Multiscale modeling for coastal cities: addressing climate change impacts on flood events at urban-scale

Abstract. This study presents an integrated modeling framework designed to bridge scales from regional to urban, enabling a detailed assessment of the impacts of future climate scenarios on three European coastal cities: Massa (Italy) and Vilanova (Spain) in the Mediterranean, and Oarsoaldea (Spain) in the Atlantic. Conducted as part of the SCORE EU Project (Smart Control of Climate Resilience in European Coastal Cities), the framework employs a novel, non-standard downscaling approach to translate large-scale atmospheric outputs from the EURO-CORDEX regional model ALADIN63 (for Historical, RCP4.5, and RCP8.5 scenarios) into high-resolution simulations of storm surges, wave climate, and river discharge using SHYFEM, WAVEWATCH III, and LISFLOOD models. The framework achieves coastal resolutions on the order of 100 m, providing time series of water levels and wave runup, which are combined into total water levels. These results, together with extreme value analysis of river discharge and projected relative sea level rise (RSLR), are used as boundary conditions for an urban-scale hydrodynamic model with resolutions as fine as 2–20 m. This multi-scale integration allows for detailed analysis of changes in flooded areas and volumes under RCP4.5 and RCP8.5 scenarios, relative to historical conditions, highlighting the influence of shifting extremes, RSLR, and site-specific features. Results show that in Massa and Vilanova, increased extreme river discharges are projected, while moderate changes in extreme water levels are overshadowed by RSLR, particularly for Massa. Oarsoaldea, well protected from storm surges, is expected to experience a slight reduction in extreme river discharge. This work demonstrates the capability of an integrated modeling framework to address climate change impacts at the urban scale. Local-scale modeling is essential: accurate flood hazard assessment in coastal cities requires high-resolution simulations to capture the influence of local topography and infrastructure, especially where global DEMs are inadequate. By linking climate projections to urban flood impacts, the framework enables a consistent evaluation of future extremes, sea level rise, and their interaction. A further key message of this study is the need to generate actionable insights to support the development of targeted and site-specific adaptation strategies. Adaptation must be tailored: only by quantifying future extremes and exposure is it possible to design effective, place-based responses.