Enhancing coastal flood mitigation through hybrid defences integrating hard engineering and nature-based solutions

Coastal flooding affects the lives and prosperity of millions of people living by the sea, and rising sea levels will only increase this risk. Coastal defences are already subject to more extreme and frequent storm events and may not be able to withstand future conditions. Consequently, designing suitable flood protection policies and schemes is becoming ever more crucial. Coastal practitioners across sectors have started to champion ‘greener’ nature-based solutions as alternatives to traditional hard coastal defences. Coastal wetlands (e.g., salt marshes, mangroves) can act as buffers and help mitigate storm impacts because their vegetation dissipates wave energy. Multiple studies have confirmed that wetlands effectively attenuate short period waves (i.e., wind waves), but their efficiency against long period waves (e.g., tidal waves, storm surges) remains in doubt. It is generally assumed that tens of kilometres of wetland width are required to achieve sufficient storm attenuation in these cases. However, coastal squeeze and urbanization often limit the creation of such large wetlands, and the necessary conversion of agricultural land causes social resistance to nature-based solutions. In this study, the effectiveness of hybrid solutions was tested as an alternative. A 2D numerical model is built in Delft3D-FM to simulate flooding in the inner Forth Estuary (UK), in an area that suffers from frequent flooding. The hybrid defence scheme comprises an existing embankment enhanced by vegetation patches of various sizes and locations in front, on top and behind the embankment. In the model, the vegetation consists of grasslands including salt tolerant plants of substantial height and density. Model simulations were designed to replicate conditions during the December 2013 storm, which devastated the study area. The results indicate that vegetation can significantly increase the energy dissipation already provided by the embankment and, in turn, reduce water depths and flood extents.

Our results also show that combining vegetation and embankment requires vegetated zones with less cross-shore width to achieve desired protection. In this specific example, this reduces the loss of agricultural land, and more generally points at limiting necessary land use conversion. It also lowers repair and maintenance costs of seawalls and dikes. The effectiveness of vegetation in storm attenuation is enhanced when it interferes with the main flow path and alters flow circulation.  As such, the location of vegetation is a key consideration when implementing these solutions. Finally, this study suggests that wet grasslands can be a viable option for flood mitigation as an alternative to salt marshes and mangroves when implemented aside of hard engineering solutions. These findings offer valuable insights for coastal managers and practitioners interested in implementing hybrid or composite defences and highlight the potential benefits of these approaches, including testing more socially acceptable solutions.