Vietnam metro is sinking, flooding easier

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Groundwater overexploitation and urbanization blamed, but one study finds rainwater can be pumped to effectively recharge the water table.

People push motorbikes through the inundated Nguyen Huu Canh Street in Ho Chi Minh City November 7. Experts blame urbanization and overexploitation of groundwater for the city's sinking, which worsens flooding due to high tides and rains. Photo by Tran Phong

Ngo Van Binh has lived in Ho Chi Minh City for more than 10 years, but last week was the first time he was able to catch a catfish in his yard.

On November 7 he and many of his neighbors did not go to work because of severe inundation caused by a heavy downpour early morning that lasted hours as a tropical depression hit the city.

"It was one of the worst inundations I have ever seen in District 12," he said.

While meteorologists pointed to the rain and high tides, experts blamed it on urbanization and overexploitation of groundwater which have sunk parts of the city, causing it to flood more readily than before.

On that day the daily life of people was severely impacted. Those who tried to go to work soon had to push their stalled motorbikes through inundated streets or hire people to push their cars.

Many people living in low-lying areas had to use sandbags to keep water out of their houses.

People even saw a man rowing a small boat on Hoa Binh Street in District 11.

Nguyen Minh Giam, deputy director of the Southern Region Hydrometeorology Station, said the tide was high at 1.55 meters when the tropical depression hit.

"The heavy rains increased the tide to 1.64 meters, only four centimeters lower than the highest-ever tide two weeks ago," he said.

Sinking city

According to a recent study by the National University Ho Chi Minh City's Geoinformatics Center, parts of the southeastern region and the Mekong Delta are sinking, with HCMC suffering the most.

Using the differential interferometric synthetic aperture radar or INSAR method, researchers found many places in HCMC sinking by up to 20 millimeters (0.8 inch) a year.

The center said the city has been sinking since 1996, with the speed increasing gradually since 2004.

According to the city Department of Natural Resources and Environment, many parts will sink a further 12-20 centimeters by 2020.

Besides geological factors, the surface is sinking also due to urbanization and dwindling groundwater, according to the agency.

Ho Phi Long, director of the Water Management and Climate Change Center, said many places in HCMC are sinking by 3 centimeters a year, three times faster than the sea-level rise.

"Higher tides are the impact of sea-level rise. The consequences are worse when the city is sinking," he said. 

Digging the ground

The Department of Natural Resources and Environment said there are more than 200,000 wells in the city pumping more than one billion of liters of groundwater every day, five times the volume envisaged in a plan approved by the prime minister.

To Van Truong, former director of the Southern Institute for Water Resources Planning, said there were only 96,000 wells in the city in 1999.

"There are more than 100,000 additional wells now and alarming overexploitation of groundwater. Groundwater is declining, leading to the sinking of the surface."

But the Saigon Water Corporation paints an even more alarming situation: the actual number of wells is much higher than 200,000, and many people are using well water in Binh Tan, Binh Chanh, and District 8, it said.

Concrete weight

Le Huy Ba, former director of the Institute for Environmental Science, Technology and Management, said rapid urbanization is the major cause of the sinking.

"The sinking in the city's eastern parts is the result of urbanization of the lowland."

He warned that the city should halt the urbanization in low areas like Nha Be, Binh Chanh, Thu Duc, and District 9, where there are not many skyscrapers yet.

"By halting development in the lowlands, the city also saves people from losing money by buying houses in areas that will be inundated in future," he said.

He called for setting binding terms for investors in the lowlands because it is almost impossible to repair the drainage system in areas that have sunk.

"They have to be responsible for the sewer system, for 30 years for instance, instead of simply handing it over once it is built and taking no further responsibility.

"Sinking underground structures have been discovered in the residential areas of Bau Cat and Phu My Hung a couple of years after they were finished.

"The government has to pay for the repairs, which means taxpayers have to pay for [others'] fault," he said.

New hope

In an effort to check the groundwater depletion, Nguyen Viet Ky, a lecturer at the HCMC University of Technology, has done a study by collecting rainwater and pumping it into the ground.

Ky said the groundwater is receding by 2-5 meters a year in many places in the city, but it is possible to fully replenish water in the layer 12-60 meters under the ground.

He proposed harvesting rainwater from house roofs.

In his test at his school's block B8 over the last year he found the area's groundwater level has increased by a meter.

Rainwater from the 400-square-meter roof is sent to a filter tank before being pumped into the ground.

"Rapid urbanization with its skyscrapers and villas offers roofs to collect rainwater to pump into the ground or use for other purposes like firefighting, watering plants, and human consumption," he said.

Many countries like Mexico, Germany, and Japan have used rainwater to combat the sinking of land, and others like Israel, the US, and Morocco, to prevent the sea from encroaching inland, he said.

Phan Minh Tan, director of the city Department of Science and Technology, said Ky offered a good model to resolve the sinking problem.

"But there should be more studies"¦ before it can be implemented widely," he said.

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