Integrating Nature-Based Solutions (NbS) for Enhanced Flood Resilience under a Changing Climate: The Case of the Cologne District, Germany

The Cologne District in western Germany has frequently experienced flooding from the Rhine River and its tributaries. Current protection measures consist of a combination of engineered, green, blue, and soft infrastructure. The district was affected by the catastrophic floods in Europe in July 2021, and climate models predict an increased risk of heavy summer rainfall, which could lead to severe flooding, especially of the tributaries of the Rhine. Nature-based solutions (NbS) are recommended to enhance flood resilience. However, there is no publicly available systematic register of existing and planned NbS. This study addresses this gap by applying a novel landscape-based approach to map, categorise, and qualitatively evaluate existing and planned NbS for flood risk mitigation, and to identify suitable areas for additional NbS implementation. Using GIS analyses, including the Topographic Wetness Index, potential intervention areas on slopes prone to rapid runoff and soil saturation were identified to inform targeted NbS implementation. Maps and assessment tables have been created for both existing and planned NbS, and potential areas for future interventions identified using public databases. The results demonstrate that multiple NbS have been implemented and are planned along the Rhine, but additional efforts are needed in the Erft and Wupper tributaries, despite several planned and implemented river restoration projects. NbS in these catchments focus on floodplains, while slopes and riparian areas, suitable for the existing urban and agricultural land uses, have not been systematically assessed for their effectiveness. With the predicted increase in extreme climate events featuring intense summer rainfall and drought periods, surface runoff may rise due to dried and hardened soils, increasing flash flood risks. We therefore recommend targeted NbS interventions on slopes. Additionally, cities such as Cologne should develop further infiltration areas to mitigate the risk of pluvial flooding.