Creating story lines on floods: relating climate-change uplift to (extreme) experienced and future flooding events

Abstract. Fluvial flooding remains one of the most significant climate-related hazards worldwide, with its impacts intensified by increasing urbanisation, land-use change, and climate change. We apply the flood-excess volume (FEV) methodology to analyse major recent flood events on the River Aire in Leeds, UK, and specifically to the 2015 Boxing Day and February 2020 floods, as a basis for evaluating the sufficiency and cost-efficacy of flood defences under current and future climate scenarios. The FEV methodology is used to assess the performance of flood-mitigation measures, integrating engineered interventions and nature-based solutions. Using UK-government climate-uplift guidance, we model how magnitude and frequency of flood events may evolve across future climate time slices. Focusing on the period 2070–2125, our results show that under a foreseen worst-case climate-change uplift of 51 %, the 2020 minor flood event may produce floods with an FEV comparable to that of the major 2015 Boxing Day flood. Such abstract and difficult-to-comprehend climate-change uplift factors are hence clarified by relating two recently experienced flooding events, still fresh in Leeds' citizens' memory, by being able to relate stories of two recent floods of different magnitude. To assess the cost-efficacy and the (in)sufficiency of flood mitigations, we compare the percentage of FEV mitigated with associated costs for future climate-adjusted flood events. Long-term resilience of a future Boxing Day flood worsened by climate-change uplift factors may be insufficient without additional adaptation. We conclude that significant upgrades may thus be required to ensure protection against climate-change enhanced extremes.