Flexible Salt Marsh Vegetation: Model-Driven Replication of Stem Biomechanics across Seasonal and Regional Scales

Abstract

Coastal protection strategies have shifted from structural measures toward more integrated approaches that combine traditional engineering elements with nature-based solutions provided by coastal ecosystems. Among these coastal ecosystems, salt marshes play a key role by offering multiple ecosystem services such as wave energy dissipation, sediment stabilization, and erosion reduction. A critical process governing their protective function is the near-stem flow field, which describes the local interaction between vegetation and hydrodynamic forcing. However, the influence of seasonal and regional variations in the biomechanical properties of salt-marsh vegetation on these flow dynamics remains largely unexplored. This thesis addresses this gap by incorporating in situ measured biomechanical properties of live stems into physical and numerical modeling approaches, enabling a detailed analysis of the fluid–vegetation interaction. The replication of live vegetation properties in physical hydrodynamic experiments is enhanced in accuracy compared to traditional surrogate materials with 3D printing utilizing flexible filament. The developed surrogates are tested in a flume under unidirectional and oscillatory flow conditions, with particle image velocimetry enabling the measurement of the flow field alterations and the stem-induced turbulence. Next, this thesis employs the fluid-structure interaction solver in the open-source model REEF3D::CFD to quantify the role of salt marsh vegetation in coastal protection numerically, implementing a two-way coupling between vegetation stems and hydrodynamic forcing from unidirectional and oscillatory flow conditions. This extends one-way coupling approaches and provides more accurate solutions compared to previous fluid-structure interaction solvers to model flexible submerged vegetation. The application of field data from various European salt marsh studies at different times during the growing season, together with the first measurements of biomechanical stem properties from a Canadian salt marsh, enables an assessment of both seasonal and regional effects on the stem-induced flow field alterations. The 3D printed surrogate stems have been confirmed as similar to the in situ measured biomechanical stem properties of live vegetation through a statistical evaluation. The fluid-structure interaction solver has been validated to replicate stem motion and forces of flexible vegetation within a maximum deviation of 10% for most tested conditions. Based on the provided extensive validation, the solver was shown to be a valuable approach to simulate various stiffnesses. The results from both numerical and physical modeling suggest that the stem-induced turbulence varies to a greater extent between the different replicated plant species than the seasonal influence within a species. Due to seasonal variations of the stems’ flexural stiffnesses across the investigated species and regions, distinctions regarding the stem bending and extent of motion have been observed, resulting in seasonal differences for the derived drag coefficients. In conclusion, the accurate replication of in situ measured biomechanical stem properties in model approaches reveals the difference in flow field alterations depending on the season and region of a salt marsh. Drag coefficients provided through single-stem simulations enable future investigations of the seasonal and regional variation of salt marshes in a simplified model at the meadow scale. Numerical models would then allow for an assessment of ecosystem services, such as the wave attenuation capacity. By incorporating seasonal field data in modeling strategies, this study enhances the understanding of fluid-vegetation interaction, contributing to reliable nature-based coastal protection strategies.