Local resources for dike revetment in managed realignments

Managed realignment is the landward relocation of flood infrastructure to reintroduce the tide on former reclaimed land. A managed realignment site is an ecological restoration site. At the same time, it forms a new type of hybrid flood defence that makes use of Natural and Nature Based Features. Nature-based flood protection is provided by the flood risk reduction capacity of the restored habitat such as saltmarshes, complemented by the landward relocated flood defence infrastructure, i.e., the realigned dike. The realigned dike is either a (reinforced) existing or newly constructed dike. The realigned dike can be built or reinforced from local resources such as the saltmarshes and formerly reclaimed land. Material from the removed dikes can also be re-used. In history, saltmarsh sods have been used as building material for the dikes themselves and for their revetment. The sods were also used as emergency dike repair material. In addition to the use of local resources as building material for flood infrastructure, the mining of local resources can simultaneously support nature restoration under the dynamic circumstances of the coast.
      We tested the historic sod technique for dike revetments at a managed realignment project in progress where the dikes were available for real size experiments. On a dike with grass revetment, we studied the erosion resistance of transplanted dike grass sods after one growth season. Grass sods were transplanted to the inner and outer dike slope. The erosion resistance of the transplanted sections was tested under calm and extreme conditions with a grass pull test, a wave impact simulator, an overflow generator, and by analysing roots development. After one growth season, we found that the vegetation of transplanted sods continued to grow and started to connect to the original dike revetment. While some erosion occurred under extreme circumstances, the grass pull test revealed the transplanted sod revetment was stronger than a bare clay revetment. The erosion resistance of transplanted sods after one growth season is promising when compared to for instance a newly seeded grass revetment.
      In conclusion, the sod application technique can provide local resources for the revetment of realigned dikes. Sod transplantation can also be used to introduce target species at for instance the dike toe. At the same time, mining of (grass) sods from former land or saltmarshes can support nature restoration and development. Our results can hereby contribute to increase our ability to design flood infrastructure with Natural and Nature Based Features.