Abstract
Debris flows in settings that have experienced net glacial erosion within the UK's Ice-scoured Quaternary domain are the result of a complex interaction of a range of geological and geomorphological factors. On the 11th of August 2016 a rainfall-triggered debris flow deposited 100 t of sediment onto local road and rail infrastructure blocking transport between town of Fort William and port of Mallaig in north-west Scotland. The debris flow occurred in an ice-scoured setting, where current 1:50,000-scale geological maps suggest that little or no sediment is expected on the valley slopes. In this study, we show how weathering and mass-wasting processes have interacted with bedrock structures to fill localised depressions with sediment on the upper parts of the slope. The intense rainfall event of August 2016 caused the destabilisation of this localised sediment, with eventual failure along bedrock joint surfaces resulting in two debris flows. This study demonstrates the combination of processes that can result in thick accumulations of sediment on slopes that are otherwise generally lacking in superficial sediment cover. These sediment accumulations have the potential to pose a significant landslide hazard in areas that might previously have been thought of as lower susceptibility. The research illustrates a need to improve understanding and representation of sediment thickness and distribution on hill slopes – particularly those that show an absence of superficial deposits at the scale of currently available geological maps.
Highlights
Rainfall-triggered debris flows are relatively common on steep mountain slopes in the Scottish Highlands (Ballantyne and Harris, 1994; Ballantyne, 2004)
Unweathered bedrock with only a thin or patchy cover of superficial deposits (Eyles et al, 1983; Booth et al, 2015; Ballantyne, 2018). These areas have generally been interpreted as lower susceptibility regions for debris flows (Dashwood et al, 2017; Freeborough et al, 2016), due to the apparent lack of source sediments represented at the scale of geological maps
The Sentinel 2 satellite image that was taken on the 17th August 2016 (6 days after the debris flows) shows the sources and pathways of the two adjacent debris flows (Fig. 3a), which clearly coincide with zones of high flow accumulation derived from the elevation dataset (Fig. 3b)
Summary
Rainfall-triggered debris flows are relatively common on steep mountain slopes in the Scottish Highlands (Ballantyne and Harris, 1994; Ballantyne, 2004). In the ‘Ice-scoured Montane’ Quaternary domain of western Scotland, many slopes are represented on geological maps as. Unweathered bedrock with only a thin or patchy cover of superficial deposits (Eyles et al, 1983; Booth et al, 2015; Ballantyne, 2018) As a result, these areas have generally been interpreted as lower susceptibility regions for debris flows (Dashwood et al, 2017; Freeborough et al, 2016), due to the apparent lack of source sediments represented at the scale of geological maps. We demonstrate the importance of understanding a broad range of geological and geomorphological controls, which can influence conditions for debris flow activity in the Ice-scoured Montane domain of western Scotland. The research illustrates a need to improve understanding and representation of sediment thickness and distribution on hill slopes – those that show an absence of superficial deposits at the scale of currently available geological maps
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