Interactions of Biogenic Structures with Waves: Bed Roughness Effects of Oyster Reefs and Mussel Beds in the Central Wadden Sea

The Pacific oyster Magallana gigas is an invasive species in the central Wadden Sea, transforming the predominant biogenic structures on the intertidal flats from blue mussel (Mytilus edulis) beds to oyster reefs. This transformation has led to a permanent increase in the topographical roughness of the biogenic structures, impacting the wave-biogenic structure interactions. While it is generally known that this ecological shift has led to increased wave attenuation, there is a lack of detailed quantification of its impact on wave energy dissipation in such biogenic structures and a comprehensive understanding of the governing processes. A deeper understanding is needed to quantify the influence of M. gigas reefs on local hydro- and morphodynamics and assess their influence on regional sediment dynamics as well as their potential application as a nature-based solution for coastal protection. This cumulative dissertation aims to advance the process-based understanding of how the transformation of the biogenic structures influences waves. Based on identified knowledge gaps, three research questions were derived to 1) quantify the topographical changes at the species-related scale, 2) identify the processes governing frictional wave energy dissipation, and 3) quantify frictional wave energy dissipation for both biogenic structures. The wave-biogenic structure interactions were investigated through controlled, reduced-scale wave flume experiments using generic surrogate models. Topographical data sampled in field campaigns informed the surrogate model design. Addressing the research questions led to four published Works, which form the foundation of this dissertation. Work #1 introduced an intra-reef structural classification of M. gigas reef surfaces in the central Wadden Sea and provided topographical roughness properties for each class relevant to hydraulic roughness. Work #2 explored how digital design and fabrication strategies can be applied to manufacture surrogate models of both biogenic structures. This work integrated in-situ data collection, data analysis, parameterization, computer-aided design, and additive manufacturing, leading to the fabrication of complex surrogate models of both biogenic structures. Works #3 and #4 report two experimental studies in which different sets of generic surrogate models (abstract and complex) were subjected to regular, non-breaking waves in a wave flume. In Work #3, abstract surrogate models were used to investigate the influence of sharp edges of roughness elements, such as the margins of oyster shells, on frictional wave energy dissipation. It was found that sharp-edged surfaces induce more substantial wave energy dissipation than bluntly-shaped surfaces, providing insights into the underlying processes. In Work #4, the wave-induced hydrodynamics of both biogenic structures were compared using the complex surrogate models developed in Work #2. The findings revealed that different topographical features act as significant roughness elements: frictional wave energy dissipation is governed by the mussel agglomerations in M. edulis beds and the sharp-edged oyster shells in M. gigas reefs, with M. gigas reefs being more effective at attenuating waves. The findings of this dissertation provide a novel framework to quantify the impact of biogenic structures on waves, particularly in the context of the transformation of M. edulis beds to M. gigas reefs in the central Wadden Sea. These insights enable the parametrization of frictional wave energy dissipation for these biogenic structures, facilitating more accurate hydro-morphodynamic modeling of soft-bottom intertidal environments. The findings also contribute to a better understanding of biogenic structures as nature-based solutions supporting existing gray coastal protection infrastructure.