Long-term reclamation of tidal flats of Chongming Island and ecological security of Yangtze estuary, China

The evolution of tidal flat wetland is closely related to the ecological security of the Yangtze estuary. Based on the method of multi-sourced data analysis, this paper aims at revealing the evolution process of the tidal flat of Chongming Island on a centennial scale and the phases of reclamation intensity change over the last thousand years, and discussing the driving human and environmental factors. The results of this study show that the landscape transformation from natural wetland to reclamation region of Chongming Island has experienced four stages: early development during the seventh to eighteenth centuries, slow reclamation from the mid-eighteenth to the mid-twentieth centuries, rapid reclamation in the 1960–1990s, and limited reclamation in the last 20 years. The proportion of natural tidal flat above 0 m in elevation (Wusong Datum) outside the sea dikes in total land area of this island dropped from about 50% of the mid-twentieth century to less than 20% of the early twenty-first century. Sediment accumulation was the main cause of tidal flat growth before the 1950s, while human reclamation after the 1950s was the driving factor of tidal flat shrinkage. The rapid urbanization of the Yangtze estuary in the last 30 years has exerted great pressure on the protection of tidal flat wetland. With the evolution trend of tidal flat from expansion to erosion, sharp decline of sediment, and shrinking of the northern branch in the Yangtze estuary, there will be limit space for further expansion of the shape of Chongming Island, and the environmental conditions that supported the past rapid reclamation were no longer held. Maintaining the present tidal flat wetlands space, and constructing ecological reclamation and coastal engineering system for promoting the dynamic balance between human reclamation intensity and tidal flat evolution will be the key to the future ecological security of the Yangtze estuary.