Nature-based flood adaptation in Kigali’s Mpazi sub-catchment
The Mpazi sub-catchment in Kigali, Rwanda has long been one of the city's most flood-prone areas characterized by steep slopes, unplanned settlements, and upstream land degradation. In 2021, Rwanda Young Water Professional, with support from the Rwanda National Commission for UNESCO, carried out a study to conceptualize and pre-design Nature-Based Solutions (NBS) for flood adaptation.
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Indicated issues
Flooding in a rapidly urbanizing city
Mpazi faces recurrent flash floods driven by heavy rainfall, high-velocity flows, sedimentation, and inadequate drainage. Steep topography and degraded upstream landscapes accelerate runoff, while unplanned informal settlements and rapid urbanization increase impervious surfaces, exacerbating flood risks. These conditions lead to significant economic impacts, with the Nyabugogo commercial area suffering annual losses of at least 178.2 million Rwf, as reported in a 2016 City of Kigali survey. Infrastructure vulnerabilities, such as overtopping channels and damage to bridges, markets, and transport corridors, further compound the issue. Environmental degradation, including soil erosion and loss of vegetation cover, intensifies these challenges in the context of Kigali's rapidly growing urban landscape.
Goals
From short-term fixes to long-term resilience
The study sought to integrate NBS with grey infrastructure for sustainable flood management in Mpazi sub-catchment. It assessed hydrological dynamics using scientific modeling, conceptualized NBS to reduce peak flows and enhance resilience, engaged stakeholders for scalability to other catchments, and promoted community awareness for proactive, nature-integrated urban planning. Beyond reducing runoff and flood risks, the aim was to show how NBS can be mainstreamed into Kigali's Master Plan, making resilience to flooding part of the city's long-term development framework.
Solutions
Nature-Based interventions for Mpazi
Based on satellite imagery, field validation and hydrological modeling, the study identified several NBS that are suitable for Mpazi. Reforestation and conservation agriculture were proposed to reduce erosion and enhance infiltration upstream. In the urbanized areas, options such as permeable paving, rainwater harvesting, bio-retention systems, and buffer zones were highlighted. Green spaces and flow control structures were also recommended to slow runoff and improve the performance of downstream grey infrastructure.
By linking these measures with Kigali's spatial planning priorities, the study illustrated how NBS could be embedded into the city's development strategies, ensuring that flood control is not treated as a one-off response but as part of long-term urban planning.
Results
Evidence for integration and scaling up
Hydrological modeling showed that the proposed NBS could reduce peak flood discharges by up to 28 m³/s, significantly improving the effectiveness of downstream infrastructure. Beyond the technical findings, the study emphasized the need to frame NBS within an economic perspective, given the scale of flood damages reported in Nyabugogo. A cost-benefit analysis would strengthen the case for NBS as cost-effective alternatives to repeated reconstruction (City of Kigali, 2016).
The study also stressed the importance of involving local residents and businesses in the design, implementation, and maintenance of NBS to ensure sustainability. Finally, it highlighted the potential for upscaling NBS to other flood-prone micro-catchments in Kigali such as Rugunga, Rwandex, and Mulindi, where similar conditions exist.
Valuation of solution
The conceptualization of NBS in Mpazi was rigorously grounded in scientific facts, including detailed hydrological assessments using tools like HEC-HMS for rainfall-runoff modeling and HEC-RAS for flood simulation. This approach ensured that proposed measures were tailored to the sub-catchment's unique hydrological behavior, such as peak discharges, runoff patterns, and flood extents.
The project in Kigali's Mpazi sub-catchment shows that NBS can significantly reduce peak flood discharges, preventing recurring damages that cost millions each year.
Find out more here: Nature-Based Flood Adaptation Project