Geomorphic Threshold and Landslide - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 150

data since 1968 (Table D.1, See Appendix D). To determine the temporal proba-
bility of rain-induced landslide events the exceedance probability of one or more
landslides were attempted by considering the landslide as random point events.
Two major discrete probability distribution models such as the

' Poisson distribu-

tion were mostly applied to calculate the exceed-

ance probability of landslide (Coe et al. 2004 ; Crovelli 2000 ). According to
'

' and the ' Binomial distribution '

Poisson distribution model the occurrences of landslide events that is experi-

'

encing ' n ' landslides during the time ' t ' could be expressed by:

n

e ðk t Þ ðk

t Þ

P ½ N L t ðÞ [ ¼ n ¼ ð 5 : 13 Þ

n !

where, ʻ = average rate of landslide occurrence.
n = 0, 1, 2, 3, n.

So, the exceedance probability or the probability of experiencing landslide
events during the time

…

' t ' could be expressed as

e t e k t PN L t ½ ðÞ [ ¼ 1 ¼ 1 PN L t ½ ðÞ ¼ 0 ¼ 1 ¼ 1 ð 5 : 14 Þ

l

1
k

where, = mean recurrence interval between successive landslide events.

In the same way, the exceedance probability could be assessed by using the
binomial probability distribution model with the help of following expression.

l ¼ and

µ

t

1
l

t

ðÞ [ ¼ ¼ 1 ðÞ ¼ ¼ ð Þ ð 5 : 15 Þ

PN L t ½ 1 PNLt ½ 0 1 1 P ¼ 1 1

To estimate the temporal probability of the landslide events in the Shivkhola
watershed, the mean recurrence interval of known landslide event years was
deduced that is 2.75 (16 known events year in 44 years). In the same way, mean
recurrence interval of known major landslide events was also deduced that is 3.66
(12 major events year in 44 years). Then, both Poisson and Binomial distribution
models were being applied to determine the exceedance probability.

5.3 Result and Discussion

5.3.1 Geo-technical Properties and Critical Slope Angle

The investigation with the temporal change in the slide scar reveals that the slope
evolution is subjected to a complex interaction between physical and anthropogenic
processes. The human actions in various developmental activities lead to the
development of geomorphic threshold in the form of slope steepness, slope height
and threshold rainfall.