Investigating the efficiency of nature-based solutions against estuarine coastal flooding under present and future conditions

Most coastal areas around the world are currently at risk of flooding, which is increasing due to sea level rise and other impacts of a changing climate. The design of appropriate flood protection policies and schemes is thus becoming more imperative. Partly in response to net zero and net gain agendas, coastal practitioners across sectors have started to champion greener nature-based solutions in place of traditional hard coastal defences. However, social acceptance is limited, and examples worldwide are too scarce to fully test and demonstrate the efficiency and societal benefits of nature-based solutions. Appropriate case studies are required to build the knowledge and evidence base needed for the implementation of nature-based solutions. In this study, the efficiency of nature-based solutions (e.g., managed realignment) against flooding is investigated for an estuarine case study in Scotland. The Forth Estuary is one of UKs most important estuarine ecosystems both for economic and ecological reasons. In recent years, flooding events have considerably affected urban areas and infrastructure along the estuary. The frequency and intensity of such events is expected to increase due to climate change and result in significant adverse impacts on local population and economy. Airth is a villagesituated in the south bank of the inner Forth Estuary. It is a residential area that covers 5500 hectares of agricultural land with some woodland as well. Part of it is designated as a conservation area because of its significant historical background. However, it is often subject to coastal and/or surface water flooding. The local authority has launched a management plan strategy for flooding mitigation seeking adaptation solutions. A 2D numerical model has been built in Delft3D-FM to determine the hydrodynamic setup in the Forth estuary. The model encompasses a large area starting from the inland tidal limit and including both the inner and outer Forth estuary. It is forced upstream by river discharge and downstream by water level time series. To account for additional flood drivers such as wave set-up, run-up, and wind-driven surges, a second model is built in SFINCS with a finer resolution and with its extents locally restrained around the Airth coast. Modelling scenarios comprise at first a series of hindcast simulations performed to reproduce the impact of three recent storm events that largely affected the local community by causing extensive inundation and flooding of properties. The simulations are then repeated with bathymetry adaptations to represent interventions (i.e., managed realignment) into the model and compare their effect against flooding. In addition, simulations with future sea level scenarios are considered to assess these interventions efficiency under a changing climate. As events of similar or higher intensity can be expected in the future, model results can give a good indication of how the system responds when the nature-based defences are in place. These can assist, advise, and direct stakeholders and local authorities to consider alternative and state-of-the-art solutions in their fight against coastal flooding impact.