BIVA-WATT: Roughness Effects of Oyster Reefs and Blue Mussel Beds in the Field, Hydraulic Model Ex-periments, and Numerical Simulations

The bioinvasion of the non-native Pacific oyster (Magallana gigas) has transformed low-relief blue mussel (Mytilus edulis) beds in the German Wadden Sea into rigid, ultra-rough biogenic structures that strongly interact with waves, currents, and sediment dynamics. This work synthesizes results from the BMBF-funded KFKI research project BIVA-WATT, which investigates how biogenic structures formed by the non-native Pacific oyster and the native blue mussel modify the hydro- and morphodynamics of the German Wadden Sea. Field campaigns, hydraulic model experiments, and numerical modelling were combined to (i) parameterize the topographical roughness and spatial configuration of mussel beds and oyster reefs, (ii) quantify their influence on wave attenuation, flow resistance, and sediment accumulation, (iii) resolve near-bed flow and force patterns around individual roughness elements, and (iv) implement coupling strategies between ecological descriptions and process-based morphodynamic models.

High-resolution structure-from-motion photogrammetry, terrestrial laser scanning, and UAV mapping revealed distinct intra-reef structural classes and showed that mature oyster reefs form large, continuous, ultra-rough surfaces with vertical accretion and sediment accumulation rates that exceed local sea-level rise. Seasonal population data demonstrated that vertical reef growth is tightly linked to temperature-driven shell production and recruitment. Two-year volumetric analyses indicated net gains in reef area and volume where suitable hard substrate is available, while lateral stability is controlled by local hydrodynamic exposure and episodic burial and re-exposure.

Scaled surrogate models of both biogenic structures were subjected to waves and currents in controlled flume experiments, demonstrating that oyster reefs dissipate roughly twice as much wave energy as mussel beds and that sharp-edged roughness elements are key drivers of turbulence production, bed shear stress, and frictional energy loss. High-fidelity LES simulations around a single oyster shell resolved vortex dynamics and force progression.

A data-based hindcast framework was developed to generate time-variant, 10 m grids from 1996 to 2020 and to transfer biologically derived roughness parameters into the MARINA model system. Together, these advances provide a basis for assessing the system-scale consequences of the ongoing shift from mussel beds to oyster reefs and for supporting informed coastal management under sea-level rise.