Image: Aerial view of Manhattan, New York, August 6, 2021 © AFP Ed JONES.
Subsidence (from the Latin subsidere, “to sink”) is a geological phenomenon that occurs when the ground sinks or sinks, often gradually, from its initial level.
Subsidence mainly affects populations living in areas close to the coast.
A study published in March 2021 in the journal Science identifies more than 200 regions threatened by subsidence in 34 countries, and estimates that this phenomenon will affect 635 million people by 2040, or 19% of the world's population. This is due to several natural and anthropogenic factors.
Natural subsidence can be caused by many geological processes.
- Plate tectonics: When the lithospheric plates move or overlap, this leads to vertical movements in the earth's crust, thus causing subsidence or subsidence of the ground.
- Volcanic activity: Volcanic eruptions and magmatic activity can also cause subsidence in regions close to volcanoes.
- Decompression of the lithosphere: When layers of rock are eroded or when glaciers melt, the underlying lithosphere can undergo decompression. This too can lead to gradual subsidence of the affected regions.
- Sedimentation and compaction: Sedimentary deposits, such as sands, clays and loose rocks, can compact over time under the effect of the weight of the upper layers. This compaction process can lead to subsidence.
- Dissolution of rocks: In some areas, soluble rocks such as limestone can be dissolved by groundwater, forming underground cavities and voids. When these cavities collapse, it can cause subsidence at the surface.
Anthropogenic subsidence is caused by human activities, water pumping, drainage of wetlands, deforestation, mining, tunnels, drilling, etc.
- Water extraction: Excessive groundwater extraction is one of the main causes of human-induced subsidence. When groundwater is pumped at a faster rate than it is recharged, the voids left by the pumped groundwater can cause the soil layers above to subsidence. The ceiling of these empty spaces can even collapse suddenly, causing a huge subsidence of the surface.
- Urbanization: Rapid urbanization and the expansion of infrastructure also contribute to the subsidence of cities. The construction of buildings, roads and other heavy structures puts pressure on the ground, which can cause soil layers to compress and gradually subside.
The natural and anthropogenic phenomena responsible for land subsidence, combined with the rise in sea level, considerably increase the risks of flooding of coastal cities.
Subsidence has become a serious problem in many parts of the world, which is why special construction techniques, such as the injection of filler materials into sunken areas, are used to mitigate the effects of subsidence.
Subsidence rates can vary greatly depending on soil geology, studies, specific assessments, areas, neighborhoods within cities, and time periods analyzed. All of this makes estimating subsidence rates complex and sometimes unreliable.
Although these rates are difficult to measure, many cities are partially or totally sinking by more than 20 mm/year.
Satellite data InSAR (Interferometric synthetic-aperture radar) shows that many coastal cities of the world are collapsing. This is revealed by a study published in Geophysical Research Letters for which the rates of 99 cities were measured between 2015 and 2020. The rate at which these cities sink can vary depending on several factors, such as the extent of groundwater extraction, local geology, urbanization, and other human activities. Subsidence rates can also vary in different parts of the same city.
Some cities are sinking faster than the rising waters due to climate change. If subsidence continues at the current rate, these cities will face flooding much sooner than predicted by sea level rise models (2 mm/yr).
The fastest subsidence is occurring in Asia, however, all continents are affected.
In 33 of the 99 cities studied, part of the city is sinking by 10 mm/year, i.e. 5 times faster than the rise in sea level. The highest subsidence rates appear in parts of Tianjin, Semarang and Jakarta.
Tianjin (northeast China): The rate of subsidence exceeds 40 mm/year (almost 20 times the average sea level rise). A subsidence of 50 mm/year is observed on the northeast just outside the city.
Semarang (Indonesia): located on the north coast of Java, the city undergoes subsidence of 20 to 30 mm/year on average. Subsidence rates in Semarang vary in different parts of the city. Studies have shown that parts of Semarang experience high sink rates, exceeding 100 mm/year.
Jakarta (Indonesia): An area on the northwest coast of the city is experiencing subsidence of up to 20 mm/year. In addition, Bekasi Regency, a suburb of Jakarta, has been characterized by subsidence of up to 50 mm/year in recent years, possibly due to groundwater extraction.
Chittagong (Bangladesh): Bangladesh's second largest city is experiencing significant subsidence (20 mm/yr) attributed mainly to excessive groundwater extraction for drinking water supply. Studies have indicated that in some parts of the city of Chittagong the sinking rate was up to 100 mm/year.
Manila (Philippines): the capital of the Philippines, is also facing problems of subsidence. Factors contributing to subsidence in Manila include excessive groundwater extraction, degradation of aquifers, tectonic movements and the geological nature of the ground. It exceeds 20 mm/year going up to 100 mm/year in certain parts of the city.
Houston (Texas, USA): Faces subsidence problems related to excessive groundwater extraction. Specific rates of sinkage may vary in different areas of the city, but some reports indicate rates of 20-30 mm/year.
Bombay (India): India's largest city faces subsidence problems mainly caused by excessive groundwater extraction, degradation of aquifers and the geological nature of the soil. Some studies have reported rates of around 20-30 mm/yr in some parts of the city.
Shanghai (China): One of the largest cities in China also faces subsidence problems of up to 20 mm/year in some parts of the city. Subsidence in Shanghai is mainly due to excessive groundwater extraction, degradation of aquifers and the geological nature of the soil.
Karachi (Pakistan): The largest city in Pakistan, also faces subsidence problems of up to 20 mm/year in some parts of the city. Subsidence in Karachi is mainly due to excessive groundwater extraction, degradation of aquifers and the geological nature of the soil.
New Orleans (Louisiana USA): Subsidence in New Orleans is mainly due to the degradation of organic and clay soils, as well as the extraction of hydrocarbons and groundwater . Subsidence rates in New Orleans vary, but parts of the city are sinking about 10 mm/year. It should also be mentioned that the city of New Orleans is particularly vulnerable to flood risk due to its location below sea level and land subsidence.
Tampa (Florida USA): subsidence of up to 6 mm/year is observed in a large area north of Tampa Bay.
Bangkok (Thailand): the capital of Thailand, faces subsidence problems of up to 10 mm/year. Subsidence in Bangkok is mainly due to the excessive extraction of groundwater, the geological nature of the soil, the consolidation of loose sediments and the increasing load on the ground caused by urban development.
However, some scientific sources report exceptional sinking rates exceeding 100 mm/year, such as in parts of the cities of Istanbul (Turkey), Tehran (Iran), Lagos (Nigeria), Jakarta (Indonesia), Taipei (Taiwan), Mexico City (Mexico), Ho Chi Minh City (Vietnam) and many more.