Scottish Road Network Landslides Study - University of Glasgow

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TYPES AND MECHANISMS
Figure 4.2 shows the predominantly coarse debris deposited by Debris Flows on the A887
Trunk Road at Invermoriston (August 1997). This is by far the most common type of flow
encountered on the road network in Scotland.
Figure 4.2 – Debris flow material on the A887 Trunk Road at Invermoriston, August
1997. The debris consists predominantly of coarse material (gravel, cobbles and
boulders), with some finer material (clay, silt and sand) and tree trunk debris.
(Photograph courtesy of Northpix.)
Pierson and Costa (1987) classified flow type landslides on the basis of the flow velocity and
sediment concentration and their rheological classification of sediment water flows is shown
in Figure 2.2. This rheological approach to classification may be particularly appropriate for
the development of remedial measures such as channels, overshoots, culverts and so forth.
Based on Pierson and Costas’ (1987) classification it is considered that most recent Scottish
flow landslides would fall within the Debris Flow category - which description is adopted as
an all-encompassing term for this work. Peat flows are discussed further in Section 4.3.4.
4.2 DEBRIS FLOWS
4.2.1 Debris Flow Materials
Debris flows usually comprise a mixture of fine (clay, silt and sand) and coarse (gravel,
cobbles and boulders) materials with a variable quantity of water. The resulting mixtures
often behave like viscous “slurries” as they flow down slope. They are often of high density,
60% to 80% by weight solids (Varnes, 1978; Hutchinson, 1988), and may be described as
being analogous to “wet concrete” (Hutchinson, 1988).
Debris flows are potentially very destructive as they cause significant erosion of the
substrates over which they flow, thereby increasing their sediment charge and further
increasing their erosive capabilities. The density and rapid movement of debris flow materials
yield a mass with significant energy (Table 4.2). This has the ability to pick-up and transport
even large and/or well secured objects, thereby giving rise to the potential for significant