Hillslope–channel sediment transfer in a slope failure event: Wet Swine Gill, Lake District, northern England

This paper describes and analyses a hillslope–channel slope failure event that occurred at Wet Swine Gill, Lake District, northern England. This comprised a hillslope slide (180 m3, c. 203 ± 36 t), which coupled with the adjacent stream, resulting in a channelized debris flow and fluvial flood. The timing of the event is constrained between January and March 2002. The hillslope failure occurred in response to a rainfall/snowmelt trigger, on ground recently disturbed by a heather moorland fire and modified by artificial drainage. Slide and flow dynamics are estimated using reconstructed velocity and discharge values along the sediment transfer path. There is a rapid downstream reduction in both maximum velocity, from 9·8 to 1·3 m s−1; and maximum discharge, ranging from 33·5 to 2·4 m3 s−1. A volumetric sediment budget quantified a high degree of coupling between the hillslope and immediate channel (∼92%: 167 m3), but virtually all of the sediment was retained in the first-order tributary channel. Approximately 44% (81 m3) of the slide volume was retained in the run-up deposit, and termination of the debris flow prior to the main river meant that the remainder did not discharge into the fluvial system downstream. These results suggest poor transmission of sediment to the main river at the time of the event, but importantly an increase in available material for post-event sediment transfer processes within the small upland tributary