Belfast Telegraph

Saturday 30 May 2015

Venice, Bangkok, New Orleans - the coastal cities that are sinking ten times faster than sea levels are rising

Venice, flooded in November 2013. Human-driven subsidence has stopped but the ground level is still falling thanks to natural factors.
Venice, flooded in November 2013. Human-driven subsidence has stopped but the ground level is still falling thanks to natural factors.
Tokyo. The centre of the most populous metropolitan area of the world (home to more than 36 million), Tokyo has struggled with subsidence for decades. Groundwater extraction peaked in the 1970s and subsidence was bought down through restrictions to a rate 1cm a year in the early 2000s.
Dhaka. The Bangladeshi capital is home to around 15 million, with 546 water pumps removing around 1.9 million cubic metres of water a day for drinking supplies - this still only meets 85 per cent of demand.
New Orleans. New Orleans may not be a megacity in terms of population (roughly 370,000) but is sinking all the same. Land reclamation in the first half of the 20th century has turned the land into a giant sink, with ground levels falling as low as 12ft below sea level.
Jakarta. The capital of Indonesia and its population of nearly 10 million are expected to sink five to six metres by the end of the century unless groundwater extraction is stopped.
Ho Chi Minh City. The capital of Vietnam, formerly known as Saigon, is expected to grow to a population of 13.9 million by 2025 and is sinking 2cm a year.
Bangkok. Land subsidence has affected the capital of Thailand for more than 35 years, with the ground's soft thick clay only exacerbating the threat from flooding.

Scientists have issued a new warning to the world’s coastal megacities that the threat from subsiding land is a more immediate problem than rising sea levels caused by global warming.

A new paper from the Deltares Research Institute in the Netherlands published earlier this month identified regions of the globe where the ground level is falling 10 times faster than water levels are rising - with human activity often to blame.

In Jakarta, Indonesia’s largest city, the population has grown from around half a million in the 1930s to just under 10 million today, with heavily populated areas dropping by as much as six and a half feet as groundwater is pumped up from the Earth to drink.

The same practice led to Tokyo’s ground level falling by two metres before new restrictions were introduced, and in Venice, this sort of extraction has only compounded the effects of natural subsidence caused by long-term geological processes.

"Land subsidence and sea level rise are both happening, and they are both contributing to the same problem - larger and longer floods, and bigger inundation depth of floods," Dr Gilles Erkens, who led the research from Deltares, told the BBC. 

"The most rigorous solution and the best one is to stop pumping groundwater for drinking water, but then of course you need a new source of drinking water for these cities. But Tokyo did that and subsidence more or less stopped, and in Venice, too, they have done that."

In Pictures: The real Atlantis? Megacities under threat from land subsidence

Unfortunately, human-driven subsidence is having a great affect than natural processes, with rapid urbanization and its associated impacts leading to increased vulnerability to floods.

Dr Erkens and his team estimate that the financial cost of structural damage and maintenance amounts to around a billion dollars annually and that parts of many megacities – including Jakarta, Ho Chi Minh City, Bangkok and Dhaka – will sink below sea level unless action is taken.

In the case of Jakarta defences such as a 30-kilometer seawall have been erected to protect the city form flooding, but if this were to break, Deltares have predicted that within 48 hours the homes of nearly one million people would be flooded.

For other cities though, even this sort of defence is futile, as rising water levels will overrun them first instead.

The tiny island nation of Maldives (formed from a double chain of 26 atolls) sits just five feet above sea level. Worst-case projections of rising water levels suggest that some 350,000 islanders will have to completely abandon their home before the end of the century, leaving behind a 2,000 year old culture for good.

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