Background

In 2011 the EA released guidance to flood managers (Environment Agency 2011), which provided information on the range of flood changes under climate change that might be expected in an average catchment in each of 12 river-basin regions across England.

This included ‘H++ river flow scenarios’ for each region (Table 3 of the EA guidance; see Table 6.1 for an example). The guidance was based on research by CEH, funded by Defra/EA (projects FD2020 and FD2648; Reynard et al. 2009 and Kay et al. 2011a), which used the UKCP09 sampled data for river basins, along with a sensitivity-based approach to estimating flood changes from climatic changes. The H++ scenarios provided in the EA guidance represent a high-end estimate of change in a type of catchment that is particularly sensitive to changes in climatic inputs (‘Enhanced-High’).

Such catchments are more likely to occur in some river basin regions than others (Figure 2 of the EA guidance), but they cannot currently be completely ruled out anywhere.

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Table 6.1: Potential changes in peak river flows for the Northumbria river basin region (Environment Agency 2011).

Note that the H++ high flow scenarios in the EA guidance, and those derived for this project, are presented as percentage changes in flows (from a baseline period of approximately 1961-2001) rather than absolute values of flows. The latter are not

appropriate for high flows as, even under the current climate, there is always a chance of a flood event occurring that is larger than any previously experienced at a particular location on a river. Also, the uniqueness of every river catchment, in terms of area, soils, geology, land cover, topography and orientation as well as climatology, means that generic absolute scenarios are impossible. When applying the H++ high flow scenarios, it is thus important that a reliable baseline flood frequency curve is developed, to which the percentage changes can be applied. This would usually be done via one of the Flood Estimation Handbook (FEH) methods, which are discussed briefly later in this chapter.

6.3 Approach

The derivation of the H++ high flow scenarios for the EA 2011 guidance for river basin regions in England was re-assessed, to decide how best to provide H++ high flow scenarios for CCRA2 which are as consistent as possible both with the H++ scenarios for other variables within CCRA2 and with the original EA guidance. In particular, an H++

range was preferred, rather than a single number as in the EA guidance. It was decided that a method similar to that used for the original EA guidance should be applied to derive the ‘H++ lower end’ numbers, thus providing regionally varying values for three time-slices (2020s, 2050s and 2080s), but that the ‘H++ upper end’ should go further into the tails of the UKCP09 distributions and be taken as the maximum across all regions of the UK (for the 2080s under the high emissions scenario). The H++ high flow scenarios thus derived are then discussed in the context of a review of other, more recent, sources of evidence (e.g. from CMIP5).

77 | P a g e The final method had to be applied to derive values for all river basin regions across the UK, not just those in England; the Adaptation Sub-Committee (ASC) requested UK-wide consistency wherever possible. This was straightforward for the West Wales river basin region, which was covered in project FD2648, and for river basin regions across

Scotland, which were covered in similar research by CEH funded by SEPA (project R10023PUR; Kay et al. 2011b), so directly equivalent numbers could be derived for these regions. For river basin regions in Northern Ireland though, there has been no equivalent research using UKCP09 scenarios and the sensitivity-based modelling

approach, and such an approach could not be fully developed within the time and budget constraints of this project. However, it was considered reasonable to assume that the range of response types in Northern Ireland is the same as that derived from modelling catchments in England, Wales and Scotland, and that the same FD2020 (average and standard deviation) response surfaces for each response type are applicable in Northern Ireland. The UKCP09 sampled data for the three river basin regions in Northern Ireland have thus been downloaded and overlaid on the ‘Enhanced-High’ response surfaces, allowing derivation of H++ high flows scenarios for Northern Ireland using region-specific UKCP09 projections, as for the rest of the UK.

What is not known is the chance of any catchment in Northern Ireland being of the

‘Enhanced-High’ type. Looking at the decision trees for England and Wales (Kay et al.

2011a) and Scotland (Kay et al. 2011b), it is likely that the best estimate of the response type of most gauged catchments in Northern Ireland would be Neutral, due to their high annual rainfall and relatively small catchment area. This is consistent with the pattern across the rest of the UK, where the best estimate of the response type for many catchments in western England, Wales and Scotland is ‘Neutral’, whereas catchments further to the east are more variable in type. Thus the H++ high flow scenarios have a lower (but currently unquantifiable) chance of occurring for any individual catchment in Northern Ireland, compared to the chance for a catchment in the Anglian, Northumbria, Thames or South-East England regions for example.

Further research is required to better identify catchment-by-catchment differences in response to climatic changes, and thus provide more catchment-specific information on the potential impacts of climate change on flood peaks. A new project to address precisely this issue is just being initiated by EA via FCERM.

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6.4 Physical limits

The concept of a probable maximum flood (PMF) for river flooding has always been controversial but the Flood Studies Report (FSR; NERC, 1975) introduced a procedure for estimating PMF based on an extension to the design hydrograph method. PMF can be defined as the flood of near-zero exceedance probability and it is assumed to be caused by the most extreme combination of antecedent catchment wetness, rainfall and runoff response possible. The concept is still used by UK reservoir engineers when assessing flood safety at dam sites (Institution of Civil Engineers, 1996). The recommended procedure relies on a statistical estimate of probable maximum

precipitation (PMP) deriving from the FSR which is routed via the unit hydrograph and losses model. The unit hydrograph time-to-peak is reduced to represent the more rapid and intensive response that may occur in exceptional conditions, and optional changes to the percentage runoff allow for higher than normal runoff from frozen ground. The estimation of PMF is gradually being superseded by the use of probabilistic risk

assessment within the reservoir industry, reflecting a general feeling that the concept of an upper limit and, more importantly, the methods in current use are outdated.