Climate change and flood frequency in the UK: An appraisal of the 20% allowance in the light of the UKCIP02 scenarios

Global warming is predicted to cause significant changes to the world’s climate, but uncertainties remain about the precise nature of these changes. This is particularly true with regard to possible changes at a regional or local level and to changes in the climate extremes that produce catchment flooding. Such changes might include more frequent short-duration, high-intensity rainfall or more frequent periods of long-duration, sustained rainfall of the type responsible for the Autumn 2000 floods. To address the uncertainty surrounding this issue, DEFRA guidance on flood defence scheme appraisal currently suggests sensitivity allowances for climate change, for example a 20% increase in peak flows over the next 50 years. The need to further develop this policy and guidance on climate change impacts is being informed by improved modelling capabilities and climate change scenarios. For example, statistical rainfall models and fine resolution regional climate model data are now available to drive the hydrological models used to investigate the potential impacts of climate change on flood flows. CEH-Wallingford has been commissioned by DEFRA and the Environment Agency to use these data sets and models to assess the impacts and this paper describes the final project results for a range of catchments across the UK.