The Hidden Threat of Subsidence in Coastal Cities
The weight of millions of buildings in New York City amounts to about 1.7 trillion pounds, which is not inclusive of the other urban infrastructures like roads and sidewalks. This weight causes the deformation of the ground, which leads to subsidence, a slow compression of the earth. While it may not sound worrisome, it is a significant subsidence that doubles the relative rise in sea level in the city. In fact, on average, subsidence rates in New York City range between 1 and 2 millimeters per year, but in some areas, they can reach up to 4 millimeters. Instances of subsidence are not exclusive to New York City, as they affect cities worldwide. Jakarta, for example, is sinking at a rate of almost a foot a year, while parts of Delaware report subsidence as high as 10 millimeters a year.
Urbanization and subsidence are linked, and the more people live and work in cities, the more the problem is exacerbated. As the world’s population urbanizes, subsidence will no doubt worsen, and sea levels will continue to rise. This poses a hidden threat to coastal cities, and projections of how much sea levels will rise in a particular area do not take into account subsidence. According to a study, 70% of the world’s population will be urbanites by 2050, and coastal cities will experience more subsidence as more people will need to extract more groundwater and will require more buildings and roads. This will increase the pressure on sediments, leading to more subsidence.
The causes of subsidence vary across different cities and regions. The collapse of drained aquifers, excessive groundwater extraction, and sediment composition are some of the significant causes of subsidence. In New York City, subsidence depends on the composition of underlying soil, which is a complex mix of materials like clay, silt, artificial fill, sand, and gravel. The building of structures on soils that are more prone to sink causes them to sink even faster.
The softer the soil, the more likely it is to compress under load, even if a structure is not built on it, and it will continue to sink under its weight. In New York City, the heavier buildings like skyscrapers that lie along the coast are anchored to the bedrock, so they do not pose much of a subsidence problem. The challenge is that spongy materials like clay and artificial fill, all along the coast, are particularly prone to compression, and seawater levels are rising.
New Research Finds that Subsidence Poses a Significant Threat
A recent study conducted by geophysicists with the United States Geological Survey finds that on average, subsidence rates in New York City range between 1 and 2 millimeters per year. However, some areas report subsidence of up to 4 millimeters. The study found the following:
Subsidence rates in New York City range from 1 to 2 millimeters per year, on average.
Subsidence is worse than the relative rise in sea levels.
Manhattan’s skyscrapers are not a significant threat as they are anchored to the underlying bedrock.
Spongy materials like clay and artificial fill, all along the coast, are particularly prone to compression, and seawater levels are rising.
Models that predict sea level rise do not account for subsidence.
While the study carried out in New York City shows that subsidence can be a significant problem to coast cities, it also highlights the need for more studies to be undertaken in other cities worldwide to determine the magnitude of this issue. As it stands, the projections of how much sea levels will rise only paint part of the picture, as they do not take into account subsidence.
Conclusion
Subsidence is a pressing issue that affects many coastal cities worldwide. Urbanization and subsidence are linked, and projections that predict sea level rise do not account for subsidence. Research has found that subsidence poses a more significant threat than the relative rise in sea levels, and it is a hidden vulnerability for coastal cities. While the causes of subsidence vary, it is a matter that requires more detailed studies to determine the magnitude of the problem and its effects on different cities worldwide.
As a society, we need to rethink our approach to urban development, taking into account the risk of subsidence and how building on certain soils could cause greater subsidence. With more people expected to live and work in coastal cities, there will be more demand for buildings and infrastructure, and groundwater extraction that will result in increased pressure on sediments and may lead to more subsidence.
Subsidence is not a problem that can be solved with one solution; it requires a collective effort to address a range of issues from water usage and extraction to land use, building materials, and infrastructure. As we build cities, we must consider the impact of our actions on the environment and how they ripen into climatic hazards for us and future generations.
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add the Millions or more buildings in New York City, and you get something on the order of 1.7 trillion pounds of weight pressing down on the earth, and that’s not counting all the other infrastructure, like roads and sidewalks. All that weight is deforming the ground, like bowling balls on a memory foam mattress, and causing a type of subsidence known as subsidence, when the earth slowly compresses.
New research finds that, on average, subsidence rates in New York City are between 1 and 2 millimeters per year, but in some places they are as much as 4 millimeters. This may not sound like a worrying figure, but compounded year on year, it is a significant subsidence that is effectively double the relative rise in sea level in the metropolis. “Sea level rises between 1 and 2 millimeters, while on average it falls between 1 and 2 millimeters,” says Tom Parsons, a geophysicist with the United States Geological Survey, co-author of a new study. paper describing the investigation. “It is a common problem in cities around the world. There seems to be a definite link between urbanization and subsidence.”
Parts of Jakarta, Indonesia, for example, are sinking from almost a foot a year. The San Francisco Bay Area could lose up to 165 square miles of coastline due to a combination of sea level rise and subsidence. And just last month, another team of researchers reported finding subsidence up and down the east coastas high as 10 millimeters a year in parts of Delaware.
The main way to cause dramatic subsidence is excessive groundwater extraction, as is the case in Jakarta; drained aquifers collapse like empty water bottles. But in New York, subsidence depends on the composition of the underlying soil. Long ago, glaciers scraped into the area, depositing sediment. Lakes also formed that deposited even more sediment. So the metropolis is built on a complex mix of materials like clay, silt, and artificial fill, which are more prone to sinking, as well as sand and gravel, which tend to resist it.
“The softer the soil, the more likely it is to compress under load,” Parsons says. “Even if you don’t build on it, it will continue to sink under its own weight. But if you build on it, it definitely sinks in pretty well.”
Parsons and his colleagues calculated subsidence rates in New York City by first adding up all that urban weight and then combining it with geologic data on the composition of different deposits. They also collected satellite data that measured minute changes in elevation to show which areas have subsided and which have remained relatively stable.
Manhattan’s skyscrapers may be the heaviest buildings in the city, but they’re anchored to the underlying bedrock, so they don’t pose much of a subsidence problem. The problem is more along the coast, where spongy materials like clay and artificial fill are particularly prone to compression, and where seawater levels are rising.
Subsidence is a hidden vulnerability for coastal cities: models that project how much sea level will rise in a given area still don’t account for it. By 2050, mean sea level in the US will go up a footand in that moment, 70 percent of the world’s population will be urbanites, compared to the current 56 percent. In coastal cities, that boom will exacerbate the problem because more people will need to extract more groundwater and will need more buildings and roads, which in turn will increase pressure on sediments.
https://www.wired.com/story/new-york-city-is-sinking-its-far-from-alone/
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