Wave-induced hydrodynamics of biogenic structures in the central Wadden Sea: Implications of the transformation from mussel beds to oyster reefs for wave attenuation

Abstract

The transformation of the predominant biogenic structures in the Wadden Sea, from blue mussel (Mytilus edulis) beds to Pacific oyster (Magallana gigas) reefs, has increased their topographical roughness, impacting the wave-biogenic structure interactions. Despite the general knowledge of increased wave attenuation due to the ecological transformation, a detailed quantification of wave energy dissipation induced by both biogenic structures and a comprehensive understanding of the governing processes remain lacking. This study systematically investigates frictional wave energy dissipation of both biogenic structures by subjecting generic surrogate models to regular, non-breaking waves in reduced-scale wave flume experiments. The results reveal pronounced wave height reductions for both structures, with oyster reefs exhibiting approximately twice the frictional wave energy dissipation (wave friction factor f_w,OR = 0.44 ± 0.30) of mussel beds (f_w,MB = 0.21 ± 0.18). Comparing near-bed velocities with topographical roughness parameters identified mussel agglomerations governing the frictional wave energy dissipation in mussel beds and oyster shells in oyster reefs. In mussel beds, form drag dominates frictional resistance under moderate hydrodynamic conditions, whereas flow separation at high intensities substantially lowers the wave energy dissipation. Conversely, oyster reefs maintain wave energy dissipation across a broader hydrodynamic range due to the sharp-edged, rigid shells. The enhanced wave attenuation by oyster reefs and their expected long-term persistence in the Wadden Sea presents an opportunity to complement existing gray coastal protection infrastructure as a nature-based solution. The parameterization of frictional wave energy dissipation presented here enables more accurate hydro-morphodynamic modeling of large-scale sediment dynamics in such soft-bottom environments.