A recent debris flow event and implications for hazard management

A high-magnitude rainstorm on 18 August 2004 triggered approximately 30 debris flow landslides in Glen Ogle, Scotland. Two of these debris flows traversed the A85 trunk road, trapping several vehicles and resulting in the airlift rescue of 57 people. The location of the initiating failure of the larger of the two debris flows was determined by the presence of a shallow depression that facilitated a concentration of hillslope runoff leading to preferential saturation of slope material, reduction in effective stress and instability. This highlights the critical control localized morphologically driven hillslope hydrology can have on debris flow susceptibility and spatial distribution. Subsequent to initiating failure the sliding mass made a rapid transition to flow mass movement, passed into a nearby gullied stream channel and moved downslope as a channelized debris flow. The sediment budget of the debris flows showed that the majority of material involved was sourced during gully propagation. Hillslope-gully coupling is essential for conceptualization of the debris flow geohazard, and the magnitude of channelized debris flow is demonstrated to be largely determined by the sediment capacity, length and gradient of the propagation gully. Accordingly, hazard management practices should be focused around gullies in areas susceptible to debris flow activity. It is suggested that geohazard mitigation may take the form of increased capacity culverts to allow the safe passage of future debris flows under the road.