New York City is sinking slowly but surely into the Atlantic Ocean. Researchers have recently shown that the city is getting 1 to 2 millimeters closer to sea level every year. Combined with sea level rise and intensified storms, this could increase the risk of devastating floods.
“Most headlines say skyscrapers are the real problem, but that’s not the case,” says Tom Parsons of the United States Institute of Geological Studies, who led the study.
Although some parts of the city are built on artificial land, obtained by pouring sediment into the water, most of the heaviest skyscrapers in the Big Apple are built on solid foundations. The slow sinking of the city towards the shore has much more to do with geology than with heavy construction. In the case of New York, the sea is actually rising faster than the land is sinking, adds Tom Parsons.
“Without sea level rise, the problem wouldn’t really arise,” he adds.
However, this is not the case for many other cities around the world. From Jakarta, Indonesia, to New Orleans, USA, many are sinking much faster than the water rises to swallow them up.
Each year, the average sea level rises by about 2.5 millimetres. In some parts of Jakarta, however, the relative annual sea level rise can be closer to 25 centimeters. This is explained by the fact that the water is not only rising: the city is sinking.
“You have to add up these two effects,” says Pietro Teatini, a civil engineer at the University of Padua, Italy, and chair of UNESCO’s Land Subsidence Initiative (LaSII). “This is called relative sea level rise: it is the rise in sea level combined with land subsidence. »
Subsidence, or land subsidence, occurs when human activity or natural forces cause parts of the Earth’s surface to sink. This can cause problems inland and on the coasts. Although subsidence has many causes, including human activities such as pumping groundwater and building cities on loose sediment, it is usually not the direct result of climate change.
Due to a frenetic accumulation of human factors and an unfortunate geography, Jakarta is one of the fastest sinking cities in the world. With over 40% of the capital’s surface area below sea level and storms likely intensifying due to climate change, flooding is becoming frequent and devastating, to the point that the Indonesian government is considering moving its capital to another island.
Humans need water. However, when cities turn to aquifers, they can unwittingly create huge subsidence issues. “I can say without a doubt that [le pompage des eaux souterraines] is the leading cause of land subsidence worldwide. It is certainly the most important for cities,” says Pietro Teatini.
The rocks and sediments of some aquifers act like sponges: they are dotted with empty spaces, called pores, which are filled with water. If water is squeezed out, the pores may settle or shrink under the weight of the earth covering them. This is why groundwater pumping can lead to soil compaction.
In parts of Mexico City, scientists say groundwater extraction is causing land subsidence of up to 35 centimeters per year.
“The population has increased and needs water. So they take water from the aquifer,” explains geophysicist Shimon Wdowinski of Florida International University. “The surface reacts by subsiding. »
Over the past century, the Mexican capital has sunk about 10 meters, deforming buildings and disrupting infrastructure. Groundwater extraction is largely responsible, but the city’s unusual geography also contributes. Much of Mexico City was built on a filled in lake, and the old lake bed is soft, waterlogged, and quite squashed.
Rock pores can also contain hydrocarbons such as petroleum or natural gas. The extraction of these resources can also cause subsidence, adds Pietro Teatini, as has happened in the Netherlands and around Ravenna, Italy.
Development at and near ground level can also cause subsidence.
In some areas of the Netherlands, for example, the ground is subsiding by almost 4 to 5 millimeters per year. According to geologist Gilles Erkens, from the University of Utrecht and Deltares, a non-profit research institution, this is largely due to human intervention in wetlands.
The draining of wetlands to gain agricultural land can lead to soil degradation. When wetlands dry up, the oxygen in the air seeps into the soil and the microbes that breathe it in begin to eat the peat, turning it into carbon dioxide for energy. This phenomenon compacts and weakens the soil, which leads to subsidence.
Although it is mainly a rural problem, some Dutch cities, such as Gouda, are built on peat soils, explains Gilles Erkens. Because peat compacts easily, he adds, it is “susceptible to subsidence under load. This is what happens in cities. »
Building heavy cities on suspended sediments is not just a problem in bogs. Many large cities, from Shanghai to Jakarta to Cairo, are located on deltas, flat and often fertile plains at the mouths of rivers which, like peat bogs, are naturally spongy.
Over time, sediments in deltas naturally compress under their own weight. Normally, regular floods compensate for this phenomenon by bringing in fresh sediments. Nevertheless, cities often confine their waterways with levees to prevent flooding, which prevents new sediment from reaching the delta. Building a dam upstream can also interrupt the supply of sediment, leading to subsidence and flooding.
“A delta is only a delta if there is enough sediment coming from inland…without the sea level being too high and the land too subsided”, explains Gilles Erkens. “These three factors currently pose a danger to many deltas. »
The Earth’s surface is in perpetual motion: sediments naturally compact, the ground deforms and tectonic plates crack and collide. Incidentally, the sagging New York City skyline has its origins in the Ice Age, not modern construction.
“During the last ice age, a huge, heavy sheet of ice pressed down on the center of the continent,” says Tom Parsons. Just as the sides of an inflatable mattress fill with air when someone lays down in the middle, that extra weight in central North America has caused their ribs to rise.
“When the ice melted, the process reversed,” he continues. “This is what causes 2 millimeters [d’affaissement] per year all along the East Coast, including New York. »
The land beneath New York City would therefore continue to sink at about the same rate, even if the metropolis’ millions of buildings were removed.
While geological forces significantly reshape the Earth’s surface over long periods of time, human activity is almost always the primary cause of major subsidence, Pietro Teatini points out. Even in places like New York, where sags are minor and largely beyond human control, tiny drop-offs accumulate, especially when combined with accelerating sea level rise. sea due to climate change.
2023-06-20 07:00:00
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