Long-term channel response to a major flood in an upland gravel-bed river

Abstract

Long-term data quantifying the response of fluvial systems to geomorphically effective events are rare, limiting our ability to validate conceptual models of geomorphic evolution. Here we present geomorphic change data for a 200 m reach of the Thinhope Burn a small (12 km2) tributary to the South Tyne, Cumbria, UK, monitored since 2003. In July 2007 a severe flood resulted in a peak discharge of 60 m3s-1, and boundary shear stress maxima of 533 Nm-2; capable of mobilising metre-size boulders. The Thinhope Burn catchment showed ‘responsive’ \nbehaviour to the event causing; full activation the valley floor and slopes, a peat slide in the headwaters, and delivery ~3077 m3 of gravel to the study reach. Ten years on, there appears to be limited evidence of recovery to the valley floor, possibly due to the effects of several wet winters. The channel and floodplain surface appear unconsolidated and vegetation-free, and as a consequence remains highly mobile during floods. Some climate change predictions for the UK uplands indicate wetter winters, that could push sensitive upland catchments closer towards ‘tipping-point’, where thresholds are crossed within the fluvial system resulting in significant morphological changes to the river channel and it’s floodplain, and producing very high bedload transport volumes. A new regime of wetter winters may also halt recovery of upland rivers where the sediment system has already been fully activated by previous floods. We conclude that a better understanding of relative catchment sensitivity and the potential implications of climate-change induced increases in flows upon those catchments is needed to plan for the potential impacts of enhanced sediment loads upon flood risk.