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approachable landslide events and amongst them 76 events were treated as reacti-
vated (not 70 m away from old slided area) and 52 as fresh events (70 m away from
the old slided area) (Appendix D, Table D.1). Out of 16 prominent landslide events
year during the period 1968
2011, 12 years had been recognized as the major
landslide hazard risk events years because in these years destructive landslide events
completely cut-off communication lines, destroyed human settlements, reduced tea
garden area and threatened human lives and properties severely. Considering 36
landslide hazard risk events which occurred within 12 landslide risk event years at
different parts of the Shivkhola
-
values were derived and probable chances of
future hazard risk events were estimated for each landslide hazard risk zone.
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7.2.2.5 Accuracy Assessment of the Landslide Hazard Risk Map
with Field Data (GPS Survey)
The accuracy assessment of the landslide hazard risk map was made by using Erdas
Imagine (8.5). Accuracy assessment is a general term for comparing the classifi-
-
cation with geographical data that are assumed to be true, in order to determine the
classi
cation process. Basically, the true data were derived for ground truth veri-
fication with the help of GPS from the existing/active 50 landslide location with risk
elements (settlement, road and tea and agriculture). Simultaneously, a set of ran-
domly selected 50 reference pixels points from the classi
ed image corresponding
to the true data (GPS record) were used for evaluating the validity of landslide
hazard risk map (Congalton
1991
).
7.3 Result and Discussion
7.3.1 Relationship Between Landslide Susceptibility
and Landslide Triggering Factors
Landslide susceptibility map of the Shivkhola Watershed was the product of an
interaction between factors and existing landslide. Slope gradient (Fig.
2.6
, Chap. 2)
of the watershed varies from very gentle gradient (around 10
°
) in the mid central and
mid-lower part to that of high (more than 60
), towards the marginal part/water
divide. Most of the landslide phenomena were found in the area of above 35
°
slope
gradient where class weight value ranges between 5.35 and 77.56 (Appendix G,
Table G.1). South, south east, north, east and north easterly facing slope (Fig.
2.7
,
Chap. 2) were registered with highest class weight values of 29.23, 23.63, 69.63, 53.
51, and 14.86 respectively. All these slope facets were associated with moderate to
high landslide susceptibility and large number landslide events. The derived
°
and
class weight values revealed that high to very high landslide susceptibility zones are
characterized by high positive and negative curvature. Lower Paglajhora, Gayabari
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