Rapid Loss of Tidal Flats in the Yangtze River Delta since 1974

Abstract

As the home to national nature reserves and a Ramsar wetland, the tidal flats of the Yangtze River Delta are of great significance for ecological security, at both the local and global scales. However, a comprehensive understanding of the spatiotemporal conditions of the tidal flats in the Yangtze River Delta remains lacking. Here, we propose using remote sensing to obtain a detailed spatiotemporal profile of the tidal flats, using all available Landsat images from 1974 to 2018 with the help of the Google Earth Engine cloud platform. In addition, reclamation data were manually extracted from time series Landsat images for the same period. We found that approximately 40.0% (34.9–43.1%) of the tidal flats in the study area have been lost since 1980, the year in which the tidal flat area was maximal. The change in the tidal flat areas was consistent with the change in the riverine sediment supply. We also found that the cumulative reclamation areas totaled 816.6 km2 and 431.9 km2 in the Yangtze estuary zone and along the Jiangsu coast, respectively, between 1974 and 2018. Because of reclamation, some areas (e.g., the Hengsha eastern shoal and Pudong bank), which used to be quite rich, have lost most of their tidal flats. Currently, almost 70% of the remaining tidal flats are located in the shrinking branch (North Branch) and the two National Nature Reserves (Chongming Dongtan and Jiuduansha) in the Yangtze estuary zone. Consequently, the large-scale loss of tidal flats observed was primarily associated with reduced sediment supply and land reclamation at the time scale of the study. Because increasing demand for land and rising sea levels are expected in the future, immediate steps should be taken to prevent the further deterioration of this valuable ecosystem.

Reclamation of Tidal Flats Within Tidal Basins Alters Centennial Morphodynamic Adaptation to Sea‐Level Rise

“…As an example, the surface area of the North Branch in the Changjiang Delta has approximately halved in the period of 1974–2018 (Li et al., 2020), predominantly due to extensive reclamation of tidal flats. A flood dominance and associated sediment import has established since the 1950s when river influences minimized owing to degenerated inflow conditions (Guo et al., 2021; Obodoefuna et al., 2020; see Figure S1 in Supporting Information SI).…”

Recent Evolution of the Intertidal Sand Ridge Lines of the Dongsha Shoal in the Modern Radial Sand Ridges, East China

“…Tidal flats, located between the average low tide and average high tide of the coastal zone, are the most important landform system in the coastal zone. They can protect the biodiversity of the coastal zone, resist storm surges, and provide habitat for migrating birds, and provide reserve land resources for coastal economic development [ 2 , 3 , 4 , 5 ]. Therefore, the tidal flats have received extensive attention from all over the world [ 6 , 7 , 8 , 9 , 10 ].…”

“…Therefore, an accurate assessment of the evolution of the Radial Sand Ridges is critical to the protection of tidal flat resources and sustainable coastal development. Remote-sensing technology provides nearly continuous monitoring of tidal flats, which are commonly used in many coastal areas around the world [ 3 , 4 , 19 , 20 ]. The most representative methods are based on airborne light detection and ranging (LiDAR) and interferometric synthetic aperture radar (InSAR) images [ 21 , 22 , 23 , 24 ].…”

“…In this study, we attempt to take the sand ridge lines of Dongsha Shoal as the research area, analyze the trends of the long-term morphological evolution of the sand ridge lines over the period 1973-2016 based on a high-density time series of medium-resolution satellite images. Our specific goals were (1) to study the long-term evolution of the location of the sand ridge lines, (2) to study the overall trend and change stage of the sand ridge lines (3) to determine the change trend of different positions of the sand ridge lines, and (4) to discuss global changes and the impact of human activities on the landform changes in Dongsha Shoal. This study will help evaluate changes in tidal flats in modern radial sand ridges in Jiangsu Province, provide support for the management of related sea areas, and provide scientific references for sustainable tidal flat development.…”

“…In most time phases, the proportion of the length of the sand ridge line on the east side of the baseline exceeds 50% and gradually increases (3). The hydrodynamic force of the Xiyang Tidal Channel continues to increase.…”