Evolution of the Jiuduansha wetland and the impact of navigation works in the Yangtze Estuary, China

Abstract

The estuarine wetlands in the Yangtze Estuary face increasing threats due to large-scale engineering projects and huge land requirements. As a Wetland National Nature Reserve and important stopover site for migratory shorebirds of the East Asian–Australasian Flyway, the Jiuduansha wetland, one of four main islands in the estuary, is of both national and international importance. Since 1998 the previously undisturbed wetland has been considerably modified by the adjacent Deep-water Navigation Channel project (DNC). In order to understand the role of the DNC for the evolution of the Jiuduansha wetland, we performed a diagnostic study from multiple perspectives, including deposition rate, area change, evolution track, geometric characteristics, and geomorphological change. By coupling GIS, geostatistics, and remote sensing techniques, this study demonstrates the impact of the DNC on the Jiuduansha wetland in a spatially explicit way. Multi-temporal bathymetric maps and Landsat images from before and after the DNC was constructed were adopted to conduct our study. We find that deposition has occurred in most parts of the wetland over the past almost 30years, accounting for 67.3% of the total area. However, over 40% of the study area shows a decreasing trend in deposition rate. The spatial distribution of deposition–erosion patterns is closely linked to the DNC. We also find that the DNC has substantial effects on the evolution of the Jiuduansha wetland. The 0, 2, and 5m isobaths in the Jiuduansha Shoal (JDS), as well as in the Jiangya Shoal (JYS), the Upper Shoal (US) and the Middle-Lower Shoal (MLS), show different response patterns to the DNC. Specifically, the south training jetty of the DNC has an “adsorption effect” on the JDS above the 2m isobaths intersecting with it. As a result, the evolution track, geometric and geomorphological characteristics of the JDS were markedly altered in the short term. Our comprehensive analysis suggests that in the future, the JDS will become more elongated in both the upstream and downstream direction along the south training jetty, and that the south side of the JDS will be eroded.