Hydrodynamic processes of a typical reef island in the northern South China Sea revealed by distributed acoustic sensing

Complex hydrodynamic processes in reef-island environments are fundamental to the evolution of their geomorphology and exert a strong influence on coastal engineering and risk management. However, conventional observation methods have a limited ability to continuously capture these processes with meter-scale spatial and second-scale temporal resolution. In this study, distributed acoustic sensing (DAS) technology was employed to enable high spatiotemporal resolution monitoring of hydrodynamic processes around Zhaoshu Island, located in the Xuande Atoll of the northern South China Sea. Based on time-frequency and frequency-wavenumber (f-k) analysis, we characterize the spatiotemporal variations of DAS recorded signals across multiple frequency bands, indicating that these signals are closely related to hydrodynamic processes driven by the coupling of tides, wind forcing, and topography: (1) signals in the 0.1–0.6 Hz range exhibit an apparent phase velocity of approximately 6 m s−1, consistent with typical ocean surface gravity waves (OSGWs). The signal energy shows a pronounced tidal modulation and a gradual attenuation from the reef crest toward the lagoon; (2) signal energy in the 2−10 Hz range exhibits temporal variations and spatial gradient similar to those of the 0.1−0.6 Hz signals, likely associated with local turbulence over the reef flat; (3) The peak frequencies of signals between 0.6−2 Hz show a negative correlation with tidal elevation, yet disappear following changes in wind direction, suggesting a connection to vortex-induced vibrations (VIV); (4) signal energy recorded by the onshore cable in the construction area is notably higher than that observed in the adjacent beach zone, reflecting the influence of coastal morphology on wave-breaking processes; and (5) pronounced wave reflection is detected at the breakwater, with an estimated reflection coefficient of 0.46–0.54 at high tide, consistent with typical values reported for rubble-mound breakwaters. This study demonstrates the unique advantages of DAS for high spatiotemporal resolution observations of reef hydrodynamics, providing new insights into the mechanisms that govern hydrodynamic processes in reef-island environments. The findings can be applied to evaluate the stability of breakwaters and reef-island slopes, as well as to guide the optimization of resource development and ecological management strategies.