A national assessment of natural flood management and its contribution to fluvial flood risk reduction

The desire to promote Natural Flood Management (NFM) has not yet been matched by implementation. In part, this reflects thelack of scientific evidence regarding the ability of NFM measures to contribute to risk reduction at the national scale. Broad scaleunderstanding, as exemplified for Great Britain in this paper, is necessary evidence for policy development and a prerequisitefor implementation at scale. This does not imply a lack of confidence in the wider benefits that NFM provide (for biodiversity,carbon sequestration, well-being and many others), but without credible quantified flood risk reduction evidence, progress hasbeen slow. This paper integrates national-scale hydrological models (using SHETRAN and HBV-TYN) and fluvial flood riskanalysis (using the Future Flood Explorer, FFE) to quantify the flood risk reduction benefits of NFM across Great Britain underconditions of future climate and socio-economic change. An optimisation of these benefits is presented considering alternativeNFM policy ambitions and other demands on land (urban development, agriculture, and biodiversity). The findings suggest NFMhas the potential to make a significant contribution to national flood risk reduction when implemented as part of a portfolio ofmeasures. An optimisation through to 2100 suggests investment in NFM achieves a benefit-to-cost ratio of ~3 to 5 (based on thereduction in Expected Annual Damage (EAD) to residential properties alone). By the 2050s, this equates to an ~£80 m reductionin EAD under a scenario of low population growth and a 2°C rise in global warming by 2100. This increases to £110 m given ascenario of high population growth and a 4°C rise. Assuming current levels of adaptation continue in all other aspects of floodrisk management, this represents ~9%–13% of the reduction in EAD achieved by the portfolio as a whole. By the 2080s, the con-tribution of NFM to risk reduction increases to ~£110 and ~£145 m under these two scenarios. These figures are based on thereduction in EAD to residential properties alone, and do not include the substantial co-benefits that would also accrue.