Work

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

Year: 2026

Type: article

Abstract: ABSTRACT 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.

Source: Journal of Water and Climate Change

DOI: 10.2166/wcc.2026.484

Authors: Dennis Ndashiru Audu, Hosea Munge Mwangi, John Mwangi Gathenya

Institution: Jomo Kenyatta University of Agriculture and Technology

Language: English

Topic: Flood Risk Assessment and Management

Subfield: Global and Planetary Change

Field: Environmental Science

Domain: Physical Sciences

SDGs: Sustainable cities and communities, Climate action, Clean water and sanitation, Life on land

Open Access status: diamond

Funders: Deutscher Akademischer Austauschdienst, Jomo Kenyatta University of Agriculture and Technology