Linking biomechanical stem properties of salt marsh vegetation to flow field alterations: A regional and seasonal analysis

Salt marshes play a crucial role in coastal defense, yet quantifying their protective contribution requires understanding the seasonal and regional variability of their biomechanical properties. Stem characteristics such as bending stiffness, diameter, and geometry determine plant motion and the resulting hydrodynamic forces under wave and current exposure. The first field-based quantification of flexural stiffness and its seasonal variation for Canadian Spartina alterniflora was obtained through in situ measurements. To link these measurements with hydrodynamic processes, the fluid–structure interaction solver in REEF3D::CFD was employed to simulate stem-scale flow field alterations. Extending the analysis to a broader geographical context, biomechanical properties of Spartina anglica from Southeast North Sea marshes were incorporated to assess the influence of seasonal and regional variation on the local flow field. Field data for S. alterniflora indicates pronounced seasonal changes, with stems increasing substantially in stiffness and size from early-season to adult stages. Comparison with S. anglica shows, for the first time, that Canadian stems are smaller and more flexible. Numerical results demonstrate that stem-induced flow modifications, expressed as turbulence intensity in currents and turbulent kinetic energy in waves, exhibit stronger regional than seasonal differences. Corresponding drag coefficients, differentiated by season and region and ranging from 1.5 to 2.0, are provided for future hydrodynamic modeling. The findings highlight that regional variation in stem biomechanics influences the flow-vegetation interaction, emphasizing the importance of cross-regional assessments for nature-based coastal protection strategies.