Numerical Simulation of Hydrodynamics and Sediment Transport for Coastal Protection with Artificial Reefs

Artificial reefs (ARs) are eco-friendly coastal protection infrastructures that mitigate wave-induced erosion while maintaining hydrodynamic connectivity and supporting ecological functions. This study evaluates the protective efficacy of a shellfish-algae reef system—a new type of AR—within the Houlong Bay coastal restoration project (Quanzhou, China) using an integrated numerical modeling approach. A coupled model system was established, incorporating MIKE 21 FM for hydrodynamics, MIKE 21 SW for waves, and MIKE ZERO ST for sediment transport, using unstructured triangular grids to resolve complex coastal topography. The model was validated against field data, including tidal currents and wave heights, showing good agreement. Pre-implementation simulations identified key coastal issues: insufficient wave attenuation in the southern fishery port segment, which results in localized erosion. Post-project simulations demonstrate that the novel integrated system—comprising shellfish-algae reefs, broad gentle beaches, and coastal vegetation—effectively reduced nearshore current speeds by approximately 0.15 m/s and attenuated significant wave heights by up to 70% during typhoon events. Short-term (1-year) sediment evolution showed mild deposition (0.1–0.8 m) at the toe of the artificial beach, which is consistent with design expectations. Long-term (10-year) simulations further confirmed coastal stability, with minimal long-term shoreline retreat (maximum 15 m) and low net alongshore sediment transport (annual average: 800 m3). This study provides a validated, data-driven reference for the design and implementation of AR-based restoration strategies in semi-enclosed bays, highlighting their dual role in erosion control and sustainable coastal management.