Climate change and river flooding: part 1 classifying the sensitivity of British catchments

Abstract

Effective national and regional policy guidance on climate change adaptation relies on robust scientific evidence. This two-part series of papers develops and implements a novel scenario-neutral framework enabling an assessment of the vulnerability of flood flows in British catchments to climatic change, to underpin the development of guidance for the flood management community. In this first part, the sensitivity of the 20-year return period flood peak (RP20) to changes in precipitation (P), temperature (T) and potential evapotranspiration (PE) is systematically assessed for 154 catchments. A sensitivity domain of 4,200 scenarios is applied combining 525 and 8 sets of P and T/PE mean monthly changes, respectively, with seasonality incorporated using a single-phase harmonic function.Using the change factor method, the percentage change in RP20 associated with each scenario of the sensitivity domain is calculated, giving flood response surfaces for each catchment. Using a clustering procedure on the response surfaces, the 154 catchments are divided into nine groups: flood sensitivity types. These sensitivity types show that some catchments are (very) sensitive to changes in P but others buffer the response, while the location of catchments of the same type does not show any strong geographical pattern.These results reflect the range of hydrological processes found in Britain, and demonstrate the potential importance of catchment properties (physical and climatic) in the propagation 2 of change in climate to change in floods, and so in characterising the sensitivity types (covered in the companion paper).

Probabilistic impacts of climate change on flood frequency using response surfaces I: England and Wales

“…Only the UKCP09 projections of monthly P changes are used here; variations in T changes are much less important for changes in flood peaks in Britain (Prudhomme et al 2013a), so the composite response surfaces (Figure 1a) incorporate all eight T/PE scenarios together. The standard deviation surfaces then include the (small) additional uncertainty from use of composite surfaces averaged over T/PE scenarios, as well as covering the uncertainty due to the range of possible catchment responses of a given type (see Section 2.4).…”

“…developed a sensitivity domain to investigate impacts of climate change on flood peaks in Britain. Prudhomme et al (2013a) implemented this domain for 154 catchments across Britain and grouped the catchment response surfaces, leading to nine 'response types' each represented by composite (average) response surfaces. Prudhomme et al (2013b) characterised families of response types by catchment properties, enabling estimation of type for any catchment where the necessary properties are available.…”

“…Develops the response type estimation method of Prudhomme et al (2013b), to allow robust application to a large set of catchments across England and Wales. b. Overlays the UKCP09 projections on the composite response surfaces of Prudhomme et al (2013a), to estimate the impacts (percentage changes in flood peaks) for each response type in regions across England and Wales. c. Combines a and b above, to produce weighted regional impacts.…”

“…Using this sensitivity framework, 154 catchments across Britain were modelled (Prudhomme et al 2013a). Details on the hydrological modelling, which included a temperature-dependent snowmelt module, are given by Crooks et al (2009).…”

“…These were presented as a 2-d response surface for each catchment and each T/PE scenario, with P harmonic amplitude (A) on the xaxis, P harmonic mean (X mean ) on the y-axis, and the colour of the square at each point representing the modelled change in RP20 for that scenario of P, T and PE change. The response surfaces of the 154 catchments were grouped by similarity (using a hierarchical clustering method), forming nine response types (Prudhomme et al 2013a). These types were approximately ordered and named by the sensitivity shown in their response surfaces: Damped-Extreme (DpE), Damped-High (DpH), Damped-Low (DpL), Neutral (Neu), Mixed (Mix), Enhanced-Low (EnL), Enhanced-Medium (EnM), Enhanced-High (EnH) and Sensitive (Sen).…”