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

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

Regionalisation of climate impacts on flood flows to support the development of climate change guidance for Flood Management

Current Defra / Environment Agency guidance (FCDPAG3 supplementary note: http://www.defra.gov.uk/environ/fcd/pubs/pagn/climatechangeupdate.pdf) requires all flood management plans to allow for climate change by incorporating, within a sensitivity analysis, an increase in river flows of up 20% over the next 50 years, and beyond. This guidance is the same for all of England and Wales, making no allowance for regional variation in climate change or catchment type. This reflects the lack of scientific evidence to resolve the spatial distribution of potential impacts on flood flows with enough confidence to set such policy regionally. The 20% allowance was first raised in 1999 for MAFF and subsequently reviewed following the release of the UKCIP02 scenarios. Although the 20% figure is a memorable precautionary target, there is the risk that it leads to a significant under- or over-estimation of future flood risk in individual catchments. Defra and the Environment Agency procured project FD2020 (Regionalisation of climate change impacts on flood flows) to provide a more rigorous science base for refreshing the FCDPAG3: supplementary note guidance. The FD2020 approach is exploring the relationships between catchment characteristics and climate change impacts on peak flows in a “scenario neutral” way. This is done by defining a regular set of changes in climate that encompass all the current knowledge from the new scenarios available from the IPCC Fourth Assessment Report. For each of the 155 catchments included in the research, this broad approach will provide multiple scenarios to produce a “vulnerability surface” for change in the metrics of peak flows (e.g. the 20-year flood flow). Some of the UKCP09 products have also been used to understand what these projections may mean for changes to peak flow. The catchment-based analysis will be used to generalise to other gauged sites across Britain, using relationships with catchment characteristics, providing the scientific evidence for the development of regional guidance on climate change allowances. Specifically the project is: Investigating the impact of climate change on peak river flows in over 150 catchments across Britain to assess the suitability of the FCDPAG3 20% climate change allowance. Investigating catchment response to climate change to identify potential similarities such that the FCDPAG3 nationwide allowance could be regionalised. Investigating the uncertainty in changes to future peak river flows from climate change. Developing an approach that has longevity beyond the project timeframe and the lifetime of the latest generation of climate model results