add or more than a million buildings in New York City, and you get something that’s about 1.7 trillion pounds on top of 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 subsidence known as subsidence, as the soil is slowly compressed.
New research shows that the average rate of subsidence in NYC is between 1 and 2 millimeters per year, but in some places it’s as high as 4 millimeters. This may not be a worrying number, but compounded year after year, significant subsidence is doubling relative urban sea level rise. “You have about 1 to 2 millimeters of sea level rise, while on average you have 1 to 2 millimeters of sea level rise,” said Tom Parsons, a geophysicist with the US Geological Survey and co-author of a new study. 2mm going down. paper research description. “It’s a common problem with cities around the world. There seems to be a clear link between urbanization and subsidence.”
For example, some areas of Jakarta, Indonesia, are sinking almost a foot a year. The San Francisco Bay Area could take up to 165 square miles of coastline due to a combination of sea level rise and subsidence. And just last month, another group of researchers reported finding subsidence up and down the east coastup to 10 mm a year in parts of Delaware.
The main cause of severe subsidence is over-exploitation of groundwater, which is the case in Jakarta; Depleted aquifer collapses like an empty water bottle. But in NYC, subsidence depends on the composition of the underlying soil. A long time ago, glaciers swept the area, depositing sediment. Lakes also formed, depositing more sediment. So the municipality is built on a complex mixture of materials such as clay, silt, and man-made fillers that are more prone to subsidence, as well as sand and gravel that tend to resist it.
“The softer the soil, the easier it is to compress under load,” says Parsons. “Even if you don’t build it, it will still sink under its own weight. But if you build on it, it definitely sinks pretty well.”
Parsons and his colleagues calculated the subsidence rate in NYC by adding all that urban weight first, then combining it with geological data on the composition of the various deposits. They also collect satellite data that measures small changes in elevation to show which areas are sinking and which are relatively stable.
Manhattan’s skyscrapers may be the heaviest of the city’s buildings, but they’re anchored to the bedrock below, so they don’t pose much of a subsidence problem. The problem is more pronounced along the coast, where porous materials like clay and man-made fillers are especially prone to compression—and where sea levels are rising.
Subsidence is a potential vulnerability for coastal cities—models that predict how much sea level will rise in a given area don’t take this into account. By 2050, mean sea level in the United States will go up one legand at that time, 70 percent The world’s population will be urban, up from 56% today. In coastal cities, that boom will exacerbate the problem as more people will need to extract more groundwater and will need more buildings and roads, which will increase pressure on sediments. .
