Like many cities, New York City is slowly sinking. But the weight of the many skyscrapers is accelerating the process in Manhattan. What is happening in the metropolis and how can the sinking be stopped?
New York City still has a lot of catching up to do to protect itself from flooding. Image source: Unsplash
764 million tons cause New York City to sink
Rising sea levels are bad news for many cities. The US metropolis of New York City, for example, is sinking by around one to two millimetres per year. This increases the risk of flooding even further. As a result, the city could be significantly more affected by flooding in the future. The combination of rising sea levels and a sinking city is fatal, and the weight of the many skyscrapers in Manhattan exacerbates the effect.
There are over one million buildings in New York City. The skyscrapers in particular are very heavy at 1.5 to 2 tons per cubic meter. One of the heaviest buildings is the famous Empire State Building made of steel, granite and limestone, weighing around 370,000 tons. This is not a problem on rock, but if a skyscraper stands on softer ground near the coast, it contributes to sinking.
According to estimates by a group of experts led by geophysicist Tom Parsons, published in the journal Earth’s Future, the city’s buildings weigh around 764 million tons. The different and very diverse types of surface in the city behave differently under load. But the clayey soils with artificial fill, which are widespread in the Brooklyn district, for example, give way slightly under weight. The subsidence potential is between 7.5 and 60 centimetres with building development – with other soils only 6 to 12 centimetres and with rock only 0.5 centimetres can be expected.
The many skyscrapers in New York City are contributing to the city sinking. In combination with rising sea levels, this is very dangerous. Image source: Unsplash
Lower Manhattan just above sea level
The scientific team warns against being too careless in dealing with the double threat of sinking ground and rising sea levels. Since 1950, the water level on the coast of New York has risen by around 23 cm. And the risk of rising sea levels is particularly high on the east coast of the USA.
Hurricanes also occur frequently here and can lead to flooding. In 2021, for example, Hurricane Ida showed that New York City’s drainage systems had major problems coping with the large volumes of water. If buildings come into contact with salt water too often, materials such as concrete and steel can weaken.
Other problems in New York City, according to the study, include a lack of floodplain standards, additional subsidence from groundwater extraction and a lack of sediment in New York Harbor. The latter used to come from the East River and the Harlem River and protected the city from strong north-easterly winds and hurricanes.
Lower Manhattan is particularly at risk, as the southern tip of the central, densely populated borough is only one to two meters above sea level. But Staten Island, where the building load is significantly lower, is also sinking due to soil quality and natural subsidence processes.
The Empire State Building alone weighs around 370,000 tons. Image source: Unsplash
Sinking into cities worldwide
90 percent of buildings in New York City are not built for flooding. Even after Hurricane Sandy in 2012, new construction did not follow floodplain standards. Instead, the city continues to sink and it seems that there are no innovative solutions yet.
A look at other cities shows that New York is not alone with this problem. Jakarta in Indonesia, for example, is also sinking very quickly – much faster than the sea level has risen. Parts of the Indonesian capital are sinking at a rate of 2 to 5 centimetres per year. And Mexico City, although not a coastal city, is sinking by up to 50 centimetres per year due to the soft ground, as the city is located on a former lake.
The first effect of sea level rise can be observed below the surface. This is because the supply lines located there, such as cables, pipes and foundations, suffer from the salt water. Storms and floods exacerbate the problem. And with increasing urbanization, the demand for groundwater increases, which damages the soil and can contribute to subsidence. It therefore makes sense to find more sustainable ways of meeting a city’s water needs.
As in many other cities, the buildings in New York are not built for flooding - and even after Hurricane Sandy, there were hardly any improvements. Image source: Unsplash
New York City must plan for rising sea levels
What can New York do now? Solutions for sinking cities always depend on the context. One approach is to declare a building freeze in order to at least slow down the sinking. This means a building freeze on soft ground. Instead, new buildings in New York City would be better built on the much more stable rocky ground.
Some cities have had success in slowing down water extraction from groundwater and aquifers. However, the most common solution to rising sea levels is to build barriers. In London, for example, there is the Thames Barrier, which is intended to protect the city from flooding from the Thames until at least 2040. Venice has designed MOSE, an intelligent gate system that closes at high tide to protect the city from flooding. And Tokyo is working with concrete dykes, walls, pumping stations and flood gates as well as early warning systems and evacuation drills.
New York City is still planning: 4 billion dollars (3.65 billion euros) are to be invested in walls, improved drainage systems and raised roads. But it is still unclear when the metropolis will actually start to tackle this urgent problem.
By the way: Read more about New York City and its five districts in our city portrait.