Costing of nature-based and grey solutions for water management: case studies in Nairobi.

Adaptation to climate change in urban Africa is a critical development issue of this century, with informal neighbourhoods particularly vulnerable to climate change impacts. Nature-based Solutions (NbS) have been put forward as an approach to enhance flood and climate resilience in urban settings. While the multiple social and environmental benefits of NbS are increasingly well documented, a key gap in the literature, particularly for African cities, is robust data on the cost of NbS compared to traditional "grey" infrastructure. In this study, the costs of two different NbS in informal settlements in Nairobi, Kenya, were compared with the costs of hypothetical equivalent "grey" solutions. Comparative Life Cycle Costing (LCC) was applied for both cases and a cost-benefit analysis (CBA) was conducted for one case where historical data was available. The LCC found in the first case that the NbS was more expensive than the grey solution, whereas in the second case the NbS was significantly cheaper. As the two NbS projects were sequential, higher costs of the first case were partly attributed to the initial learning process of the implementing organisation to build knowledge on how to create context-appropriate solutions through thorough testing and participatory design. While the cost of any solution is context dependent, we conclude that NbS here can show competitive costs, particularly when considering co-benefits highlighted by the CBA. [ABSTRACT FROM AUTHOR]