Neumann, J.; Scheid, C.; Dittmer, U.
(2024):
Urban drainage systems are generally designed to handle rainfall events only up to a certain intensity or volume. With climate change, extreme events that exceed the design storms and consequently result in flooding are occurring more frequently. Nature-based solutions (NBSs) have the potential to reduce the pressure on urban drainage systems and to increase their resilience. This study presents an approach to compare and evaluate the effectiveness of NBSs for flood mitigation using a coupled 1D/2D model of surface and sewer flow. The study analyzes the effect of infiltration systems (dimensioned to return periods of T = 5 and 100 years), various green roofs, and tree pits considering the different degrees of implementation. The NBSs are represented as LID elements according to SWMM. As expected, the mitigation effect of NBSs declines with increasing rainfall intensities. However, infiltration systems dimensioned to T = 100 years achieve almost three times the flood reduction compared to systems dimensioned to T = 5 years, even during extremely heavy rainfall events (100 mm), resulting in a reduced total flood volume of 15.1% to 25.8%. Overall, green roofs (excluding extensive green roofs) provide the most significant flood reduction (33.5%), while tree locations have the least effect.
The water balance of urban areas can highly differ from natural areas, like forests. This deviation can be an indicator for water-related issues in urban areas, like floods, droughts and heat stress. To quantify the deviation of the water balance from a natural reference case, a new parameter ΔW was introduced to summarize the deviation of the components evaporation, surface runoff and infiltration. The open source water balance model ABIMO was further developed to simulate these three components for urban areas. ABIMO has been applied to the city of Berlin including a successful model validation for the surface runoff within the combined sewer system of Berlin. Simulations using ABIMO and computations of ΔW gave promising results for Berlin, and can be used to locate areas of high deviation in water balance as hotspots for the implementation of green and blue infrastructure. The model is openly available including R packages for its application.