A hazardous channelized debris flow in Glen Ogle, Stirlingshire

On 18 August 2004 an intense rainstorm generated 31 debris flows in Glen Ogle, Stirlingshire. Two channelized debris flows traversed the A85 trunk road trapping 20 vehicles and resulting in the helicopter airlift rescue of 57 motorists and passengers. The failure zone of the largest of these debris flows occurred within a shallow hillslope depression highlighting the importance of topographic controls on hillslope hydrology and connectivity between open hillslopes and stream channels for the generation of channelized debris flows. Five years after the 2004 event, channel sediment recharge from localized slumping and slope wash on over-steepened gully walls was observed. Sediment recharge is negligible in comparison to the abundant boulder and cobble sized channel lag deposits from the 2004 debris flow. But the abundance of coarse debris within the channel and the potential for instability in gully walls and on the adjacent hillslope indicate that the gully retains a residual susceptibility to the occurrence of hazardous accumulative channelized debris flow. It is suggested that the geohazard should be mitigated by increasing culvert capacity to allow the safe passage of future debris flows under the road.