Flood risk mitigation for an urban catchment: findings from a grey, nature-based solutions and hybrid strategy approach

Flow diagram showing rainfall-driven flood modelling using HEC-RAS, comparing baseline, nature-based solutions, structural measures, and hybrid scenarios, with reductions in flood depth, velocity, and discharge across the catchment. Rapid urban expansion and informal settlements are increasing exposure to flood hazards in urbanising catchments such as the Nairobi River Catchment. The 2024 El Niño event caused widespread flooding, affecting critical infrastructure and highlighting the urgent need for effective mitigation strategies. This study evaluates three flood management approaches using the HEC-RAS model: (1) modified channels as grey infrastructure, (2) soil organic matter enhancement and reforestation as Nature-based Solutions, and (3) a hybrid strategy. Model performance showed strong agreement with observed flows (R2 = 0.88; NSE = 0.75). Spatial datasets, including Global Forest Change and Land Productivity Dynamics, were used to identify intervention zones and parameterise scenarios. Results show that NbS reduced peak discharge by 37–75% and delayed peak flow by about one hour. Grey infrastructure achieved the greatest impact, reducing flood depths by 55–60% and peak discharge by 48–75%, but increased flow velocities. The hybrid strategy provided the most balanced hydraulic performance, combining runoff attenuation with improved channel conveyance to deliver consistent reductions in flood magnitude and enhanced flow regulation. These findings demonstrate that integrated approaches combining engineered and nature-based interventions offer a resilient pathway for managing urban flood risk under increasing climatic and developmental pressures.