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subsidence

Subsidence – stopping the slide

October 3, 2013 by Timothy Beardson

China has been suffering from subsidence for decades. The reasons vary, but its principal cause is the excessive drawing off of groundwater. Others important causes include oil drilling, (including offshore), mining, neo-tectonic movement, underground real estate development and high-rise construction. Most obviously, it manifests itself in such phenomena as sinking land levels – sometimes slowly and sometimes suddenly – in sinkholes and in tilting buildings.

Subsidence affects over half the provinces of the country. It has been noted in Shanghai since at least the 1920s, for example, and the 1950s in Tianjin. The Northern China plain is the largest affected area; in fact it is the largest subsidence-prone area in the world. However, the effects are well-known as far south as Guangzhou and Shenzhen. Indeed it has even been noted in Heilongjiang in the far northeast and Tibet in the west.

Shanghai has fallen more than two metres since 1921. Tianjin has fallen by even further in recent years. The city of Cangzhou in Hebei has sunk by two-and-a-half metres in the last 40 years. Across China over 50 cities have seen land levels fall by more than 200mm. Out of 655 cities in China in 2012, it was estimated that more than 400 rely heavily on groundwater for their supplies. This is particularly true in northern China which has been described as the world’s largest piece of contiguous water-short land.

Railway officials have asked local governments not to extract groundwater along the route of the Beijing-Shanghai railway for fear of destabilisation. Engineers recommend that subsidence has to be kept below 5mm in total if trains are to continue travelling at 240mph. The high-speed railway line in Hubei was hit by this problem last year. There is serious concern about the threat of subsidence to Beijing airport, the Beijing subway and other vital infrastructure.

In Shanghai, although groundwater abstraction is the most important factor in causing subsidence, as elsewhere, the heavy weight of newly-built skyscrapers bears perhaps 40 per cent of the responsibility. As early as 2005 it was estimated there were 7,000 tall buildings in the city, with 2,000 more planned or under construction, although definitions vary.

Furthermore the city has a massive four billion square feet of subterranean real estate. In Beijing observers have made a link between subway expansion and the level of subsidence.
The most modern form of land subsidence measurement (“interferometry”) gives new data on the degree of slippage in Shanghai. The 1990s seem to have witnessed an increasing degree of subsidence, averaging well over 15mm a year. Equally important, the new techniques give figures at variance to – and sometimes higher than – the official ones. This might not need to be construed as sinister, merely that more advanced equipment is now available.

The more sophisticated calculations are usually the work of one-off research projects and thus not regularly available. The official Shanghai city figures show around 10-11 mm of subsidence in 2000-2002, falling to 6-7mm in 2010-11. If correct, this is a creditable reduction, but does not meet the goal set by the city authorities of 5mm by 2010 and of course it means the city is still sinking.

China is not alone in experiencing massive subsidence. Many other countries are also affected; a US study in 2012 rated Shanghai the eighth most affected city in the world. More critical in extent of subsidence were three US locations – Long Beach/Los Angeles, the San Joaquin Valley and Houston.

It is very difficult to create international comparisons as data are often so old and not always comparable. However, a fact used frequently in China is that subsidence affects over 36,000 square miles. In America, with a similar-sized land mass, the official figure is 17,000 square miles. However the Chinese figure is unrevised since 2005, despite widespread claims that the subsidence process is continuous, irreversible and occasionally “fast”. Similarly, the US figure seems to date back to at least 1991 and yet is still cited in official publications in 2013, almost a quarter century later.

This simply seems to confirm our longstanding view that official data in most countries, but particularly in China, should be treated with care.

China is taking measures to address the problem of subsidence. This is particularly true in Shanghai where groundwater monitoring systems have been installed since the 1990s, limits have been placed on high rise development. The city also puts 60,000 tons a year of water into its aquifers and has increased tree coverage. It is also making water more expensive: this summer a 30 per cent rise in water rates was announced.

In looking to the future, it is difficult to be wholly sure that all the right steps have been taken. There is still far too much abstraction of groundwater across China. We don’t know the up-to-date subsidence rates using the best techniques. There is a sense that Shanghai has made important strides but sea levels appear to be rising and the land is still falling. There is probably too much focus on the symptoms rather than the causes. Overall the researchers are still saying that despite helpful measures, nationwide subsidence will not be reversed but will continue to expand.

This is a major problem that needs to be put right – and can be. A more nuanced approach to real estate construction in geologically sensitive areas would be beneficial. A greater focus on water conservation would permit a tougher stand against local governments continuing to tap groundwater excessively. More measures can be taken.

Tags: Subsidence,


One comment on “Subsidence – stopping the slide”

  1. Johnd523 says:

    Very well written article. It will be helpful to anyone who usess it, as well as yours truly aedgdekckded

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