On Communicating Cost-Effectiveness of Flood-Mitigation Schemes

Flood risk protection measures are designed to reduce intensity, frequency and extent of feared events. For any category of measures ranging from classical civil engineered measures to Nature-Based Solutions (NBS), being able to assess their physical and technical capacity remains a starting key issue and requirement. It is essential both to design effective solutions and also to analyze their reliability during their lifetime. For hydraulic applications, the analysis of this capacity consists in checking that proposed solutions are able to evacuate flood water discharge or to store water volume. The protocol described in this paper provides an easy-to-understand framework to assess and represent the effect of measures on the considered flood event and to compare it with their relative costs. It can therefore be considered as a basis to help decision-making within the risk management process and also as a contribution to the analysis of the safety and reliability of planned measures. The protocol enables rapid a priori, as well as thorough a posteriori, comparisons to be made of the efficacy of various flood-mitigation options and scenarios. We have considered a concept called ``dynamic flood-excess volume'' (dFEV or FEV) and revisited it in a three-panel graph comprised of the (measured) in-situ river-level as function of time, the rating curve and the hydrograph, including critical flooding thresholds and error estimates. FEV is the amount of water in a river system that cannot be contained by existing flood defences. The new tool deliberately eschews equations and scientific jargon and instead uses a graphical display with FEV displayed as a (dynamic) hypothetical square lake two metres deep. This square-lake graphic is overlaid with the various mitigation measures necessary to capture the floodwaters and how much each option will cost. The tool is designed to help both the public and policymakers grasp the headline options and trade-offs inherent in flood-mitigation schemes. It has already led to better understanding and decisionmaking regarding flood defences in the UK, Slovenia and France, particularly where a number of alternatives are being considered. Three realistic cases -from the UK, Slovenia and France- will be reviewed, including insights on dealing with uncertainty and on the communication of multiple benefits of Nature-Based Solutions, followed by a Socratic-method dialogue.