Exploring the Evolution of Salt Marshes: Investigating the Parametrization of Biogeomorphological Processes across Time Scales with Numerical Modelling

Salt marshes play a vital role in climate adaptation by offering crucial ecosystem services, such as coastal protection from rising sea levels and intensified storms. The cross-shore width or lateral extent of salt marshes directly relates to their wave attenuation potential and varies spatiotemporally due to lateral retreat and expansion. Predicting the lateral extent for sufficient flood protection is therefore crucial but complex. Salt marshes are highly dynamic systems governed by intricate biogeomorphological interactions influenced by external forcings (e.g., waves, tides, sediment availability) and internal characteristics (e.g., vegetation species traits, soil characteristics). Thus, to ensure accurate projections of marsh evolution, it is imperative to evaluate the influence of those processes’ variability across various time scales on salt marsh development, particularly on the lateral extent. The aim of this study is to determine to what degree spatially and/or temporally averaged processes suffice to describe salt marsh development and which processes are essential to consider spatiotemporally variable.
We investigate the biogeomorphological development of marshes over 50 years, considering interacting biophysical processes operating at different time scales. A coupled, state-of-the-art numerical model, combining Delft3D Flexible Mesh for flow, morphology, and wave simulation with a dynamic vegetation module, is used to simulate marsh development (Figure 1). The model is based on marshes in the Dutch Wadden Sea. The vegetation module includes seasonal vegetation establishment, growth, and decay. The model also includes parametrizations for spatiotemporally variable critical bed shear stress for erosion and cliff erosion processes at the marsh edge. External forcings consist of tides and temporally varying waves. Individually assessing their effects on salt marsh development offers insights into their significance. Such insights are key for building trust in predictions made using numerical models for salt marshes' morphological evolution. Thus, this research offers a foundation for assessing salt marsh resilience to future stresses, such as identifying thresholds for marsh retreat and expansion, and their effective implementation in flood protection strategies.