No abstract

De nos jours, les Solutions Fondées sur la Nature (SFN) représentent un levier important des politiques publiques de réduction d’inondation. Afin de faciliter l’analyse coûts-bénéfices des investissements dans les SFN, cet article propose la première fonction de transfert fondée sur une méta-analyse. Cette fonction permet d’identifier et d’évaluer la valeur des SFN pour la réduction des risques d’inondation dans un contexte de bassin-versant. Nous avons analysé 199 observations provenant de 62 études menées en Amérique, en Europe et en Asie-Océanique depuis le milieu des années 1990. Nos estimations montrent que les SFN liées à la restauration écologique du lit de la rivière et des zones humides sont les plus valorisées. Quant aux bénéfices, nos résultats montrent que le principal avantage de la régulation de l’eau n’est pas significatif alors que les co-bénéfices tels que les services de récréation, d’appréciation esthétique et de provision de nourriture et de matériels sont positifs et significatifs. Les tests de validité de la fonction de transfert – au sein de l’échantillon et dans le contexte du bassin versant de la Brague – montrent une Moyenne Absolue des Pourcentages d’Erreurs (MAPE) comprise entre 5 % et 66 % avec une médiane de 17 %. Cela suggère que la fonction de transfert peut raisonnablement être utilisée pour estimer les bénéfices des SFN pour réduire des risques d’inondation.

<p>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/m<sup>3</sup> 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.</p>

Cet article évalue l’efficience économique de stratégies de prévention du risque d’inondation en vue d’éclairer la décision de financement des acteurs publics. Le cas de la protection contre les crues « éclair » (courtes et violentes) de la Brague et des inondations associées de la basse vallée à Biot et Antibes (Alpes-Maritimes) est pris comme exemple. Dans cette perspective, nous proposons une double analyse coût-bénéfice (ACB) dans le contexte particulier de la loi Gemapi (gestion des milieux aquatiques et protection contre les inondations) et des stratégies dites « SFN » (solutions fondées sur la nature). L’ACB top-down éclaire les décisions des acteurs nationaux et régionaux tandis que l’ACB bottom-up répond aux préoccupations des acteurs locaux en évaluant l’acceptabilité sociale des stratégies étudiées.

Evidence are dearly needed to understand under which conditions it is relevant for decisions makers to invest in NBS for water-related risk. This chapter presents the methodological framework developed for the economic assessment of NBS for water-related risks and its application to seven case studies. We particularly develop methods for the evaluation of implementation and opportunity costs, the assessment of the reduction of damage costs, and the monetary value of co-benefits. 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 generally not sufficient to cover investment and maintenance costs. Co-benefits represent the largest share of the value generated by NBS strategies. The overall cost-benefit analysis implemented in four cases, is positive in three case studies and negative in one. This confirms the importance to carry out thorough economic assessments for the elaboration of Natural Assurance Schemes.