Spatial Variability and Drivers of Salt Marsh Cliff Erosion in the Dutch Wadden Sea: From Observation to Simulation

Abstract

Lateral erosion of salt marshes via cliff retreat is a primary cause of global marsh loss, driven by interactions between hydrodynamics, sediment, and vegetation. While previous studies show a linear relationship between wave power and cliff retreat, models based on this relationship are largely unvalidated for short-term (sub-yearly) and spatially variable predictions, limiting their practical application. This study addresses this gap by integrating a process-based subgrid cliff erosion model into a 2DH hydrodynamic model, validated using high-resolution UAV-derived DEMs over 9 months in a Dutch Wadden Sea marsh. The site featured 20–60 cm high cliffs retreating at ∼0.9 m/yr. Once calibrated, the model captured overall erosion trends and volumes, confirming the linear wave power-retreat relationship when averaged across the marsh. However, it did not account for local effects, like artificial structures, cliff undercutting, or mass failure, highlighting the need for site-specific calibration and better understanding of underlying processes. Simulations under varying hydrodynamic conditions assessed marsh cliff vulnerability. Water levels determine the location and timing of cliff exposure, while wave energy dictates retreat magnitude. Yearly storm surges caused the most significant erosion, whereas more extreme surges (5–10 years return-period) submerged the marsh edge, limiting further erosion. This research shows that the linear wave-power relationship is scale-dependent: it can be calibrated for longer-term, marsh-averaged erosion volumes when sufficient local data support a linear relationship, but its applicability is limited for within-marsh, shorter-term forecasting. Accurate simulation and effective coastal management require further understanding of the processes and implementation of non-linear erosion mechanisms.