Geoscience Reference
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5.3.2 Calculated Critical Rainfall to Initiate Debris Slide
The calculated critical rainfall (Table 5.4 ) of two major landslide prone parts of the
Shivkhola watershed is 105.88 mm/day (Tindharia T.E.) and 88.93 mm/day (Lower
Paglajhora).
5.3.2.1 Return Period of Rainfall
The return period of the total of the catastrophic rainfall and average daily rain of
catastrophic days was calculated on the basis of the rainfall data during 2001
2006
-
following Gumbel ( 1954 ) (Table 5.5 ).
T
¼
ð
N
þ
1
Þ=
m
ð 5 : 16 Þ
T Return period
N No of years
m Rank in ascending order
(Gumbel 1954 )
5.3.2.2 Probabilistic Recurrence Interval of Rainfall
The calculation of recurrence interval of the total of the catastrophic rainfall and
average daily catastrophic rain of days recording more than the calculated threshold
rain is done by log probability law following Chow ( 1951 ), ( 1954 ) and Schwab
et al. ( 2002 ).
Xc
¼
x1
ð
þ
CvK
Þ
ð 5 : 17 Þ
Xc Calculated rainfall
X mean value
Cv Coef
cient of variation
K
Log probability Frequency Factor (calculated from the table of Chow 1954 ).
The daily average catastrophic rain (more than the calculated threshold) that can
be experienced at a recurrence interval of 20 years (with 5 % probability) is
164.97 mm and that at a recurrence interval of 5 years (with 20 % probability) is
131.793 mm (Table 5.6 ).
The calculation shows that 105.88 and 88.928 mm daily rainfall is the threshold
rain for Tindharia and Paglajhora respectively and the analysis of return period
shows that 120.7 mm daily rainfall can occur at a recurrence interval of 1.4 years
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