Impacts of Land Subsidence on Flood Inundation: An Example of the "2022.8" Levee-Breach in Raoyang River of Liaohe Delta

Since the beginning of the flood season in 2022, nearly 90% of the area of the Raoyang River Basin has experienced torrential to heavy rainfall, causing the largest flood in this basin since 1951. A levee-breach flood of the Shusi section of the left embankment of the Raoyang River occurred at 10:30 am on 1 August, triggering widespread flooding and inundation, and causing serious damage to houses, crops and oil production facilities in cities along the river and their surrounding areas. In addition to the huge amount of rainfall brought by the heavy rainfall, land subsidence may have been an important factor contributing to the inundation of the "2022.8" levee-breach flood in the Raoyang River. Objectives: Analyzing the spatial and temporal distribution of land subsidence in the Liaohe delta, and investigating its impacts on the inundation extent of the "2022.8" levee-breach of the Raoyang River. Methods: We adopted the coherence-based SBAS (Small Baseline Subset, SBAS) method to process the time series of ALOS-1 and Sentinel-1 SAR data, and combined with the levelling data to obtain the distribution and evolution of land subsidence in the study area in different historical periods. We used surface deformation monitoring data from two GNSS continuous stations in the study area to validate the accuracy of the InSAR-derived surface deformation. Based on three GF-3 SAR acquisitions, we applied the water index to extract the levee-breach flood inundation of Raoyang River and to compute the affected area. The distribution of land subsidence was superimposed on the inundation extent of the flooding to emphasize the importance of taking into account land subsidence in the assessment of flood risk. Results: (1) oil extraction caused an elliptical land subsidence funnel in Shuguang oilfield, with a maximum rate of -212 mm/a at the subsidence center. (2) the Raoyang River passes through the Shuguang oilfield subsidence funnel area, the flood inundation extent caused by this dike breach highly overlaps with the ground subsidence area, and the rate of subsidence at the breach of the embankment is -150 mm/a. Conclusions: Land Subsidence plays an important role in the occurrence of levee-breach and flood development. On the one hand, the elevation loss caused by land subsidence destroys the safety and stability of embankment, makes it easier for the oil pipeline to be immersed under the water level, and causes seepage damage on the joint surface of the pipeline through the embankment, which triggers the occurrence of the breach; on the other hand, the elevation loss caused by land subsidence results in the formation of a subsidence funnel on the land surface, and reduces the river's capacity to discharge floodwaters, which leads to water more easily to get out of the river, and to be retained in the subsidence funnel basins, increasing the extent and depth of flood inundation. Our study highlights the importance of considering surface subsidence in flood risk assessment, providing a scientific reference for regional flood protection and planning.