Evolution of off-shore tidal flats in the South Yellow Sea Wetland Natural Heritage Site from 1984 to 2020

The coastal tidal flats of the South Yellow Sea are an important stopover for global bird migration and have been listed as World Natural Heritage Site. However, they have been in the process of turning change under the influence of river diversion, marine dynamics and human activities. This study selected seven tidal flat sections, including Sheyang Estuary, Xinyanggang, Chuandonggang, the inner and outer sides of radial sandbars, and the inner and outer sides of Jinggang in the World Natural Heritage Site of Yellow (Bohai) Sea Migratory Bird Habitat (Phase I) along South Yellow Sea. By using Landsat images from 1984 to 2020, the exposed width of the tidal flats was obtained, and the positions and shapes of the tidal flats in different periods were reconstructed based on the tide level information during the imaging period, thereby obtaining the evolution trend of the tidal flats. The results show that, except for Jinggang section, the tidal flats along the coast of this natural heritage site have been in a shrinking state since the beginning of this century, with the width of the tidal flats narrowing and the slope becoming steeper. This trend is more obvious on the northern side than on the southern side. The tidal flats on both sides of the radial sandbar outside the coast also show a significant retreat trend. The tidal flat sections on the inner and outer sides of Jinggang in the south, however, showed a significant siltation and extension trend during the second decade of this century. The retreat of tidal flat wetlands is a threat to the habitat space of rare birds and the stability of the tidal flat ecosystem within the natural heritage site. In order to cope with this challenge, it is necessary to strengthen the unified management of coastal wetlands, promote the subsequent application for World Natural Heritage status of the coastal wetlands, build artificial wetlands and take engineering protection measures to protect the coastal wetland environment, and expand the habitat space for rare birds.