correlation between the subsidence and the reclamation of agricultural land, often rice fields, for building via the dumping of aggregate to create dry, raised areas on which to build. We illustrate our findings with examples where developed areas are co-incident with areas of subsidence, we show the relationships between the stages of the ground loading and the rate of the resulting subsidence. Ultimately, we extract rates of motion for each year following ground loading. This has been completed for a sufficient number of locations allowing the construction of curves to define how the subsidence rate declines as the consolidation process occurs. This relationship therefore enables an understanding of subsidence rate with time which has clear applications in the planning of future developments on thick superficial geological deposits.