Vietnam’s incredible sinking cities

The increasing use of groundwater is causing parts of Hanoi to sink by two to four centimeters every year, a study has found.
The study, conducted by the Hanoi Institute of Construction Science, Technologies and Economics, examined data collected from 10 monitoring stations around the capital city since 1991.
The data confirms “the reduction of groundwater has caused ground surfaces to sink at well sites,” a representative of the institute said.
Other factors, including Hanoi’s massive building boom, are contributing to the subsidence problem, the institute’s representative said.
In its report, the institute called for more monitoring stations to be set up throughout the city to collect more comprehensive data.
The data could be used to compile a “sinking map” of Hanoi as well as to forecast subsidence levels around the city.
Hanoi, the country’s political and economic hub, has experienced a population explosion.
By last December, the city had 3.4 million residents, a population density of nearly 3,500 people per square kilometer.
The study showed that in 2006 the city drew about 700,000 cubic meters of groundwater a day from underground aquifers.
Rainfall and surface water, which replenish the underground water systems, do not replace what the city has been taking.
The 10 monitoring stations were located at local water wells which were built on various types of soil.
More subsidence was recorded at wells built on weaker soils.
In Thanh Cong and Ngo Si Lien areas, for example, the subsidence rates were more than 41 millimeters and 27 millimeters a year, respectively.
The subsidence rates were lower around wells built on stronger soil and near the Hong River, which supplies the groundwater.
The study recommended future wells should be built near the Hong River.
And in order to reduce the city’s reliance on groundwater, the city should also harvest water from the Da River, the report said.
Groundwater depletion and construction have also been blamed for land subsidence and collapsed buildings in Ho Chi Minh City.
A conference last month heard that up to 600,000 cubic meters of groundwater was being pumped from underneath HCMC daily, only about a third of which was replaced naturally every year.
HCMC Construction, Science and Technology Association Deputy Chairman Phan Phung Sanh said taking too much water out of underground aquifers made the ground vulnerable to collapse when foundation work was carried out in the vicinity.
“The deeper the foundation, the more dangerous the collapse can be,” he told the conference.
Sanh called for better management of groundwater to minimize future subsidence.