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1. Extension of the urban study area is identified through the Copernicus Land Monitoring Service European Urban Atlas 2018 dataset⁵, which provides reliable, inter-comparable, high- resolution land use and land cover data for 785 Functional Urban Area (FUA) for the 2018 reference year in EEA39 countries. Extension of the core urban area for each city is used to define the analysis domain.
2. Base digital elevation model (DEM) describing surface topography for the 20 cities is the European Digital Elevation Model (EU-DEM, v. 1.1) at 25 m ground resolution from Copernicus Land monitoring Service⁶. For each city, EU-DEM is resampled at 5 m to enable the buildings extrusion process. Moreover, EU-DEM is hydrologically conditioned through a stream burning operation (a common flow enforcement technique used to correct surface drainage patterns derived from digital elevation models). In this sense, the EU-Hydro - River Network Database from Copernicus Land Monitoring Service is used to depress by – 10 m elevation along river network.
3. For the city of Koln, Milan and Pamplona, the analyses are duplicated also using high- resolution DEM. Specifically, it has been adopted a LiDAR-Derived DEM at 1 m for Koln and Milan, and a LiDAR-Derived at 2 m for Pamplona. For these additional elaborations, the analysis domain is the same of LiDAR. Regarding a potential stream burning operation, the high resolution of LiDAR does not require any stream burning operation, as the river network is well represented in the dataset.
4. To consider presence of obstacles (buildings) that reduces effective storage capacity over DTM surface, Open Street Map ⁷ (OSM) vector buildings database is retrieved for the area of interest. Building polygons have been used to extrude a conventional elevation of + 10 m over original elevation inside each building to simulate the obstacle to water storage.
5. Regarding Safer_RAIN assumptions, such a model is adopted by considering, on the safe side, impermeable surface (neglecting soil infiltration capacity that reduces rainfall runoff volumes). It is also neglected the sewage network capacity to collect and drain rainfall out of the domain, although this assumption is closer to reality the more pluvial events exhibit short duration and high intensity.

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