Land subsidence of the Yellow River Delta in China driven by river sediment compaction.

Many of the world's largest deltas are sinking due to multiple natural and anthropogenic causes. This is particularly evident for the modern Yellow River Delta (YRD) in China, which is one of the most dynamic coastal systems on Earth. The YRD has experienced complicated patterns of accretion and erosion as well as significant compaction settlements. However, spatiotemporal variability and the long-term settlement rates law in this complex delta system remain poorly understood. Evidently, the surface settlement is supposedly controlled by a long-term natural compaction process of sediments. We first combined the Small Baseline Subset (SBAS) Interferometric Synthetic Aperture Radar (InSAR) method with a geotechnical model and geological, geomorphological, hydrogeological, and geotechnical data to assess the delta-wide long-term spatiotemporal settlement triggered by the consolidation and compaction of river sediments. The combination of satellite and field observations allows us to gain insights into the primary processes controlling surface movement. A total of seventy-five SAR images acquired by ERS and Envisat from 1992 to 2010 are used to generate three independent interferometric subsets calibrated with leveling to unveil spatiotemporal settlement variability. The densely distributed spatiotemporal measurements enable us to acquire the characteristics of the spatiotemporal variations of land movements. Moreover, the annual average settlement rates are observed within a range of 0 to >30 mm. Results suggest that the relative inland stability of the delta contrasts with the decreasing coastal margin trend at an average annual rate of 15 mm. Moreover, the variability is significantly correlated with the delta evolution and variations in subsoil architecture. A comparative analysis has also been conducted between time series InSAR measurements and the theoretical estimates of settlement derived from the geotechnical model. The strong agreement between the InSAR measurements and the geotechnical modeled results indicates that long-term settlement (in a decade-to-century scale) is primarily driven by the compaction of river sediments. The more the delta sub-lobe was newly formed, the more significant the settlement. Decreasing trends in annual settlement rates from approximately 70 mm to 0 mm in the long-term deposit compaction process are also identified. These findings are useful to understand the YRD morphological evolution and may provide insights into the changes in other deltas worldwide.