Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 28

Considerable efforts have been made to understand the triggers for landsliding in
natural systems, with quite variable results. Simon et al. ( 1990 ) found

that storms with a total precipitation of 100 200 mm, about 14 mm of rain per hour

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for several hours, or 2 3 mm of rain per hour for about 100 h can trigger landslides

in that environment. Corominas and Moya (1999) investigated the upper basin of
the Llobregat River, Eastern Pyrenees area and found that without antecedent
rainfall, high intensity and short duration rains triggered debris flows and shallow
slides developed in colluvium and weathered rocks. A rainfall threshold of around
190 mm in 24 h initiated failures whereas more than 300 mm in 24

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48 h were
needed to cause widespread shallow landsliding. With antecedent rain, moderate
intensity precipitation of at least 40 mm in 24 h reactivated mudslides and both
rotational and translational slides affecting clayey and silty-clayey formations. In
this case, several weeks and 200 mm of precipitation were needed to cause land-
slide reactivation.
Rapid changes in the groundwater level along a slope can also trigger landslides.
This is often the case where a slope is adjacent to a water body or a river. When the
water level adjacent to the slope falls rapidly the groundwater level frequently
cannot dissipate quickly enough,

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cially high water table. This
subjects the slope to higher than normal shear stresses, leading to potential insta-
bility. In some cases, failures are triggered as a result of undercutting of the slope by
a river, especially during a flood. This undercutting serves both to increase the
gradient of the slope, reducing stability, and to remove toe weighting, which also
decreases stability. For example, in Nepal this process is often seen after a glacial
lake outburst flood, when toe erosion occurs along the channel. Immediately after
the passage of flood waves extensive landsliding often occurs. This instability can
continue to occur for a long time afterwards, especially during subsequent periods
of heavy rain and flood events.
The second major factor in the triggering of landslides is seismicity. The passage
of the earthquake waves through the rock and soil produces a complex set
of accelerations that effectively act to change the gravitational load on the slope. So,
for example, vertical accelerations successively increase and decrease the normal
load acting on the slope. Similarly, horizontal accelerations induce a shearing force
due to the inertia of the landslide mass during the accelerations. These processes are
complex, but can be suf

leaving an arti

cient to induce failure of the slope. These processes can be
much more serious in mountainous areas in which the seismic waves interact with
the terrain to produce increases in the magnitude of the ground accelerations.
Some of the largest and most destructive landslides known have been associated
with volcanoes. There are two main types of volcanic landslide: lahars and debris
avalanches, the largest of which are sometimes termed as flank collapses.An

example of a lahar was seen at Mount St Helens during its catastrophic eruption on
May 18, 1980. The lahar killed more than 2,000 people as it swept over the towns
of El Porvenir and Rolando Rodriguez at the base of the mountain. Debris
avalanches commonly occur at the same time as an eruption, but occasionally they
may be triggered by other factors such as a seismic shock or heavy rainfall. They
are particularly common on strato volcanoes, which can be massively destructive

fl