Why cities sink

By A.A.K.

IF A city sinks into the ground but nobody notices, is it really sinking? After many years of tempting fate Jakarta, Indonesia’s capital, found its answer in 2007 when a regular (if especially high) tide triggered a massive flood: 76 people were killed and nearly 590,000 made homeless. The process of land subsidence is slow, often invisible and affects most citizens only rarely. But a World Bank report from 2013 reckoned that by 2050 “risks from sea-level rise and sinking land” will cost the world’s large coastal cities up to $1trn a year. According to data released by NASA and the European Space Agency earlier this month, parts of a 715km aqueduct in California sank more than 60cm between 2013 and 2016. The Oroville dam, at the head of the aqueduct, has spent most of February threatening to flood communities below. Why do cities sink?

The gradual subsidence of most cities has several causes, both man-made and natural. It starts with the hundreds of millions of people migrating into urban centres in search of better jobs and higher standards of living. This puts pressure on certain pockets of the planet to keep up with the intensified demands for basic human sustenance. The indiscriminate use of groundwater, a scourge of rapidly-expanding cities, is a prime contributor. Dried-up lands compress under their own weight. In this sense it costs too little to sink tubewells and pump up the precious stuff at will. In America groundwater extraction without commensurate recharge is responsible for 80% of subsidence. In China more than 50 cities have subsided because of heedless pumping. Some have been pressed under the additional weight of enormous skyscrapers; they are judged to be responsible for 30% of metropolitan Shanghai’s surface subsidence. There are natural causes too. A disproportionate number of cities were founded on pristine river deltas. Sedimentary bedrock compresses too slowly to notice during a human lifetime. In the past such subsidence would have been offset by a perennial supply of silt deposited by tributary rivers. Today silt is much more likely to be trapped upstream by dams, mined for building material or dredged to clear shipping lanes.

Unlike global warming, the problem of a sinking city is local, and eminently solvable. Gilles Erkens, a geologist with Deltares Research Institute in the Netherlands, points out that in Tokyo regulations enacted in the 1960s to restrict groundwater extraction have stopped land subsidence entirely. Bangkok, which was built over a swamp and had been sinking 5cm per year at the turn of the 21st century, has started to recover, slowly, since adopting judicious guidelines for the use of groundwater. Jakarta is still losing 7.5cm every year—or 25cm in some areas. In the teeth of a crisis, it has adopted a different tack: building a massive seawall, which should someday be 24 metres high and 40km long. Venice is built on such soft soil that it can expect to keep sinking whatever it does; so far as Venetians can hope to minimise the loss, they must convert to lighter building materials, stop heavy lorries from plying certain highways and expand horizontally, if at all. For more than a decade it has been mulling a plan to build inflatable gates on the seabed, which could be raised to close off the lagoon during high tides. Some ideas are wackier than others: Sweden has considered physically moving a whole town, intact, across 3km of Lapland. Kiribati, a tiny Pacific island nation (that has more to fear from climate change than subsidence) wants to become the world’s first floating republic.

As when facing the challenges posed by climate change, politicians tend to balk at most proposals to stop land subsidence. They have to keep an eye on voters, who bear the brunt of most massive public expenditure in the short term through high taxes. Over the span of multiple political cycles however, a sinking city can have consequences that are not just costly but also deadly. Underground pipelines can tilt marginally or crack under pressure, causing water and sludge to vomit onto the earth’s surface. This can disrupt a city’s transportation networks, sewage systems and other hydraulic infrastructure, like the sluice gates that control water levels in canals. Asking a few hundred million people to step aside, so that workmen can tend to an invisible problem underfoot, will never be easy. Expensive, unexciting and unpopular, the war against land subsidence needs support all the same.