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Photo: the weight of about 1.1 million buildings in Pixabay is causing New York to slowly sink.

Write | Megan Battle (Meghan Bartels)

Translation | Zeng Xinxin

Editor | 2 / 7

By 2020, New York City, with a population of 8.8 million, is currently the most densely populated city in the United States. In order to accommodate so many people, residential and commercial buildings are increasing year by year. The weight of more than a million buildings is sinking the land and causing relative sea levels to rise, increasing the risk of flooding, according to a new study published in the Future of the Earth (Earth's Future) on May 8.

Jack Osterman, a geophysicist at Columbia University who was not involved in the study, said: "most people believe that melting ice is causing global sea level rise. But when we talk about a particular location, this is only part of the reason for the relative sea level rise here." Land subsidence caused by various factors is another important factor to consider, she said. Osterman added: "if you stand on the coastline, whether it is land subsidence or sea level rise, it will lead to flooding."

The weight of a simulated city in this new study, scientists are trying to understand how the weight of the city itself causes relative sea level rise, and New York City happens to have a lot to study. " We entered the weight of all the buildings in the city, "said the lead author of the study, geophysicist Tom Parsons (Tom Parsons) of the U.S. Geological Survey." for example, in lower Manhattan, we are building a mountain here, and the weight of all the buildings is pressing down the land. "

The first step for the researchers was to analyze the weight of the city, which meant they counted all the buildings in New York City-all 1,084,954 buildings in five boroughs. Parsons and his colleagues mapped the city on a grid and looked at a database that includes the area and total height of each building in the city. According to the building code, the researchers estimated the weight of each grid square and estimated the total weight of all buildings in New York City at about 764 billion kilograms. "the weight is not entirely accurate, but it gives us a general idea of how dense the building is," Parsons said. " (to simplify the calculation, the research team did not take into account the weight of other transportation infrastructure on the surface. )

Next, the researchers mapped the geology of New York City. In areas such as Midtown Manhattan, where bedrock is shallower below the surface, there is less loose sediment that can be compressed, so there is less subsidence affected by weight. On the south coast of Brooklyn, however, the area has been expanded by artificial filling. However, the artificial fill is composed of a variety of materials, which is not as dense and stable as the bedrock, and is easily affected by the upper pressure. In other areas, the geology is somewhere in between.

The researchers input the distribution of buildings and geological conditions into the model to predict the response of different geological environments to overlying pressure, thus finding areas where the weight of the city is particularly serious.

Based on the results of the model, the researchers also compared satellite data to see how much the city had actually sunk in the past decade. The researchers found that New York has sunk by an average of 1mm to 2mm a year over the past decade, and that urban weight is the main reason for the overall settlement of the city. Especially in the areas where the building weight is concentrated on the loose sediment, the settlement is particularly serious.

The settlement rate of New York City shown by satellite data (top left), the model simulated settlement rate of New York City (top right) and the geological environment of different areas of New York City (bottom) (image source: original paper) although there are many factors affecting urban settlement, this study is not conclusive, but this finding is the first step in understanding how the world responds to relative sea-level rise. "as [the researchers] emphasized," Osterman said.... the comparison between data and models is very complex, and there are a lot of things we don't understand. " He added that the study was basically based on a rough estimate of settlement based on the city's weight, rather than an accurate calculation.

Another limitation of the study is that scientists are unable to simulate the city's development over the past 400 years to fully capture and predict its weight and any associated subsidence over the next few years. Physicist Kathleen Jones (Cathleen Jones, not involved in the study) at NASA's Jet Propulsion Laboratory (JPL) said: "they model the overall settlement based on the initial load of all buildings, as if they were built at the same time in some way and magically appear on uncompressed soil or rock at the same time. Obviously, these buildings were built at different times, so this part of the model is not in line with reality, "she added, adding that this is only her own point of view and does not represent NASA or JPL. However, Jones also pointed out that the team's long-term settlement estimates are closer to reality and match satellite observations.

Although the new study only studies New York City, it is a wake-up call for people around the world: coastal cities everywhere are facing some problems. According to calculations, Manhattan is sinking at a rate of 2.1 millimeters a year, reaching 4.5 millimeters a year in some areas.

At present, about 40% of people live within 100 kilometers of the coastline, and nearly 70% of people are expected to live in cities by 2050. Studies have found that 44 of the 48 densely populated coastal cities are sinking faster than sea levels are rising. Such figures mean that cities vulnerable to rising sea levels may also be threatened by the weight of their own buildings. Although the bedrock in much of New York City is stable enough, other cities are at greater risk. "most of the world's coastal cities are expanding rapidly," Parsons said. "given the changes in population distribution, this may be a growing concern."

Original text link:

Https://www.scientificamerican.com/article/new-york-city-is-sinking-under-its-own-weight/

Related papers:

Https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022EF003465

This article comes from the official account of Wechat: global Science (ID:huanqiukexue)

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