Flash Flood Risk Management – A Training of Trainers ... - ReliefWeb

Day 3
Figure 15: Types of landslides
a. Fall b. Topple c. Slide
Talus
d. Spread
e. Flow
Original Position
Session 14
Moving Mass
Source: Deoja et al. (1991)
Flows. Flows move like a viscous fluid, sometimes very rapidly, and can travel several miles. Water is not
essential for flows to occur, although most flows form after periods of heavy rainfall. The different types of
flow include earth flow, mud flow, debris flow, debris avalanche, and creep. Mud flows contain at least 50%
sand, silt, and clay particles. A debris flow is a slurry of soil, rock, and organic matter combined with air and
water. Debris flows usually occur on steep gullies. Creep is a very slow, almost imperceptible flow of soil and
bedrock.
Formation and types of landslide dams
In general, high landslide dams form in steep-walled, narrow valleys because there is little area for the
landslide mass to spread out (Costa and Schuster 1988). Commonly, large landslide dams are caused by
complex landslides that start as slumps or slides and transform into rock or debris avalanches. Excessive
precipitation and earthquakes are the most important triggers that can initiate landslides that form dams
(Figure 16). Other mechanisms include stream under-cutting and entrenchment.
Landslide dams can be classified geomorphologically according to their orientation relative to the valley floor
(Swanson et al. 1986, in Costa and Schuster 1988) and factors causing their formation (Figure 17,
Table 10).
Modes of failure of landslide dams and triggering factors
A natural landslide dam differs from a constructed dam in that it is made up of a heterogeneous mass of
unconsolidated or poorly consolidated material and has no proper drainage system to prevent piping and
control pore pressure. It also has no channelised spillway or other protected outlet; as a result, landslide dams
commonly fail by overtopping (Figure 18) (after which the overflow water erodes the dam and breaching
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