Sinking Cities – Ho Chi Minh City (Vietnam)

Over the last 20 years, the population of Ho Chi Minh City has doubled to 8.8 million, making the city one of the fastest growing economic centers of Asia. It is expected that by 2035, the city will host more than 12 million inhabitants. Sustaining this population amount brings major challenges for city expansion, water supply and climate change resilience. Considering the geographic location of Ho Chi Minh City – from a few meters above to around sea level, close to the sea and in a river delta – the region is very vulnerable to land subsidence. Subsidence affects both the urban infrastructure (roads, sewerage, metro, bridges, etc.) and the hydraulic infrastructure (embankments, sluices) and leads to increased flood risk. Currently, parts of the city are already subject to regular tidal floods.

The effects of flooding in Ho Chi Minh City [source: Geomatics Center, Dinh et al 2016]

The relevance

Effective countermeasures can only be taken when the cause and extent of the land subsidence is properly understood. Sensar has cooperated with local parties (Geomatics Center and the Vietnam National University Center of Water Management and Climate Change (VNU-WACC)), WaterLand Experts and Partners for Water to accurately map the magnitude and extent of the land subsidence in Ho Chi Minh City with state-of-the-art InSAR data. Using Copernicus satellite data, we have mapped the height changes over 2 years (2017 – 2019) with weekly intervals. The result is available as an interactive 4D visualization, created together with our partner Klaas Nienhuis.

The 4D visualization

In the interactive demo, the surface displacement over time is shown. All movements are relative to the first observation and are given with respect to a stable reference point. Only observations from the ground level were used (buildings are removed). The deformation on the outer edges of the city is clearly visible. This can be explained by the fact that the old city is built on a strong layer of sand and sandstone, but is surrounded by weaker soils (clay and loam). Due to the rapid expansion of Ho Chi Minh City, the surroundings become increasingly more populated and the extraction of groundwater has increased manifold in order to sustain the growing population, industry and agriculture. The combination of loading weak soils with buildings and infrastructure and a decreasing groundwater table leads to severe subsidence in the outer parts of the city, reaching values up to ≈80 mm/yr. Combined with a sea level rise of approximately 3 mm/yr, the region faces a serious challenge.

Insight in future flood risks

Our partners from VNU-WACC and WaterLand Experts have analyzed the subsidence data for the purpose of estimating future flood risks of Ho Chi Minh City. Floods are caused by the high discharge of the river systems, both downstream and upstream tidal effects, and severe rainfall. Based on the observed subsidence rates around the Sai Gon River and Dong Nai/Soai Rap estuary, a projection of the future topographic elevation is made. This shows that some areas, such as Districts 8 and 12, are expected to sink below sea level in 2050 and, consequently, are subject to floods that can reach more than 2 meters depth. Other areas, like District 7 and Nha Be, are estimated to be subject to smaller floods reaching 0.4 – 0.8 meters in 2050. Insights of this detail level were not available before, but due to the extent, abundance and quality of satellite-based subsidence measurements this is now possible. These projections allow for informed policy making to mitigate future subsidence and to protect the most vulnerable areas and infrastructure.

Projected floods in 2050. The model has considered the effect of subsidence on the DEM-height by using the latest subsidence rates observed by Sensar. Source: VNU-WACC.