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

Day 3
occurs). In most documented cases, the breach
results from fluvial erosion of the landslide
material; the head cutting typically originates
at the toe of the dam and moves progressively
upstream to the lake. When it reaches the lake,
breaching occurs. The breach commonly does
not erode down to the original level of the river
bed, as many landslide dams contain some
coarse material that is not swept away. Thus,
smaller lakes can remain after dam failure.
A landslide dams that have steep upstream
and downstream faces and that also have high
pore-water pressure are susceptible to slope
failure. If a dam has a narrow cross-section or
progressive slope failure, the crest may fail,
leading to overtopping and breaching. Nearly
all faces of landslide dams are at the angle
of repose of the material or less; however,
because they are formed dynamically, slope
failures are rare. A special type of slope failure
occurs when there is lateral erosion of the dam
by a stream or river.
Figure 18: Modes of failure of landslide dams
% of all events
70
60
50
40
30
20
10
0
60
50
Overtopping Piping Slope failure Physically
controlled
(did not fail)
Modes of failure
Source: Based on Costa and Schuster (1988)
Figure 19: Causes of landslides that form dams
Session 14
RM 14.2: Process of landslide
damming and factors that cause
landslide dams
Both natural and anthropogenic factors can
initiate dam-forming landslides. The most
important natural processes are excessive
precipitation (rainfall and snowmelt) and
earthquakes.
Source: Based on Costa and Schuster (1988)
Globally, about 50% of dam-forming landslides
are caused by rainstorms and snowmelt, and
about 40% are caused by earthquakes (Figure 19). In the HKH region, volcanic eruptions are rare, so most
landslide dams are caused by rainfall, snowmelt, and earthquakes.
% of all events
40
30
20
10
0
Rainstorm
and snowmelt
Earthquakes
Modes of failure
Volcanic
eruptions
Others
Landslide dams are frequent in steep, narrow
valleys bordered by high rugged mountains
(Table 11). This setting is widespread in
geologically active areas such as the HKH
where earthquakes are common and mountain
slopes are steepened by glacial activity. Steep,
narrow valleys are dammed by a relatively
small volume of material; thus, even small
mass movements can cause landslide dams.
Large landslide dams are caused by complex
landslides that start as slumps of slides and
become rock or debris avalanches.
Table 11: Factors causing landslide dams
Natural
Anthropogenic
High relief
Undercutting of river banks
Deforestation
Improper land use
Weak geology
High weathering
• agriculture on steep
slopes
• irrigation of steep slopes
Intensive rainfall
• overgrazing
High snowmelt
• quarrying
Poor sub-surface drainage
Construction activities
Seismic activities
Source: Shrestha (2008)
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