New York’s buildings weigh a total of 760 million tons. The weight compresses the ground beneath the city. At the same time, a phenomenon from the past is causing the city to sink. For residents, this means a high risk of flooding.
The mass of tall buildings in New York City slowly causes parts of the city to sink into the ground – sometimes by up to 60 centimeters. That’s the result of an analysis by a team led by Tom Parsons of the US Geological Survey, for which they added up the mass of all the buildings in the city. As the working group writes in the journal Earth’s Future, New York weighs a total of around 760 million tons.
How much the ground sinks in an area depends on how stable the material is underground – so the ground settles very unevenly under the weight of the buildings. The study aims to provide information on the role played by the mass of the building in the risk of flooding. Many coastal cities around the world are currently sinking much faster than sea levels are rising so that this effect currently plays a central role in the risk of flooding.
Entire coastal cities are sinking – the risk of flooding is increasing
It has long been known that buildings settle under their own weight and must be taken into account when planning high-rise buildings in particular. However, the long-term extent of such settlement processes and the large-scale effects on the terrain height of entire cities have not yet been investigated, the experts write. Large-scale subsidence is a major problem in many coastal regions, which can have very different causes.
The Indonesian capital Jakarta is currently sinking by almost eleven centimeters a year because groundwater is being pumped out of the depths. Other reasons for ground subsidence are large-scale compensatory movements after the end of the last ice age, oil and gas production or the natural settlement of sediments in river deltas.
New York is sinking – because glaciers are melting
On average, New York City is sinking at a rate of one to two millimeters per year. The city is among the top ten major cities in the world most at risk from flooding in terms of both population and property at risk. The sinking of the city as a whole is due to the fact that the glaciers of the last ice age dented the earth’s crust farther north, thereby raising the regions farther south like the other end of a seesaw. After the glaciers have disappeared, this hump is slowly sinking again.
Local subsidence plays a greater role in the consequences of flooding. To determine this, Parsons’ team divided the city into a grid of squares measuring 100 by 100 meters and calculated for each of these fields how great the load and thus the pressure on the ground is and how strongly the underlying material is compressed becomes. It then compared these results with satellite data. Unsurprisingly, it turns out that the greatest subsidence occurs where the subsoil is sandy or loamy, or where loose sediments lie beneath the buildings.
Overall, according to the working group, the results are inconsistent. While some areas in Brooklyn and Queens may actually experience the simulated settlements from the building load, satellite data shows that other areas are also sinking sharply regardless of building development. Presumably other factors play a role here, which make a much larger contribution to the lowering, for example groundwater extraction or erosion through drainage on the surface. Another problem is that most of the subsidence through a building takes place relatively soon after its construction and is therefore relatively difficult to prove.
2023-05-21 06:48:00
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