Geomorphic impact and system recovery following an extreme flood in an upland stream: Thinhope Burn, northern England, UK

Quantitative assessments of the impacts of extreme floods on channel morphology are rare. Real Time Kinematic (RTK) GPS surveys of a 500-m reach of the Thinhope Burn, an upland gravel-bed stream in the UK, taken in 2003 and 2004 permitted an assessment of geomorphic work whilst the channel was at steady-state. A large flood that occurred on 17 July 2007 resulted in a catastrophic impact to the Thinhope Burn valley floor. The reach was re-surveyed after the event in 2007, and again in 2008 and in 2011. Digital elevation models were produced from the survey data, which allowed the spatial patterns of erosion and deposition and volumetric changes between surveys to be established. A total of 5202 m3 of deposition and 2125 m3 of erosion was recorded in the reach following the flood event. Field walking of the catchment and comparison of aerial photographs for 2003 and 2007 suggested that most of the material mobilised had originated from existing sediment held in terraces and paleoberms on the valley floor. Although slope failures were evident, including peat slides in the headwaters, delivery of sediment from coupling zones to the channel was thought to play a secondary role in the geomorphic response shown by the channel. Similarly, large volumes of erosion and deposition were found after resurveys in 2008 and 2011, suggesting that the system was still in its relaxation phase. The results obtained in this investigation coupled with historical information on the flood history of Thinhope Burn dating back to 1766 suggested that rare large floods are the geomorphically effective flows in the catchment.