Geomorphological processes following the construction of an offshore artificial island in the radial sand ridges of the South Yellow Sea

Offshore artificial islands represent a novel way to develop and utilize marine space resources. Previous research has focused on predicting erosion and deposition distributions following the islands construction, utilizing laboratory experiments and numerical simulations. However, the absence of long-term continuous monitoring has hindered systematic studies on the geomorphological processes of these islands. A 20-years of underwater terrain monitoring scheme was conducted before and after construction of the Rudong Artificial Island in Jiangsu Province, China. The results were combined with on-site observations of tidal currents, suspended sediment concentrations, and surficial sediment to investigate the evolution of the water channel-sand ridge system and the development process of local scour around the artificial island. The results indicated that after the construction of the artificial island, its surrounding water channels showed an overall evolution trend of the north channel shifting towards the north, the middle channel widening and moving south, and the south channel extending westward and southward. Scour has been observed at the four corners of the artificial island, with the peak depths of 7.3 m, 5.6 m, 13.7 m, and 12.2 m at the southwest, northwest, northeast, and southeast corners respectively. The scour in the southwest and northwest corners reached an equilibrium within five years. However, in the southeast corner, continuous erosion of the shoal on the southeast side of the artificial island enhanced the local hydrodynamic force, leading to further scour development. The large-scale scour in the northeast corner was mainly caused by changes in the local flow field following the construction of water intake pipelines and their protection projects. These geomorphological changes are the joint results of the natural evolution of the radial sand ridges and local erosion and deposition from engineering construction. Currently, the sandbanks surrounding the island are still undergoing dynamic evolution following engineering construction, and local scour around the island has not yet reached equilibrium.