Enhancement of coastal flood mitigation by implementing hybrid flood defences integrating hard engineering and nature-based solutions

Due to the energy dissipation induced by vegetation, restoration of natural ecosystems such as salt marshes is often considered an attractive nature-based solution against coastal flooding as an alternative to hard engineered defences. Even though this vegetation's capacity to dissipate energy has been well affirmed in the literature for short waves, it is still disputed for long waves (i.e., storm surges and tides) resulting in significant uncertainty on the potential effectiveness of mitigating the impact of storm surges. Furthermore, restoration or creation of coastal habitats often face challenges due to space limitations and social resistance linked to loss of valuable land. Hybrid schemes, integrating existing hard defences and natural solutions (e.g., vegetation zones), could be a good alternative to overcome such problems. However, evidence to support their implementation and design is lacking. In this study, a modelling chain in Delft3D-FM has been implemented to test the efficiency of a hybrid solution consisting of an embankment and vegetation zones in a case study in the inner Forth estuary (Scotland, UK). The flood protection effectiveness of the proposed solution is assessed for the storm Xaver (December 2013) that greatly affected the area. The model results indicate that the proposed hybrid scheme could considerably reduce the land loss for coastal protection and that the effect of vegetation depends on design criteria such as position and orientation relative to the prevalent current flow. These results provide valuable insight towards applicability, design and implementation of hybrid schemes.