Decadal shifts and future projections of the Han River Delta coastline

The Han River Delta, the sixth largest delta in China, has received decreasing amounts of fluvial sediments over the last decades. Here, we use satellite images for 1981–2020 and a conceptual geometric model to investigate recent and future evolution of the delta coastline. Satellite images were able to capture the transition from a more natural coastline with sandy beaches to an artificial structure since 2015. This transition has been accompanied by rapid seaward migration of a portion of the artificial coastline due to land reclamation. Furthermore, most of the coastline has remained stable (showing no significant advance or retreat), except for coastlines that are dominated by mangrove forest. The coastline has not always remained stable. Before the 1980s, the coastline was continuously advancing. Modeling results reveal that a significant reduction of fluvial sediment, mainly caused by upstream dam emplacement, had a key role in the shifting status of the delta coastline evolution. Additionally, model results suggest that if fluvial sediment supply had remained consistent with pre-dam levels, the more abundant fluvial sediment supply could counteract the effects of a maximum sea-level rise scenario for decades. However, a sustained decrease of the fluvial sediment supply, at rates similar to the less abundant sediment supply at present, is expected to dominate the evolution of the delta coastline under a conservative sea level rise scenario. In the case of a medium or maximum sea level rise scenario, the future evolution of the delta coastline will be dominated by the rate at which sea level rises, rather than a further decrease of riverine sediment. The large-scale land reclamation has a negligible impact on the advance or retreat rate of the delta coastline. These findings offer valuable insights for the management of delta coastlines.