As African cities expand in flood-prone areas, floods become more frequent, costly, and destructive. Faced with this reality, traditional drainage infrastructure is no longer enough. A study conducted by Practical Action and the Ecole Polytechnique de Thiès (EPT), under the Project for Strengthening Community Resilience to Floods through Ecosystem-Based Adaptation (PRECABE) in Thiès, Senegal, shows that investing in nature-based solutions not only reduces the risk of flooding but also generates economic, social, and environmental benefits.
Every rainy season, the same scene unfolds in many African cities: flooded neighbourhoods, impassable roads, disrupted economic activities, and thousands of families facing losses they cannot afford to absorb. In the face of these events, the response is almost always the same: build more canals, retention basins, and dikes.
For a long time, nature-based solutions have been seen as environmental alternatives that benefit nature, biodiversity, or urban aesthetics, but rarely as strategic investments. This perception helps explain why green and blue infrastructure (such as rain gardens, vegetated swales and wetlands) remain marginal in African urban policies, despite its recognised potential to reduce climate risks. By contrast, grey infrastructure remains a common response to urban flooding.
Traditional engineering approaches, such as drainage canals, retention basins and dikes, remain essential to urban flood management. These are examples of grey infrastructure: human-built structures designed to collect, store or convey stormwater. Yet, as cities expand and rainfall patterns change, these systems can face increasing pressure. Nature-based solutions (NbS), by contrast, use or restore natural processes to help address societal challenges. Rather than viewing these approaches as alternatives, it is worth exploring how they can complement traditional ones to strengthen urban resilience.
Could nature-based solutions help address some of these limitations by slowing runoff, increasing water infiltration and retaining rainwater? And rather than choosing between conventional infrastructure and nature, could combining the two offer a more effective and sustainable approach to reducing flood risk? This is the question explored by the PRECABE project in Thiès, Senegal, through an economic assessment of ecosystem-based adaptation measures and their potential to deliver benefits beyond flood prevention.
Nature-based solutions (NbS) are actions that protect, restore or sustainably manage natural ecosystems to address societal challenges. In urban areas, they can include restoring wetlands, planting trees, creating rain gardens and establishing vegetated channels to help manage stormwater and reduce flood risks. Beyond their environmental value, these solutions can also deliver social and economic benefits. Yet, nature-based solutions are still often viewed primarily as environmental interventions rather than strategic investments. One reason is the limited availability of economic evidence showing how their costs and benefits compare with those of conventional flood mitigation infrastructure. This is precisely the question that the PRECABE project, implemented in Thiès, Senegal, sought to explore.
The PRECABE project sought to go beyond assessing the environmental benefits of nature-based solutions. It aimed to answer a key question for policymakers: can restoring natural ecosystems and integrating vegetation into urban spaces be an economically sound investment in reducing flood risk? To answer this question, the researchers assessed the costs and potential benefits of several approaches, including restoring the Nguinth wetland, reforesting the Thiès plateau, and creating vegetated swales and rain gardens. The analysis considered not only their potential to reduce flood damage, but also their wider environmental and social benefits.
To answer this question, the researchers adopted an integrated approach combining territorial diagnosis, mapping of flood risk areas, cost-benefit analysis of nature-based approaches, comparison with grey infrastructure, and consultation with local stakeholders, including local authorities, technical services and community members.
This approach reflects an important evolution in the way we approach climate adaptation. It is no longer just a question of measuring the costs of implementing a solution, but of assessing the economic and financial losses it avoids, the services it provides, and the value it creates for communities.
According to the 2026 cost-benefit analysis carried out as part of the PRECABE project, the restoration of the Nguinth wetland in Thies could protect between 300 and 500 homes, benefit 15,000 to 25,000 inhabitants and generate annual economic benefits estimated at between 212 and 398 thousand USD. With an initial investment of about 572 thousand USD, the payback time is estimated at between two and three years. But these numbers only tell part of the story.
The results of the cost-benefit analysis conducted by the research team under the PRECABE project show that implementing nature-based solutions, such as restoring the Nguinth wetland, contribute to improving water quality, reducing the heat island effect, sequestering carbon, strengthening biodiversity, creating green jobs, and increasing the attractiveness of neighbourhoods. Rain gardens and vegetated valleys can reduce flood damage while generating local jobs and sustainably improving the living environment. In other words, the same investment simultaneously produces economic, social, and environmental benefits.
Before PRECABE, the question we were invariably asked in Thiès was: why plant trees when we need pumps and canals? Two years later, following the EPT’s hydrological studies and the rain gardens set up with the communities, the question has changed. People now ask: where shall we put the next ones? To me, that’s proof that NbS work when it’s the local people themselves who are asking for more.
Mamadou Touré, Thématique Lead Climate and Resilience
Nature-based solutions can complement grey infrastructure by addressing different aspects of urban flood risk. While drainage systems collect and convey stormwater, wetlands, reforestation, vegetated swales and rain gardens can slow runoff, increase infiltration and retain water before it reaches drainage networks. Combining these approaches can therefore help manage flood risks while delivering wider environmental and social benefits. The question is not whether to choose concrete or nature, but how to combine appropriate solutions according to local needs and conditions.
Integrating green infrastructure into urban planning requires close collaboration between local authorities, technical services, researchers, the private sector and local communities. Land availability is also an important consideration in Thiès. Rapid and unplanned urbanisation has put pressure on natural areas, led to the occupation of flood-prone zones and disrupted natural water flows. In some of these areas, existing drainage infrastructure may be insufficient to manage flood risks. Urban planning therefore needs to protect suitable spaces for green infrastructure while ensuring that nature-based solutions complement existing drainage systems.
But making space for green infrastructure is only part of the challenge. These solutions also require regular maintenance to remain effective over time. This does not necessarily make them more maintenance-intensive than grey infrastructure, but it does require clear responsibilities between public authorities and other local stakeholders, with community involvement where appropriate.
As the risks associated with climate change intensify, the urgency is not simply to invest more, but to invest in solutions that can reduce risk while delivering multiple benefits. In this context, the return on investment in climate adaptation should not be measured only in financial terms. It can also be reflected in avoided flood damage, the protection of lives and livelihoods, and improvements in environmental and social well-being. Investing in nature-based solutions can therefore be part of a broader strategy to make every adaptation investment deliver greater value for people, communities and public finances.
The experience of Thiès shows that climate adaptation requires us to think differently about public investment. Rather than choosing between grey infrastructure and nature-based solutions, cities can explore how both approaches complement each other to reduce flood risks while delivering wider economic, social and environmental benefits.
The real question may no longer be whether we can invest in nature. It is whether we can still afford not to do so.
Related article:
Reducing urban flood risk through nature-based adaptation and resilience in Senegal