Cities Are Sinking Under the Weight
of Urban Development
A new study quantifies what big buildings are doing to the
ground beneath San Francisco and other cities, as sea levels rise.
By Linda Poon
February 23, 2021, 12:23 PM EST Updated on February 23,
2021, 3:55 PM EST
Millennium Tower (left) in San Francisco has sunk 16 inches since construction was completed ten years ago. Other big buildings like it are contributing to sinking ground across the city, a new study warns.
In late 2020, engineers began working on a $100 million
project to stop San Francisco’s Millennium Tower from tilting and sinking
further into the ground. Tenants of the beleaguered luxury condo had learned
four years earlier that the 58-story high-rise had sunk some 16 inches in over
a decade. But the tower’s predicament is only part of a larger problem, and not
just for the Bay Area: Cities around the world are sinking under the weight of
their own urban development — at the same time that sea levels are rising.
A new study seeks to quantify how much the sheer weight of
the built environment contributes to the sinking of cities, a geological
phenomenon known as land subsidence. While urbanization is just one small cause
of this phenomenon among several, the paper in the journal AGU Advances
estimates that its impact is only likely to grow as people move to cities in
greater numbers. As a result, densely packed cities are likely to sink faster
than less developed areas.
Study author Tom Parsons, an earthquake seismologist at the
U.S. Geological Survey, looked at the San Francisco Bay Area as a case study of
this impact. He estimates that the collective weight of all of the San
Francisco region’s buildings is roughly 1.6 trillion kilograms, or 3.5 trillion
pounds. That alone may have caused the land to sink by as much as 80
millimeters, or more than three inches, over time as the city grew.
That is a conservative estimate, says Parsons. “It's pretty
much impossible to try to account for every single item in a city,” he says.
“But I figured that most of the stuff that we have in cities is inside
buildings.”
His model for calculating those numbers uses public data
about the height and base outline of each building in the San Francisco Bay
Area, and makes broad assumptions about their makeup and the contents inside.
It doesn’t include other mass sources like roads, bridges and other paved
areas, nor does it factor in humans.
Still, the pressure of buildings alone is enough to
contribute to the sinking of the region. Previous research in 2018 suggested
that coastal areas in the densely packed region around San Francisco, with a
population of more than 7 million, are sinking at a rate of nearly 2
millimeters a year. Some areas are settling by as much as 10 millimeters
annually. The region also faces threats of catastrophic flooding as sea levels
are expected to rise by nearly a foot by 2050, and by three feet by the end of
the century.
While factors like groundwater pumping, erosion and the
shifting of tectonic plates are often the major causes of subsidence, Parsons’s
paper zooms in on the San Francisco region to explain how urbanization also
plays a small but not insignificant role. “In a super-urbanized area like
downtown San Francisco, there’s a lot of pressure in a small area,” he says.
It’s enough to cause the Earth’s rocky outer crust, or the lithosphere, to
deform and sink, though only by a few millimeters each year.
Then there’s the fact that many of the city’s buildings are
built on soil, which compresses more easily, causing the ground to sink even
more over time. Engineers usually plan for this effect. “When they first put up
a building, it’s going to settle. It could settle 8 or 10 millimeters right
away,” says Parsons. “Over time, the buildings, especially the big heavy ones,
can continue to settle
indefinitely as the soil underneath starts to creep and flow
down, especially if the soil is clay rich.”
The impact of all this downward pressure may be small
compared to the other contributors of land subsidence. But Parsons argues that
the weight of urban development will be increasingly significant all over the
world as people continue migrating to cities, which will in turn see more
development to accommodate the population increase. By one estimate, some 70% of the world’s
population will live in large urban areas by 2050.
“It’s not a huge concern at the moment relative to other
things, but the issue is going to grow as we continue to pile on massive
developments,” he says. “Especially in developing countries, where there could
be a 50% increase in the amount of people flocking into coastal regions.
Construction is going to follow.”
The paper points to Lagos, Nigeria, where the urban
population of more than 14 million is expected to double over the next three
decades. The city is currently sinking at between 2 and 87 millimeters (or 3.4
inches) per year. Parts of the city that are along the coast, where “heavy
structures are built on poorly consolidated sediments,” Parsons writes, are
seeing higher rates of subsidence.
In developed cities, there isn’t much that can be done to
reverse the effect of urbanization. In Indonesia, leaders finally pushed
forward a plan to move the capital city out of Jakarta in part to lessen the
strain on the overcrowded city, but it’s already the fastest sinking metropolis
in the world. Excessive groundwater pumping has led the city to settle, or
sink, around 10 centimeters per year, with some coastal areas, where poorer
residents face chronic flooding, are sinking at some 25 millimeters a year.
“If you're intent on urbanizing a place, it’s not really
something you can do much about,” Parson says. “It’s just gravity pushing down,
you can’t really fight that.” There are some mitigating factors to consider,
however. Where you build and what land you build on matters in the long run.
Cities may want to reconsider plans to build heavily near the coast, or atop of
reclaimed land, which is susceptible to subsidence.
They may also want to rethink their urban drainage system.
When planners pave over soil, it typically reroutes drainage away from their
traditional river sources and right into the largest nearby body of water,
whether it be an ocean or a bay. So “a lot of the settlements that would
normally accumulate in the delta just get sent right out into the ocean,” he
says. That causes the ground to continually compact over time, and accelerate
the sinking of the city.
The key is to think about all this ahead of time. Otherwise,
“as cities get denser and denser, and build taller and taller, there could be
some issues going forward.”
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