Rapid urbanization and climate change pose increasing flood risks to cities in Sub-Saharan Africa (SSA). This study developed and tested a method to assess how Nature-Based Solutions (NBS) implemented across peri-urban areas can reduce downstream urban flooding, using the city of Nakuru in Kenya as a case study. Coupling the Wflow hydrological model with the SWMM hydraulic model shows that external runoff is a major contributor to urban flooding in Nakuru, accounting for approximately \(70\%\) of the total flood volume for a \(T=5\) year event, in all four simulation scenarios.
Three types of NBS were modeled: reforestation, terracing, and reservoirs. Among these, reservoirs proved most effective, reducing the flood volume for a \(T=5\) event by \(17\%\). The cumulative NBS reduced the flood volume by \(19\%\), indicating a limited impact of reforestation and terracing. This limited impact is partly due to their small-scale implementation and simplified modeling of their effects. The study revealed that a catchment-scale approach is necessary to fully utilize the potential of NBS for urban flood mitigation. To have a tangible impact on urban flooding, NBS implementation should scale to the size of the upstream catchment areas, not to the size of administrative boundaries.
In conclusion, NBS hold potential for sustainable urban flood management in SSA. Their success depends on catchment-wide integration in flood risk management, improved data and modeling practices, and continued research on their context-specific effectiveness and model implementation.