Recommendations for the development of erosion-resistant tidal marshes to enhance nature-based flood risk mitigation.

There is growing interest in nature-based hybrid flood risk mitigation, in which wetland ecosystems, such as tidal marshes, are an integral part. Tidal marshes can attenuate wave loading on defense structures, reduce breach depth in case of defense structure failure, attenuate storm surges, reduce shoreline erosion, and grow vertically with sea-level rise. Marshes are currently being restored and created for flood risk mitigation and/or for other ecosystem services, making it crucial that they can persist in the long term and therefore develop erosion-resistant sediment beds. In this paper, we first discuss the key physical and biological processes and properties that drive the development of erosion-resistant sediment beds in tidal marshes, showing that vegetated, consolidated sediment beds, effectively functioning food webs, and a wide intertidal zone with a variety of ecosystems are highly important for the realization of marsh restoration/creation projects for flood risk mitigation. Second, we discuss how the location (i.e., either seaward or landward of a defense structure) and the physical conditions (i.e., elevation relative to the tidal range and the strength of local wave forcing) under which tidal marsh restoration/creation takes place can determine which interventions will be most suitable. Possible interventions include: sediment nourishment, stabilization structures, managed realignment, transitional polder, regulated tidal exchange, vegetation planting/transplanting, and/or creek excavation. The location and the chosen restoration or creation intervention will strongly affect how fast marsh establishment and erosion resistance development will occur. Overall, our study reveals that successful marsh restoration/creation for nature-based flood risk mitigation requires (1) selecting the most appropriate marsh restoration or creation intervention for a specific location, and (2) giving the marsh enough time to develop before it is needed for flood risk mitigation. The incorporation of marshes within nature-based flood risk mitigation strategies should not only be considered at an individual site scale but also upscaled to large landscape scale consequences for flood risk mitigation, to ensure a fully functioning wider ecosystem with good connectivity between ecosystems.