Some key topographic and material controls on debris flows in Scotland

Abstract

Debris-flow phenomena were investigated at six study sites across upland Scotland using a combination of laboratory and field-based analyses. In agreement with previous research, higher spatial frequencies of debris-flow paths were measured in areas underlain by coarse-grained intrusive igneous and arenaceous sedimentary bedrocks compared with metamorphic and extrusive igneous geologies. A strong relationship between critical state friction angle of sampled initiation zone soils and spatial frequency of debris-flow paths suggests that this trend is attributable to generally lower shear strengths in sandier hillslope material generated from coarser grained bedrocks. Topographic controls on debris-flow susceptibility are demonstrated by higher numbers of debris-flow paths at sites with persistently steep upper slopes (≥30°) and a higher occurrence of potential initiation zones. Strong correlation between debris-flow magnitude and slope length shows that longer mass movements tend to produce higher volumes of material and terminal deposits that travel further at the slope foot. In the cases studied here this reflects greater opportunity for accumulation of fresh material during the transport phase, particularly in the case of long channelized flows. The highest levels of hazard are likely to occur where these topographic and material characteristics conducive to heightened susceptibility and magnitude coincide.