Vietnam's southernmost province, Cà Mau, is in danger of sinking below sea level in the next few decades, unless drastic measures are taken urgently. This warning was released last week by the Vietnamese Ministry of Agriculture and Rural Development in co-operation with the Norwegian Geotechnical Institute (NGI).
- Our research shows that the main reason is over-exploitation of underground water, said Dr. Philos Kjell Karlsrud of NGI at a Workshop held on 19 June in Cân Tho, the largest city in the Mekong delta.
NGI has been conducting a study of possible causes of apparent land loss in the Cà Mau province since May 2012, at the request of the Ministry of Agriculture and Rural Development in Vietnam and the Norwegian Ministry of Foreign Affairs. Cà Mau is shaped as a peninsula at the southern tip of the country, with more than 1.2 million inhabitants, covering an area of 4,350 square kilometers.
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Satellite picture showing coastline in 1989, 1999 og 2012
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NGI's interpretation of satellite photography data confirms that significant retreat of the coastline of 100 to 1400 m has occurred over the past 13 years. From the Vietnamese side, it was indicated that the causes of the land loss could be changes in sediment transport and erosion processes along the coastline, deterioration of the protecting Mangrove forest, and possibly sea level rise due to climate changes.
NGI's study has revealed however, that significant ground water pumping takes place in the province. Every day, approximately 370,000 cubic meters of water are pumped from 100,000 drilled wells in the Cà Mau province. An upper capping soft clay layer effectively prevents natural recharge of ground water to the deeper aquifers from which the ground water pumping takes place. The ground water pumping therefore cause a reduction in pore pressures at the base of the soft clay layer, triggering significant consolidation settlements in the clay, and thereby subsidence or sinking of the ground surface.
The preliminary calculations done by NGI show that the subsidence may already have reached between 30 and 70 cm in Cà Mau. The subsidence may reach 80 to 150 cm within the next two decades, but will also continue to increase after that. Such subsidence induced by ground water pumping is a well known problem, and has already been experienced in many areas around the world. As examples, Karlsrud mentioned subsidence of up to about 1.5 m in the Bangkok metropolis, up to 2 m in Shanghai and 4m in parts of Jakarta.
¿Since the average surface of the Cà Mau province is only 1 to 1.5 meter above sea level, the consequences of such subsidence will be catastrophic, and make the province inhabitable within a few decades,¿ said Kjell Karlsrud. Future sea level rise due to climate changes will further enhance the problem, but the predicted sea level rise during this century is much smaller than the predicted subsidence due to ground water pumping. NGI warns that several provinces in the Mekong Delta region may be facing a similar risk of subsidence. Other studies have indicated that parts of Ho Chi Minh city is presently sinking 2-3 cm per year due to ground water pumping.
To stop the subsidence or sinking of the ground surface, NGI has recommended the local authorities to stop or greatly reduce the present groundwater pumping, and find alternative ways of supplying pure water. One option could be to build water purification plants utilizing river water, and/or to collect pure rainwater in suitable storage facilities.
To verify the rate and extent of the predicted subsidence, NGI In cooperation with the local authorities have proposed to implement detailed site investigations to verify the general geologic conditions in the area, and to establish a monitoring program involving measurement of ground water pressures and surface settlements across the Ca Mau region. NGI hopes that this work can start later this year. "It is urgent that this issue is dealt in an expedient manner before the consequences become too serious", says Karlsrud.
Following http://www.ngi.no/
