Sediment source connectivity, torrent erosion and sediment delivery during a record-breaking UK flood

The significance of channel and hillslope sediment sources to sediment delivery during extreme floods in steep landscapes is difficult to evaluate due to the local nature of many mountain storms and lack of quantitative geomorphologic measurements. Using a rapid-appraisal sediment budget framework, which captured key evidence on sediment source contributions and; torrent erosion and deposition in the immediate aftermath of an extreme flood event, we present a unique record of detailed sediment balance calculations for multiple mountain catchments. This study focusses on record-breaking Storm Desmond (December 2015) which impacted the Helvellyn mountain massif in Northern England, UK. The storm was the largest in a 150 year local rainfall series and set a new UK 48-hour rainfall record of 405 mm (falling in 38 hours, 4-6 December). The steep mountain catchments studied here flank Thirlmere Reservoir (a major water supply asset, providing c. 11% of North-West England’s water) and range in size from 0.14 to 1.31 km2, have catchment slopes 0.34-0.67 m m-1 and channel slopes 0.4-0.75 m m-1. The torrents consist of steep, boulder-mantled / bedrock channels which are incised in to glacial and colluvium deposits. Lower slopes are forested and here a forest road network and major highway traverse the base of the main torrents. During the flood these roads intercepted sediment resulting in local deposition of debris cones / fans and extensive damage to built infrastructure. Geomorphological surveys enabled the volumes of erosion and deposition to be estimated and individual catchment sediment budgets to be constructed. Shallow hillslope landslides, local debris flows and extensive channel erosion (bank erosion and bed scour) supplied the majority of sediment. Channel erosion accounted for over 80% of the erosion and the net catchment sediment delivery was 60% with specific sediment yields of up to 2000 t km-2. Hillslope landslides were only weakly coupled to channels. Boulder size surveys, comparing the recent flood deposits with historic deposits (dated using lichenometry) show the 2015 flood was as least as large as any previous event in the area. Results provide an important baseline and modern analogue for understanding sediment delivery processes during extreme floods in temperate, steep mountain environments