Geoscience Reference
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due to their large size. The most famous debris avalanche occurred at Mount St
Helens during the massive eruption in 1980. The debris avalanche had a volume of
about 1 km 3
180 mph), and covered an
area of 62 km 2 (24 sq mi), killing 57 people (Table 1.3 ).
The major causes of the landslides that may induce the slope materials to move
downward in the representative mountain watershed of the Shivkhola Watershed of
Darjiling Himalaya are:
(0.24 cu mi), traveled at 50
80 m/s (110
-
-
Average inclination of rocks is generally steep towards the roadside (60
70
°
).
￿
-
Weathering due to diurnal and seasonal ranges of temperature causes rapid
disintegration and decomposition of the rock body.
￿
Due to intensity of local rainfall, the feldspar present in quartzite, decompose to
form kaolin which helps rock particle to slide down.
￿
Continuous and heavy rain during monsoon period reduces cohesion of slope
materials.
￿
Percolation of high intensity rain water through joints and cracks increases the
hydrostatic pressure within the soils and bring a change in the consistency of the
soil.
￿
Table 1.3 Basic causes of landslide
Geological causes
Morphological
cause
Physical causes
of landslide
Human intervention
Weathered materials
e.g. heavy rainfall
Slope angle
Intense rainfall
Excavation
Sheared materials
Uplift
Rapid snow
melt
Loading
Jointed or ssured
materials
Rebound
Prolonged
precipitation
Draw-down
Adversely orientated
discontinuities
Fluvial erosion
Rapid
drawdown
Land use (e.g. construction
of roads, houses etc.)
Permeability contrasts
Wave erosion
Earthquake
Water management
Material contrasts
Glacial erosion
Volcanic
eruption
Mining
Rainfall and snow fall
Erosion of lat-
eral margins
Thawing
Quarrying
Earthquakes
Subterranean
erosion
Freeze-thaw
Vibration
Working of
machinery
Slope loading
Ground water
changes
Water leakage
Vegetation
change
Soil pore water
pressure
Deforestation
Erosion
Surface runoff
Land use pattern
Seismic activity
Soil erosion
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