Nature-based mitigation of shoreline erosion risks in tidal marshes created by managed realignment vs. sediment nourishment

Tidal marshes provide many valuable ecosystem services and can play an important role in nature-based flood risk mitigation along low-lying coasts and estuaries, by attenuating waves and increasing erosion resistance. There is an effort around the world to restore or create tidal marshes, but it remains unknown how different marsh restoration and creation techniques affect the development of erosion resistant sediment beds, which is essential for their contribution to long-term erosion and flood risk mitigation. Here, we compared sediment shear strength and erosion resistance under very high flow velocity (i.e. as may occur during the breach of a dike or seawall behind the marsh) of a managed realignment site versus a sediment nourishment site, restored and created respectively, and assessed the effects of tidal inundation, sediment characteristics, and vegetation. Managed realignment consists of the landward relocation of flood defence structures like seawalls and dikes and tidal flooding of low-lying land, creating a sheltered environment for tidal deposition of fine-grained sediments, while sediment nourishment is the seaward placement of mostly more coarse-grained dredged sediment. This study showed that at both sites sediment shear strength and erosion resistance were higher at vegetated locations than at unvegetated locations. In addition, at the managed realignment site, tidal inundation duration affected shear strength negatively, while bulk density affected shear strength positively. At the sediment nourishment site, sediment grain size was the most important driving factor of shear strength and erosion resistance, besides vegetation presence: a decreasing shear strength and erosion resistance were observed with increasing sediment grain size. Managed realignment and sediment nourishment both have advantages and disadvantages for the fast development of an erosion resistant sediment bed. Managed realignment will likely lead to fine-grained, cohesive sediments, which are in this case colonised by dense, but slowly establishing, mud-loving Spartina vegetation. In contrast, sediment nourishments are typically done with more coarse-grained, non-cohesive sediments, which are in this case colonised by initially sparse, fast establishing Salicornia vegetation. Dense Spartina increased erosion resistance more than sparse Salicornia. If we plan well ahead of time and temporarily support their development, restored or created marshes have time to become erosion resistant, thereby enabling sustainable use of marsh ecosystem services for long-term nature-based flood risk mitigation.