Saigon is sinking: Land subsidence an alarming issue in southern Vietnam: expert

Ho Chi Minh City has been sinking for decades, posing a serious risk to the local economy and livelihoods, experts warned.

Alarming numbers regarding subsidence in Ho Chi Minh City have been published recently in a study by a group of scientists at the Ho Chi Minh City University of Technology, led by Le Van Trung.

The scientists looked at remote sensing images of the area taken between 1992 and 2010, with updates added in 2016, to observe changes to land levels.

The research showed that Ho Chi Minh City has been suffering from complete or partial subsidence for 20 years. It first appeared in District 6 and Binh Thanh, but has spread to districts 7, 8, 9, 11 and 12, as well as Tan Phu, Binh Tan, Binh Thanh, Go Vap, Hoc Mon, Binh Chanh and Nha Be.

From 2002-2010, the problem seemed to largely disappear, apart from in District 9, Binh Thanh, Thu Duc and Hoc Mon where it accelerated. Binh Chanh District recorded the highest subsidence of 309mm during that time.

"This is an alarming issue that requires solutions to ensure the sustainable development of Ho Chi Minh City, especially drainage solutions," Trung, the lead researcher, said.

Land subsidence occurs when large amounts of groundwater are withdrawn from aquifers, bodies of permeable rock that can contain or transmit groundwater. The water is partially responsible for holding the ground up. With the absence of it, the ground sinks.

In Ho Chi Minh City, the problem has been attributed to excessive extraction of groundwater, as well as rapid urbanization that has placed tremendous pressure on the ground while preventing rain from filtering down to the underground aquifers, which are buried beneath concrete and asphalt.

According to Trung, local and foreign experts have been warning of subsidence for years, but local authorities do not consider the situation "critical".

Trung said that while the impacts of subsidence in Ho Chi Minh City are insignificant compared to other big cities such as Mexico and Shanghai, authorities should be on the alert.

"It is estimated that by 2070, sea levels will have risen by 50cm. Without swift action, the subsidence combined with rising sea levels will put enormous pressure on the city's drainage system and flood defences," Trung warned.

To reduce subsidence in the southern city, Trung and his research team said authorities should halt the urbanization of low-lying, weak land, in addition to limiting the construction of new underground water projects.

Authorities should also ensure a sufficient supply of surface water to meet the demand from new urban and industrial areas. At the same time, the city should look at how to funnel rainwater into aquifers and lakes, instead of letting it build up and cause floods.