Alternative nature-based solutions for flood protection in a macrotidal estuary under a changing climate.

Estuaries are complex and dynamic systems where physical and biological processes overlap with social and economic activities. Increasing coastal hazards and human pressure threaten the fragile equilibrium of these ecosystems. The combination of fluvial and coastal processes increases the probability of flooding in estuaries. Expanding urban development in these low-lying areas increases their exposure (i.e. the population, and the number and value of the coastal assets), which impacts the vulnerability of coastal communities and businesses. Traditionally, hard-engineered structures (i.e. grey defences) have been used to protect the coast from flooding risk. Nature-based solutions (i.e. green solutions) are now increasingly promoted to help remediate the growing costs and long-lasting impacts of grey defences, and to address the need for solutions that can balance the benefits for both nature and society and bridge social and economic interests. These green solutions have the potential to deliver both flood risk reduction, and other co-benefits such as habitat provision, spaces for recreation, or climate regulation. However, the shift towards green solutions is hindered by social and political resistance, by the difficulty to assess the co-benefits they offer, and by knowledge gaps in the level of flood protection they can provide under a changing climate. Here, we explore management options that can mitigate or worsen flood hazards both now and in the future for the Ribble estuary, a macrotidal estuary in North West England. The Ribble estuary includes Hesketh Out Marsh, one of Englands most important estuarine bird habitats and one of the biggest (322ha) completed managed realignment schemes in the UK. We will present coastal inundation modelling results using the SFINCS (Super-Fast Inundation of CoastS, Deltares) model under a series of what-if scenarios including alternative interventions, and future sea level rise. We tailored the experimental design by considering the coupled human-environment estuarine system. The chosen scenarios encompass a range of plausible interventions focusing on the managed realignment site of Hesketh Bank (e.g. no intervention vs managed realignment alternatives). We reviewed historic events with strong local narrative and legacy (e.g. near-miss event for storm Desmond in 2015). We chose the events so that they cover a range of compound fluvial and coastal hazards. We built on these historic events to explore mid to long term flooding risk under changing climate. For each scenario, we propagated nearshore the offshore conditions with the Delft3d model (Deltares). We then used the Delft3d outputs as inputs to the SFINCS model. The outcome is a library of flood maps, which can be overlapped with vulnerability or exposure data. This evidence can support coastal managers both on present day coastal management and on adaptation planning for environmental resilience.