Is-it worth investing in NBS aiming at mitigating water risks? Insights from the economic assessment of NAIAD case studies

The economic advantage of NBS solutions aiming at mitigating water-risk is widely put forward as an argument for their development. There is nevertheless limited scientific evidence to support this argument. This paper therefore elaborates a methodological framework for the economic assessment of NBS and presents its application to three NAIAD case studies (the Lez catchment, France; Rotterdam, the Netherlands and Brague catchment, France). Robust methods are particularly applied for the estimation of the benefits associated with NBS. Physical models coupled with damage estimation models are developed to estimate the avoided damages generated by NBS. A diversity of ecosystem service valuation methods are also applied to evaluate the monetary value of NBS co-benefits: contingent valuation (Brague), choice experiment (Lez) and direct valuation methods (Rotterdam). We estimate the cost of implementation and maintenance mainly through the transfer of values coming from studies in similar contexts. Proxies are used to estimate the opportunity costs associated with the development of NBS. Finally, these estimations are compiled in a cost-benefit indicator allowing the estimation of the economic efficiency of NBS strategies. The study confirms that the cost of implementation and maintenance of NBS strategies is lower than the cost of grey solutions for the same level of water risk management, emphasizing the better cost-effectiveness of these solutions. Benefits in terms of avoided damages are however not sufficient to cover investment and maintenance costs. The cost–effectiveness of NBS strategies, which are combinations of individual NBS measures, may be improved by combining cost effective individual NBS measures. There is indeed a very large heterogeneity of cost-effectiveness of individual NBS measures (cost/m³ of water retention). Results also reveal that co-benefits represent the largest share of the value generated by NBS strategies. It is therefore of utmost importance that co-benefits are integrated in the economic valuation of NBS for them to be judged economically efficient. This conclusion must be taken into account in the elaboration of NBS funding strategies.There is finally no clear-cut conclusion on the overall economic efficiency of NBS throughout the case studies. Lez reveal a positive cost-benefit analysis, while Rotterdam and Brague cases do not. Results are therefore case-specific and confirm the importance to carry out thorough economic valuations of a diversity of strategies at each sites, including NBS, grey and hybrid solutions, in order to identify the most adequate strategy for water risk management and to address territorial challenges.