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- Author: Tom Ough
- Rolle, BBC Future
2 hours ago
The ground beneath New York City is sinking, in part due to the bulk of all of its buildings — and it’s not the only coastal city affected by this problem.
Can these concrete jungle regions still be saved in the face of rising sea levels?
On September 27, 1889, workers put the finishing touches on the Tower Building. It was an 11storey building, which with its steel structure is considered New York’s first skyscraper.
The construction marked the beginning of a wave that still hasn’t stopped.
In the 777 km² that make up the city of New York, there are 762 million tons of concrete, glass and steel, according to estimates by researchers from the United States Geological Survey (USGS, acronym for English).
While this number includes some generalizations about building materials, this quantity does not include the lighting fixtures, fixtures, and furniture found in these buildings.
Nor does it include the transport infrastructure that connects them, nor the 8.5 million people who live there.
All that weight has an extraordinary effect on the ground it’s built on. According to a study published in May, this soil is sinking by 1 to 2 mm per year, partly due to the pressure exerted on it by city buildings. And that’s worrying for experts.
That might not seem like much, but over a few years that adds up to significant problems for a coastal city.
New York has been suffering from the sinking since the end of the last ice age.
After the weight of the ice sheets is relieved, part of the land on the east coast is expanding while other parts of the coastal zone, including the part where New York City lies, appear to be calming down.
“This relaxation leads to subsidence,” says Tom Parsons, a research geophysicist at the USGS Pacific Coastal and Marine Science Center in Moffett Field, California, and one of the four authors of the study.
But the sheer weight of the city’s built environment makes that sinking worse, says Parsons.
And this is a global phenomenon. New York City, Parsons says, “can be seen as an example to other coastal cities in the United States and the world that are experiencing increasing immigration, urbanization, and rising sea levels.”
There are a variety of reasons for the demise of coastal cities, but a major factor is the mass of human infrastructure pressing down on the land.
The scale of this infrastructure is enormous: in 2020, the mass of manmade objects exceeded that of all living biomass.
Can anything be done to prevent these cities some with hundreds of millions of people from sinking under the sea?
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Indonesia’s capital, Jakarta, is increasingly prone to flooding due to the combined effect of subsidence and rising sea levels.
Some cities around the world — like Jakarta, the capital of Indonesia — are sinking much faster than others.
“In some cities, we’re seeing subsidence of a few centimeters a year,” says Steven D’Hondt, professor of oceanography at the University of Rhode Island in Narragansett.
At this rate, the city is sinking much faster than sea level is rising.
“To achieve that, ice melt would have to increase significantly.”
In addition to coauthoring the New York study, D’Hondt is one of three authors of a 2022 study that used satellite imagery to measure subsidence rates in 99 coastal counties around the world.
“If subsidence continues at the current rate, they will face severe flooding much sooner than projected,” wrote D’Hondt and his colleagues PeiChin Wu and Matt Wei, both of the University of Rhode Island.
Southeast Asia topped the list of cities experiencing the fastest declines. Parts of Jakarta are sinking between 2 and 5 cm per year.
Besides Jakarta, which will be replaced as the capital by a city 1,996 km away, there were Manila (Philippines), Chittagong (Bangladesh), Karachi (Pakistan) and Tianjin (China). These cities are already suffering from infrastructure damage and frequent flooding.
Though Mexico City isn’t on the coast, it’s now sinking at an incredible 50 centimeters a year thanks to the draining of underground aquifers by the Spanish when they occupied it as a colony.
Research suggests it could be another 150 years before the sinking stops.
However, coastal cities are the focus of D’Hondt and his colleagues’ research. For example, much of the Indonesian semarang is sinking at 2 to 3 cm per year, while a significant area north of Tampa Bay, Florida is sinking at 6 mm per year.
Some degree of this subsidence happens naturally, Wei says. However, it can be greatly accelerated by humans not only by the stresses on our buildings, but also by our groundwater extraction and our deepsea oil and gas exploration.
The relative contribution of each of these phenomena, Wei says, “varies from location to location, making understanding and managing coastal subsidence a challenging task.”
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As the humanmade environment continues to grow, it increases pressure on the ground and rocks below, which can lead to subsidence.
But we have to deal with it. The rising water wreaks havoc long before it reaches the flood barriers: it’s a tide that sinks all the boats.
According to D’Hondt, the first effects of relative sea level rise occur below the surface.
“You’ve buried utilities, infrastructure, and building foundations, and then seawater starts working on those things long before you see them above ground.” And storms are driving the water deeper and deeper into cities.
The solutions vary depending on the local causes of settlement.
One obvious approach, albeit with its own problems, is to stop building. As Parsons explains, the laying of soil under buildings “is usually completed within a year or two of construction.” Although much of New York City has bedrock of shale, marble, and gneiss, these rocks exhibit a some degree of elasticity and rupture, which accounts for part of the sagging. But the loamy soil and manmade fill materials, particularly found in lower Manhattan, could lead to the largest subsidence, say Parsons and his colleagues. Therefore, ensuring that the largest buildings stand on the sturdiest bedrock can help mitigate the downtrend.
Another solution, at least for some places, is to delay groundwater abstraction and aquifer extraction. Parsons and his colleagues warn that increasing urbanization is likely to lead to an increase in groundwater abstraction and will require even more construction to accommodate population growth. Finding more sustainable ways to meet the city’s water needs and maintain water tables can be helpful.
However, the most common approach is a chaotic and imperfect program to build and maintain flood defenses such as levees. Tokyo’s adaptation to land subsidence occurs in two ways. The city constructed physical structures such as concrete dikes, dikes, pumping stations, and sluices. These are combined with social measures such as evacuation tests and an early warning system.
To some extent, this subsidence is occurring naturally, but it can be significantly accelerated by humans not only through the stresses on our buildings, but also through the abstraction of groundwater and our oil and gas production in the deep sea.
Sometimes it is the residents themselves who intervene. A 2021 study documented how residents of Jakarta, Manila and Ho Chi Minh City took their own informal actions. This includes raising floors, moving equipment, and, in Manila, building makeshift bridges between houses in swampy areas.
Other useful tools include catch basins: large reservoirs located underground that release rainwater at a controlled, slow rate. Martin Lambley, drainage expert at pipe manufacturer Wavin, says abatement tanks should be combined with natural elements such as ponds, sinkholes (gravel pits from which water slowly drains), and ditches (swamp pools).
“The challenges we face today are drastically different than when urban sewage and drainage systems were introduced,” he says.
The higher the altitude, the more innovative solutions we will see. In 2019, the United Nations hosted a roundtable on cities on water that can take the form of floating structures.
Finally, mitigating climate change by eliminating greenhouse gas emissions would at least partially prevent or slow the melting of the polar ice caps, thereby slowing sea level rise.
“I think governments need to be concerned,” says D’Hondt. “If they don’t want to see a massive loss of infrastructure and economic capacity in a couple of decades, they need to start planning now.”