A framework for morphological impact assessment of a small sandy beach under climate What-If Scenarios and Nature-based Solutions

Wave climate influences the morphodynamics of sandy beaches and is the main driver of coastal impacts. Since impacts depend strongly on local characteristics, a framework for morphological impact assessment must harmonize climate scenarios with the status of key features such as river discharge and seagrass meadows. This study aims to provide a framework for assessment based on the combination of What-If Scenarios (WIS) and Nature-based Solutions (NbS), representing a broad range of estimates for impacts under different climates. In this work, the framework is tested on the Ropotamo Beach (Western Black Sea, Bulgaria), a well-conserved sandy beach extensively used by local communities. The WIS and NbS were explored with the XBeach model across 200 numerical experiments, incorporating variations in river discharge and wave climate. Future wave conditions were derived from regional projections forced with CORDEX-EU11 for 2070-2100 and compared with a historical baseline (1975-2005). The WIS explored scenarios for the 95th percentile of significant wave height and wave peak period, while the NbS explored the capacity of seagrass meadows to buffer impacts. Results summarize a comprehensive set of risk metrics quantifying erosion and hazards induced by circulation. Seagrass meadows significantly reduced nearshore erosion and strong currents by 94% across the majority of scenarios, demonstrating their effectiveness under projected climate change conditions. The novelty of this study lies in integrating local features within a Digital Twin of the Ocean (DTO) framework to evaluate coastal morphological impacts. This approach highlights the potential of NbS to enhance coastal resilience and support sustainable shoreline management in regions lacking data.