Distribution characteristics and evolution trend of severe land subsidence areas in Tianjin City

Tianjin has a long history of land subsidence, which has gone through six different stages since 1923. Although as of 2020, the land subsidence in a large area of Tianjin has been mostly controlled, there are still serious subsidence areas with an annual subsidence of over 50mm. From large-scale treatment to precise prevention and control of small areas, the distribution characteristics of ground subsidence in Tianjin have shown a new situation, and the prevention and control of land subsidence is also facing new requirements. To accurately grasp the development law of ground subsidence and implement precise policies under the new situation, this paper summarizes the distribution characteristics and evolution rules of severe land subsidence areas in Tianjin by collecting and analyzing the data of ground subsidence leveling, groundwater level, and groundwater extraction in Tianjin from 2010 to 2020. The results show that the land subsidence of Tianjin has experienced three periods during the ten years from 2010 to 2020: a fluctuation period in subsidence from 2010 to 2012, a steady improvement period from 2013 to 2016, and a rapid slowdown period from 2017 to 2020. During these periods, the average amount of land subsidence dropped by 37%, and the area of severe subsidence decreased by 67%. The changes in subsidence were closely related to the amount of groundwater extraction. As of 2020, five severe subsidence areas are concentrated in the southwest of Tianjin, with a distribution range that is similar to the groundwater funnel of the deep water-bearing group.