Analysis of relative sea level change of the Tianjin coast in recent 25 years using satellite altimetry and GNSS observations

Objectives:  In recent 40 years, the risk of sea level rise in China's coastal areas is further increased with the acceleration of sea level rise, especially in serious land subsidence areas (e.g., Tianjin, Shanghai). However, it is difficult to know the real relative sea level (RSL) change at Tianjin coast due to time frame and subsidence correction of the public tidal data. To solve this issue, we propose an analysis method of RSL change by using the data of Satellite Altimetry and Global Navigation Satellite System (GNSS).   Methods:  The method is executed based on the idea of the collocating observation of GNSS and tide gauge. Meanwhile, to obtain RSL in different areas of Tianjin coast, we simulated 4 GNSS and tide gauge co-stations. Firstly, the absolute sea level (ASL) change and vertical land motion (VLM) of tide gauge stations are determined by using the data of satellite altimetry and co-located GNSS observations, respectively. Then, the relative sea level rise of Tanggu and four virtual tide gauge stations is calculated. Finally, the feasibility of our method is discussed based on multi-year leveling data.   Results:  The results show that the RSL rate was 13.45±0.45 mm/a at Tanggu tide station in the past 25 years, the RSL rate of four fictitious stations varied from 11.15±0.44 mm/a to 19.17±0.45 mm/a, and the mean rate along the Tianjin coast was 15.09±0.45 mm/a. Vertical land motion and its non-uniform distribution were the main influencing factors of the RSL rise and its regional differences, with the contribution rate more than 70%.   Conclusions:  Our research provides a new and feasible method for analyzing the RSL rise of Tianjin coast, however, it is still necessary to encrypt the tide observation facilities along the coast and retain and release the original tidal data, which can better serve the monitoring and research of sea level in Tianjin coastal area.