Unravelling foreshore ecosystem dynamics: Applications for ecosystem-based coastal defence

Hard engineering structures such as dikes, commonly used for coastal protection, are often associated with negative impacts on natural functioning and biodiversity of coastal ecosystems. Hybrid nature-based defences, which combine conventional hard engineered barriers with coastal ecosystems such as marshes, may offer a more sustainable, ecologically valuable and cost-effective alternative to hard engineering alone. However, uncertainties about the actual effectiveness of these nature-based measures still hampers their practical implementation. In this thesis I focussed on coastal ecosystem dynamics in relation to their coastal protection role, as well as investigating nature management strategies to optimize both the coastal protection services and the ecological value of these ecosystems. Overall, this thesis highlights that salt marshes effectively reduce wave heights reaching the dike and wave run-up onto the dikes, even if the marsh vegetation is short. Furthermore, marsh vegetation traps sediment and creates a cohesive top layer that is highly resistant to fast water flow, which is beneficial in reducing the breach depth in case of a dike failure. Only pioneer vegetation growing at sandy places is not erosion resistant, comparable to bare mudflats. Grazing by hare, geese and cattle can reduce the lateral erodibility of fine-grained marsh soils. The resulting slow cliff retreat is beneficial for flood safety, as marsh width is needed to attenuate waves. However, intensive cattle grazing can over-compact the soil, which may negatively impact the marsh resilience to sea level rise. Thus, to be beneficial, cattle grazing should be extensive. Finally, marsh expansion may be promoted by stabilizing and trapping sediment on the connected fronting tidal flats. More specifically, we found that foreshore ecosystems such as seagrass beds with high root density can reduce soil erosion, and that there is potential to change the tidal flat morphology by using artificial reefs.