Major cities are drowning due to skyscrapers and groundwater pumping out: how does this threaten the world and what to do - ForumDaily
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Major cities are sinking because of skyscrapers and groundwater pumping: what threatens the world and what to do

On September 27, 1889, the workers completed the construction of the tower. It was an 11-story building, which, thanks to its steel frame, is considered the first skyscraper in New York. The building is long gone - its place on Broadway was occupied in 1914, and this construction marked the beginning of a building boom that has not stopped to this day. Years later, it became known that skyscrapers are not as safe for cities as previously thought. What is going on, according to the publication with the BBC.

Photo: IStock

USGS researchers estimate that 300 square miles (777 square kilometers) of New York contain 762 million tons (1,68 trillion pounds) of concrete, glass and steel. While this figure includes some generalizations about building materials, this huge tonnage does not include fixtures, fittings, and furniture inside these buildings. This does not include the transport infrastructure that connects them, nor the 8,5 million people who inhabit them. And that's just New York.

All this mass has an extraordinary effect on the ground on which it is built. This land, according to a study published in May, is sinking 1-2 mm (0,04-0,08 inches) a year, due in part to the pressure exerted on it by city buildings. Add land subsidence to sea level rise and the relative sea level rise is 3-4 mm (0,12-0,16 inches) per year. This may not seem like much, but in a few years it will create serious problems for the coastal city.

New York has already been suffering from subsidence since the end of the last ice age. Shedding the weight of the ice sheets, some of the east coast land is expanding while other parts of the coastal landmass, including the chunk that New York sits on, are sinking.

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But the sheer weight of urban development exacerbates this subsidence, says Tom Parsons, research geophysicist at the USGS Pacific Coastal and Marine Science Center in Moffett Field, Calif., and one of the study's four authors.

Not only New York

And this is a global phenomenon. New York, according to Parsons, "can be seen as analogous to other coastal cities in the United States and around the world, whose population is growing due to people immigrating to them, who are associated with urbanization and face rising sea levels."

There are many reasons why coastal cities are sinking, but it is the mass of human infrastructure that is pushing against land that plays a role. The scale of this infrastructure is enormous: in 2020, the mass of man-made objects exceeded the mass of all living biomass.

Can anything be done

Some cities around the world, such as Jakarta, the capital of Indonesia, are sinking much faster than others.

"In some cities, we're seeing subsidence of several centimeters a year," says Stephen D'Hondt, professor of oceanography at the University of Rhode Island Narragansett. At this rate, the city is sinking much faster than the sea level is rising.

In addition to co-authoring the New York study, D'Hondt is one of three authors of a 2022 study that used satellite imagery to measure subsidence rates in 99 coastal cities around the world.

“If subsidence continues at the same pace, these cities will experience severe flooding much sooner than anticipated,” D’Hondt and colleagues Pei-Chin Wu and Matt Wei of the University of Rhode Island wrote.

Southeast Asia is high on the list of cities experiencing the fastest subsidence. Parts of Jakarta are subsiding at 2–5 cm (0,8–2 in) per year. Along with Jakarta, there were Manila (Philippines), Chittagong (Bangladesh), Karachi (Pakistan) and Tianjin (China). These cities are already suffering from infrastructure damage and frequent flooding.

Meanwhile, although not on the coast, Mexico City is sinking an astounding 50 cm (20 inches) per year due to the Spanish draining its underlying aquifers when they occupied it as a colony. Studies have shown that it could take another 150 years before this sinking stops - or about 30 m (98 ft) of additional subsidence.

But it is coastal cities that are the focus of D'Hondt and colleagues' research. Most of Semarang in Indonesia, for example, is sinking 2-3 cm (0,8-1,2 inches) per year, while a large area in northern Tampa Bay, Florida is sinking 6 mm (0,2 inches). ) in year.

Matt Wei believes that some level of this drawdown occurs naturally. However, it can be significantly accelerated by man - not only due to the load of buildings, but also due to the extraction of groundwater, as well as deep-seated oil and gas. The relative contribution of each of these events, according to Wei, "varies from place to place, making it difficult to understand and address coastal subsidence."

Rising water causes damage long before it reaches land. The first effects of relative sea level rise, according to D'Hondt, appear below the surface of the earth.

“We have underground engineering communications, underground infrastructure, underground foundations for buildings. Water reaches these materials long before we see it above ground,” notes D'Hondt.

Solutions vary depending on the local causes of subsidence

One obvious approach, although with its own problems, is to stop construction. As Parsons explains, ground subsidence under buildings "is usually complete within a year or two of construction." Although much of New York City is made up of shale, marble, and gneiss, these rocks have a certain amount of elasticity and cracking that partly explains the subsidence. But clay-rich soil and artificial fills, which are especially common in Lower Manhattan, can cause the biggest subsidence. Thus, placing the largest buildings on the most durable will help reduce the downward trend.

Another solution, at least for some places, is to slow down groundwater abstraction and extraction from underground aquifers. Parsons and colleagues warn that growing urbanization will likely lead to more groundwater being extracted and coupled with even more construction to cope with the growing population. Therefore, more sustainable ways to meet the city's water needs and maintain groundwater levels should be sought.

However, the most common approach is a messy and imperfect program of construction and maintenance of flood defenses such as breakwaters. Tokyo's adaptation to land subsidence has two aspects. The city built physical structures—concrete dams, pumping stations, and locks. They are combined with social measures such as evacuation rehearsals and an early warning system.

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Sometimes residents themselves intervene. A 2021 study documented how the populations of Jakarta, Manila and Ho Chi Minh City were taking their own unofficial measures. Among them are raising floors, moving household appliances, and (in Manila) building makeshift bridges between houses in a swampy area.

Other useful tools include buffer tanks, which are large tanks that sit underground and release storm water at a controlled, slow rate. Martin Lumbly, drainage expert at pipe company Wavin, says shock-absorbing tanks should be combined with natural features such as ponds, ponds and wetlands.

“The problems we face today are fundamentally different from those that existed when city sewers and drainage systems were first introduced,” he says.

In 2019, the UN held a roundtable on floating cities, which can take the form of pontoon structures. Finally, halting climate change by eliminating greenhouse gas emissions will prevent or delay at least some of the melting of the polar ice sheets, slowing sea level rise.

“I think governments should be concerned,” D'Hondt said. “If they don’t want a massive loss of infrastructure and economic potential in a few decades, they need to start doing something right now.”

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