Scottish Road Network Landslides Study - University of Glasgow

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INFORMATION SOURCES
soils, such as the lodgement 8 tills common in Scotland, are less susceptible to debris flow as
bonds between particles provide cohesion and impede flow (Ballantyne, 1986). This can also
be explained in terms of lithology. Where rocks yield sand-rich soils on weathering, such as
the Torridonian sandstone of the NW Highlands and the granites of the Cairngorms, debris
flow activity is more common (Strachan, 1976; Ballantyne, 1981). Tivy (1962) and
Ballantyne (1984) suggest that areas underlain by schist, shale or greywacke, such as the
Southern Uplands, yield clay- and silt-rich soils and are subject to debris flows only rarely.
However, on-the-ground experience indicates that there is a comprehensive history of
instability, including in the form of debris flows, in many areas underlain by schist. Good
examples of such instability are the A83 in the vicinity of the Rest and be Thankful, A83
Loch Shira, A890 Stromeferry and the A87 at Invermoriston.
However, clay content is an important constituent in the mobilisation of flows. Although
debris flows are rarely initiated in these soils, a cohesive debris flow has the potential for
longer run-out distances. Clay impedes soil water movement and hence increases the
possibility of soil saturation (Innes, 1983b). This fluid matrix is highly mobile and capable of
travelling long distances, and Innes (1983b) found that debris flows in deep tills “may be two
or three orders of magnitude larger” than in areas of thin cover. However, in the grain-tograin
interactions of cohesionless (or granular) debris flows (Bagnold, 1954; Takahashi, 1978;
1980; 1981) energy is dissipated more rapidly and therefore, run-out is shorter.
Channel/slope geometry is an important control on the nature of debris flows. While confined
flows will often travel further, relatively unconfined flows (floodplains/large U-shaped
valleys) will frequently spread out to a greater degree forming a large lobate geometry. Where
flow run-out is confined to tight valleys it will usually terminate close to the source, but the
flow itself may incise deep channels (up to 5m) (Yarnold, 1993; Berti et al., 1999).
Plant roots play a critical role in stabilising colluvium 9 against failure on hillsides.
Furthermore, vegetation cover provides interception of rainfall and encourages
evapotranspiration, thus reducing both direct and indirect infiltration into the soil which can
de-stabilise colluvium. Removal of vegetation by deforestation and heather burning increases
the possibility of debris flow (Bovis, 1993; Benda and Dunne, 1997) by increasing water
ingress into the soil. The effects of deforestation are known to endure for up to 10 years, with
an associated elevated likelihood of instability during that time.
3.1.4 Hazard Identification, Assessment and Management
The body of literature on hazard identification, risk assessment and management of debris
flows grows as our understanding of the phenomenon increases. Knowledge of debris flows
may not allow us to prevent debris flows. However, with sensible hazard identification,
assessment and management some degree of control is possible. The following paragraphs
identify some approaches to the identification, assessment and management of debris flows.
These issues are discussed further in later sections of this report.
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Lodgement tills are formed by a ‘plastering-on’ process at the base of an ice bed. They are normally compact
and rich in fine particles usually exhibit preferred orientation of the larger particles which may indicate the
direction of ice movement. Shear planes, joints and fissures are frequently found in lodgement tills.
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Colluvium is material that has been transported down slope by the action of water and/or gravity and includes
hillwash, scree (talus) and other materials.