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| Beijing’s bustling Chaoyang district |
China’s capital is known for its horrendous smog and occasional
sandstorms. Yet one of its major environmental threats lies underground:
Beijing is sinking.
Excessive pumping of groundwater is causing the geology under the
city to collapse, according to a new study using satellite imagery that
reveals parts of Beijing – particularly its central business district –
are subsiding each year by as much as 11 centimetres, or more than four
inches.
The authors of the study warn that continued subsidence poses a
safety threat to the city of more than 2o million, with “a strong impact
on train operations” one of the predictions.
The study on Beijing’s subsidence has been published in the
peer-reviewed journal Remote Sensing and is based on InSAR, a type of
radar that monitors land elevation changes.
It was written by a team of seven researchers, including three who
explained their findings to the Guardian: Chinese academics Chen Mi
[Capital Normal University, China] and Li Zhenhong [Newcastle
University, UK], and Spanish engineer Roberto Tomas [University of
Alicante, Spain].
“We are currently carrying out a detailed analysis of the impacts of
subsidence on critical infrastructure (eg high-speed railways) in the
Beijing plain,” they said in an email to the Guardian. “Hopefully a
paper summarising our findings will come out later this year.”
Beijing sits in a dry plain where groundwater has accumulated over
millennia. As wells are drilled and the water table drops, the
underlying soil compacts, much like a dried-out sponge.
The study finds that the entire city is sinking but the subsidence is
most pronounced in Beijing’s Chaoyang district, which has boomed since
1990 with skyscrapers, ringroads and other development. The researchers
say the uneven nature of the subsidence in some areas poses risks to
buildings and other infrastructure.
Tens of thousands of water wells are thought to exist in and around
Beijing, many of them used in farming and landscaping. The state has
regulatory power over installation of wells but is inconsistent in
applying it, according to one leading Chinese environmentalist.
“There are some rules but the enforcement is doubtful,” said Ma Jun,
director of the Institute of Public and Environmental Affairs in
Beijing. Ma said he wasn’t surprised subsidence was relatively high in
the Chaoyang district given its rapid growth of recent decades. He
expected it to keep moving east as the city sprawled in that direction.
In
2015 China inaugurated a mega-engineering project aimed at mitigating
Beijing’s water crisis. The state completed construction of the
South-North Water Diversion, a £48bn, 2,400km network of canals and
tunnels, designed to divert 44.8bn cubic metres of water to the capital.
Even before the canal began delivering water, Beijing was easing up
on some groundwater pumping. In January of 2015 the Chaoyang district
announced plans to phase out 367 water wells, reducing the use of 10m
cubic meters of underground water.
Experts say it is still too early to know if the canal’s water
deliveries will help recharge the aquifer and slow Beijing’s rate of
subsidence. In the meantime concerns about impacts to buildings and rail
systems continue. To prevent derailments a 2015 study recommended that
China ban new water wells near completed high-speed rail lines [PDF].
Other cities around the world are experiencing subsidence caused by
excessive water pumping or other factors. Mexico City is sinking by up
to 28cm a year and Jakarta is subsiding at a similar rate. Bangkok is
dropping annually by as much as 12cm, similar to Beijing, according to
the Remote Sensing researchers.
Source: The Guardian International
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