Combination of large- and small-scale nature-based solutions for enhancing the performance of urban drainage systems

Abstract

Urban flooding poses increasing risks to cities, requiring solutions that integrate hydrological effectiveness with governance feasibility. This study evaluates the combined application of large- and small-scale nature-based solutions (NBS) to improve urban drainage system performance. Using the Storm Water Management Model (SWMM), three stakeholder-defined NBS combinations were tested under design storm scenarios for a selected study area in Cali, Colombia. The prioritization of measures was guided by Multi-Attribute Value Theory (MAVT), which incorporated input from municipal authorities, planners, and academia through structured scoring of hydrological, technical, and societal criteria. The calibrated and validated SWMM model showed that combined NBS interventions reduced peak runoff by up to 32 percent in major flood events and enhanced delay of peak flow compared to baseline conditions. Simulations under multiple rainfall return periods confirmed the robustness of the results. In addition to hydrological performance, the study highlights co-benefits of NBS such as biodiversity support, urban cooling, and recreational potential, which strengthen their relevance for integrated planning. Unlike optimization-based approaches, the proposed framework emphasizes stakeholder-driven prioritization, ensuring that selected measures align with local governance capacity and community preferences. The findings demonstrate that combining large- and small-scale NBS can provide resilient and socially grounded solutions for urban flood risk management.