Cliff Erosion Observations and Modelling at the Wierum Salt Marsh in the Dutch Wadden Sea::The Importance of Local Water and Site-Specific Calibration

Salt marshes play a vital role in coastal resilience, offering ecosystem services such as carbon sequestration, biodiversity, wave attenuation, and shoreline stabilisation. In low-lying regions like the Netherlands, their function as nature-based flood defences is increasingly significant under conditions of accelerating sea-level rise and increasing storm intensity. The long-term effectiveness of this role depends on maintaining stable marshes. A key driver of marsh loss is the lateral retreat due to cliff erosion at the seaward marsh edge, resulting from an interplay between hydrodynamics, sediment, and vegetation properties. Although prior studies have established a linear correlation between wave power and cliff retreat rate, models based on this relationship are often unverified for capturing annual and spatial variability, both of which are essential for effective coastal management. This study integrates a sub-grid cliff erosion module (Bendoni et al., 2019) into a 2D depth-averaged hydrodynamic framework. The model is calibrated and validated using high-resolution UAV-derived digital elevation data over a 9-month period from the Wierum salt marsh in the Dutch Wadden Sea.<br/>Results show that, when locally calibrated, the integrated framework accurately captures the spatial variability and magnitude of cliff erosion, confirming the linear relationship between wave power and erosion. Furthermore, the results highlight the importance of water levels in determining when and where erosion can occur. Therefore, at this study site with high-marsh cliffs, moderate storm surges induce the most significant lateral retreat, while extreme surges submerge marsh edges, limiting erosion. However, localised erosion mechanisms such as undercutting as a driver for bulk erosion remain unresolved. These insights highlight the necessity of site-specific calibration and the importance of considering different local erosion mechanisms. The proposed framework can offer a valuable tool for predicting salt marsh vulnerability at management-relevant timescales.