Regionalised impacts of climate change on flood flows: rationale for definition of climate change scenarios and sensitivity framework. Milestone report 2. Revised November 2009

Abstract

The primary objective of FD2020 ‘Regionalising the impacts of climate change on flood flows’ was to assess the suitability of current FCDPAG3 guidance given the advances in climate change science since its publication. PAG3 requires an allowance of 20% to be added to peak flows for any period between 2025 and 2115 for any location across Britain. This guidance was considered a precautionary value and its derivation reflected the evidence available at that time. FD2020 has been designed to increase this evidence base, and it is anticipated that the research will lead to the development of regional, rather than national, guidelines for changes to peak flows due to climate change. A scenario-neutral approach based on a broad sensitivity analysis to determine catchment response to changes in climate as chosen for FD2020. The method separates the climate change that a catchment may be exposed to (the hazard) from the catchment response (change in peak flows) to changes in the climate (the vulnerability). By combining current understanding of climate change likelihood (the ‘hazard’) with the vulnerability of a given catchment, it is possible to evaluate the risk of flood flow changes. The vulnerability of a catchment is to be characterised in two steps: first, the response of a set of catchment’s to a range of climatic changes are modelled, then analysed for similarity, and characterised according to catchment properties. This is done by defining a sensitivity framework of changes to the mean and seasonality of precipitation and temperature and modelling the response of each catchment within this fixed framework. To properly understand the relationship between catchment properties, climate changes and changes in flood flows, it is essential that the considered scenarios capture the range of potential climatic changes expected to occur in Great Britain, including the large GCM (Global Climate Model) uncertainty. This means the vulnerability assessment (or the conclusions of the modelling exercise and regionalisation study) will be as robust as possible, and provide a sound science-base for subsequent policy guidance to the flood management community. This project report describes the rationale and the development of the climate change scenarios used in the project FD2020. The objective of this module of work was to develop a methodology to conceptualise how a catchment’s vulnerability (in terms of change in its flood regime under climate change) could be evaluated. This requires the identification of a range of climate change scenarios to be used in a comprehensive yet manageable evaluation of future river flood flows, which was guided by, but not limited to, current predictions of future climatic changes. This methodology is also designed to characterise the climatic change hazard, so that it can be compared with the catchments vulnerability to changes. Previous climate change studies relied only on projections from a few global (GCM) and regional climate models (RCMs), and thus could only capture a very limited part of the GCM uncertainty. The IPCC AR4 now provides data from 17 GCMs, all considered equally plausible representations of future climates