Land subsidence, caused by excessive groundwater extraction, has always been a problem awaiting a solution in Tianjin, China. Although the subsidence in the urban area is somewhat controlled, the subsidence issue has not been effectively solved in the suburban area. Recently, it has become apparent that it requires urgent attention.
Sinking cities are urban environments that are in danger of disappearing due to their rapidly changing landscapes. The most significant contributors to these cities becoming unlivable are sea-level rise, intensifying storms, storm surges, land subsidence, and accelerated urbanization.
On March 24, Geophysical Research Letters published research about Subsidence in Coastal Cities Throughout the World Observed by Interferometric Synthetic Aperture Radar (InSAR). They measured subsidence rates in 99 coastal cities worldwide between 2015 and 2020. They concluded, “If subsidence continues at present rates, these cities will be challenged by flooding much sooner than projected by sea-level rise models. The most rapid subsidence occurs in South, Southeast, and East Asia. However, rapid subsidence is also happening in North America, Europe, Africa, and Australia.”
According to this research, four populous cities with fast subsidence are Tianjin (China), Chittagong (Bangladesh), Manila (Philippines), and Karachi (Pakistan). The combined population of these four cities is 59 million. The maximum subsidence rate in Tianjin exceeds 40 mm/yr (line-of-sight [LOS] displacement—almost 20x mean sea level rise), in Chittagong and Manila exceeds 20 mm/yr LOS (nearly 10x average sea-level rise), and Karachi exceeds 10 mm/yr (approximately 5x average sea level rise). All the fast-subsiding areas are either residential or industrial, suggesting that the subsidence is likely related to excessive groundwater extraction.
In Tianjin, the land subsidence rate gradually spread from the urban area of Tianjin to the suburbs, and high subsidence zones are distributed around the junction of the Wuqing, Xiqing, and Beichen districts. As the subsidence continues, it could cause the slow lowering of ground surface elevation resulting in critical infrastructure damage.
To control Tianjin suburbs’ ground subsidence, the Chinese Academy of Sciences, after Ground Subsidence Analysis based on Sentinel-1A Data using MT-InSAR methods, suggested three measures:
- Improve the regional ground subsidence monitoring network dynamically;
- Optimize the monitoring methods and improve the level of survey;
- Strengthen the supervision of laws and regulations, and increase publicity and education.