## 3 Experiments
### Remote Sensing Investigations of Large Rivers Deltas  
#### 3.5.1 The Yangtze River Delta  
The Yangtze River is the third-largest river globally in terms of length and is one of the world's most important river deltas. The coastal vulnerability of the Yangtze River Delta is currently amplified due to the combined impacts of non-linear land subsidence related to groundwater extraction [259], instabilities of underground infrastructure constructions [182], large-scale land reclamation of ocean areas [397], an accelerated rate of SLR [406], tidal evolution [407], and natural hazards [408; 409]. Shanghai, one of the world's largest megacities with 24 million inhabitants, is located within the Yangtze River Delta zone and has an average population density of over 2000 inhabitants per square kilometer. Additionally, a continuous supply of fine-grained suspended sediments from the Yangtze River has yielded the development of extensive tidal flats along the deltaic coast. Large dams, like other large rivers, intensively regulate the Yangtze River. The sediment supply and the high total discharge rate in the Yangtze River Delta have promoted the development of sizeable ocean-land reclamation projects in the intertidal and wetland areas [410; 411]. The Yangtze River Delta is also experiencing highly rapid economic and population growth over the last fifty years. Rapid industrial development and urbanization have caused widespread ground subsidence in the delta region, not only in Shanghai but also in the mid-sized cities and many nearby small cities and towns. Homogeneous subsidence in the Shanghai suburban area and exceptionally rapid subsidence along metro lines, highways, and elevated roads have also been observed by InSAR-based time series measurements ([182; 412]). The detected land subsidence is mainly caused by groundwater exploitation and rapid urbanization [412]. Shanghai has also performed numerous large-scale ocean-reclamation projects [413]. The new lands are subject to long-lasting subsidence phenomena, which have been extensively studied in recent years using InSAR technology [240; 314; 415; 416; 417; 414; 417]. These investigations have clarified how to land subsidence of the reclamation projects combined with SLR can pose severe threats to population centers in the future [308; 314; 418]. The newly reclaimed lands may also advance the shoreline dozens of kilometers inland. Simultaneously, it is difficult to predict how the subsidence trend will continue, and the magnitude of cumulative land deformation depends highly on the localized drivers and responses. RS technology permits us to monitor the land continuously and apply corrective measures. However, the penetration of these technologies into local administrations is still lacking and has to be further promoted.  
The mean Yangtze River water discharge and mean sediment flux of the Yangtze River had declined since 2003 when the Three Gorges Dam began operation [419]. The decline of water and sediment discharges is attributable to climate change and human activities, such as decreased precipitation, construction of reservoir dams, and soil conservation. On the other hand, the Yangtze River Delta, especially the Yangtze Estuary, is highly susceptible to SLR. In particular, it has been predicted that 40% of the terrestrial area of the Chongming Dongtan Nature Reserve, at the mouth of the Yangtze Estuary, will likely be submerged by 2100, given the expected 0.88 m of SLR [420]. The effects of SLR might be even worse due to ground subsidence [421].