Geomorphic Threshold and Landslide - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 144

(cumulative rainfall, antecedent rainfall, intensity and duration of rainfall) in rela-
tion to landslide phenomena has been attempted to identify in the present study.
Starkel ( 1972 ) for the

first time, observed the geomorphic effects of an extreme
rainfall event in the eastern Himalaya (mainly Darjiling), India. Froehlich et al.
( 1990 ), investigated the same area and found that shallow slides and slumps on
steep slope segments occured when 24 h rainfall reaches 130 150 mm or contin-

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uous 3 days rainfall totals 180 200 mm. Campbell ( 1975 ), Cotecchia ( 1978 ), Caine

( 1980 ), Innes ( 1983 ), Pomeroy ( 1984 ), Cannon and Ellen ( 1985 ), Neary and Swift
( 1987 ) Keefer et al. ( 1987 ), Cannon ( 1988 ), Kim et al. ( 1991 ), Li and Wang ( 1992 ),
Ceriani et al. ( 1992 ), Larsen and Simon ( 1993 ), Wilson and Wieczoreck ( 1995 ),
Wieczorek ( 1987 , 1996 ), Wieczorek et al. ( 2000 ), Terlien ( 1997 , 1998 ), Crosta
( 1998 ), Crozier ( 1999 ), Glade et al. ( 2000 ), Crosta and Frattini ( 2001 ), Aleotti
( 2004 ), Guzzetti et al. ( 2004 ), Gabet et al. ( 2004 ), Giannecchini ( 2006 ), Glade
( 1998 ), Zezere et al. ( 2005 ), Cardinali et al. ( 2006 ) and Dahal et al. ( 2006b ) tried to
establish rainfall-intensity thresholds for predicting the slope failure accurately.
Caine ( 1980 )

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first established worldwide rainfall threshold values for landslides.
Montgomery and Dietrich ( 1994 ) introduced a physically based model for the
topographic control on shallow landsliding in terms of geomorphic threshold.
Recently Guzzetti et al. ( 2007 ) reviewed rainfall thresholds for the initiation of
landslides worldwide and proposed new empirical thresholds based on the statis-
tical analysis of the relationship between rainfall and landslide occurrences. They
de

ned intensity-duration threshold as:

90D 0 : 79 I ¼ 73 : ð 5 : 1 Þ

where, I is the hourly rainfall intensity in millimeters (mm hr-1) and D is duration in
hours.
Brunsden et al. ( 1981 ), Manandhar and Khanal ( 1988 ), Dhital et al. ( 1993 ),
Upreti and Dhital ( 1996 ), Gerrard and Gardner ( 2000 ), Dhital ( 2003 ), Dahal et al.
( 2006a ), and Dahal ( 2006 ) while other works, such as Caine and Mool ( 1982 ),
Dhakal et al. ( 1999 ), De Vleeschauwer and De Smedt ( 2002 ), De Smedt ( 2005 )
focused mainly on landslide risk assessment in Himalayan terrains by analyzing
physical properties of landslides and debris flows, effects of regional and local
geological settings, and recommendations for environmental-friendly preventive
measures.
In the Shivkhola watershed of Darjiling Himalaya, the physical processes and
human actions (formation of road-cut benches and concentration of human settle-
ment) are active on the slope in a systematic interactive combination which make
the slope steeper than repose angle and thus the instability is introduced into the
system. The present study attempts to identify the critical values of rainfall, slope
height and slope angle beyond which there is a greater probability of slope insta-
bility. The formation of road-cut benches to develop communication network
lengthens the steep slope, removes the lateral and basal support, and disturbs the
soil, favours in

ltration and through-flow, helping in the increase of wet soil depth.

All those changes and their combined manifold after-effects help to generate