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New York's skyscrapers are causing it to sink - what can be done about it?

Leroy N. Soetoro <[email protected]>

Date: Fri, 2 Jun 2023 21:56:48 -0000 (UTC)

https://www.bbc.com/future/article/20230523-new-yorks-skyscrapers-are-
causing-it-to-sink-what-can-be-done-about-it?utm_source=bbc-
news&utm_medium=right-hand-slot

The ground under New York City is sinking partly due to the sheer mass of 
all its buildings ? and it isn't the only coastal city to be suffering 
this fate. As sea levels also rise to meet these concrete jungles, can 
they be saved?

On 27 September 1889, workers put the finishing touches to the Tower 
Building. It was an 11-storey building that, thanks to its steel skeleton 
structure, is thought of as New York City's first skyscraper. The Tower 
Building is long gone ? its plum spot on Broadway was taken in 1914 ? but 
its erection marked the beginning of a construction spree that still has 
not stopped.

On the 300sq miles (777sq km) that comprise New York City sit 762 million 
tonnes (1.68 trillion pounds) of concrete, glass and steel, according to 
estimates by researchers at the United States Geological Survey (USGS). 
While that figure involves some generalisations about constriction 
materials, that prodigious tonnage does not include the fixtures, fittings 
and furniture inside those million-odd buildings. Nor does it include the 
transport infrastructure that connects them, nor the 8.5 million people 
who inhabit them.

All that weight is having an extraordinary effect on the land on which it 
is built. That ground, according to a study published in May, is sinking 
by 1-2mm (0.04-0.08in) per year, partly due to the pressure exerted on it 
by the city buildings above. And that is concerning experts ? add the 
subsidence of the land to the rising of sea levels, and the relative sea 
level rise is 3-4mm (0.12-0.16in) per year. That may not sound like much, 
but over a few years it adds up to significant problems for a coastal 
city.

New York has already been suffering subsidence since the end of last ice 
age. Relieved of the weight of ice sheets, some land on the Eastern 
Seaboard is expanding, while other parts of the coastal landmass, 
including the chunk on which New York City lies, seem to be settling down. 
"That relaxation causes subsidence," says Tom Parsons, a research 
geophysicist at the Pacific Coastal and Marine Science Center of the USGS 
in Moffett Field, California and one of the four authors of the study. 

But the enormous weight of the city's built environment worsens this 
subsidence, Parsons says.

And this is a global phenomenon. New York City, says Parsons, "can be seen 
as a proxy for other coastal cities in the US and the world that have 
growing populations from people migrating to them, that have associated 
urbanisation, and that face rising seas".

There is a wide range of reasons for why coastal cities are sinking, but 
the mass of human infrastructure pressing down on the land is playing a 
role. The scale of this infrastructure is vast: in 2020 the mass of human-
made objects surpassed that of all living biomass. (Learn more about how 
concrete has become the material that defines our age.)

Can anything be done to halt these cities ? which between them have 
hundreds of millions of residents ? from sinking into the sea?

Some cities around the world ? such as Jakarta, capital of Indonesia ? are 
sinking far faster than others. "In some cities, we're seeing subsidence 
of a few centimetres a year," says Steven D'Hondt, professor of 
oceanography at the University of Rhode Island in Narragansett. At this 
rate, the city is sinking far faster than sea levels are rising to meet 
it. "We'd have to increase ice melt by an order of magnitude to match 
that." 

As well as being a co-author on the New York study, D'Hondt is one of 
three authors of a 2022 study that used satellite images to measure 
subsidence rates in 99 coastal cities around the world. "If subsidence 
continues at recent rates, these cities will be challenged by severe flood 
events much sooner than projected," wrote D'Hondt and his colleagues Pei-
Chin Wu and Matt Wei, who are both at the University of Rhode Island. 

Southeast Asia featured heavily in the list of cities suffering the most 
rapid subsidence. Parts of Jakarta are subsiding at between 2cmm-5cm (0.8-
2in) per year. Alongside Jakarta, which is being replaced as Indonesia's 
capital by a city being constructed 1,240 miles (1996km) away, were Manila 
(Philippines), Chittagong (Bangladesh), Karachi (Pakistan) and Tianjin 
(China). These cities are already suffering infrastructure damage and 
frequent flooding.

Meanwhile, although it is not on the coast, Mexico City is sinking at an 
astonishing 50cm (20in) a year thanks to the Spanish draining its 
underlying aquifers when they occupied it as a colony. Research has 
suggested it could take another 150 years before that sinking halts ? and 
round 30m (98ft) of additional subsidence.

You might also like:

Will Venice succumb to the sea?
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But it's the coastal cities that are the focus of the study by D'Hondt and 
his colleagues. A large portion of Semarang in Indonesia, for example, is 
sinking at 2-3cm (0.8-1.2in) per year while a significant area in the 
north of Tampa Bay, Florida, is subsiding at 6mm (0.2in) annually. 

Some level of this subsidence happens naturally, Wei says. However, it can 
be greatly accelerated by humans ? not only by the load of our buildings, 
but by our extraction of groundwater and our production of deep-lying oil 
and gas. The relative contribution of each of these phenomena, says Wei, 
"vary from place to place, making it a challenging task to understand and 
address coastal subsidence".

But address it we must. Rising water causes damage well before it starts 
crashing over flood barriers: it is a rising tide that sinks all boats.

The first effects of a relative rise in sea level, says D'Hondt, take 
place below the surface. "You've got buried utility lines, buried 
infrastructure, buried foundations for buildings, and then, the seawater 
starts working with that stuff long before you see it above ground." As 
this goes on, storms bring water ever further into cities.

The solutions vary according to the local causes of subsidence.

One obvious approach, albeit one that comes with its own problems, is to 
stop building. As Parsons explains, the settling of the ground beneath 
buildings "is generally complete a year or two after construction". 
Although much of New York City has bedrock of schist, marble and gneiss, 
these rocks have a degree of elasticity and fractures in them that account 
for some of the subsidence. But the clay-rich soil and artificial fill 
materials that are particularly prevalent in lower Manhattan can cause 
some of the largest amounts of subsidence, says Parsons and his 
colleagues. So ensuring the largest buildings are positioned on the most 
solid bedrock could help to reduce the downward trend.

Another solution, at least for some places, is to slow the withdrawal of 
groundwater and extraction from underground aquifers. Parsons and his 
colleagues warn that increasing urbanisation will likely increase the 
amount of groundwater being extracted and combine with even more 
construction to cope with the growing population. Finding more sustainable 
ways of supplying the city's water needs and maintaining groundwater 
levels could help.

However, the most common approach is a messy and imperfect programme of 
constructing and maintaining flood defences such as sea walls. Tokyo's 
adaptation to land subsidence is two-pronged. The city has built physical 
structures like concrete dykes, seawalls, pump stations and flood gates. 
These are combined with social measures like evacuation rehearsals and an 
early-warning system.

Some level of this subsidence happens naturally, but it can be greatly 
accelerated by humans ? not only by the load of our buildings, but by our 
extraction of groundwater and our production of deep-lying oil and gas
Sometimes it is residents themselves who step in. A 2021 study documented 
how residents of Jakarta, Manila and Ho Chi Minh City have taken their 
own, informal measures. These include raising floors, moving household 
appliances and, in Manila, building makeshift bridges between houses in 
swampy areas.

Other useful tools include attenuation tanks: large tanks that sit 
underground and release stormwater at a controlled, slow rate. Martin 
Lambley, a drainage expert at the pipe manufacturing company Wavin, says 
that attenuation tanks should be combined with natural elements like 
ponds, soakaways (rubbly pits from which water drains slowly) and swales 
(marshy basins). "The challenges we face today differ drastically from 
when urban sewers and drainage systems were first introduced," he says. 

We might see more innovative solutions as the waters rise. In 2019, the UN 
held a roundtable discussion on floating cities, which might take the form 
of pontoon structures. Finally, stopping climate change by eliminating 
greenhouse gas emissions would prevent or delay at least some melting of 
the polar ice caps, slowing sea level rise.

"I think that governments need to be concerned," says D'Hondt. "If they 
don't want to have massive loss of infrastructure and economic capacity in 
a few decades, they need to start planning right now." 



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