Geoscience Reference
In-Depth Information
factors have recognized Lower Paglajhora, Tindharia and Shiviter as high to very
high landslide hazard risk zone in the Shivkhola Watershed.
The thickness of the soil and that of the saturated soil during monsoon are
measured to be 4.5 m (Shiviter T.E.) and 7.25 m (Lower Paglajhora) and 1.28 m
(Shiviter T.E) and 1.30 m (Lower Paglajhora) respectively at the back wall of the
landslide scar though it is a bit lower further upslope on steeper section. The wet soil
buck density is measured to be 1.96 g/cc and density of water is 1.07 g/cc. The angle
of internal friction varies from 19
. The calculated
critical rainfalls of two major landslide prone parts of the Shivkhola watershed are
105.88 mm/day (Shiviter T.E.) and 88.93 mm/day (Lower Paglajhora) after Borga
et al. (
1998
). Following Chow (
1951
,
1954
,
1964
) the calculated rainfall of
90.539 mm which is less than the critical rainfall of those two places at the
recurrence interval of 1.01 year with 99 % probability.
The determined safety factor (FS) of all three locations are less than
°
to 23
°
with an average of 21
°
which is
measured considering the stress parameters such as major principal stress (
'
1
'
r
1
),
minor principal stress (
r
3
), normal stress (
r
n
), shear stress (
s
), angle of internal
friction (
ʦ
), cohesion (C), shear strength (
ʻ
), and rupture angle (
ʱ
) applying Direct
Shear Test Mechanism and developing Mohr
s Stress Circle (Table
7.9
). All the
above mentioned parameters have recognized Paglajhora, Tindharia and Shiviter as
signi
'
cant unstable sections in the Shivkhola watershed. Not only that the slope
steepening developed by road-cut benches and toe erosion, plying of heavy loaded
vehicles and its enormous pressure on more fragile slope materials, depletion of
forest cover in a rapid pace, continuous and regular orographic rainfall in rainy
season, easy percolation of water through fragmented rock-soil composition and
increase pore water pressure have caused destructive slope failure and damaged
human structure and disrupted normal life by cutting-off the communication lines at
all these three locations and have treated them as most signi
cant landslide hazard
risk prone sectors of the Shivkhola Watershed.
A comparative study has been made here to establish the interrelationship
between all the landslide triggering factors (slope angle, slope curvature, slope
aspect, lithology, drainage density, upslope contributing area, settlement density,
road contributing area and land use and land cover.) considered in the present study
as well as to
figure out the different levels of abstraction of all the parameters in
landslide hazard risk zones (Table
7.8
). Study revealed that high and very high
landslide hazard risk zones covering the places of Pahlajhora sinking zone,
Tindharia, Shiviter, and 14 Miles Bustee are closely associated with 35
slope
angle; north-east, south-east, north, east and south slope aspect; high posivite and
high negative curvature; drainage density of 3.5
45
°
-
6.5 km/km
2
; upslope area of more
than 5 km
2
; high settlement density; moderate to high road contributing area; and
land use/land cover of
-
settlement,
road, degraded forest and open forest
(Table
7.10
).
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