Sediment Transport Dynamics of Barchan Dunes Formed by Coral Sand from the Xisha Islands, South China Sea: Insights from Water Tunnel Experiments

Coral reefs in the South China Sea, which underpin coastal protection and serve as natural laboratories, are rapidly degrading under climate change and human pressures. Barchan dunes are key tracers of sediment transport, yet their formation and migration in coral-sand island systems remain largely uncharacterized. We conducted closed-loop water tunnel experiments using coral sand from the Xisha Islands to quantify subaqueous barchan dynamics under precisely controlled flows; high-resolution plan-view videos captured dune evolution, and Particle Image Velocimetry (PIV) was used to reconstruct the flow fields. Compared with spherical glass bead controls, irregular and porous coral particles produced dunes that were more stable and resistant to erosion. Finer coral sand maintained its morphology over a wider range of flows, whereas coarser fractions seldom formed stable horns. The dimensionless dune speed Fr d scaled as a power law with Re, Fr d −Re n . These results isolate the role of particle irregularity and porosity in governing barchan transport and morphology, contributing to a mechanistic understanding of sediment flux controls on coral-reef islands and helping to inform coastal protection and reef-restoration planning.