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Table 6.4 Frequency ratio analysis for dry, semi-saturated and saturated soil condition
Safety
factor
Landslide
susceptibility
Area in
sq km
Percentage
of area
Landslide
frequency
Frequency
ratio
Frequency ratio analysis under dry condition
0.158
0.40
Very high
0.320933
1.58
8 (33.33 %)
21.09
-
0.40
0.70
High
5.045979
24.88
9 (37.5 %)
1.50
-
0.70
-
1.00
Moderate
6.136645
30.25
5 (20.83 %)
0.67
1.00
1.50
Low
6.602401
30.51
2 (8.33 %)
0.27
-
1.50
2.83
Very low
2.188175
10.79
0 (0.00 %)
0.00
-
Frequency ratio analysis under semi-saturated condition
0.158
-
0.40
Very high
0.88796
4.38
9 (37.5 %)
8.56
0.40
0.70
High
7.691486
37.88
10 (41.67 %)
1.10
-
0.70
1.00
Moderate
6.393539
31.47
4 (16.67 %)
0.53
-
1.00
-
1.50
Low
4.15209
20.44
1 (4.16 %)
0.20
1.50
2.83
Very low
1.169058
5.76
0 (0.00 %)
0.00
-
Frequency ratio analysis under saturated condition
0.158
-
0.40
Very high
2.430299
11.97
12 (50 %)
4.17
0.40
0.70
High
7.985823
39.36
8 (33.33 %)
0.85
-
0.70
1.00
Moderate
6.406416
31.58
3 (12.5 %)
0.39
-
1.00
-
1.50
Low
2.763299
13.60
1 (4.17 %)
0.31
1.50
2.83
Very low
0.708297
3.50
0 (0.00 %)
0.00
-
greater than dry condition (Fig.
6.4
). 37.88 % area of the basin is dominated by high
landslide susceptibility and 31.47 % area is registered with moderate landslide
susceptibility and equal chances of landslide occurrence phenomena. About 25 %
of the watershed is characterised with low to very low landslide susceptibility
condition. Frequency ratio value under semi-saturated condition revealed that the
probability of landslide occurrence was very high in the area of very high landslide
susceptibility which was followed by high, moderate and low. Under dry condition
moderate, low and very low landslide susceptibility area did not have experienced
major landslide events and so derived frequency ratio values tent to
'
0
'
.
6.5.3.3 Stability Analysis Under Saturated Condition
Under the complete saturated soil condition pore water pressure becomes very high
and reduces the cohesive strength of the soil and mountain slope become most
unstable. The values of safety factor under saturated condition ranges between 0.13
and 2.30. The area of very high landslide susceptibility had been increased in
comparison to dry and semi-saturated condition. More than 50 % area of the
Shivkhola watershed is attributed by high to very high landslide susceptibility.
31.585 % areas are under moderate landslide susceptibility where the probability of
landslide activities is low. Under the saturated condition 11.97 % area with very
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