Identifying the drivers of sediment strength and erosion resistance in natural and restored tidal marshes

Tidal marshes can contribute to nature-based coastal protection by reducing both wave loading onto the shore and erosion of the shoreline. To implement such nature-based coastal protection requires knowledge on how to restore or create tidal marshes in such a way that they quickly become highly stable and erosion resistant. Hence, we aimed to identify the drivers controlling the rate by which sediment strength and erosion resistance build up in natural and restored (managed realignment, sand suppletion, and controlled reduced tide) tidal marshes, using three different approaches. That is, we quantified sediment strength and erosion resistance (1) at natural marsh locations of different age, (2) at a restored marsh in sediment layers of different age, and (3) in a controlled experiment with pots filled with sandy or muddy sediment subjected to four different tidal regimes, with pots either left bare or planted with a sparsely or a densely growing marsh pioneer species. Sediment strength and erosion resistance were measured by a broad range of techniques, including shear vane, penetrologger, and flumes.

Our results revealed several important drivers affecting the development of sediment strength and erosion resistance in tidal marshes. Firstly, a densely growing pioneer species (e.g., Spartina anglica) increased sediment strength faster than a sparsely growing pioneer species (e.g., Scirpus maritimus). Secondly, a smaller tidal inundation frequency as well as lower sediment water content  increased sediment strength and erosion resistance. Lastly, lower sedimentation rates led to stronger consolidation, and thus higher sediment strength, in deeper sediment layers. Overall, our research shows that to create erosion resistant sediment beds in future marsh restoration projects, one should ideally aim for densely vegetated tidal marshes with well-drained, cohesive sediments at relatively high intertidal elevation, where sedimentation rates are moderate. These conditions provide the highest chance of resulting in highly erosion resistant tidal marshes that can serve within a reasonable amount of time as a nature-based coastal protection strategy.