Salt marshes for nature-based flood defense: Sediment type, drainage, and vegetation drive the development of strong sediment beds

In face of sea-level rise and increasing risks for storm impacts on shorelines, there is a growing demand for developing nature-based flood defenses, for example by restoring or creating salt marshes in front of engineered structures such as dikes. However, salt marshes can only optimally provide flood defense if their sediment beds are erosion resistant, even under very high flow velocities. It remains unknown how fast sediment strength develops in marshes restored or created for nature-based flood defense. Therefore, this study investigated how 1) sediment type, 2) tidal drainage depth and duration, and 3) pioneer vegetation species drive the development rate of sediment strength. A controlled experiment was set up with pots filled with two sediment types, which were either left bare or planted with Spartina anglica or Scirpus maritimus, two dominant salt marsh pioneers in NW Europe. All treatments were subjected to four different tidal regimes with different tidal drainage depth and duration. The results showed that sandy mud (with a 37% silt and clay fraction) led to much stronger sediments than fine mud (with a 77% silt and clay fraction). Sediment strength was higher in the treatments with deeper tidal drainage depth and longer drainage duration. The presence of vegetation increased sediment strength and this effect was stronger with Scirpus maritimus than with Spartina anglica. Plant roots increased sediment strength directly, and the presence of vegetation also seemed to increase sediment strength through enhanced evaporation and transpiration. From these results it can be concluded that to restore or create erosion resistant salt marshes for flood defense, it is essential to ensure that marshes can form at relatively high elevations from well-draining sand-mud mixtures, thereby also ensuring vegetation growth.