Land subsidence: the flooding threat in coastal cities, a case study in Ho Chi Minh city

Duy Truong Tuan, Frits Dirks, Phi Ho Long, Ger de Lange, Ruben Dahm

Monday 29 june 2015

17:36 - 17:39h at North America (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Adaptation measures, Poster pitches

Parallel session: Poster pitch: 2I: FloodRisk - Adaption

Many rapidly developing coastal cities, like Jakarta, Manila, Shanghai and Ho Chi Minh City, face flooding threats caused by a variety of reasons, some natural and some man-made. In many cases, land subsidence is posing a much bigger threat to the city than sea level rise. This phenomenon is not yet widely recognized and therefore attracts insufficient attention for remediation. The example of Ho Chi Minh City is elaborated where the effect of land subsidence is 3 to 10 times bigger than the effect of expected Sea Level Rise. Due to heavy rainfall, high river discharges and high tides, HCM City is experiencing frequent flooding of streets and wards, which is not life-threatening but causing nuisance and also significant economic damage. It is recognized that this situation is further aggravated by the impact of climate change, which is causing sea level rise and changes in rainfall distribution over the year. What is not widely recognized is the fact that land subsidence is occurring at an alarming rate and posing an even bigger threat to the city. The HCM City flood center, SCFC, has developed an Integrated Flood Management Strategy. In the preparation of that strategy, also an analysis is made of all uncertainties affecting the future flood risk, including sea level rise, land subsidence, precipitation, etc. To analyze the possible land subsidence, a model was developed, based on an existing groundwater model, to make land subsidence projections. Analysis shows that land subsidence is mostly caused by controlled and uncontrolled groundwater extraction for drinking and industrial water supply. Projections of future subsidence show very disturbing figures for Ho Chi Minh City, which are much larger than the expected sea level rise, even for the worst case (high emission) scenario. Based on this analysis, it is concluded that it is of great economic interest for HCMC to give high priority to activities aimed at 1) collecting more knowledge on land subsidence and 2) reducing the land subsidence as much as possible. Priority should be given to the reduction of groundwater usage in the vulnerable areas and developing a strategy towards renewable water resources. Other coastal cities in the region, like Jakarta, Manila, Shanghai and many others, face similar problems and can learn from the Ho Chi Minh City case to set up a strategy to cope with land subsidence.