This research investigates how salt marshes contribute to both wave energy dissipation and spectral period transformation, advancing their role as a nature-based solution for coastal protection. Using laboratory simulations with a scaled barren foreshore, salt marsh and dike model, we examine the interactions between vegetation, water depth, and wave properties under varied conditions, including storm scenarios with irregular waves. Results indicate a case specific threshold at which the salt marsh model attenuates energy optimally, as for very shallow water depths wave energy is predominantly dissipated by the barren foreshore. The spectral wave period T m − 1 , 0 increases when waves propagate from deep to shallow water depths, as a result of wave breaking and generation of infragravity waves. The presence of salt marsh vegetation further enhances this effect by preferentially damping high frequency components. This highlights that an increase in T m − 1 , 0 in vegetated environments may not always correspond to an increased hydrodynamic load on the dike.

Hard coastal structures such as dikes covered with asphalt or placed block revetments have been widely used in the past for coastal protection in densely populated deltas around the world. Nonetheless, in recent years the effectiveness of hard structures has been questioned in light of the inevitable effects of climate change and their static nature. Decades of research on how salt marshes can play a role within a comprehensive coastal protection scheme suggest that these low environmental impact structures (Maza et al., 2015) might have the capability of dissipating wave energy and hence be technically and formally considered within hybrid coastal erosion and flood protection systems (Borsje et al., 2011). However, only very few studies investigated wave attenuation by real salt marsh vegetation in large-scale laboratories (Ghodoosipour et al., 2022; Maza et al., 2015; Möller et al., 2014) and none of them addressed extreme hydrodynamic design conditions in terms of wave energy and water levels. As a result of this knowledge gap, salt marshes in The Netherlands and all around the world have never been formally considered within the coastal flood protection systems and the underlying risk assessment. With this contribution our aim is to provide an overview of the first worldwide large-scale test focused on the interaction between a salt marsh (i.e. vegetation and shallow foreshore) and extreme hydrodynamic conditions, the adopted measurement techniques and the preliminary results in terms of wave damping, erosion and removed biomass.