The study presents a novel framework for undertaking climate change impact studies, which can be used for testing the robustness of precautionary climate change allowances. It explores the relationship between climate change impacts on peak flows and catchment characteristics in a scenario-neutral framework. A national climate change sensitivity domain, defined to encapsulate magnitude and seasonality of changes in precipitation and temperature in Great Britain as currently projected by the Global Climate Models developed for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), serves as a framework for an extensive impact study on 154 catchments. The response of the catchments to these changes are analysed for a selected flood indicator, the 20-year return period flood (RP20), and mapped into the climate change domain, thus defining a vulnerability pattern for each catchment. When compared with the likelihood of future climate change scenarios (the climate ‘hazard’), an estimation of the climate change risk can be made easily. The probabilistic risk of exceeding a specific design allowance, such as an allowance of a 20% increase in flood peak, can be assessed for individual catchments or by regions. Using projections of 16 IPCC-AR4 GCMs for the 2080s based on the A1B emission scenarios, the study showed that 10.9% of the projections suggest RP20 will increase by at least 20% for the Enrick at Mill Tore, and across GB, 15% of the projections generate a RP20 increase of 20% or greater for at half of the 154 study catchments. Such conclusion can help evaluate the robustness of the tested allowance
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