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more from 1, showing most irregularity in shape. Besides, sub-basin 2 and 4 are
also indicating the lower values of circularity index and consequently revealing
another two irregular segments of the siva-khola watershed. The sub-basin 3, 5 and
6 are more regular.
The ranks were assigned to the sub-watersheds according to their forms and
relative shapes in ascending order of magnitude favouring easy drainage. Sub-basin
6 was thus ranked V for maximum form factor. All the ranks for Form Factor,
Elongation ratio, Circularity Ratio, Elipticity Index, Compactness Coef
cient and
length of overland flow were assigned accordingly and
finally cumulative value
were assigned as the composite ranking for drainage ef
ciency. Thus sub-basin 5
and 1 are more ef
cient in drainage followed by sub-basin 4 (Table 3.8 ).
3.5 Conclusion
Rainfall is the signi
cant landslide triggering factor in the Shivkhola Watershed as
most of the landslides are rain-induced. Not only that, all the recorded landslides
occurred during the excess moisture period. The analysis of drainage network of
1972 and 1987 depicts that there is continuous branching of channels and these are
developed through headward erosion and sharpening of interfluves between two
sub-watersheds. Sub-watershed 1 and 6 are attributed with maximum number of
stream, stream length, and mean area at each orders which are the indicators
of more surface run-off and potential erosion and slope failure. Maximum number
of stream confluence points is found in sub-watershed 1 and 6 which is followed by
3, 2, 4 and 5. The potential slope failure zones are extended along the drainage lines
and concentrated at the source points. The greater drainage density is an indicator of
potential instability. At the zones near Paglajhora, mid-central areas, numbers of
drainage channels from the upper marginal watershed region meet the main stream.
Here drainage density is maximum indicating the excess surface water. The max-
imum LPI is recorded within the range between 5.00 and 6.50 km/km 2
and is
5.00 km/km 2 showing the effects of intensity of drainage density
on slope instability. In the saucer shaped basin, the concentration of flow at the
central lower portion caused the maximum density but due to gentle slope, the
landslide occurrences are less. The moderate density on steeper slope caused havoc
on slope instability due to cumulative effects of other triggering factors. The surface
water accumulates in a cumulative rate away from the water divide as more Upslope
Contributing Area (UCA) helps in accumulation of more water that indicates more
surplus moisture and instability of slope and soil. Upslope Contributing Area of
20.98 km 2 is registered at the lower most portion of every sub-watershed. The more
contributing areas are registered along the main river which experience maximum
length and thus maximum flow. The percentage of occurrence of landslides and its
distribution among the different groups of contributing area shows increasing
Landslide Potentiality Index (LPI) with increasing value of upslope contributing
area. The regions of upslope contributing area of less than 5 Km 2 experiences the
followed by 3.50
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