On Wednesday 18 August 2004, the A85 in Glen Ogle, north of Lochearnhead, Stirlingshire, was blocked by what was widely reported as two landslides (Figure 1). There had been considerable rainfall in the area prior to the event. Fifty seven people were trapped on the roadway and either left the scene on foot or were rescued by helicopter. No injuries were reported. The landslides that caused the disruption on the A85 originated in two-streams, one leading to a culvert under the road and a second 435 m further down the road.
This landslide was a shallow planar movement which developed into a debris flow. This landslide is National Landslide Database ID 10881/1.
Specialist staff from the BGS Landslides Response Team carried out a survey of the landslide. The BGS routinely responds to major landslide events in this way, sending specialist staff to the scene of landslide events to collate data from landslides whilst they are relatively fresh and often active, offering invaluable information.
The uppermost part of the slope failure was identified by the presence of a curved backscarp and depletion zone within superficial materials (Figure 2). Detachment of material appeared to have occurred close to the superficial–bedrock interface, at a depth from surface of between 0.6 and 1.8 m. The width of the displacement zone was approximately 8 m. The length of the surface of rupture appeared to have been constrained to 14 m by the presence of a topographic bench formed by bedrock.
Immediately downslope of this bench the ground surface appeared to be intact, with vegetation inclined downslope — considered to be an indicator that the displaced mass travelled over this area, leaving it relatively undisturbed. The failure occurred within a pre-existing depression in which vegetation was noticeably lighter in colour than surrounding areas. The depression was interpreted as a source areas and track of one or more previous failures. The material involved at this part of the failure was a head deposit.
The displaced mass from this failure appeared to have moved in a shallow, almost planar movement. Several smaller failures of similar style were also noted in this area, each exposing bedrock.
The exposed backscarp was relatively clear of debris, with only a few scattered blocks of broken head observed, this is thought to indicate that the slide mass disaggregated and became a flow soon after initial failure.
At the base of the slope, some 40 m upslope from the roadway, the natural channel abruptly changes direction. At the junction with the roadway, the channel stream is normally diverted through a stone culvert (Figure 3).
A significant amount of debris bypassed this, overwhelming the banks of the gulley (at this point estimated to be approximately 4 m in height) and flowed directly downslope. The majority of the debris fan was deposited to the east of the main channel outlet, following the direction of the gulley at the roadway (Figure 1).
The following interpretation of events has been inferred:
Winter, M G, Heald, A P, Parsons, J A, Shackman, L. and Macgregor, F. 2006. Photographic Feature: Scottish debris flow events of August 2004. Quarterly Journal of Engineering Geology and Hydrogeology, Feb 2006; 39: 73–78.
Winter, M G, Macgregor, F, and Shackman, L (ed). 2008. Scottish Road Network Study: