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Source: Intellectuals
Author: Li Shanshan
The pattern of land subsidence in large and medium-sized cities in mainland China. Each circle represents a city, and the three parts of the circle are three subsidence indicators: the proportion of area of all subsidence areas (P3 mm), the proportion of area of areas with a subsidence rate faster than 1 cm/year (P10 mm), and the 5th percentile subsidence rate (V5th) of all areas within the city. The gray section circles five urban areas with relatively rapid subsidence rates. The figure below on the left shows the statistical values of the three indicators at the national scale (i.e., all cities combined).
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The urban surface can move downward for a variety of reasons, which is called land subsidence. This process is generally slow, but long-term development can lead to damage to building foundations, cracking walls, broken pipes, etc., so urban land subsidence is also known as a "chronic disease" that restricts urban development.
However, urban land subsidence is a phenomenon that almost all countries in the world cannot escape.
In Houston, USA, more than 8,000 square kilometers of subsidence have been subsided, with some areas accumulating more than 3.5 meters, in Mexico City, a large area has reached a staggering rate of 40-50 centimeters per year, in Iran, more than 90% of the country's land area is facing subsidence, with a land subsidence rate of up to 6 cm/year, and in 2022, Indonesia decided to move its capital due to the rapid land subsidence in the capital Jakarta - at that time, the subsidence rate in parts of Jakarta reached almost 15 centimeters/ According to estimates at the time, nearly 27% of Jakarta, with a population of 10 million, would be submerged by sea by 2025.
Recently, the international academic journal Science published a research paper titled "A national-scale assessment of land subsidence in China's major cities". Researchers from 28 institutions, including the Chenghuan Institute and the Beidou Research Institute of South China Normal University, used satellite remote sensing data to assess the degree of land subsidence in 82 large and medium-sized cities across the country. The study found that 44.7% of these cities had an average annual subsidence rate of more than 3 mm/year, and 15.8% of the area was faster than 10 mm/year.
The study attracted widespread attention when it was published. Yesterday, the topic of "nearly half of China's largest cities are sinking" even rushed to the hot search list of social media, is this really the case? Why does subsidence occur? How should we deal with land subsidence? What efforts is the mainland making?
To answer these questions, The Intellectuals contacted the authors of the article to further explain the study, which is closely related to the land of China.
Urban land subsidence in China
And not worse than other countries
"The problem of land subsidence on the mainland is no more serious than that of other countries and regions. The authors of the article, Tao Shengli and Hu Xie, researchers at Peking University's School of Urban and Environmental Sciences, told Intellectuals.
"In terms of median, the median rate of land subsidence in the country's 82 large and medium-sized cities is about 2 millimeters per year, which is not high compared to other countries. For example, the journal Science reported in 2022 that more than 90% of Iran's area is subsided, with an overall subsidence rate of about 6 cm per year, and a new Nature study shows that between 2007 and 2020, the median annual subsidence rate of 32 cities in the coastal region of the United States is concentrated in 2 ~ 6 mm/year, and most of them exceed 3 mm/year. ”
"Even considering the extreme value (quintile) of the rate of land subsidence in all 82 large and medium-sized cities in the country, we find that the rate is 2.2 centimeters per year. Compared to the world's faster subsidence rates of 40-50 cm per year in large areas of Mexico City and about 36 cm per year in Tehran, the capital of Iran, 2.2 cm/year is still low. ”
"In fact, the headline of many reports is wrong: 'Nearly half of China's cities are sinking', and strictly speaking, the claim that 'cities' are sinking is unscientific: almost every city has a sinking and rising area, and what matters is the proportion of area and the rate of subsidence. Just because part of a city is sinking doesn't mean that the whole city is sinking, or that all of the city's inhabitants are exposed to the risk of land subsidence. Our study is based on the statistics of population and land subsidence per unit area. ”
"While comparing with international cities, it is also important to note that different countries have different stages of economic development. Land subsidence is often associated with economic development. Some industrial areas in the United States were heavily subsided, in the twenties and thirties of the last century (e.g., San Jose, California, where some areas subsided by 1.2 meters from 1912 to 1933) and in the fifties and sixties (Pasadena, Houston, where some areas subsided by 2.3 meters between 1943 and 1973). In Tokyo, Japan, in the 50s of the last century, it even reached a sedimentation rate of 27 cm/year. The mainland's economy is developing rapidly, but the rate of subsidence is not as serious as it once was."
In the face of urban land subsidence,
How do we respond?
How to deal with urban land subsidence? First of all, we must understand why cities are subsided. The researchers mentioned in the paper that the first factor affecting subsidence is the natural geological environment in which the city is located.
"For example, if a building is built on soft soil, it will settle slowly due to its own weight. Therefore, the building should be surveyed in detail before construction, and the foundation depth should be designed reasonably."
Another major factor in subsidence is groundwater extraction, which is also the main cause of land subsidence in some Chinese cities, represented by the Beijing-Tianjin-Hebei region. In particular, the article mentions that many of the measures taken by the mainland government, such as the South-to-North Water Diversion, are reversing urban land subsidence in the north. In 2012, the Ministry of Water Resources issued a nationwide targeted document, the National Plan for the Prevention and Control of Land Subsidence (2011-2020). After the water transfer project of the South-to-North Water Diversion Project, land subsidence in many places in Beijing and Tianjin has stopped or even begun to rise. Shanghai, on the other hand, has long enforced strict building standards, reduced groundwater extraction and accelerated groundwater recharge year by year. At present, the subsidence area in the center of Shanghai has basically disappeared, which is quite difficult for a coastal city built on an alluvial delta. The effect of these policies in our country is obvious. ”
In the study of urban land subsidence, cities in coastal areas tend to receive more attention. Coastal cities are often built on alluvial plains with soft geology, and the geological conditions are inherently prone to subsidence. Coastal cities around the world are often economically developed and densely populated, which also means that the demand for water resources is huge, and there is often a risk of over-extraction of groundwater. But coastal cities don't just face troubles: they also have to deal with rising sea levels.
"In fact, the study of urban land subsidence has received increasing attention in recent years, and one of the main triggers is climate change: climate change is causing sea levels to rise, so more research has begun to focus on whether coastal cities are at risk of inundation. This risk increases if coastal cities are also experiencing land subsidence. For example, a study titled "Disappearing cities on US coasts", just published in Nature in March 2024, shows that by 2050, about 1,000-1,300 square kilometers of land in 32 coastal cities in the United States could be inundated due to the dual effects of land subsidence and sea level rise, affecting 5-270,000 people
Population".
"Our study makes a similar exploration: assuming that the current rate of urban subsidence remains constant, and assuming the maximum possible level of sea level rise, in a hundred years, large and medium-sized coastal cities on the mainland will have up to 26% of the ground
The relative elevation will be lower than the sea level of the same period. Note that a relative elevation below sea level does not necessarily mean that you will be submerged. The article mentions that this is because our country has built a huge coastal dam system, and shows the dam construction site in Shanghai as a saying
Bright. In fact, the mainland coast
The scale of the dam is rare in the world." Tao Shengli and Hu Xie said.
Continuous monitoring of sedimentation is the beginning of the search for a solution
The original intention of this study is to respond to the needs of national land monitoring, to identify where mainland cities are subsided, and then to take targeted measures. In a review article published in the current issue of Science, Robert J. Nicholls, an expert on climate change and land subsidence at East Angli University, commented: "In order to guide the implementation of reasonable measures to prevent or mitigate their consequences in inland and coastal areas, accurate mapping of urban land subsidence is essential...... Continuous monitoring of settlement is a great achievement, but it is only the beginning of the search for a solution......"
This large-scale monitoring of land surface deformation benefits from the development of satellite remote sensing technology. The data used in the subsidence study of China's large and medium-sized cities came from the European Space Agency's (ESA) remote sensing satellite Sentinel-1, which is freely available to researchers around the world, using observations from Sentinel-1 from 2015 to 2022. Sentinel-1 was launched in 2014 and carries a C-band synthetic aperture radar
(SAR) antennas, SAR sensors measure the round-trip propagation distance from the sensor to the ground by the time difference between the time difference between emitting and receiving electromagnetic waves, thus measuring surface deformation.
Although the mainland SAR satellite started late, it has developed rapidly. The GF-3 satellite, launched in 2016, was the first civilian C-band SAR satellite in mainland China, followed by a series of civil SAR satellites such as L-band Land Exploration No. 1, S-band Environmental Disaster Reduction No. 2, and L-band Land Exploration No. 4.
In the future, domestic satellite data will play an increasingly important role in the monitoring of land subsidence in the mainland and even the world.
The increasingly powerful satellite network provides us with a more accurate understanding of the deformation of the land in China, which reflects the health of the land below the ground, and all this will be the beginning of our search for solutions.
The papers mentioned in the article are available at: https://www.science.org/doi/10.1126/science.adl4366