Human activity and natural forces cause many places on the Earth’s surface to sink, threatening many densely populated cities .
New York City is slowly sinking into the Atlantic Ocean. Recent researchers have shown that the city is sinking 0.1 to 0.2 centimeters per year. Combined with rising sea levels and intense storms, this can increase the risk of severe flooding. According to Tom Parsons, a researcher at the US Geological Survey, while most of the city is built on man-made soil, most of the heaviest skyscrapers here are built on solid foundation rock. The city’s subsidence has more to do with geology than construction. Sea levels are rising faster than the city is sinking. However, many other cities around the world from Jakarta, Indonesia to New Orleans are sinking much faster than sea levels are rising, according to National Geographic .
Skyscrapers in Manhattan, New York. (Photo: Ecowatch).
Each year, the average global sea level rises by about 0.3 cm. But in some parts of Jakarta, the annual relative sea level rise can be as close as 25 centimeters. The reason is not only due to the high tide, but the city is also declining. “We need to combine these two effects,” said Pietro Teatini, a civil engineer at the University of Padova, Italy. “This is what people still call relative sea level rise , which means sea level rise combined with land subsidence.”
Land subsidence occurs when human activity or natural forces cause many places on the Earth’s surface to lower. This situation can cause problems for both land and coastal areas. Land subsidence has many causes (including human activities such as pumping groundwater and building cities on soft sediments), which are not a direct result of climate change. Due to a number of unique geological and factors, Jakarta is among the fastest sinking cities in the world . With more than 40% of the city now below sea level and increasingly violent storms due to climate change, flooding in Jakarta has become so frequent and severe that the Indonesian government is planning to relocate the capital to the island. other island.
Cities create huge subsidence problems when mining underground aquifers. According to Teatini, groundwater pumping is the main problem causing land subsidence worldwide. Rocks and sediments in underground aquifers act as sponges, filled with holes filled with water. As the water is sucked away, those holes can shrink under the weight of the soil above. This is why pumping groundwater causes the soil to compact.
In Mexico City , scientists say groundwater extraction causes the ground to sink 36 centimeters per year. “Population increases demand for water. So they draw water from groundwater and the ground responds by subsidence,” said geophysicist Shimon Wdowinski of Florida International University.
Over the past decade, the Mexican capital has sunk about 10 meters, buckling buildings and destroying infrastructure. The main cause is groundwater extraction, but the city’s unusual geography also contributes. Most of Mexico City was built on a leveling lake, the old lake bed is very soft, soggy and easily squeezed. These rock holes can be filled with hydrocarbons such as oil or gas. Exploitation of this resource also causes subsidence as in the Netherlands and around Ravenna, Italy.
In many places in the Netherlands , the ground is sinking 0.4 – 0.5 cm/year. Geologist Gilles Erkens of the University of Utrecht and Deltares, a nonprofit research institute, says the main cause is human intervention in the marshes. Dredging swamps for arable land can cause land to disappear. As the marsh dries up, oxygen from the air enters the soil and bacteria begin to eat the peat and convert it to carbon dioxide for energy. The process causes the ground to shrink and weaken, leading to subsidence.
Although this is mostly a rural problem, some Dutch cities like Gouda are built right on peat land, according to Erken. Because peat is easy to compact, it is very susceptible to subsidence by load. Building heavy cities over soft sediments is not just a problem in peatlands. Many major cities, from Shanghai, Jakarta to Cairo, lie on river deltas, the fertile flat land at the mouth of the river having a soft, peat-like character.
Alluvium in the river delta is compressed under its own weight over time, but frequent flooding will compensate through the provision of new silt. Cities often build dykes along rivers to prevent flooding, but also prevent new silt from reaching the delta. Dam construction in the upper reaches of the river also impedes the transport of silt, leading to subsidence and flooding.