El debate acerca del uso de los sistemas internacionales y su relación con los sistemas nacionales

corresponding key flood response patterns are built results in the quantification of flood changes from 4,200 different scenarios of rainfall and T/PE change.

Using the flood response patterns (either obtained from modelling of the flood regime of a catchment, or approximated from the catchment descriptors and assigned flood response type) it is possible to quickly identify the sets of scenarios generating flood changes greater/lower than a given threshold.

Combining this information with current understanding of the climate change hazard, it is possible to estimate the proportion of scenarios in each category, i.e. the risk of exceeding a threshold of change in flood peaks due to climate change. By repeating this process, it is possible to compare the risk of exceeding alternative thresholds. This provides a powerful tool for policy-makers, allowing the analysis of the resilience of catchments to climate change for any number of possible allowances.

The analysis can be done on a national basis, as presented in Figure 7.4 and Figure 7.5, or regionally. In Figure 7.4 (16 AR4 GCMs) and Figure 7.5 (11 UKCP09 RCMs), the sets of impacts obtained from the flood response patterns for each catchment (for all 8 T/PE scenarios) are summarised in terms of their exceedance of a given set of climate change allowances (chosen at 10%

intervals between 0% and 100%) for the four flood indicators. That is, for a given catchment, flood indicator and set of climate change scenarios, the proportion of considered climate change scenarios that are greater than X% is calculated, where X is 0, 10, 20, …100. These proportions are plotted against the allowance X on the graphs in Figure 7.4 and Figure 7.5, with each cross for a given value of X representing one of the 155 project modelled catchments.

The 10^th, 30^th, 50^th (median), 70^th and 90^th percentiles over these catchments are also indicated for each value of X.

Looking at the median (solid) line in these graphs (i.e. with half the catchments plotted above this line and half plotted below) the level of risk falls off quickly (at all four return periods) as the allowance is increased. For instance, under the set of GCM scenarios (Figure 7.4), for half of the modelled catchments, around 50% of climate change scenarios generate an impact greater than that 10%

allowance, whereas only around 15% of scenarios generate an impact greater than 20% (the current national allowance). If the allowance is taken as 30%, practically no scenarios result in impact greater than this threshold for half of the catchments. Similar conclusions apply to the set of RCM scenarios (Figure 7.5). However, there are a number of catchments where the decrease in risk with increasing allowance is much slower (the points above the median line), particularly when considering changes in higher return period flows, and under the RCM scenarios. Using this type of assessment it is possible to make an informed decision for new allowances after deciding on the level of risk that is acceptable:

i. What proportion of catchments is to be protected?

ii. What proportion of climate change scenarios are permitted to exceed the allowance

Section 7: Application of the FD2020 methodology 95

Figure 7.4 Summary of the impacts obtained from the catchment response patterns (for all 8 T/PE scenarios), relative to a given set of allowances, for 16 AR4 GCMs (2080s, A1B emissions scenario). Each cross for a given value of the allowance represents the results for one catchment (not including any extra uncertainty allowances). The 50^th, 30^th and 70^th, and 10^th and 90^th percentiles (solid, dashed and dotted lines respectively) are shown for each value of the allowance

Section 7: Application of the FD2020 methodology 96

Figure 7.5 As Figure 7.4 but for the 11 UKCP09 RCMs (2080s, A1B emissions scenario)

Section 7: Application of the FD2020 methodology 97

The results in Figure 7.4 and Figure 7.5 do not include the uncertainty due to the assumptions made in developing the sensitivity framework. The uncertainty analysis (Section 6) showed that some types of catchment have a greater level of uncertainty than others. The flood response type of each modelled

catchment can be used to add the appropriate extra uncertainty allowance according to the flood response type and return period (Table 7.1). Figure 7.6 illustrates how the results in Figure 7.4 are altered when this extra uncertainty is included (where in this case the flood response type has been determined by the grouping methodology, Section 4, rather than via regionalisation, Section 5).

In this case, there is a much slower decrease in the proportion of catchments and scenarios for which the different allowance thresholds are exceeded, suggesting the need for a higher allowance to achieve the same level of protection. For instance, without the extra uncertainty added, for half of the modelled catchments 15% of (GCM) scenarios produce changes in the 20-year flow above the 20% allowance (Figure 7.4). When the uncertainty is added this value increases to nearly 50% of scenarios exceeding the 20% allowance in half the catchments (Figure 7.6).

One of the objectives of the FD2020 project was to assess whether a national climate change allowance for flood risk was appropriate, or whether this allowance should be different depending on catchment type (i.e. is the vulnerability to climatic change different for catchments with different

characteristics?) or catchment location (is the climate change hazard different for different regions of Britain?). Figure 7.7 (for the GCMs) and Figure 7.8 (for the RCMs) are the same as Figure 7.6 but with the probability of exceedance of an allowance threshold coloured according to the catchment’s flood response type (obtained by the grouping methodology, Section 4, rather than

regionalisation, Section 5). This probability does appear to vary by catchment type, and in particular, catchments with Enhanced or Sensitive flood response types seem to be much more likely to have an above average level of risk.

As the risk is a combination of vulnerability (flood response pattern, depending on catchment type) and hazard (climate change scenarios, depending on catchment location), the conclusions obtained from Figure 7.7 and Figure 7.8 could depend on the specific set of catchments modelled in the project. In particular, some flood response types might not be present in a region so the risk associated with the corresponding vulnerability-hazard combination will not be covered in such summary plots. A nationwide assessment of the

vulnerability of British flood regime to climate change would be necessary to evaluate with more accuracy current climate change flood risk allowance and evaluate the importance of catchment type in their resilience to fixed flood risk allowances.

Section 7: Application of the FD2020 methodology 98

Figure 7.6 As Figure 7.4 but including the appropriate extra uncertainty allowances for each catchment, according to their flood response type (as determined by the grouping methodology, Section 4, rather than

regionalisation, Section 5)

Section 7: Application of the FD2020 methodology 99

Figure 7.7 As Figure 7.6 (for 16 AR4 GCMs ) but with catchments

separated/colour-coded by their flood response type (as determined by the grouping methodology). Flood response types, plotted from left to right for each value of the allowance: Extreme (brown), Damped-High (red), Damped-Low (orange), Neutral (green), Mixed (gold),

Enhanced-Low (cyan), Enhanced-Medium (blue), Enhanced-High (purple), Sensitive (magenta). As in Figure 7.4, the 50^th, 30^th and 70^th, and 10^th and 90^th percentiles (solid, dashed and dotted lines respectively) over all catchments are shown for each value of the allowance

Section 7: Application of the FD2020 methodology 100

Figure 7.8 As Figure 7.7 but for the 11 UKCP09 RCMs (2080s, A1B emissions scenario)

Section 7: Application of the FD2020 methodology 101