The majority of climate change impact analyses are scenario-led using the outputs from one or more Global (GCM) or Regional Climate Models (RCM). There are two main weaknesses of this approach. First, a full understanding of the inter-relationships between climate changes, catchment properties and changes in flood flows cannot be obtained. Second, no insight is gained into what might occur if something happens other than the exact projections of the climate model-based scenarios, so that when new scenarios are released, new impact studies have to be performed. This implies that any policy derived from this scenario-led evidence is equally time-limited. To overcome this issue, this project took a different approach, basing the methodology on a wide-ranging sensitivity analysis, and as such is scenario-neutral and not dependent on any one set of climate change scenarios. The approach investigates catchment response to changes in climate by imposing the same changes to a set of catchments across Britain. This allows those catchments that respond in a similar manner to be grouped together, or “regionalised”, into flood response types. To ensure the results are robust, and any subsequent policy guidance long-lasting, the framework has been designed to investigate changes in climate that encompass current knowledge of future climate change available from the GCMs of IPCC Fourth Assessment Report and RCM used to derive the suite of UKCP09 products.

The method allows any catchment, including those not modelled as part of this project, to be allocated to a flood response type according to its catchment properties, and hence its vulnerability to climate change assessed. The research has also provided a range of other catchment, and scenario-specific tools, for assessing the risk of change in peak flows, and these are illustrated in this report.

The research has led to a number of key findings in relation to the project objectives. First, the catchment-based analysis suggests that the current allowance can no longer be considered precautionary as a change of 20% does not encompass the majority of catchment changes in flood flows. Second, there is strong evidence that catchment response to climate change (in terms of change in flood flows) is influenced by catchment properties. This implies that a single national allowance for climate change might not be appropriate and that more “regionalised” allowances, depending on catchment type, could be developed.