New York’s Buildings Weigh 760 Million Tons: Higher Risk of Flooding

New York is continuously sinking due to the weight of the buildings, which weigh around 760 million tons. This greatly increases the risk of flooding in many parts of the city.

Moffett Field (U.S.A.). A study by the University of Rhode Island (URI) recently published in the Geophysical Research Letters was published shows that numerous coastal cities are sinking rapidly. Because the sea level is rising at the same time, the risk of flooding is constantly increasing. There are many reasons for the subsidence of the ground.

As an example, the capital of Indonesia, Jakarta, is currently experiencing an annual deepening of almost eleven centimeters due to the extraction of groundwater from deeper layers of the earth. Additional influences that can lead to surface depression are extensive compensatory movements following the end of the last ice age, oil and gas extraction, and the natural compaction of sediments in river delta regions.

Buildings weigh 760 million tons

It is also known that large buildings slowly settle. 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. scientist of United States Geological Survey (USGS) used New York City as an example to analyze whether and to what extent the weight of the buildings influences the subsidence of the ground.

According to the publication in the specialist magazine Earth’s Future the researchers led by Tom Parsons first added the weight of the city’s buildings. In total, these weigh 760 million tons. The extent of soil depression is strongly dependent on the stability of the subsoil, which leads to significant fluctuations. Overall, the metropolis of New York City records a subsidence rate of one to two millimeters per year. However, the considerable weight load from the imposing high-rise buildings means that certain parts of the city sink by up to 60 centimeters every year.

Higher risk of flooding in New York

With regard to the number of inhabitants potentially affected and the material goods at risk, the city is one of the ten major cities in the world most at risk of flooding. The general subsidence process of the city results from geological processes associated with the last Ice Age. Back then, glaciers pushed down the Earth’s crust in more northern regions, causing it to bulge in more southern regions. With the retreat of the glaciers, this bulge is slowly beginning to recede and the risk of flooding is increasing.

Local subsidence exacerbates flooding

Parsons’ team developed an approach to quantitatively study how the effects of flooding are affected by city subsidence. To do this, they divided New York into a grid of squares with a side length of 100 meters. For each of these spaces, they calculated the load exerted on the floor and the resulting compression of the underlying material. These calculations were then compared with satellite data. Unsurprisingly, the strongest subsidence occurred where the soil consists of sand or clay or where loose sediments form the foundations of the buildings.

According to the research group, the study results are inconsistent. While simulated subsidence was actually observed in certain areas of Brooklyn and Queens, in other areas the satellite data revealed severe subsidence, independent of building development. Other influences seem to play a role here that make a significant contribution to subsidence, such as groundwater extraction or surface erosion through drainage. In addition, there is the challenge that the majority of building subsidence takes place shortly after the construction of the building and is therefore difficult to prove.

Geophysical Research Letters, doi: 10.1029/2022GL098477

Earth’s Future, doi: 10.1029/2022EF003465