Geomorphic Threshold and Landslide - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 156

Table 5.6 Amount of rain
fall at certain probability and
with speci c return period
(after Chow 1951 , 1954 )

P % T (Years) K Xc (mm)

99 1.01 − 2.001 90.539

50 2 − 0.083 128.507

20 5 0.083 131.793

5 20 1.759 164.971

1 100 2.669 182.985

following Gumbel ( 1954 ) and 128.507 mm daily rain has a recurrence interval of
2 years with 50 % probability following Chow ( 1951 ), ( 1954 ). That means there is
every possibility for the generation of geomorphic threshold for initiation of slide
due to hydrologic factor. At Paglajhora the critical rainfall is 88.93 mm which is
less than the estimated rainfall of 90.54 mm at the recurrence interval of 1.01 year
with 99 % probability. So it can be inferred that Paglajhora is a place of higher
probability of rainfall triggering landslide phenomena in every rainy season (Maiti
2007a , b ).

5.3.2.3 Relationship Between Major Landslide Events and Triggering
Antecedent Rainfall in the Shivkhola Watershed (Based on July
Rainfall)

A relationship between antecedent cumulative rainfall and landslide events of 1993,
1998, 2003, 2007 and 2010 was established on the basis of the data recorded from
earlier research works done by Ghosh et al. ( 2009b ) and the collection of rainfall
data from nearby Selim Hill Tea Estate by author himself. Only two days ante-
cedent cumulative rainfall of 211.3 mm invited the slope failure at several places of
Tindharia and Gayabari and Mahanadi (Table 5.7 ) in 1993. The 1998 landslide
event took place due to 300

- 600 mm cumulative rainfall in the past 2/3 days. The

2 days antecedent cumulative rainfall of 390 mm was responsible for 1998 land-

slide events. The major event of 2003 happened due to incessant rainfall of 500 mm
in 2 days. 17 and 18th July, 2007 received rainfall of 124.5 and 100 mm respec-
tively. These 2 days

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antecedent cumulative rainfall of 224.5 mm caused havoc
slope failure at Tindharia and Upper and Lower Paglajhora. Again 2007 faced
landslide events on 8th September when 6, 7 and 8th September ' s antecedent
cumulative rainfall amount was 275 mm.
In 2010, major and prominent landslide events happened as a result of 5 days

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rainfall of 345 mm at 14 Mile near lower Paglajhora, Nurbong, Gitingia, and
Shiviter. Antecedent Cumulative rainfall induced landslide analysis shows that the
continuous and uniform rate of minimum amount of rainfall (approx. less than
80 mm/day) for few consecutive days can cross the geomorphic threshold and can
introduce slope instability condition.