Life-Cycle Impacts of Artificial Islands on Shoreline Evolution: A High-Frequency Satellite-Based Assessment

Offshore artificial islands are increasingly constructed along sedimentary coasts, yet their life-cycle impacts on adjacent beaches remain poorly quantified. Here we analyze 21 years of high-frequency satellite observations to assess how the building and removal of two adjacent islands (Ridao and Yuedao) altered shoreline evolution at Riyue Beach, China. A total of 884 Landsat and Sentinel-2 images were processed with sub-pixel shoreline detection, georeferenced against a stable coastal highway and corrected for tidal elevation to derive mean water shoreline positions along 19 transects. Results show that island emplacement triggered rapid salient growth (62–86 m yr−1) opposite the structures and temporary erosion on their flanks. A full tombolo formed on the lee side of Ridao within four years. As the salient widened, the former eroding flanks switched from an “erosional shadow” to a “secondary shelter” and began to re-accrete. The study also reveals lateral coupling between the islands; combined with previous work, it encompasses a critical D/L (offshore distance/alongshore length) threshold of 0.44–0.9 for salient–tombolo formation. Rather than perpetual dredging, we recommend accepting the impending landward connection of Ridao Island. This strategy would eliminate maintenance costs and provide a practical reference for the sustainable management of artificial island shorelines.