Part 1: Saigon facing land subsidence, rising flood
Part 2: A fifth of Saigon may be inundated: expert
Tuoi Tre interviewed Phi about the issue.
When did subsidence begin in Ho Chi Minh City?
HCMC began sinking about 15 years ago. However, the land subsidence rate has increased in recent years. The worst affected areas include Hiep Binh Chanh Ward in Thu Duc District and Phu Dinh Wharf in Districts 6 and 8, with a sinking rate of 2.5 cm a year.
I attribute the subsidence in Hiep Binh Chanh to the development of high-rise buildings, especially along Kha Van Can Street. Parts of Districts 6 and 8 are suffering the same problem due to excessive exploitation of underground water as tap water has failed to meet demand.
What are the consequences of the land subsidence in the long term?
It will paralyze the city’s drainage system, which will sink, while high tides will rise higher than before. There will be no way for floodwaters to flow out. Subsidence due to underground water exploitation usually happens over a wide area.
HCMC has worked out a policy to stop exploiting underground water over the last several years, but many areas have not been provided with tap water
In recent years the city has taken more action than before to stop the exploitation of groundwater. However, the policy has not been thoroughly implemented in some portions because of faucet water shortage. As a result, land subsidence will continue in the next few years.
It will only abate after becoming stable because land subsidence is periodic and lags behind effective action. This lag means that if we stop exploiting underground water now, the land will continue sinking for a while after that.
Bangkok has stopped underground water exploitation over the last 20 years, but it is still experiencing subsidence now. However the phenomenon has stabilized and now moves in a horizontal direction. It is hard to determine when the subsidence will pause, as it depends on geologic structures and the supplementation of underground water.
Before halting their subsidence problems, areas of Bangkok and Shanghai sank by over one meter and two meters, respectively.
If HCMC stops exploiting underground water now, will the subsidence take about 20 years to stabilize?
Based on study results since 1996 and the highest sinking level of 2.5cm a year, HCMC has already sunk by about 40cm in some area. Specifically, the subsidence rate has reached 70-80cm on Nguyen Huu Canh Street in Binh Thanh District over the last few years. It is worrying as the land subsidence is broadening.
What will happen when HCMC subsides by 1-2m like Bangkok or Shanghai?
I have made a model to simulate the 1-2m subsidence and found that the city drainage and sluice system would be neutralized, rendered it unable to drain water or control high tides.
The simulated region covers 5,000 hectares, stretching along the Tau Hu-Ben Nghe Canal to Districts 1, 5 and 10. If the area sinks 1-2m, 20 percent of its surface will be flooded and people have to pump out 10 million cubic meters of water.
The HCMC area is ten times larger than the above simulated region, thus the floodwater volume will be proportionally higher. This means that every time rains the city will have to deal with tens of million of cubic meters of water, and every house will be forced to resort to pumps.
What should HCMC do now to limit land subsidence and flooding?
The city should restrict construction on low-lying areas and prohibit exploitation of underground water and invest in a faucet water system. If these measures are implemented now, HCMC will sink by only 50-60 cm in the next 20 years before potentially halting the process.
Another measure is to build reservoirs in low-lying areas to regulate the flow of water. Some regulating reservoirs can contain the 10 million cubic meters of water and then gradually release it, otherwise the city will have to suffer inundation first and then pump the water out.
Ho Long Phi, director of the HCMC Center for Water and Climate Change Management
Why has construction of regulating reservoirs been referred to for a long time without any actual work being done?
The city initially placed an order to the Steering Center of the Urban Flood Control Program to outline a plan but it took a lot of time. The city thus then tasked the Department of Planning and Architecture to do the job. At present, relevant sides have basically reached an agreement. If the city soon approves the plan, the project will be carried out in the next few months.
Where will the city choose to build the reservoirs?
The city should map places where water flows into. In my opinion, the reservoirs should be built in low-lying areas and places with low-roof houses.
Earlier the city leveled the surface in several areas for urban construction, and is now looking for land to build reservoirs. Are these moves at odds with each other?
It was a mistake for the city to fill in ponds and lakes to build residential and urban areas. For instance, the Binh Phu residential area and Radar Station housing estate in District 6 used to be lowland areas which contained water running from some portions of the inner city.
Since these areas were flattened to build residential areas, water has flown into their vicinity, causing flooding.
Although it is rather late, HCMC has determined to build reservoirs to contain rainwater and regulate high tide and prevent land subsidence as well as rises in the water level caused by climate change.