A high quality safety assessment of levee systems requires a good prediction of when the grass cover of levees fail. Current methods relate the onset of failure to the peak in momentum or energy of the flow, instead of the peak in momentum transfer or energy transfer to the grass cover. The critical velocity necessary in the current methods is thereby difficult to quantify. In line with determining the peak in momentum transfer to the grass, here is shown that the onset of damage of the grass cover can be related to the peak normal stresses acting on the grass cover during wave overtopping. The peak in momentum transfer is thereby assumed to be located at the point of reattachment of the flow with the landside slope. The method is validated against the results of two wave overtopping experiments and benchmarked against the cumulative overload method. An advantage of this method is thereby that both the time and location of the onset of damage can be predicted.@en

Coastal flood managers seek to anticipate future flood risk and as a result consider the adaptation of flood defences. Instead of crest heightening, dikes can be adapted to include hydrodynamic reducing vegetated foreshores to form a nature-based hybrid flood defence, for instance; at managed realignments. In this study we investigated the potential of vegetated revetments as a natural continuous connection between the realigned dike and restored foreshore. We applied the historic grass sod transplantation technique with the aim to improve our understanding of the strength of a transplanted sod revetment. In Living Lab Hedwige-Prosperpolder, dikes were available for in-situ experiments during managed realignment preparations. We transplanted grass sods and studied erosion resistance after one growth season. Our results show transplanted sod vegetation continued to grow and started to attach to the clay layer. While erosion occurred under extreme wave impact and overflow, the sod pulling method revealed individual sod strength. In conclusion, sod transplantation is a good technique to source local material for green realigned dike revetments. A vegetated dike revetment can hereby create a natural continuous connection between the realigned dike and foreshore, which benefits flood protection as well as flora and fauna.@en