Rising tides, sinking cities

Imagine a house of cards, in an aquarium that is partiallly filled with water.

On top of this house of cards is a city. This city could be Bangkok, it could be Mexico City. Or it could be any number of cities in 45 out of the 50 US states that are at risk of sinking due to groundwater consumption.

Picture the city pumping water out of the aquarium until there is no water left in the aquarium. Consequently, the house of cards begins to collapse with the city on top of it. The residents of the city realise that they are sinking and try to pump water back into the aquarium to no avail. They end up on top of a collapsed house of cards in an aquarium with no water.

This is the analogy on how groundwater consumption affects major cities provided by Dr Eloise Kendy, a freshwater scientist at the Nature Conservancy, a leading environmental agency that aims to conserve land and water. The water filling up the spaces in the house of cards is the water that fills up the aquifers - bodies of rock underground that store groundwater - and the city on top could be one of several major cities peppered across the globe.

Think Beijing, Jakarta, Ho Chi Minh, Venice and Tokyo. All these cities have been affected from excessive groundwater consumption that is causing them to sink. Groundwater consumption is also prevalent in Turkey, with major tourist centres such as Konya suffering from this sinking effect.

The rate at which these cities are sinking is severe, according to a 2013 report published by Deltares. Deltares is an independent institution based in the Netherlands, that researches subsurfaces and water, using information it collects to work with governments, businesses and other research institutes.

The report shows that between 1990 to 2013, Jakarta has experienced a lowering of its land by 200cm. Certain areas of New Orleans have dropped 113cm. Coastal cities, like New Orleans, are prone to a higher rate of sinkage due to the presence of softer soil. A recently published study found that Beijing is sinking at a rate of nearly 11cm a year.

Although the rates vary, one thing is clear: cities across the world are sinking lower while sea levels are rising. This means that cities are due to go underwater sooner than we think.

How cities slump

Sinking cities happen because of something called land subsidence. "Land subsidence which is the sinking of the land means that the general elevation of a city is getting lower over a period of time," explain s Gilles Erkens, a senior geologist at Deltares. "If the city used to be at one metre above sea level it slowly sinks to 90cm above sea level."

As the global population rises exponentially, the demand for water increases. "If you have more people in the city, you need more water," says Gilles. Last year, the UN warned that continuous overconsumption of water will lead to an increasing number of regions suffering from water scarcity.

Groundwater is pumped out in order to meet this increasing demand. The gradual depletion of water from the aquifers below a city reduce the water pressure holding a city up, causing the soil above the the aquifers to compact and depress, bringing down the elevation of the city.

"You extract water from the sandy layers. The surrounding clay layers deliver water to the sand layers, and so they try to compact. The water usually provides counter pressure to the weight of the soil and during groundwater extraction the pressure is reduced," explains Gilles.

Sinking cities should not be confused with sinkholes, which usually develop when water dissolves soluble rocks such as limestone in the earth, leaving nothing to support the layer of ground above. Sufficient pressure can lead to the ground caving in. This is distinctly different from land subsidence where soil still holds up the overground land regardless of being compacted.

Sinkholes occur quite frequently. In 2007 and 2010, massive sinkholes opened up in Guatemala, leading to the deaths of 20 people in both incidents. The state of Florida is also known for its frequent sinkhole appearances and CCTV footage of sinkholes appearing in various parts of China consistently appear online.

The costs of a sinking city

The soil compacts and the land sinks but what happens next?

As Eloise explains, "There are definitely monetary costs. It affects water supplies, utility, infrastructure, it costs millions of dollars annually. It is estimated to have cost China 1.5 billion dollars and Netherlands 3.5 billion euros a year."

The damage in infrastructure comes from the different rates at which the land sinks. As Gilles explains, "this is when you have differential land subsidence, where the land is sinking at different rates. If one part of the land under the house is stable and the other is slowly subsiding, then a house could crack and so would have to be demolished and rebuilt."

When a city sinks, it means the area will be more susceptible to flooding. "You will see frequent flooding, prolonged, and deeper floods which usually happen when there is a storm or something that causes flooding," says Gilles, noting that "land subsidence does not cause flooding but it makes it worse."

The damage done is more than just monetary loss to government and private industries. In 2012 cracks were reported to appear in the business district of Shanghai, at the construction site for the Shanghai Tower project. Families are also at risk of being affected due to a higher possibility that flooding will occur more frequently coupled with the risk of land sinking at a differential rate which could cause cracks in infrastructure and private homes.

The ground beneath our feet

Although groundwater consumption is usually the prime cause of land subsidence, there are a variety of other factors that exacerbate the problem.

Rising sea levels caused by the increase in global temperatures increase the risk of flooding in coastal cities.

It has also been argued by scientists that the rapid development in construction of big cities, such as Beijing, places increased pressure on the ground making it easier for the soil to compact.

Other factors such as earthquakes, caused by the shifting of tectonic plates, could destabilise the ground thereby facilitating and increasing the rate at which the soil compacts.

Groundwater depletion appears to be the most glaring factor that contributes to land subsidence, despite shifts in tectonic plate movements.

As Gilles notes, "There is relatively a high amount of subsidence through plate tectonics but not as high as the rate achieved through groundwater depletion," citing Jakarta as an example.

Can we turn the tide?

No. Land subsidence is irreversible but it can be stopped or slowed down through what Gilles calls "mitigation".

"To stop groundwater consumption, you need to stop pumping water out, increase the recharge. Stop taking (water) out of the aquifers and start putting more in," says Eloise.

The other way of dealing with land subsidence is what Gilles calls "adaptation" which is accepting the problem and then changing the infrastructure to suit it.

This can be done by increasing the rate of infiltration.

"It is hard because we don't have natural soil or land but impermeable roads." The presence of roads built with inorganic matter make it impossible for water to seep back down into the ground. "We can change that through natural or green infrastructure which is just a fancy word of saying unhardened surfaces," Eloise says.

The practical approach to tackling land subsidence varies depending on the city and their budget.

The most successful so far has been Tokyo which has managed to completely stop the process of land subsidence by completely ceasing to extract groundwater. It chose instead to depend on reservoirs constructed in the mountains to provide a piped water supply to the city.

This plan is not without its costs, however. As Gilles explains "It is a big, expensive investment. The benefits will only come later."

Other cities have not been so lucky in terms of their financial capabilities. They choose instead to tax groundwater.

Bangkok for example has made groundwater more expensive than the piped water supply through changes in regulations.
"Bangkok has reduced groundwater extraction by placing a tax on groundwater, making it more expensive than the piped water supply," says Gilles.

While this approach has worked in the central part of the city, the suburbs have not prospered as much, largely due to the fact that they are populated with factories that opt to use groundwater because of its cooling properties.

Groundwater also proves to be a more consistent supply of water than a piped water supply.

A 2013 report by Deltares suggests developing an integrated technique towards dealing with the issue.

This involves addressing land subsidence by incorporating governance into policy changes, giving decision makers the power to make informed decisions (costs and benefits), and exchanging knowledge on how to avoid unsuccessful practices when it comes to dealing with land subsidence.

Think back to that half submerged, metal house of cards.

It is on the verge of collapsing. The water is being drained out, the cards are shifting.

This can be changed by the slightest shake or the next drop of water that is pumped out. It does not have to be this way.

Through comprehensive policies that address excessive groundwater consumption through established practice as in Bangkok or Tokyo -- or through a creative, novel approach, -- infrastructure can be protected from collapsing, the risk of flooding can be decreased and the rate at which coastal cities go underwater can be reduced. The house of cards does not have to fall.

Author: Azaera Amza