New York City, it turns out, is sinking under its own weight

Scientists have discovered that New York City appears to be sinking at a rate of a few millimeters yearly. But why?
Christopher McFadden
New York City.
New York City is slowly sinking.


In a fascinating new study published in the journal Earth's Future, a team of scientists has found that New York City is slowly sinking under its weight. The problem is due to a common natural phenomenon called subsidence, where heavy things, like buildings, gradually settle over time or when dramatic changes in the Earth result in things sinking into the ground.

This can be for various reasons, but sudden movements in soft sediment or heavy loads pushing down on soft deposits are prime examples. If the calculations from the study are true, then the city is slowly sinking at a rate of about 1 to 2 millimeters per year. This doesn't sound like much, but it should be noted that this is an average; some parts are sinking much faster.

So much so that they are equivalent to the rebound rates of the Earth's crust from glacial melting. The changes to the city's foundation, home to over 8 million people, may pose a threat to its low-lying areas. Therefore, the researchers note, it is crucial to invest in developing mitigation strategies to address the increasing risk of flooding and rising sea levels.

However, constructing massive sea walls may not be the most optimal solution. "The point of the paper is to raise awareness that every additional high-rise building constructed at coastal, river, or lakefront settings could contribute to future flood risk," write geologist Tom Parsons of the United States Geological Survey and his colleagues at the University of Rhode Island.

The research, conducted by Parsons and their team, involved determining the total mass of over a million buildings in New York City. The results showed that the cumulative weight of these structures amounted to 764 billion kilograms or 1.68 trillion pounds. The researchers then divided the city into a grid of 100 by 100-meter squares and calculated the downward pressure caused by the weight of the buildings, taking into account the force of gravity.

The estimates provided only take into account the weight of the buildings and their belongings, not the paved surfaces such as roads, sidewalks, bridges, railways, and other structures in New York City. Despite these limitations, the latest calculations have improved on previous observations of sinking in the city by factoring in the intricate geological composition of the area, which includes sand, silt, clay lake deposits, and exposed bedrock.

After analyzing satellite data on land surface height, the researchers compared their models and mapped out the estimated subsidence in New York City. The team cautioned that the issue could worsen due to increased urbanization, such as groundwater drainage and pumping.

"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," the researchers conclude.

You can read the study for yourself in the journal Earth's Future.

Study abstract:

"New York City faces accelerating inundation risk from sea level rise, subsidence, and increasing storm intensity from natural and anthropogenic causes. Here we calculate a previously unquantified contribution to subsidence from the cumulative mass and downward pressure exerted by the built environment of the city. We enforce that load distribution in a multiphysics finite element model to calculate expected subsidence. Complex surface geology requires multiple rheological soil models to be applied; clay rich soils and artificial fill are calculated to have the highest post-construction subsidence as compared with more elastic soils. Minimum and maximum calculated building subsidence ranges from 0 to 600 mm depending on soil/rock physical parameters and foundation modes. We compare modeled subsidence and surface geology to observed subsidence rates from satellite data (Interferometric Synthetic Aperture Radar and Global Positioning System). The comparison is complicated because the urban load has accumulated across a much longer period than measured subsidence rates, and there are multiple causes of subsidence. Geodetic measurements show a mean subsidence rate of 1–2 mm/year across the city that is consistent with regional post-glacial deformation, though we find some areas of significantly greater subsidence rates. Some of this deformation is consistent with internal consolidation of artificial fill and other soft sediment that may be exacerbated by recent building loads, though there are many possible causes. 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."

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