Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 181

underlying rock (Bell, 1998). Debris slides are usually composed of mud or other
fine-grained sediments with coarser-grained material such as gravel and boulders
carried in suspension; they move rapidly downslope.
The mass movements occurring in flows resemble that of a viscous fluid
(Fig. 7. 1 ). In this situation intergranular movement is more important than shear
surface movement. As a consequence, slip surfaces are either not present or
short-lived. The boundary between a debris flow and the underlying layer can be
distinct or alternatively characterised by a zone of plastic flow. Although some
waterisnecessary to induce most flow events, dry flows can occur, although
these usually consist of rock fragments and result from slides or falls. Dry flows
are usually rapid and short-lived and often occur in rugged mountain terrain.
Wetflowsusually occur in fine-grained sand and silt with or without coarser-
grained material which is highly saturated. Hence they result essentially from
an excess of water. Debris flows differ from mud flows in that the former have a
higher percentage of coarse-grained material than the latter and usually occur
after heavy rainfall or sudden thaw of frozen ground. Debris flows have a high
density and have the capacity to carry large fragments such as boulders. Mud
flows generally occur where large volumes of rapidly running water mix with
debris to form a pasty mass. Mud flows have the capacity to move down shal-
low slopes. In these situations a forward thrust develops due to the undrained
loading at the rear of the flow where the basal shear surface is inclined steeply
downwards. Not only does this process result in further instability of the load-
ing, it also results in increased pore-water pressure along the back part of the
slip surface.
Earth flows result from increased pore-water pressure causing a decrease in
shear resistance. Saturated flow material can result in the frontal part of the
flow being recognised by a bulging lobe that may split into several tongues of
flow that move in a steady rolling motion. Earth flows may form at the toe of
rotational slides and represent a transitional movement between a slide and a
flow. They may be rapid or slow moving depending on the water content of the
moving mass. High water content usually results in fast earth flows, and the
opposite is true of lower water contents. Earth flows, particularly ones with low
water content, can continue to move for long periods of time (up to years) (Bell,
1998).
Magnitude of historical landslides
One of the largest landslides in historical times occurred in 1911 at
Usoy in the Parmir Mountains of Russia. The landslide was generated by an