Sea-level rise, ENSO, and coastal flood exposure in Hainan Island: Insights from Delft3D modeling for urban adaptation

Hainan Island, located in the northern South China Sea, represents a rapidly urbanizing tropical island. This study develops a coupled Delft3D hydrodynamic–morphodynamic (Flow–Sed–Mor) model to quantify flood inundation under projected sea-level rise (SLR) and ENSO-driven sea-level anomalies. The model is forced by the SSP3–7.0 scenario, corresponding to an estimated 1.1 m SLR by 2100, to evaluate multi-hazard flood exposure across Hainan’s coastal zones and assess the amplification effects of ENSO-related variability. Model findings show that the western and northern lowlands experience an inundation expansion exceeding 40 % under the 1.1 m SLR scenario, while reef-fringed eastern coasts exhibit limited permanent flooding, highlighting the importance of geomorphological controls on flood resilience. La Niña phases lead to transient sea-level rises up to 15 cm in the South China Sea with a lag of approximately six months, intensifying flood risks during astronomical high tides. Enhanced nearshore hydrodynamics under SLR increase suspended sediment concentrations by 2–3 times, accelerating shoreline retreat in soft-sediment western coasts. Exposure analysis indicates that industrial zones face up to fourfold higher asset risk compared to urban centers. A Coastal Urban Defense Model is proposed, integrating hard infrastructure, ecosystem-based solutions (mangroves, reefs), and adaptive zoning to enhance resilience under accelerating climate-driven SLR. These results emphasize the necessity of incorporating ENSO-driven variability and long-term SLR into flood management frameworks to enhance climate resilience in tropical coastal regions. • ENSO causes sea level fluctuations in the South China Sea with ∼6-month lag. • A 1.1 m SLR by 2100 intensifies tides and sediment erosion in western Hainan. • Dongfang, Ledong, and Haikou face high flood exposure under future SLR. • The Coastal Urban Defense Model integrates climate, geomorphology, and urban risk.