Urban development induced subsidence in deltaic environments: A case study in Hanoi, Vietnam

Hanoi has experienced rapid urbanisation over the last few decades, putting intense pressure on its natural resources, such as groundwater, but also on the local authorities to meet demand for infrastructure, housing and public amenities. Recent studies using Interferometric Synthetic Aperture Radar (InSAR) measured rates of subsidence in Hanoi documenting the evolution of the subsiding areas. These studies have primally attributed the high rates of subsidence to the increased extraction of groundwater. In this study we use Sentinel 1 InSAR data for six years between July 2015 and January 2021 to examine subsidence patterns across Hanoi and link them to the development of urban areas. We find that although groundwater extraction undoubtedly plays a significant role, there is a clear spatial and temporal link between new development and the areas of subsidence. The use of historical optical satellite imagery allows the evolution of the development to be linked to the InSAR ground motion time series. A correlation exists between subsidence and the reclamation of agricultural land, often flooded 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 newly 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. The collected rates of subsidence for over 40 locations of new development allows us to determine the rates of subsidence due to the consolidation process. This relationship enables an understanding of subsidence rate with time which has clear applications in the planning of future developments on thick superficial geological deposits.