Dec. 6, 2018 – A recent study based on analyzed satellite data shows that the Iranian capital is sinking at a rate of 10 inches per year.
Researchers found that some parts of the city are sinking into the ground at a rate of up to 25 centimeters (nearly 10 inches) a year. The researchers also found that the sinking areas are expanding and could reach the city’s international airport.
The research by two scientists from the GFZ German Research Center for Geosciences at Potsdam was accepted for publication by the scientific journal Remote Sensing of Environment. The findings were also published on the website of the journal Nature. The researchers used satellite tracking data from 2003 to 2017 to track the rate of land subsidence in Tehran. A previous study linked the sinkage to the depletion of the groundwater aquifers under the city, which are being pumped to irrigate the fields surrounding the capital and to supply water to the city’s 13 million residents.
The new study describes the extent of the trouble. The western Tehran Plain, which includes the western city’s urban areas, farmland and suburbs, is sinking at a rate of 25 centimeters per year.
The Varamin Plain, an agricultural area southeast of the city, is sinking at a similar pace. The international airport, southwest of Tehran, is also sinking, albeit at a slower rate of 5 centimeters a year. Overall, the researchers estimate, about 10 percent of Tehran’s urban area is affected by the sinking.
The subsidence rate in Tehran is among the highest documented in an urban area, although this is a widespread global problem. Earlier studies have shown that Jakarta, the capital of Indonesia, is sinking at a rate of 20 centimeters a year, while California’s San Joaquin Valley, whose cities include Stockton and Fresno are located, is sinking by as much as 60 centimeters a year.
Previous work had shown that Tehran is sinking and had linked the sinkage to the depletion of groundwater aquifers, which are being sucked dry to irrigate nearby farmland and serve greater Tehran’s 13 million or so residents.
Huge fissures — several kilometers in length and up to four meters wide and deep — have opened up in the land to the southeast of Tehran, some of which are threatening to topple power-transmission lines and buckle railways.
And the growth of underground cracks sometimes produces sudden sinkholes. “One farmer I met was locked up for hours when the ground gave way beneath him and he fell into a six-meter-deep crack,” says Ali Beitollahi, head of engineering seismology at the Building and Housing Research Center in Tehran.
Such farmland is becoming inviable, because the cracks drain irrigation water from the surface and leave crops parched.
A combination of population growth — the city’s population has doubled in the past 40 years — droughts and large dams, which capture rainwater and prevent aquifers from recharging, has exacerbated the problem.
The authorities are fighting a losing battle as they try to regulate water extraction. Beitollahi believes that some 100,000 illegal wells have been blocked across Iran, but an estimated 30,000 are still in operation across Greater Tehran.
The researchers say the subsidence may be irreversible. Groundwater measurements show that the land does not rise back up when the groundwater increases, after a heavy rain, for example. The findings show that the rock under the soil also loses some of its porosity when the ground sinks. This change could lead to another worrisome phenomenon – more intense flooding because rainwater is not seeping into the ground.