A multi-pass InSAR analysis of the estuarine alluvial Chongming Island ground displacements with implications on flood risk

Chongming Island of Shanghai, the largest estuarine alluvial island of China, has been kept seaward expanding due to continuous riverine sediment deposits and extensive land reclamation. Moreover, the island is highly vulnerable to long-term ground deformations and rising sea levels. To evaluate the influence of coastal ground deformation on flood risk, we investigated long-term ground deformation on Chongming Island over the last decade using RADARSAT-2 and Sentinel-1A satellite datasets collected from 2012 to 2023. The independently achieved ground deformation time series, representing the projection of ground displacement along the radar line of sight (LOS) direction (recovered using traditional multi-temporal interferometric SAR algorithms), were then jointly combined. The results show that the eastern region of Chongming Island has been experiencing significant ground subsidence over the last decade, with a maximum amplitude exceeding 20 mm/year. The significant ground subsidence in the eastern island occurred at a rate at least 2.5 times that of the concurrent sea level rise (SLR). Furthermore, we analyzed the coastal flood risk on Chongming Island under the comprehensive effects of local ground subsidence, SLR, and storm surges. To this aim, a hydrodynamic model was applied to simulate various inundation scenarios on Chongming Island after failures of different seawall sections. As the major outcome of our work, under the simulated scenario (e.g., the worst case), the total flood inundation area in 2042 was estimated to exceed 50% of the island. Under this worst scenario, almost all of the salt marsh vegetation in Chongming Dongtan Nature Reserve would be flooded after the relative SLR. The loss of elevation in the low-lying island further aggravates the coastal flooding risk. Therefore, preserving the stability of eastern and northern seawalls is essential to the security of Chongming Island in the context of the increasing flooding risk in the low-lying river estuary.