Climate‐change driven increased flood magnitudes and frequency in the British uplands: Geomorphologically informed scientific underpinning for upland flood‐risk management

Abstract

AbstractUpland river systems in the UK are predicted to be prone to the effects of increased flood magnitudes and frequency, driven by climate change. It is clear from recent events that some headwater catchments can be very sensitive to large floods, activating the full sediment system, with implications for flood risk management further down the catchment. We provide a 15‐year record of detailed morphological change on a 500‐m reach of upland gravel‐bed river, focusing upon the geomorphic response to an extreme event in 2007, and the recovery in the decade following. Through novel application of two‐dimensional (2D) hydrodynamic modelling we evaluate the different energy states of pre‐ and post‐flood morphologies of the river reach, exploring how energy state adjusts with recovery following the event. Following the 2007 flood, morphological adjustments resulted in changes to the shear stress population over the reach, resulting in higher shear stresses. Although the proportion of shear stresses in excess of those experienced using the pre‐flood digital elevation model (DEM) varied over the recovery period, they remained substantially in excess of those experienced pre‐2007, suggesting that there is still potential for enhanced bedload transport and morphological adjustment within the reach. Although volumetric change calculated from DEM differencing does indicate a reduction in erosion and deposition volumes in the decade following the flood, we argue that the system still has not fully recovered to the pre‐flood state. We further argue that Thinhope Burn, and other similarly impacted catchments in upland environments, may not recover under the wet climatic phase currently being experienced. Hence systems like Thinhope Burn will continue to deliver large volumes of sediment further down river catchments, providing new challenges for flood risk management into the future.