Effects of oyster reefs on back-barrier tidal flats on the local hydro- and morphodynamics

In response to the global decline of native bivalve populations, non-native Pacific oysters (Magallana gigas) are increasingly colonizing former habitats of native bivalves. In the Wadden Sea, M. gigas reefs replaced blue mussel beds (Mytilus edulis) as the predominant biogenic structure on the intertidal mudflats. These reefs, covering 2 – 6% of the tidal basin area, attenuate flow energy through frictional dissipation, affecting local hydro- and morphodynamics. Despite their potential to influence intertidal mudflat elevation and function as nature-based coastal protection against sea level rise, the spatio-temporal effects of oyster reef-induced frictional dissipation remain underexplored. This study evaluates the impact of oyster reef expansion in back-barrier tidal flats on hydro- and morphodynamics.

A generic tidal basin model was developed using the Delft3D framework, synthesizing average morphological and sedimentological characteristics of the seven tidal basins sheltered by the German East Frisian islands. The model features a convex-up hypsometry, five sediment fractions (mean grain size of d_50,GTB  = 205 μm), and a fixed sediment roughness (Manning coefficient of n = 0.023 m^- 1/3s), closely mirroring the input parameters. Oyster reef coverage scenarios were modeled for 2% (current average), 6% (current maximum), and 10% (projected future) of the tidal basin area. Reef roughness was parameterized by applying a drag coefficient C_D = 0.025 and roughness length z₀ = 7.8 mm. The distribution of oyster reefs within the tidal basin is determined by evaluating potential areas for reef distribution based on abiotic stressors (e.g., aerial exposure time and bed shear stress) and utilizing the Cahn-Hilliard equation to create realistic spatial patterns. A generic neap-spring tidal cycle, developed using the key tidal constituents for sediment transport, was applied at the seaward boundary.

The generic tidal basin and hydrodynamic boundary conditions are utilized to project the impact of oyster reefs on hydro- and morphodynamics. The results reveal substantial impacts of these reefs on hydrodynamic patterns and magnitudes. Furthermore, the oyster reefs cause alterations in sediment transport patterns and the resulting sea-bed level changes. The effects vary across scenarios, highlighting the diverse impacts of these reefs under spatio-temporally varying conditions.

The model presented provides a framework to estimate the biomorphodynamic feedback resulting from the bioinvasion of the Pacific oyster in the Wadden Sea, advancing the understanding of ecohydraulic processes, particularly in relation to sediment transport pathways. The results thus suggest that the presence of oyster reefs may contribute to the vertical growth of the intertidal mudflats of the Wadden Sea, providing a natural countermeasure to accelerating sea level rise.