Saltmarshes provide many vital ecosystem services, like wave dampening and carbon sequestration. These services are under pressure since saltmarshes are eroding worldwide. The erosion mechanics of saltmarshes are not well understood, especially during extreme storm conditions. Full-scale flume experiments were conducted on a real-life transplanted Dutch Wadden sea saltmarsh with a common brushwood dam protecting the marsh edge, to test the erosion resistance during extreme wave conditions. Little erosion occurred on the marsh platform during 40 h of exposure to various wave conditions, with most of the significant erosion occurring at the seaward edge of the marsh. Erosion was initiated by damaging and uprooting of the vegetation, exposing the substrate and leading to gradual erosion. Uprooting events frequency and erosion rates were higher during high wave conditions and water depths (H_m0 = 2 m and d_m = 4 m). The brushwood dam did not protect the saltmarsh from eroding at the cliff. Our results demonstrate the erosion mechanics under extreme storms conditions and that uprooting may lead to instability of the saltmarsh on long-term.

Saltmarshes are a promising nature-based alternative for conventional flood protection. However, saltmarshes can erode under storm conditions, whereby the seaward edge of the saltmarsh often forms a vertical cliff. Despite its importance, the effect of storm conditions on erosion at the saltmarsh cliff remains understudied, especially when waves traverse over a cliff. This research investigates the complex flow patterns around a saltmarsh cliff non-intrusively using Particle Image Velocimetry (PIV) conducted through a series of scaled monochromatic wave flume experiments. We adopted realistic foreshore configurations (e.g. cliff heights) and hydraulic loading conditions from the Dutch Wadden Sea. Results show two local near-bed velocity maxima on top of the saltmarsh, created during different wave phases by water depth contraction, wave transmission and interaction between flow and vortices that are shed from the cliff. Under the wave crest, high onshore-directed near-bed velocities were measured at approximately 2.5–4 times the cliff height onshore from the cliff. Under the wave trough, high offshore-directed velocities were found at the marsh edge. Both onshore- and offshore-directed velocities increase with increasing cliff height, larger wave height or lower water depth. Vegetation on top of the marsh reduces both the incoming and outgoing velocities in front of the cliff. Increasing the cliff height resulted in a greater reduction in velocities by the vegetation. These results demonstrate how local near-bed velocity maxima and location are influenced by the presence of a cliff and the interaction with vegetation on top of the saltmarsh. This research highlights the vulnerability of the cliff even during inundation of the cliff and will help to implement saltmarshes as nature-based solutions for flood defence.