Effects of island–reef topography on the hydrodynamic loads of a pile–net enclosure: An experimental study

Pile–net enclosures are frequently deployed near islands and reefs, where waves can be transformed by topography before reaching the structure. However, how such topographic effects modify the hydrodynamic loads on a complete enclosure and their spatial distribution among individual piles remains unclear. In this study, physical model experiments were conducted under a horizontal bed and two representative island–reef topographies to examine wave transformation and structural loading. The results show that the reef-flat topography generates local wave amplification for longer-period waves, with the maximum normalized wave height reaching 1.82–2.33, whereas the topography without a reef flat reduces the lee-side wave height by 28.2%–72.1%. These contrasting wave fields lead to different structural responses. The reef-flat topography increases the total wave force to 2.9 times the horizontal-bed value at T = 1.10 s, while the topography without a reef flat reduces it to 0.4–0.5. The largest topography-induced changes in individual-pile loads occur from the windward to the lateral region, where the inline load exceeds 3.4 times the horizontal-bed value near θ = 75° and the transverse load reaches five times the reference value near θ = 45°. Harmonic analysis further shows that larger second-harmonic loads occur mainly in the lateral region, where phase differences cause part of the local amplification to be cancelled during spatial superposition. These findings show that island–reef topography should be considered in the hydrodynamic assessment and siting of pile–net enclosures.