URI scientists examine how the weight of the city’s 1 million buildings are causing subsidence
 |
| The New York City skyline. (Photo by Christian Lendl) |
In a new paper, scientists from the University of Rhode Island Graduate School of Oceanography found that New York City is sinking at a rate of 1 to 4 millimeters per year. Even though this descent is consistent with natural subsidence in the region, the researchers found some areas of significantly greater subsidence rates, which may be related to the massive weight of the buildings.
The study, led by
Tom Parsons of the U.S. Geological Survey with GSO professors Meng (Matt) Wei and Steven D’Hondt, and GSO
Ph.D. student Pei-Chin Wu,
is part of a growing body of research on how waterfront cities are facing not
just rising waters, but sinking land.
It’s hardly news that global sea levels are rising due to climate
change, a threat facing coastal communities around the world, but this new
research highlights how human infrastructure, including buildings, can increase
vulnerabilities in unexpected ways.
“Coastal flooding results from sea level rise and sinking of the shore,” said D’Hondt. “So subsidence will lead directly to coastal flooding even in the absence of sea level rise. Where both processes are occurring, coastal inundation will happen much faster than predicted by sea level rise models.”
Subsidence has been occurring where New York City now sits since
the end of the last ice age. As the ice sheets retreated, the land once covered
by the ice, north of the city, began to rise and the land that was just south
of the ice, where New York City is today, began to sink. That response to
melting of the ice sheets continues today.
The research team wanted to better understand the impact of the city’s immense building infrastructure on this gradual descent. To answer this question, the team first estimated the collective weight of more than a million buildings in New York City, amounting to 1.68 trillion pounds, and then calculated the downward pressure caused by the weight of those buildings.
How
the land reacts to this pressure is not uniform, as the underlying geology of
New York City varies from solid bedrock to sand and clay deposits, which can
compress more easily. The team also used radar images of the Earth’s surface
collected from orbiting satellites to map elevation changes in the region.
The results indicate that in recent years, all of New York City is sinking about 1 to 2 millimeters per year due to post-glacial relaxation. However, some parts of the city, particularly where buildings are built on fill and sediment, are sinking faster, up to 4 millimeters per year.
While the massive weight of the buildings causes some of the subsidence, not all of the increased subsidence rate can be directly attributed to the buildings. According to the study, subsidence due to buildings usually stops within a year or two of construction.
Continuation of subsidence past that time is largely
due to other processes, such as withdrawal of groundwater. In a city where sea
level is projected to rise between 8 and 30 inches by 2050, the additional
subsidence only increases its vulnerability to coastal storms.
“Anywhere that will eventually be flooded by the combination of
sea level rise and coastal subsidence will probably be temporarily flooded by
storm surge long before it’s permanently flooded,” said D’Hondt. “By lowering
the land relative to sea level, subsidence increases the probability of
flooding by a storm surge.”
New York City is not alone, however, as Wei, D’Hondt and Wu
previously documented in research on subsidence in other coastal cities.
“In most of them, part of the coastal area is sinking as fast or
faster than sea level is rising,” said D’Hondt. “New York is comparable in this
respect. However, many cities, mostly in east or southeast Asia, have areas
sinking much faster than the fastest sinking areas in New York.”
As in New York City, the weight of urban development in those
cities contributes to subsidence for a short period of time after each building
is constructed. The subsidence that occurs in those cities on longer timescales
is mostly due to other human activities like the extraction of groundwater or
oil and gas.
Coastal cities are growing rapidly, and the combination of construction densification and sea level rise is making them increasingly vulnerable to flooding.
According to the paper, New York is emblematic of
growing coastal cities all over the world that are observed to be subsiding,
meaning there is a shared global challenge of mitigation against a growing
inundation hazard.
Both Wei and D’Hondt hope that the research will prompt coastal
communities to account for subsidence in the plans they are making today to
prepare for rising sea levels and storm surge.
“City planners, emergency managers and other decision-makers
should look at the data in more detail, identify locations that show higher
subsidence rates and incorporate subsidence into their planning,” said Wei.
“The silver lining of this unfortunate situation is that
thoughtful planning for recovery from flooding by major storms can be used to
pre-adapt coastal regions for the long-term consequences of sea level rise and
coastal subsidence,” said D’Hondt.
